New results reported in two papers in Nature from the CLOUD experiment at CERN imply that the pre-industrial climate may have had brighter and more extensive clouds than previously thought, sharpening our understanding of the impact of human activities on climate. CLOUD (Cosmics Leaving Outdoor Droplets) is designed to understand how aerosol particles form and grow in the atmosphere, and the effect this has on clouds and climate. It comprises a 26 m3 vacuum chamber containing atmospheric particles, into which beams of charged pions are fired from the Proton Synchrotron to mimic the seeding of clouds by galactic cosmic rays.
The increase in aerosols and clouds since pre-industrial times is one of the largest sources of uncertainty in climate change, according to the Intergovernmental Panel on Climate Change. The new CLOUD results show that organic vapours emitted by trees produce abundant aerosol particles in the atmosphere in the absence of sulphuric acid. Previously, it was thought that sulphuric acid – which largely arises from burning fossil fuels – was essential to initiate aerosol particle formation. CLOUD finds that oxidized biogenic vapours dominate particle growth in unpolluted environments, starting just after the first few molecules have stuck together and continuing all the way up to sizes above 50–100 nm, where the particles can seed cloud droplets.
The experiment also finds that ions from galactic cosmic rays enhance the production rate of pure biogenic particles by a factor of 10–100 compared with particles without ions, which suggests that cosmic rays played a more important role in aerosol and cloud formation in pre-industrial times than they do in today’s polluted atmosphere.
CLOUD, which has produced a series of high-impact publications following its first results in 2011, is the first experiment to reach the demanding technological performance and ultralow contaminant levels necessary to be able to measure aerosol nucleation and growth under controlled conditions in the laboratory.
CERN’s pioneering AWAKE facility, which aims to drastically reduce the scale of particle accelerators, received its first beam on 16 June. The milestone signals the next stage of commissioning for the novel experiment, which aims to use plasma wakefields driven by a proton beam to accelerate charged particles to high energies over very short distances. The proton beam had to travel around 800 m before entering a 10 m-long plasma cell, which is empty during the current commissioning phase, and then carry on downstream to several detectors. The test was a success, with protons striking the detector straight away, and the team now plans to finalize installation of the experiment, the laser and the full plasma cell. AWAKE hopes to start collecting physics data by the end of the year.
The origin of some of the heaviest chemical elements is due to rapid neutron capture, but the precise location where this cosmic alchemy takes place has been under debate for several decades. While core-collapse supernovae were thought to be the prime production site, a new study suggests that elements heavier than zinc originate from the merger of two neutron stars. Such a dramatic event would have been responsible for the extreme heavy-element enrichment observed in several stars of an ancient dwarf galaxy called Reticulum II.
Nuclear fusion in the core of massive stars produces elements up to and including iron, which is a stable nucleus with the highest binding energy per nucleon. Building heavier nuclei requires energy to compensate for the loss of nuclear binding and is therefore almost impossible to achieve experimentally. But under certain conditions, stars can produce heavier elements by allowing them to capture protons or neutrons.
The relative abundance of certain elements therefore tells researchers whether nucleosynthesis followed an s- or an r-process.
Neutron capture, which is unaffected by Coulomb repulsion, occurs either slowly (s) or rapidly (r). Slow neutron captures occur at a pace that allows the nucleus to undergo beta decay prior to a new capture, and therefore to grow following the line of nuclear stability. The r-process, on the other hand, causes a nucleus to accumulate many additional neutrons prior to radioactive decay. The relative abundance of certain elements therefore tells researchers whether nucleosynthesis followed an s- or an r-process. The rare-earth element europium is a typical r-process element, as are gold, lead and uranium.
For the r-process to work, nuclei need to be under heavy neutron bombardment in conditions that are only found in dramatic events such as a core-collapse supernova or in mergers of two neutron stars. The supernova hypothesis has long been the most probable candidate for the r-process, whereas other scenarios involving rarer events, such as encounters between a neutron star and a black hole, have only been considered since the 1970s. One way to distinguish between the two hypotheses is to study low-metallicity galaxies in which the enrichment of heavy elements is low. This enables astrophysicists to determine if the enrichment is a continuous process or the result of rare events, which would result in stronger differences from one galaxy to the other.
Alexander Ji from the Massachusetts Institute of Technology, US, and colleagues were lucky to find extreme relative abundances of r-process elements in stars located in the ultra-faint dwarf galaxy Reticulum II. Although nearby and in orbit around the Milky Way, this galaxy was only recently discovered and found to be among the most metal-poor galaxies known. This means that Reticulum II formed all of its stars within about the first three-billion years after the Big Bang, and is therefore only enriched in elements heavier than helium by a few generations of stars.
High-resolution spectroscopic measurements of the nine brightest stars in Reticulum II carried out by the team indicate a very strong excess of europium and barium compared with iron in seven of the stars. These abundances exceed by two-to-three orders of magnitude those in any other ultra-faint dwarf galaxy, suggesting that a single rare event produced these r-process elements. The results also show that this event could be a neutron-star merger, but not an ordinary core-collapse supernova. Although it is not possible to conclude that the majority of our gold and uranium comes from neutron-star mergers, the study certainly gives more weight to such a hypothesis in the 60 year-long debate about the origin of r-process elements.
In July 1956, in a brief paper published in Science, a small team based at the Los Alamos National Laboratory in the US presented results from an experiment at a new, powerful fission reactor at the Savannah River Plant, in South Carolina. The work, they wrote, “verifies the neutrino hypothesis suggested by Pauli”. Clyde Cowan, Fred Reines, Kiko Harrison, Herald Kruse and Austin McGuire had demonstrated for the first time that it was possible to detect neutrinos, setting in motion the new field of neutrino physics. The key ingredients were an intense source and a big detector, with more than a touch of ingenuity and patience.
More than two decades previously, in 1930, Wolfgang Pauli had proposed that the “energy crisis” in nuclear beta decay – presented by the continuous energy spectrum of the emitted electron – would be solved if the decaying nucleus also emitted a second, undetected particle. This would allow the energy released to be shared between three objects, including the recoiling nucleus, and so yield electrons with a range of energies, just as observed. The new particle had to be neutral and have a relatively small mass. Pauli called his proposal “a desperate remedy”, in part because he thought that if such a particle did indeed exist, then it “would probably have long ago been seen”.
Nevertheless, Enrico Fermi took the possibility seriously and based his seminal work on beta decay, published in 1934, on a point-contact interaction in which a neutron decays to a proton, electron and (anti)neutrino: n → p e–ν. Soon afterwards, Hans Bethe and Rudolf Peierls calculated the cross-section for the inverse reaction in which a neutrino is absorbed, but when they found a value of about 10–44 cm2, the pair concluded that no one would be able to detect neutrinos (Bethe and Peierls 1934). What they did not count on was the discovery of nuclear fission – which on a macroscopic scale produces copious numbers of neutrinos – or the ingenuity of experimentalists and, later, accelerator physicists.
Notoriously, nuclear fission was first applied in the atomic bombs used towards the end of the Second World War. A few years later, in 1951, Fred Reines, a physicist who had worked on the Manhattan Project at Los Alamos, began to think about how to harness the neutrinos produced during tests of atomic bombs to make a direct detection of the elusive particle. He was soon joined in this strange pursuit by Clyde Cowan, a fellow researcher at Los Alamos, after they were stranded together at Kansas Airport, where the conversation turned to the “supreme challenge” of detecting neutrinos.
Reines had an idea to place a detector close to a bomb-test tower and use the timing of the detonation as a “gate” to minimise background. But what kind of detector? He and Cowan decided on the recently developed medium of liquid scintillator, which could both act as a target for the inverse beta-decay reaction ν p → e+ n, and detect the emitted positrons via their annihilation to gamma rays. It was an audacious plan, not only in taking advantage of a bomb test but also in scaling up the use of liquid scintillator, which until then had been used only in quantities of about a litre. Reines and Cowan named it “Project Poltergeist”, to reflect the neutrino’s ghostly nature.
Remarkably, the Los Alamos director gave approval for the experiment. However, in late 1952, Cowan and Reines were urged to reconsider the more practical idea of using antineutrinos from a nuclear reactor. The challenge was to work out how to reduce the backgrounds, because the antineutrino flux from a reactor would be thousands of times smaller than that from a nuclear explosion. Reines and Cowan realised that in addition to looking for positron annihilation, they could also detect the neutrons through neutron capture – a process that is delayed for several microseconds, thanks to the neutron’s random walk through a medium prior to interacting with a nucleus. In particular, the addition of cadmium to the detector would increase the likelihood of capture and lead to the emission of gamma rays. The signature for inverse beta decay would then be a delayed coincidence between two sets of gamma rays: one from the positron’s annihilation and the other from the neutron’s capture.
The detector for Project Poltergeist contained 300 litres of liquid scintillator with added cadmium chloride, viewed by 90 photomultiplier tubes, and was set up in 1953 at a new reactor at the Hanford Engineering Works in Washington State. This initial experiment showed a small increase in delayed coincidences when the reactor was operating compared with the situation when it was turned off, but it was set against a cosmic-ray background that was more than 10 times higher than the expected signal rate (Reines and Cowan 1953).
This tantalising result encouraged a still more determined effort, with a new detector design that was basically a sandwich with three layers of liquid scintillator and two layers of water with added cadmium chloride to act as the target (figure 1). Positrons produced in a neutrino interaction would be detected almost immediately via two back-to-back gamma rays in the adjacent scintillator tanks, which would be followed a few microseconds later by another burst of gamma rays in the same two scintillator tanks, this time from neutron capture.
The second experiment ran at the newly completed Savannah River Plant for a total of 1371 hours in 1956 and, when the reactor was on, it recorded nearly three delayed coincidences per hour (Cowan et al. 1956). After completing many checks, on 14 June 1956 Reines and Cowan sent a jubilant telegram to Pauli in Zurich, informing him that they had “definitely detected neutrinos from fission fragments by observing inverse beta decay of protons”. At the time, Pauli was in fact at a meeting at CERN, to where the telegram was forwarded, and he reportedly interrupted the meeting to read out the good news, later celebrating with a case of champagne (Reines 1979).
The move to accelerators
At the time of the neutrino’s discovery, laboratories such as CERN and Brookhaven were on their way to building proton synchrotrons that would have sufficient energy and intensity to form beams of neutrinos via decays of pions and kaons produced when protons strike a suitable target. The muons produced in the decays could be stopped by large amounts of shielding, allowing only neutrinos to penetrate to experiments beyond. At Brookhaven, this led to the discovery at the Alternating Gradient Synchrotron (AGS) in 1962 that the neutrinos produced in association with electrons (as in beta decay) are different from those produced in association with muons (as in pion decay): a second type of neutrino, the muon neutrino, had been discovered.
In 1963, an ingenious way to produce neutrino beams of greater intensity first came into use at the Proton Synchrotron (PS) at CERN, where Simon van der Meer had described his concept of the neutrino horn a couple of years earlier (van der Meer 1961). Because neutrinos are electrically neutral, they cannot be focused into a beam using magnets, so he devised instead a way to focus the parent pions and kaons using magnetic fields set up by currents circulating in a metallic cone-shaped “horn” (CERN Courier June 2011 p24). The device concentrated neutrinos produced as the charged particles decayed in flight into a beam, and because it could focus either positive or negative particles, it produced an almost pure beam of neutrinos (from positive parents) or antineutrinos (negative parents). A second technical innovation at CERN enabled the horn to become a formidable device: the technique of “fast ejection”, devised by Berend Kuiper and Günther Plass, could direct all of the protons from one cycle of the PS onto the target at the mouth of the horn (Kuiper and Plass 1959). By mid-1963, thanks to these innovations, CERN had what was at the time the world’s most intense neutrino beam.
In the 1970s, the combination of the neutrino beam from the PS and Gargamelle – the large bubble chamber built at the Saclay Laboratory by a team led by André Lagarrigue – led to the discovery of weak neutral currents (CERN Courier September 2009 p25), thereby providing crucial experimental support for the unification of the weak and electromagnetic forces. The neutrino experiments with Gargamelle also produced key evidence about the existence of quarks and, in particular, their fractional charges (CERN Courier April 2014 p24). Then, in 1977, the Super Proton Synchrotron (SPS) became the source of neutrino beams at higher energies, and for the next 21 years a series of experiments in CERN’s West Area used neutrinos in experiments covering a broad range of physics, from neutral currents and the quark structure of matter through quantum chromodynamics to neutrino oscillations (CERN Courier December 1998 p28).
Around that time, physicists at Fermilab were closing in on a third neutrino type. The DONUT experiment (Direct Observation of the NU Tau) detected neutrinos produced at the Tevatron, and in 2000, the collaboration announced the discovery of the tau neutrino. Although experiments at CERN’s Large Electron–Positron collider had already established from precise measurements of the Z boson that there are three light neutrino types, the observation of the tau neutrino completed the leptonic sector of the Standard Model.
Ten years later, CERN was again setting records for neutrino beams, with the CERN Neutrinos to Gran Sasso (CNGS) project, which directed an intense beam of muon-neutrinos (νμ) to two experiments, ICARUS and OPERA, in the Gran Sasso National Laboratory in Italy about 730 km away. CNGS followed the same principle as CERN’s early record-breaking beam, this time with protons from the SPS. Following first commissioning in 2006 (CERN Courier November 2006 p20), the facility ran for physics from 2008 to the end of 2012, and achieved a maximum beam power of 480 kW – the most powerful at the time. A total of 18.24 × 1019 protons were delivered on target, and the OPERA experiment detected 19,500 neutrino events – with five among them identified as a tau neutrino (ντ), thereby firmly establishing the direct observation of νμ→ ντ oscillations (CERN Courier July/August 2015 p6).
A bountiful legacy
Since the first glimpses of antineutrino interactions 60 years ago in reactor experiments, experiments have gone on to detect neutrinos and antineutrinos produced in a variety of ways – both in beams created at particle accelerators and also naturally by reactions in the Sun, interactions of cosmic rays in the Earth’s atmosphere and, most recently, astrophysical processes. We now know that neutrinos exist not only in three flavour eigenstates – electron (νe), muon (νμ) and tau (ντ) – but also in different mass eigenstates (ν1, ν2 and ν3) with very small masses, and that they can oscillate from one flavour to another through quantum-mechanical mixing (see “Japan eyes up its future”).
Reactor experiments – in particular Double Chooz in France, the Daya Bay Reactor Neutrino Experiment in China (figure 2) and the Reactor Experiment for Neutrino Oscillation (RENO) in South Korea – are still as relevant now as they were in Cowan and Reines’ day. Modern nuclear power plants produce about 1020 electron antineutrinos (νe) per second and experiments based on the same liquid-scintillator concept continue to provide essential contributions to neutrino physics by looking for the “disappearance” of the νe.
Sixty years after the first detection of the neutrino, and more than 80 years after the particle was tentatively predicted, experiments with neutrinos continue to have a leading role in particle physics. Today, experimentalists around the world are vying to determine precisely the mixing parameters of the neutrino, including the masses. The measurements may prove to hold the answers to some key questions in the field – ensuring that the “supreme challenge” of creating and detecting neutrinos will remain a worthwhile and exciting pursuit for the foreseeable future.
When CERN was founded in 1954, the neutrino was technically still a figment of theorists’ imaginations. Six decades later, neutrinos have become the most studied of all elementary particles. Several new and upgraded neutrino-beam experiments planned in Japan and the US, in addition to the reactor-based JUNO experiment in China, aim to measure vital parameters such as the ordering of the neutrino masses and potential CP-violating effects in the neutrino sector. In support of this effort, CERN is mounting a significant R&D programme called the CERN Neutrino Platform to strengthen European participation in neutrino physics.
CERN has a long tradition in neutrino physics. It was the study of neutrino beams with the Gargamelle detector at CERN in 1973 that provided the first evidence for the weak neutral current, and in the late 1970s, three experiments – BEBC, CDHS and CHARM – used a beam from the SPS to further unveil the neutrino’s identity. A milestone came in 1989, when precise measurements at the Large Electron–Positron Collider showed that there are three, and only three, types of light neutrinos that couple to the Z boson. This was followed by searches for neutrino oscillations at NOMAD (also known as WA96) and CHORUS (WA95) during the 1990s, which were eventually established by the Super-Kamiokande collaboration in Japan and the Sudbury Neutrino Observatory in Canada. More recently, from 2006 to 2012, CERN sent a muon-neutrino beam to the ICARUS and OPERA detectors at the Gran Sasso National Laboratory, 732 km away in Italy. The main goal was to observe the transformation of muon neutrinos into tau neutrinos, which was confirmed by the OPERA collaboration in 2015.
Following the recommendations of the European Strategy for Particle Physics in 2013, CERN inaugurated the neutrino platform at the end of 2014. Its aim is to provide a focal point for Europe’s contributions to global neutrino research by developing and prototyping the next generation of neutrino detectors. So far, around 50 European institutes have signed up as members of the neutrino platform, which sees CERN shift from its traditional role of providing neutrino beams to one where it shares its expertise in detectors, infrastructure and international collaboration.
“The neutrino platform pulls together a community that is scattered across the world and CERN has committed significant resources to support R&D in all aspects of neutrino research,” says project leader Marzio Nessi. Specifically, he explains, CERN is using the organisational model of the LHC to help in
developing an international project on US soil and to contribute to neutrino programmes in Japan and elsewhere. “This is precisely what CERN is about,” says Nessi. “The platform provides a structure at CERN to foster active involvement of Europe and CERN in the US and Japanese facilities.”
In December 2014, CERN and the Italian National Institute for Nuclear Physics (INFN) took delivery of the 760 tonne ICARUS detector, which formerly was located at Gran Sasso. The detector is currently being refurbished by the neutrino platform’s WA104 team and in 2017 it will be shipped to Fermilab in the US to become part of a dedicated short-baseline neutrino (SBN) programme there. This programme was approved following unexpected results from the LSND experiment at Los Alamos National Laboratory in the 1990s, which hinted at the existence of a fourth – possibly “sterile” – type of neutrino. The result was followed up by the MiniBooNE experiment at Fermilab, which also saw deviations – albeit different again – from the expected signal.
ICARUS will be installed just behind the previous MiniBooNE site, some 600 m downstream from the source of the beam at Fermilab’s booster ring. It will be the farthest of three detectors in the line of the beam after the Short Baseline Neutrino Detector (SBND, which is currently under design) and MiniBooNE’s successor MicroBooNE (which is already operational). All three detectors employ liquid-argon time projection chambers (LAr-TPCs) to study neutrino oscillations in detail. ICARUS comprises two 270 m3 modules filled with liquid argon: when an energetic charged particle passes through its volume it ionises the liquid and a uniform electric field causes electrons to drift towards the end plates, where three layers of parallel wire planes oriented at different angles (together with the drift time) allow researchers to reconstruct a 3D image of the event.
The refurbishing campaign at CERN concerns many parts of the ICARUS experiment: the photomultipliers, the read-out electronics, the cathode plane and the argon recirculating system. Moreover, it will benefit from European expertise in automatic event reconstruction and the handling of large data sets. Finally, the unique cryostat in which ICARUS will be placed is also being assembled at CERN. “Improving the performances of a detector already successfully operating in the Gran Sasso underground laboratory is extremely challenging in many respects,” says ICARUS technical co-ordinator Claudio Montanari. “Indeed, in order to make it fully functional to operate on surface, many different aspects including data acquisition, background rejection, timing and event reconstruction needed to be rethought.”
Rapid progress made in understanding neutrino oscillations during the past 15 years has also provided a strong case for long-baseline neutrino programmes. A major new international project called DUNE (Deep Underground Neutrino Experiment), which is estimated to begin operations by approximately 2026 as part of Fermilab’s Long Baseline Neutrino Facility (LBNF), will take the form of a near and a far multikiloton detector. The far detector will consist of four 10 kt active LAr-TPC modules sited in a 1.5 km-deep cavern at the Sanford lab in South Dakota, 1300 km away, at which neutrino beams with unprecedented intensities will be fired through the Earth from Fermilab. While the three experiments in the SBN programme will look for the disappearance of electron and muon neutrinos to search for sterile neutrinos, they will also serve as a stepping stone to the large LAr modules required by LBNF. The LBNF/DUNE experiment will allow not just the neutrino-mass hierarchy to be determined but also CP violation to be looked for in the leptonic sector, which could help to explain the missing baryonic matter in the universe.
The CERN Neutrino Platform is building two large-scale prototypes – single-phase and double-phase ProtoDUNE modules – to enable LAr detectors to be scaled up to the multikiloton level. The cryostat for such giant detectors is a particular challenge, and led physicists to explore a novel technological solution inspired by the liquified-natural-gas (LNG) shipping industry. CERN is currently collaborating with French firm Gaztransport & Technigaz, which owns the patent for a membrane-type containment system with two cryogenic liners that support and insulate the liquid cargo. Although this containment system has the advantage of being modular, the challenge in a particle-physics setting is that the cryostats not only have to contain the liquid argon but also all of the detectors and read-out electronics.
Global connection
While the single-phase ProtoDUNE detector uses technology that is very similar to that in ICARUS, a second neutrino-platform project called WA105 aims to prototype the new concept of a “dual-phase” LAr time projection chamber (DLAr-TPC), which is being considered for one or more of the DUNE far-detector 10 kt modules. In a DLAr chamber, a region of gaseous argon resides above the usual liquid phase. Ionisation electrons drift up through the detector volume and are accelerated into the gaseous region near the top of the cryostat by a strong electric field. Here, large electron multipliers amplify the signals, while the anode collects the charged particles and provides the spatial read-out. “The ProtoDUNE tests foreseen at the CERN Neutrino Platform represent the culmination of more than a decade of R&D towards the feasibility of very large liquid-argon time projection chambers for next-generation long-baseline experiments,” says André Rubbia, co-spokesperson of the DUNE collaboration.
ProtoDUNE and WA105 are planned to be ready for test beam by 2018 at a new EHN1 test facility currently under construction in the north area of CERN’s Prévessin site. Most of the civil-engineering work to extend the EHN1 building is complete and all components are under procurement or installation, with staff expected to move in towards the end of the year. The test facility was financed by CERN, with two beamlines due to be commissioned in late 2017.
As ICARUS prepares for its voyage across the Atlantic, and the detectors for the next-generation of US neutrino experiments takes shape, the CERN Neutrino Platform is also working on components for Japan’s neutrino programme (see “NOvA releases new bounds on neutrino mixing parameters”). The Baby-MIND collaboration aims to construct a muon spectrometer – a state-of-the-art prototype for a would-be Magnetized Iron Neutrino Detector (MIND) – and characterise it in a charged-particle beam at CERN. The system will be assembled at CERN during the winter and tested in May next year, before being shipped to Japan in the summer of 2017. Once there, it will become part of the WAGASCI experiment, where it will contribute to a better understanding of the systematics for the T2K neutrino and antineutrino oscillation analysis. Baby-MIND was approved by the CERN research board in December last year as a Neutrino Platform experiment. “Other projects for the Japanese neutrino programme are also under discussion,” says Baby-MIND spokesperson Alain Blondel of the University of Geneva.
Finally, in June it was decided that the CERN Neutrino Platform will also involve a neutrino-theory working group to strengthen the connections between CERN and the worldwide community and help to promote research in theoretical neutrino physics at CERN. “Fundamental questions in neutrino physics, such as the existence of leptonic CP violation, the Majorana nature of neutrinos and the origin of neutrino masses and mixings, will be at the centre of research activities,” explains group-convener Pilar Hernández. “The answers to these questions could have essential implications in other areas of high-energy physics, from collider physics to indirect searches, as well as in our understanding of the universe.”
The CERN Neutrino Platform offers a unique opportunity to build a strong European neutrino community, with immediate physics potential coming from the short-baseline experiments at Fermilab in the US and the new near detector at T2K in Japan. The platform is also making a major contribution to the infrastructure of Fermilab’s Long-Baseline Neutrino Facility (LBNF), including the design and construction of a large LBNF cryostat to be placed underground at the Sanford Underground Research Facility, new large detector prototypes and generic R&D on new detectors and data handling. CERN and Europe will therefore participate fully in the construction, commissioning and physics exploitation of the new high-intensity facility. In addition to R&D for the LBNF/DUNE cryostat, the neutrino platform currently has five approved participants:
• WA104, ICARUS far detector for Fermilab’s short-baseline programme;
• WA105, the engineering prototype for a double-phase LAr-TPC;
• PLAFOND, a generic R&D framework;
• ProtoDUNE, the engineering prototype for a single-phase LAr-TPC;
• BabyMIND, a muon spectrometer for the WAGASCI experiment.
In 1976, neutrinos did not yet have the prominent role in particle physics that they play today. Postulated by Pauli in 1930, they had been said to be undetectable due to their tiny interaction probability, and were only first observed in the mid-1950s by Fred Reines and Clyde Cowan using a detector located close to a military nuclear reactor. In 1962, researchers at Brookhaven National Laboratory discovered a second type of neutrino, the muon neutrino, but the third (tau) neutrino would not be seen directly for a further 38 years. On the other hand, the theory of electroweak interactions mediated by the W and Z bosons was firming up, and measurements of neutrinos played a significant role in this context.
The first naturally generated neutrinos, originating from cosmic-ray collisions in the Earth’s atmosphere, were observed in 1965 in deep gold mines located in South Africa and India. Also in the late 1960s, Ray Davis was beginning his famous solar-neutrino observations. The time was right to start thinking seriously about neutrino astronomy.
Enter DUMAND
Plans that ultimately would shape the present-day neutrino industry blossomed in September 1976 at a meeting in Waikiki, Hawaii. It was here that some of the first ideas for large detectors such as the gigaton DUMAND (Deep Underwater Muon and Neutrino Detector) array, which eventually morphed into the present-day IceCube experiment at the South Pole, were envisioned. The technology for smaller water-filled detectors such as IMB (Irvine–Michigan–Brookhaven) and, later, Kamiokande in Japan, were also laid out in detail for the first time. Moreover, totally new concepts such as particle detection via sound waves or radio waves were explored.
The first organised stirrings of what was to become DUMAND took place at a cosmic-ray conference in Denver, Colorado, in 1973, which led to a preliminary workshop at Western Washington University in 1975. It was at this meeting that the detection of Cherenkov radiation in water was chosen as the most viable method to “see” neutrino interactions in a transparent medium, with some deep lakes and the ocean near Hawaii considered to be good locations. This detection principle goes back to Russian physicists Moisey Markov and Igor Zheleznykh in 1960: water provides the target for neutrinos, which create charged particles that generate a flash of light in approximate proportion to the neutrino energy. The water also shields the many downward-moving muons from cosmic-ray interactions in the atmosphere. With light detectors, such as a basketball-sized photomultiplier tube, one could register the light flash over large distances.
The vision of building devices larger than any yet dreamed about, and placing them in the deep ocean to study the cosmos above, attracted many adventurous souls from around the US and elsewhere, a number of whom dedicated years to realising this dream. One of us (JL) joined Reines, along with Howard Blood of the US navy and other ocean-engineering aficionados – in particular, George Wilkins, who was responsible for the first undersea fibre-optic cables. Inventor of the wetsuit and former bubble-chamber developer Hugh Bradner became another driving force behind the project. Theorists, cosmic-ray experts, particle physicists, astronomers and astrophysicists all played important roles (see photograph). The event captured a spirit of adventure and worldwide co-operation, particularly concerning interactions between US and Soviet colleagues, and the special nature of our unusual international physics collaboration brought a certain level of spice to relations.
Searching the sky
A significant problem of the era was to know what sources of neutrinos the detectors should be looking for. Astrophysicists and astronomers seldom thought about neutrinos at the time, and neutrinos were neglected in calculations of radiation and power in the universe. They were, however, included in studies of solar burning and supernovae, and it was from these efforts that neutrinos began to appear in the astronomer’s lexicon. For the first time, a survey of possible astrophysical sources of lower-energy neutrinos (typically 1–100 MeV) was produced at the 1976 meeting, which were organised by Craig Wheeler into seven possible steady sources and eight potential burst sources.
For the steady sources, only solar neutrinos and terrestrial radioactivity appeared practical for detecting neutrinos. For the bursting neutrino sources, galactic gravitational collapses clearly yielded the most available total power for neutrinos. The others – namely type I supernovae, solar flares, gamma- and X-ray bursts and mini-black-hole evaporation – did not seem to have enough power and proximity to be competitive. Even today, these sources have not been observed in terms of neutrinos.
There was also great interest in targeting high-energy (TeV and above) neutrinos, but predictions of the strengths of the potential sources were plagued by huge uncertainties. It was known that the number of cosmic rays impinging upon the Earth decreases as a function of energy up to values of around 1020 eV, whereupon the spectrum was predicted to show a cut-off known as the Greisen–Zatsepin–Kuzmin (GZK) limit caused by high-energy protons being degraded by resonant collisions with the cosmic microwave background. Veniamin Berezinsky of the Lebedev Institute in Moscow put forward some prescient models of ultra-high-energy neutrino generation, in particular, those generated in GZK processes, and also gave first estimates of upper bounds on neutrinos from star formation in the early universe. Credible sources of neutrinos with energies far beyond the TeV scale were probably too far ahead of people’s visions at the time, while neutrino cross-sections and even the production dynamics were not well defined at the highest energies.
By the end of the Hawaii workshop, which lasted for two weeks, everyone considered low-energy neutrino detection to be the most worthwhile cosmic-neutrino-detection goal. Aside from solar neutrinos, it was also agreed that neutrinos from supernova collapses were the most likely to be seen (as they later were in 1987, when a burst of neutrinos was observed by the IMB, Kamiokande and Baksan detectors). But it was also realised that this effort requires kiloton detectors with threshold sensitivities of about 10 MeV to guarantee a few supernovae per century. Regarding the detection of high-energy neutrinos in the TeV range, as expected from acceleration processes in galactic and extragalactic objects, everyone understood the necessity of detectors in the megaton to gigaton class. This was so far beyond the reach of technology at the time, however, that people realised they had to start small and work upwards in target mass.
Interestingly, neutrinos generated in the atmosphere with energies in the GeV range were regarded as the least interesting target. Nobody at that time would have expected that precisely these neutrinos, together with solar neutrinos, would demonstrate for neutrino oscillations and lead to the award of the 2015 Nobel Prize in Physics.
Considering the vast target volumes required for high-energy neutrino astronomy, it became clear that the most promising – and most affordable – detection method was to register the Cherenkov light in natural water. With the intensity of Cherenkov light being around 30 times weaker than that from a scintillator, however, the design of the optical detectors became paramount. One group of attendees from the 1976 workshop aimed for the use of wavelength shifters that absorb blue light and re-emit green light, allowing the use of modest photodetectors, while another pushed for the development of photomultipliers larger than the 25 cm-diameter versions available at the time – as did in fact transpire. The one serious alternative to optical Cherenkov light detection, building on a concept developed by Gurgen Askaryan in 1957, was to utilise the pulse of sound made by neutrino progeny after neutrino collision with a nucleus of water or another medium such as ice.
Following the 1976 event, annual neutrino workshops were held for about a decade, eventually blending with DUMAND collaboration meetings. The 1978 workshop, held in La Jolla, California, took place in three sessions over a six-week period, and attracted more neutrino converts from physics, astrophysics and ocean engineering. The following year’s event, which was held at Khabarovsk and Lake Baikal in Siberia, offered some physicists their first chance to interact with Soviet physicists who had not been able to travel. Indeed, by the end of 1979, international politics and in particular the Soviet–Afghan war had forced the separation of the Russian and US DUMAND efforts. Russian DUMANDers decided to push ahead with a detector array deployed from ice in the world’s deepest freshwater lake, Lake Baikal, and one of us (CS) joined the Baikal collaboration in 1988. A few years later, the first underwater neutrino events were identified in Lake Baikal, and the principle of this detection technique was finally proven (followed by more statistics from AMANDA at the South Pole and ANTARES in the Mediterranean Sea). The heroic efforts of Russian physicists through difficult times have continued for 35 years, and Baikal researchers have just deployed the first subunit of a cubic-kilometre array similar to IceCube.
The formal outcome of the 1976 workshop was a joint resolution and plans for ocean studies and further workshops. The major vision for the high-energy DUMAND detector itself (see diagram) was an array of bottom-moored strings carrying some 22,000 optical detectors distributed in a volume slightly larger than one cubic kilometre – quite similar to the eventual IceCube array. The DUMAND project carried out many ocean studies and also accomplished some physics offshore in Hawaii, measuring muons and the lack of large bursts.
Alas, DUMAND was cancelled by the US Department of Energy in 1995, prior to starting full deployment. One can debate the causes – the failure of the first deployed string was certainly one aspect. It may be noted, however, that the Superconducting Supercollider had just been cancelled and that the main funding agencies were simply not supportive of non-accelerator research until after SuperKamiokande’s discovery of neutrino oscillations in 1998. The DUMAND project was also ahead of its time in terms of its detection scheme, the use of undersea fibre optics and robotic module deployment.
The DUMAND legacy
Attendees of DUMAND’76 realised the importance of the venture, but probably came away with varying levels of belief in its practicality. Perhaps the greatest legacy of the 1976 workshop was bringing natural neutrino studies to the attention of a wide research community, and astrophysicists in particular. The breadth of ideas that this allowed, and the spirit of interdisciplinarity and international co-operation, has continued in the neutrino community. Moreover, initially still-born alternatives for neutrino detection have been revived in the last two decades.
The neutrino-detection method using acoustic signals was taken up in the late 1990s and early 2000s using military hydrophone arrays close to the Bahamas and to Kamchatka, as well as dedicated test set-ups in Lake Baikal, the Mediterranean Sea and in Antarctic ice – namely, the South Pole Acoustic Test Setup (SPATS). More profitably, the radio technique of detecting high-energy neutrino interactions via the negative charge excess in a very-high-energy particle shower induced by a neutrino interaction has allowed stringent upper limits to be placed on neutrino fluxes at the highest energies. The Askaryan Radio Array (ARA), the balloon-borne ANITA project and ARIANNA all focus on radio detection in Antarctic ice. Indeed, ANITA has just reported the first candidate for a super-high-energy neutrino event emerging from the Earth at a large angle.
Probably the most important lineage descending from the 1976 workshop is in the exploitation of the large water Cherenkov detectors located underground: IMB (1983), Kamiokande (1985) and, most notably, SuperKamiokande (1996). Some other experiments can be claimed as at least partial progeny of the early DUMAND enterprises, such as NESTOR in the Mediterranean, the Sudbury Neutrino Observatory (SNO) in Canada and KamLAND in Japan. The links become more tenuous, but the people engaged all owe much stimulation and experience to the explorations of these problems and detector solutions 40 years ago.
As for DUMAND itself, its spirit lives on in present-day neutrino observatories. IceCube at the South Pole is certainly the most extreme realisation of the DUMAND concept, and DUMAND spherical phototube modules are still the archetypal unit for this and many other detectors, such as ANTARES and GVD in Lake Baikal. A further array currently being installed deep in the Mediterranean, KM3NeT, has varied the principle by arranging many small phototubes inside of the glass sphere instead of a single large tube, while IceCube is exploring multiphototube modules as well as wavelength-shifter solutions for its next-generation incarnation.
Following IceCube’s discovery of the first extraterrestrial high-energy neutrinos in 2013, we are finally realising the decades-old dream of seeing the universe in neutrinos. Today, with thousands of researchers undertaking neutrino studies, the revelations for particle physics from neutrinos seem to be unending. The experimental road has been full of surprises, and neutrino physics and astronomy remain some of the most exciting games in town.
Japan has been a leader in the global neutrino community since the 1980s, breaking ground (both literally and figuratively) with multiple generations of massive underground experiments. These experiments, which although sited in Japan are built and operated by international collaborations, went on to make some of the most surprising discoveries in the history of particle physics. In doing so, they pointed the way to new experiments, and garnered the most prestigious accolades for Japanese physicists and their international partners.
Today, Japan is undertaking two major projects – T2K and Hyper-Kamiokande – to delve deeper into the neutrino’s properties. These and other global neutrino projects were the subject of discussions at the Third International Meeting for Large Neutrino Infrastructures, which took place at the KEK laboratory on 30–31 May (see panel below).
From Kamiokande to Super-K
Japan’s neutrino odyssey began with the Kamioka Nucleon Decay Experiment (Kamiokande), a 3000 tonne water Cherenkov experiment in the Kamioka mine in Japan’s Gifu prefecture, which started collecting data in search of proton decay in 1983. Although the experiment did not observe proton decay, it did make history with novel observations of solar neutrinos and, unexpectedly, 11 neutrino interactions from a supernova (SN1987a). These observations led to the 2002 Nobel Prize in Physics for Masatoshi Koshiba of the University of Tokyo, shared with the late Ray Davis Jr, and paved the way to a second-generation experiment.
Following Kamiokande’s success, in the 1990s, the late Yoji Totsuka led the construction of a 50,000 tonne water Cherenkov detector called Super-Kamiokande (Super-K). Like its predecessor, Super-K is also a proton-decay experiment that became famous for its measurements of neutrinos – both solar and atmospheric. Atmospheric neutrinos come from high-energy cosmic-ray interactions in the Earth’s atmosphere, predominately from charged-pion decays that result in a two-to-one mix of muon and electron neutrinos that can pass straight through the Earth before interacting in the Super-K detector.
Although the cosmic-ray flux impinging on the Earth is isotropic, Super-K data indicated that the flux of atmospheric neutrinos is not. In 1998, the Super-K collaboration showed that muon neutrinos coming from above the detector outnumbered those coming from below. The muon neutrinos that travel from the other side of the Earth transform into tau neutrinos and effectively disappear, because they are not energetic enough to interact and produce charged tau particles. This process, called neutrino oscillation, can only happen if neutrinos have mass – in contradiction to the Standard Model of particles physics. The discovery of atmospheric-neutrino oscillations led to the 2015 Nobel Prize in Physics for Super-K leader Takaaki Kajita and also Arthur McDonald of the Sudbury Neutrino Observatory (SNO) in Canada, for the concurrent observations of solar-neutrino oscillations.
From KEK to Kamioka to T2K
Two new experiments, K2K and KamLAND, were built in Japan to follow up the discovery of neutrino oscillations. The KEK-to-Kamioka (K2K) collaboration built at the KEK laboratory an accelerator-based neutrino beam aimed at Super-K, 250 km away, and also a suite of near-detectors. The collaboration solved several technical difficulties to confirm that nature’s most elusive particle was being created in their accelerator beam and was definitely interacting in Super-K, with careful comparisons from the near detectors at KEK showing that some muon neutrinos were indeed disappearing as they travelled, just as the Super-K collaboration predicted.
The Kamioka Liquid Scintillator AntiNeutrino Detector (KamLAND) experiment was built in the cavern that originally held Kamiokande, and offered sensitivity to electron antineutrinos from Japan’s nuclear reactors. KamLAND found that the neutrinos were indeed oscillating in a manner exactly consistent with the solar-neutrino oscillation observed by SNO and Super-K. These two international experiments in Japan, K2K and KamLAND, confirmed that neutrino oscillations were the explanation for the surprising observations of Super-K and SNO. But all of these experiments had seen only the disappearance of neutrinos, and it was therefore time for an experiment to observe the appearance of neutrinos.
In 2009, the Tokai-to-Kamioka (T2K) collaboration, comprising 500 scientists from 11 nations, built an experiment to observe the appearance of electron neutrinos in a muon-neutrino beam. The concept is similar to that of K2K, but with higher beam power and higher precision in the near detectors. To achieve higher beam power, T2K uses the new accelerator complex at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai Village on the east coast of Japan, about 70 km from KEK. The neutrino beam is directed in an off-axis configuration, which allows precise control of the neutrino energy spectrum, towards the Super-K detector 295 km away. In 2011, T2K reported the appearance of electron neutrinos from a muon-neutrino beam, which was the first significant neutrino-flavour appearance signal. For this result, which completed the picture of neutrino oscillations with the three known Standard Model neutrino flavours, T2K and K2K founding spokesperson Koichiro Nishikawa was awarded a share of the 2016 Breakthrough Prize in Fundamental Physics, along with all T2K and K2K collaboration members. The Super-K, KamLAND, SNO and Daya Bay collaborations also shared in the award.
As often happens in particle physics, the previous generation’s discovery becomes the current generation’s tool to search for new discoveries. In 2014, T2K began to operate with a muon antineutrino beam. By comparing oscillations of antineutrinos with oscillations of neutrinos, it might be possible to find a clue to one of the most profound mysteries in science: why does the universe appear to be composed entirely of matter, when it is believed that equal quantities of matter and antimatter were created in the Big Bang? Differences between the oscillations of neutrinos and antineutrinos could provide the answer to this, and, if they exist, these differences would be an example of CP violation (CPV).
In late June 2016, the T2K collaboration submitted a proposal to J-PARC requesting an extension of its neutrino (and antineutrino) data run that would give the collaboration significant (potentially 3σ) sensitivity to CPV by 2025. The physics reach of this extended run has been boosted by news that MEXT, the Japanese science funding agency, has approved the first step of an upgrade of the main-ring accelerator at J-PARC. This facility will house a new power supply system with which the repetition rate of the main ring will be doubled, resulting in 750 kW beam power for T2K with the potential to exceed 1 MW.
To Hyper-K and beyond
For discovery-level sensitivity to CPV (5σ or above), the next-generation water Cherenkov detector, Hyper-Kamiokande, is currently being designed. To reduce costs while maximising physics potential, the Hyper-K detector design has changed from the original concept of horizontal cylinders to vertical cylinders similar to Super-K, taking advantage of newly developed high-efficiency photo sensors. The international Hyper-K collaboration was formed in 2015, with agreements between the University of Tokyo’s Institute for Cosmic Ray Research (ICRR), which runs the Kamioka Observatory, and KEK.
Hyper-K, when exposed to the 1 MW J-PARC neutrino beam, will make precise measurements of neutrino and antineutrino oscillations as it searches for CPV with high significance, as well as perform the most sensitive searches for proton decay yet. Hyper-K construction could start as early as 2018, with physics data-taking in 2026. Combined with long- and short-baseline programmes in the US, Japan’s next generation of neutrino observatories should help physicists to answer most of the remaining questions about neutrino oscillations.
Planning ahead: Third International Meeting for Large Neutrino Infrastructures
A major upgrade to the T2K experiment and the ongoing design of the Hyper-K detector were among many large neutrino projects discussed at the Third International Meeting for Large Neutrino Infrastructures, which took place at the KEK laboratory on 30–31 May. The event followed the previous instance of the meeting at Fermilab in April 2015 and aims to strengthen global co-ordination on large neutrino infrastructures, not just in Japan but the world over.
The third meeting, which was organised by KEK, ICRR, Fermilab, the Astroparticle Physics European Consortium (APPEC), the ICFA Neutrino Panel and the IUPAP Astroparticle Physics International Committee (APPIC), evaluated progress made since last year, discussed strategy toward the realisation of next-generation large neutrino infrastructures, and reviewed the programme of supporting measurements, prototyping and R&D.
The first day of the event addressed major accelerator-based programmes worldwide, focusing on Hyper-Kamiokande and the DUNE experiment planned in the US. The second day was devoted to examining the non-accelerator physics potential of the various large neutrino infrastructures, for which it was agreed that closer co-ordination is needed.
Presenting progress towards its road-map document, the ICFA Neutrino Panel discussed long-term opportunities such as the Neutrino Factory and ESSnuSB. It also identified 2020 as the approximate date when the future of sterile-neutrino searches and cross-section measurement programmes should be defined, and recommended that experiments such as nuSTORM and IsoDAR be evaluated by then. It was proposed that the next “Infra” meeting be held in Europe in 2017.
The breakthrough results from the Super-Kamiokande and SNO experiments, which showed that neutrinos oscillate between their three flavours, marked the start of nearly two decades of tremendous progress in neutrino physics. The basic features of the three-flavour neutrino oscillation framework have been fleshed out, and the NOvA experiment in the US – which presented its latest results at the Neutrino 2016 conference in London earlier this month – is poised to address many of the remaining unknowns related to neutrino masses and their mixing.
Among these is whether neutrinos obey a “normal” or “inverted” mass hierarchy: that is, whether the mass eigenstate with the least νe content (called ν3) is the heaviest or lightest of the three (see “A portal to new physics”). A second set of questions relate to the flavour admixture of the ν3 state. Past experimental data are consistent with ν3 being equal parts νμ and ντ, in addition to a small amount of νe. But is there a new symmetry that underlies this apparent νμ/ντ equality? And if the equality breaks down as measurements improve, which flavour will dominate? A third major unknown is whether neutrinos violate CP symmetry, as allowed by the complex phase δ of the leptonic mixing matrix.
Addressing the unknown
NOvA was conceived to address these unknowns using two detectors together with the intense beam of muon neutrinos provided by Fermilab’s NuMI neutrino source. NOvA’s 300 tonne near detector, which is located 1 km downstream of the neutrino source, measures the rate, energy spectrum and flavour composition of the neutrino beam prior to significant flavour oscillations, while the 14,000 tonne far detector is located 810 km downstream in northern Minnesota. The detectors are identical in their structure, consisting of 4 × 6 cm liquid scintillator-filled PVC cells in alternating planes in order to provide two orthogonal 2D views of particle trajectories.
NOvA has been collecting data with the NuMI beam since February 2014, and full operations began the following October upon completion of the far detector (CERN Courier July/August 2014 p30). As of May 2016, the experiment has accumulated 16% of its planned total. The results released at Neutrino 2016 are based on this data set, and highlighted here are the measurements of νμ →νμ (corresponding to muon-neutrino survival) and νμ →νe (electron-neutrino appearance).
To identify the flavour of an interacting neutrino, researchers look for tell-tale signs of a muon or an electron in the recorded event. Muons produced in charged-current νμ interactions in NOvA leave long straight tracks of detector activity that can span hundreds of cells (see the image of a muon-neutrino interaction in the NOvA detector). Electrons, in contrast, create more compact electromagnetic showers with well-characterised longitudinal and transverse profiles. An important background to both the νμ and νe charged-current channels comes from neutral-current interactions, whereby the neutrino exits the detector and leaves behind only a hadronic recoil system. Neutral pions in these recoil systems can mimic electrons, while charged pions can mimic muons.
To keep this background at bay in the νμ→ νμ measurement, each recorded track is assigned a muon likelihood based on key track features such as overall length and the rate of energy deposition. Additionally, each event must be far enough from the detector edges to ensure that the entire final state has been recorded and that the event is not due to an incoming cosmic ray. For the latest data, the NOvA team predicts 470 selected νμ charged current interactions in the far detector if neutrino oscillations do not occur. Only 78 such interactions were observed, in line with the well-established result originally observed by Super-Kamiokande that muon neutrinos are indeed oscillating into other flavours as they travel.
Non-maximal mixing
The real value of these data, though, comes from examining the precise energy dependence and amplitude of the νμ disappearance signal, which depends most strongly on the mass splitting |Δm232| and the mixing angle θ23. The ranges of these parameters allowed by the latest NOvA data are shown above. NOvA’s results are consistent with prior measurements but show an intriguing preference for non-maximal mixing – that is, a preference for sin2θ23 ≠ 0.5 and thus a break in the ν3 state’s apparent flavour symmetry. Whether this preference becomes conclusive or fades away will be addressed as NOvA continues to accumulate data.
On the νe appearance side, distinguishing signal and background is a trickier affair due to their stronger mutual similarities and to the low probability for νμ→ νe oscillations. For its latest 2016 νe analysis, the NOvA team has developed an event-classification algorithm based on techniques from the image analysis community, notably convolutional neural networks and deep learning. A total of 33 candidate νe events were isolated in the latest data, which is far above the expected background of eight, and therefore represents a clear observation of νe appearance – in line with data from T2K.
The νμ→ νe oscillation probability, and therefore this measurement, is a function of several key unknowns that NOvA is pursuing. The probability will be 40 –70% higher for a normal mass hierarchy than for an inverted one, with the opposite correspondence holding for antineutrinos. This dependence stems from the so-called matter effect arising from neutrinos scattering off electrons in the Earth, and the effect is intentionally made large in NOvA by choosing the longest-distance baseline possible. Next, the phase δ of the leptonic mixing matrix can either increase or decrease the νμ→ νe probability by a similar amount, and this effect is also opposite for neutrinos and antineutrinos. Finally, the probability increases or decreases for neutrinos and antineutrinos alike, in step with sin2θ23. This last dependence is complementary to the behaviour of the νμ→ νμ channel, which is better at detecting non-maximal mixing but cannot on its own distinguish which way the ν3 flavour mixing breaks.
The present electron-neutrino appearance results, which point to a probability on the higher end of the range, have already started carving up parameter space. But NOvA must collect both neutrino and antineutrino data to disentangle all the above effects, particularly in light of possible non-maximal mixing. The first large antineutrino run for the experiment is slated to begin next spring.
In addition to accruing neutrino and antineutrino exposure for the flagship oscillation measurements, this summer’s results also included a first look at the total neutral current rate in the far detector, for which a deficit could suggest mixing with light sterile neutrinos. No deviation is seen thus far, but with both detectors operating smoothly and the NuMI source running at high power, NOvA is set to play a central role in illuminating the neutrino sector in the coming years.
• CERN Courier went to press just as Neutrino 2016 got under way. Other expected highlights of the conference include new neutrino oscillation measurements from the T2K experiment in Japan.
The 1998 discovery that neutrinos can oscillate between different flavours, by the Super-Kamiokande experiment in Japan and subsequently by the SNO experiment in Canada, marked a turning point in our understanding of elementary particles. For many theorists, it represents the first hard particle-physics evidence for the existence of new degrees of freedom (d.o.f.) beyond the known fundamental particles and, most probably, of new physics beyond the Standard Model (SM). The hunt to uncover the new theory is the main focus of the neutrino theoretical community.
First postulated by Pauli in 1930, neutrinos have always played the role of the elusive particle. Their interactions were soon understood thanks to Fermi’s beta-decay theory, but searching for them seemed more like science fiction, at the time. In 1946, however, Pontecorvo suggested that nuclear reactors and the Sun are copious sources of neutrinos, and proposed a radiochemical method for the detection of neutrinos.
A decade later, Reines and Cowan established the neutrino’s existence by performing a reactor neutrino experiment, and the search for astrophysical neutrinos began soon afterwards. Ray Davis Jr and collaborators later detected solar neutrinos, reporting in 1972 a flux that was significantly smaller than predicted by the most sophisticated solar models developed by John Bahcall. This “solar-neutrino puzzle” was eventually explained in terms of the proposal by Bruno Pontecorvo in 1957 and 1958, applied by him to solar neutrinos in 1967, that neutrinos may oscillate between their different types. Combined with the 1962 proposal by Maki, Nakagawa and Sakata – inspired by the discovery of the muon neutrino in the well-known Brookhaven experiment – that there exists mixing between flavour and massive neutrino states, the stage was set to catch and “see” an oscillating neutrino.
Proving this elegant theoretical picture took a further 36 years of experimental innovation. In the end, it relied on the observation of atmospheric neutrinos produced by collisions of cosmic rays in the upper atmosphere, which produce showers of pions and kaons whose subsequent decays give muon and electron neutrinos. Atmospheric neutrinos were first detected in 1965 by the Kolar Gold Fields experiment in India and another experiment at the East Rand gold mine in South Africa. Then, in 1998, in a momentous discovery, Super-Kamiokande showed that muon neutrinos disappear as a function of distance travelled (see figure 1). Just a few years later, thanks to measurements of the total solar-neutrino flux, the SNO experiment confirmed that solar neutrinos can transform from electron neutrinos into muon and tau neutrinos. These two milestones and subsequent results, which have been recognised by a number of prestigious awards, ushered in a mesmerising period of new results that continues to this day.
Neutrino oscillations are the transformation of a neutrino from one flavour into another as it propagates. It is a fundamentally quantum-mechanical process arising from a misalignment of flavour states, νe, νμ and ντ, which describe neutrinos in production and detection, compared with the mass eigenstates, ν1, ν2 and ν3. At the source, a flavour neutrino is the coherent superposition of mass states, which propagate with different phases due to their different masses. As neutrinos travel, the shift in phase results in a different combination of flavour neutrinos. The existence of neutrino oscillations necessarily requires neutrinos to have masses and to mix, which is different from the prediction of the SM (at least in its minimal form). This is why it is widely believed that neutrino oscillations are the first and so far only concrete evidence for physics beyond the SM provided by particle-physics experiments.
The decade following the discovery of neutrino oscillations did not disappoint. In 2002, the KamLAND experiment in Japan reported the first oscillations of man-made neutrinos, produced by nuclear reactors, while the K2K and MINOS experiments detected neutrinos from accelerator-produced beams. Together with the ongoing T2K and NOvA experiments, as well as atmospheric-neutrino observations at Super-Kamiokande, these experiments have established that there are two mass-squared differences that drive different neutrino oscillations and imply the existence of at least three neutrino mass eigenstates. The data also show that neutrino mixing is described by a 3 × 3 unitary matrix parameterised by three well-measured angles: θ12, θ23 and θ13.
We have now an incredibly rich picture of neutrino properties that was unthinkable at the time of the Super-Kamiokande results and that is very different from that of the quarks. But despite these immense achievements, we are still in need of crucial pieces of information to reach a complete understanding of neutrino properties.
The most important question about neutrinos concerns the type of masses they have. So far, all the known fermions are of the Dirac type: their particles and antiparticles have opposite charges and they possess a Dirac mass that arises from the coupling to the Higgs field. Neutrinos could behave in the same way, but because they are electrically neutral it is possible that neutrinos acquire mass via a different mechanism. Indeed, neutrinos and antineutrinos might be indistinguishable, constituting what is called a Majorana particle after Ettore Majorana who proposed the concept in 1937. Unlike Dirac fields, which have four components, Majorana fields have only two d.o.f. Such a particle cannot possess any charge, not even a lepton number.
A matter of conservation
The question of the nature of neutrinos is therefore intrinsically related to the conservation of the lepton number. In the SM, the lepton number is a global accidental symmetry that happens to be preserved thanks to the gauge symmetries and particle content, but it does not have a dynamic role because there are no associated gauge bosons. The question arises whether the ultimate theory of particles and their interactions is lepton-number violating or not. The most promising way to answer this question is to search for neutrinoless double-beta decay, whereby certain nuclei spontaneously undergo two beta decays at once, without producing any neutrinos. This process directly violates lepton-number conservation and would imply that neutrinos are Majorana particles, motivating a broad international experimental programme (see panel below).
A second major question is whether the CP symmetry is violated in the lepton sector, as it is in the quark one. CP violation is one of the three key ingredients in baryogenesis and leptogenesis, which are needed to dynamically explain the observed matter–antimatter asymmetry of the universe (see panel below). There are three possible sources of CP violation in the lepton sector: the Dirac phase, which is the analogue of the one in the quark sector, and two Majorana phases that appear only if neutrinos are Majorana particles. If neutrinos are Dirac particles, the latter can be rotated away as is done in the quark sector.
The first hints of leptonic CP violation came recently from combining data from China’s Daya Bay experiment with measurements at long-baseline accelerator facilities, in particular T2K and NOvA. These seem to indicate a preference for a nonzero value of the CP-violating Dirac phase (see figure 2). It is too early to tell, but very ambitious plans – including the proposed Deep Underground Neutrino Experiment (DUNE) in the US and T2HK in Japan – aim to settle the issue by allowing both neutrino and antineutrino oscillations to be studied. The latter behave differently if Dirac CP violation is present, with oscillations that are being enhanced or suppressed, depending on the values of the Dirac phase.
The other mixing parameters, namely the three mixing angles, are already quite well-determined. Angle θ13 went from being unknown just over four years ago to being the best-measured, thanks to results from the Daya Bay as well as RENO and Double Chooz experiments, while the JUNO experiment in China plans to reach a sub-per-cent accuracy for the θ12 angle after a few years of operation. θ23 is particularly interesting because it could be exactly maximal, therefore pointing towards a symmetry in the lepton-flavour sector, or could deviate from this by several degrees. Current and future long-baseline oscillation experiments will have the best chance of determining θ23, which will be critical for disentangling the different models proposed to explain the observed mixing pattern.
Massive considerations
As for the values of the neutrino masses themselves, we already have a very precise measurement of the absolute values of the two mass-squared differences – which differ by a factor of about 30 (figure 2). But we still lack key pieces of information, namely which neutrino is the lightest, defining the neutrino mass ordering, and what its mass scale is. The sign of the solar mass-squared difference is determined by solar-neutrino oscillations, but that of the atmospheric one is unknown. If it turns out to be positive, corresponding to m3 > m1, neutrino masses exhibit the so-called “normal” ordering. The alternative scenario, m3 < m1, implies an “inverted” ordering (figure 3).
Knowing the mass ordering and scale is important for theorists because different theoretical models predict different patterns, and also for experimentalists searching for specific signatures. It strongly affects the rate of neutrinoless double-beta decay, substantially impacting on the prospects of discovering the Majorana nature of neutrinos, while in the early universe heavier neutrinos suppress the growth of large-scale structures at small scales. The ordering of the masses also changes the way in which neutrinos propagate over long distances in media such as the Earth, due to weak interactions with the background of electrons, protons and neutrons. This gives neutrinos an effective mass that modifies their energies and the mixing: neutrino oscillations are enhanced for normal mass ordering and suppressed for inverted ordering, with the opposite happening in the case of antineutrinos.
Experiments such as the long-baseline experiment NOvA, which measures a neutrino beam produced 810 km away at Fermilab, exploit these effects to hunt for the neutrino mass ordering (see “NOvA releases new bounds on neutrino mixing parameters”). With DUNE, which will operate at a distance of 1300 km, and new atmospheric-neutrino observatories such as PINGU, ORCA and INO, as well as JUNO, we expect to resolve this issue in the next 5–10 years.
However, even knowing the neutrino mass ordering still leaves open the question of the overall neutrino mass scale. So far, we know that neutrino masses cannot be too large. They are restricted to be smaller than 2.2 eV by the Troitsk and Mainz experiments, and well below this limit if one considers cosmological observations, which suggest a conservative bound on the sum of the masses of around 0.7 eV in the standard cosmological model. The KArlsruhe TRItium Neutrino (KATRIN) experiment currently being commissioned in Germany aims to determine the absolute mass scale by searching for a small deformation of the electron energy spectrum in beta decays and will be sensitive to neutrino masses as small as 0.2 eV. It is expected to take data very soon.
The standard neutrino picture comprises three neutrino flavour states and correspondingly three light-mass eigenstates. In many extensions of the SM this is not the case because new degrees of freedom and/or new interactions can be added (see panel below). The simplest extension is that of sterile-neutrinos, which do not experience SM interactions. The corresponding nearly sterile neutrino mass could take any value, from the very small to the GUT scale, but in many phenomenological studies is around the eV scale and therefore could induce short-baseline oscillations.
Results from the LSND and MiniBooNE experiments in the US, as well as some reactor neutrino ones, have hinted at precisely such a signal. But the results are still controversial, and there is tension with other searches of sterile-neutrinos from short-baseline muon neutrino experiments. New short-baseline reactor and radioactive-source neutrino experiments, in addition to the dedicated short-baseline accelerator programme at Fermilab involving the MicroBooNE, ICARUS and SBND detectors, will shed light on these results and possibly hunt for nearly sterile-neutrinos with even smaller mixing angles. A positive signal would be groundbreaking, forcing us to rethink the theoretical framework for light neutrinos and posing new questions about the nature, masses, mixing and CP-violating properties of the new states.
Indeed, neutrinos remain the most intriguing and elusive of all known fermions and are an ideal portal to explore new physics beyond the Standard Model. Despite impressive progress in the past 20 years, going from not knowing if neutrino oscillations took place to having measured most of the oscillation parameters with great precision, many key phenomenological and theoretical questions remain open and urgently require answers. Fortunately, a broad and exciting experimental programme is under way and, as is often the case in research, the focus of our theoretical work could change in an instant. With the LHC now into its high-energy run, for example, it is possible that we will discover entirely new particles and phenomena beyond the SM. We would then need to establish what connection – if any – exists between this and the already new physics of neutrino masses. Or perhaps neutrino masses come from a secluded sector, possibly at energy scales so high that we cannot test it directly. These and many other questions, informed by the current wealth of new and upcoming experiments, promise to keep neutrino theorists occupied for the foreseeable future.
Two-neutrino double beta decay (DBD) is a very rare Standard Model process that causes two neutrons simultaneously to decay into two protons and two electrons, with the emission of two electron antineutrinos. If neutrinos are Majorana particles, however, instead of being emitted, the Majorana neutrinos can mediate a new process called neutrinoless double-beta decay (NDBD), which is not allowed in the Standard Model. Observing this process would be groundbreaking because it would imply that the lepton number is violated and provide crucial information about neutrino masses.
More than a dozen experiments worldwide are searching for NDBD which, like DBD, can be observed in nuclei in which ordinary beta decay is kinematically forbidden. Because NDBD produces no neutrinos to carry off energy, all events will be concentrated at the end point of the two-electron energy spectrum – unlike the case for DBD, in which the spectrum is a continuum. Being an extremely rare process, NDBD searches require sufficiently large detector volumes, very good energy resolution, a location deep underground and extremely low backgrounds.
A number of different experimental techniques are being employed. Liquid-scintillator detectors such as KamLAND-Zen in Japan and SNO+ in Canada offer large target masses, and currently KamLAND-Zen provides the strongest bound on NDBD with a half-life greater than 1.1 × 1026 years. Germanium detectors such as GERDA and MAJORANA are more compact and ensure very good energy resolution, while planned experiments such as SuperNEMO and DCBA can track both electrons and could reconstruct their angular distribution. Time projection chambers, such as nEXO in the US and NEXT in Spain, can simultaneously track the electrons and allow large target volumes, while bolometers such as CUORE and AMoRE benefit from very high energy resolution.
Despite this impressive armoury, NDBD hunters are at the mercy of the neutrino masses and mixing parameters (see main text). The NDBD rate depends crucially on the combination of masses and mixing parameters, the so-called effective Majorana mass parameter. If neutrinos exhibit an inverted mass ordering, the predicted lower bound on the decay rate will be just within reach of the next-to-next generation of experiments. If they adopt the normal mass ordering, the decay rate could be anywhere between the current bounds and zero, if a specific cancellation between the three massive neutrinos is at work (see figure).
The origin of neutrino masses and mixing is still unresolved, and necessarily requires new degrees of freedom and new interactions. The simplest extension of the Standard Model assumes the existence of right-handed (RH) neutrinos, which behave as singlets with respect to the Standard Model gauge group. Unless specific symmetries are imposed, Yukawa couplings with the lepton doublet and the Higgs will be allowed and the lepton number will be preserved. Dirac masses therefore arise for neutrinos as they do for all the other known fermions, but this mechanism provides no insight as to why neutrino masses are so small (the Yukawa coupling needed would be 12 orders of magnitude smaller than that of the top quark). One could simply accept such extreme fine-tuning as a fact of nature, but this would naively lead one to expect the same mixing in the lepton sector as in the quark one and a similar mass ordering, neither of which is observed.
The alternative option is that neutrinos are Majorana particles. Majorana neutrinos will have a mass term in the Lagrangian that breaks lepton-number conservation. Although this mass term is forbidden by the gauge group of the SM, it could arise as the low-energy realisation of a higher-energy theory. This can explain both the existence of neutrino masses and their smallness, because a strong suppression is induced by the new heavy scale. Theorists are working hard to understand what the new theory at high energy might be. The ultimate theory behind neutrino masses must also explain the observed mixing structure, the presence of CP violation (if observed), and why the lepton sector contains large angles that are different to the quark sector. Many approaches have been proposed, for instance the use of continuous or discrete flavour symmetries, but no unique underlying principle has yet been identified.
The simplest and most studied extension beyond the SM for neutrino masses is the “see-saw type I” mechanism. Because RH neutrinos are completely neutral with respect to the SM gauge symmetries, they could be much heavier than the other known fermions. The Lagrangian would then contain both a Yukawa coupling with the Higgs, as for the quarks, and a Majorana mass term, M, for the RH neutrinos. Once the neutral Higgs boson gets a vacuum expectation value, light masses for the neutrinos arise that are proportional to the square of the Yukawa couplings and suppressed by M. Taking an order-one Yukawa coupling and M of around 1014 GeV, we obtain a sub-eV neutrino mass scale as required by the data and, because the lepton number is violated by M, the light neutrinos will be Majorana particles.
This is by no means the only way to give origin to neutrino masses. First of all, since the RH neutrino masses can take any value, the scale of the see-saw mechanism could be lowered even below the electroweak scale, allowing some models to be tested at the LHC. Typical signatures are same-sign dileptons with no missing energy, indicating lepton-number violation, and flavour-violating multi-lepton events. Several searches have been conducted by the LHC’s ATLAS and CMS collaborations, but so far no positive hint has been found. The heavy particles responsible for the see-saw mechanism could also be different: a fermion triplet in see-saw type III and a scalar triplet in see-saw type II models. Some models, such as radiative and R-parity-violating supersymmetric models, do not invoke the see-saw mechanism at all.
With so many possibilities, clearly one needs to hunt for other beyond-SM signatures to try to identify the origin of neutrino masses. Leptogenesis is a key one. To generate dynamically a baryon asymmetry in the early universe, the three Sakharov conditions need to be satisfied: lepton or baryon number violation, C and CP violation, and an out-of-equilibrium state (satisfied by the expansion of the universe). The see-saw mechanism can satisfy all of these conditions. In the early universe, RH neutrinos got out of equilibrium once the temperature dropped below their mass. Thanks to their decays into leptons and Higgs bosons, a net lepton asymmetry could arise if the rate in one channel and the conjugated one are different due to CP violation. This asymmetry would then be converted into a baryon asymmetry by non-perturbative SM effects. Observing CP violation in future neutrino-oscillation experiments and lepton-number violation in neutrinoless double-beta-decay searches would therefore provide strong hints that leptogenesis is at the origin of the baryon asymmetry of the universe.
The IceCube experiment at the South Pole has been one of the pioneers of the field of neutrino astronomy. During a seven-year-long construction campaign that ended in 2010, the 325 strong IceCube collaboration transformed a cubic kilometre of ultra-transparent Antarctic ice into a giant Cherenkov detector. Today, 5160 optical sensors are suspended beneath the ice to detect Cherenkov light from charged particles produced when high-energy neutrinos from the cosmos interact with nuclei in the detector. So far, IceCube has detected neutrinos with energies in the range 1011–1016 eV, which include the most energetic neutrinos ever recorded (see image of the proposed Gen2 array). However, we do not yet know where these neutrinos come from. For this reason, the IceCube collaboration is developing designs for an expanded “Gen2” detector.
IceCube observes astrophysical neutrinos in two ways. The first approach selects upgoing events by using the Earth to filter out the large flux of cosmic-ray muons. At low energies (below 100 TeV), the measured flux of muon neutrinos is consistent with an atmospheric origin, whereas at higher energies, a clear excess of events with a significance of 5.6σ is observed. The second approach selects neutrinos that interact inside the detector. A total of 54 cosmic-neutrino events with energies ranging from 30–2000 TeV were detected during four years of operation, excluding a purely atmospheric explanation at the level of 6.5σ. Although there is some tension between the results from the two approaches, a combined analysis finds that the data are consistent with an at-Earth flux equally shared between three neutrino flavours, as is expected for neutrinos originating in cosmic sources.
Towards a new detector
Despite multiple searches for the locations of these sources, however, the IceCube team has yet to find any statistically significant associations. Searches for neutrinos from gamma-ray bursts and some classes of galaxies have also come up empty. Although these observations have disfavoured many promising models of the origin of cosmic rays, the ultimate goal of neutrino astronomy is to detect multiple neutrinos from a single source. This requires many hundreds of events, which would take an array of the scale of IceCube at least 20 years to detect.
To speed up data collection, an expanded IceCube collaboration is planning a greatly enhanced instrument (see image of the proposed Gen2 array) with multiple elements: an enlarged array to search for high-energy astrophysical neutrinos; a dense infill array to determine the neutrino properties (PINGU); a larger surface air-shower array to veto downgoing atmospheric neutrinos; and possibly an array of radio detectors targeting neutrinos with energies above 1017 eV. Most importantly, thanks to the clarity of the Antarctic ice, we would be able to increase the instrumented volume of this next-generation array by a factor of 10 without a corresponding increase in the number of deployed sensors – or in the cost. The Gen2 proposal would therefore see an instrumented volume of approximately 10 km3 comprising strings of optical modules, but with improved hardware and deployment methods compared with IceCube.
For the in-ice component PINGU (Precision IceCube Next Generation Upgrade), the Gen2 collaboration is exploring a number of optimised designs for the optical modules, as well as longer strings deployed with improved drilling methods. Photomultipliers (PMTs) with higher quantum efficiency will be used, as is already the case for DeepCore in IceCube, and pressure spheres with improved glass and optical gel will improve sensitivity by transmitting more ultraviolet Cherenkov light. Some designs include more than one phototube per optical module (see image), while more radical concepts envision the addition of long cylindrical wavelength shifters to improve information about the photon arrival direction. Many-PMT designs were pioneered by the KM3NeT collaboration, which is proposing to build a cubic-kilometre-sized European neutrino Cherenkov telescope in the Mediterranean Sea, but are also attractive to IceCube.
The increased complexity of these approaches would be offset by new electronics, and increased computing power will allow the use of more sophisticated software algorithms that better account for the positional dependence of the optical properties of the ice and the stochastic nature of muon energy loss. This will result in improved pointing and energy resolution of both tracks and showers and better identification of tau neutrinos. IceCube has produced a white paper for the Gen2 proposal (arXiv:1412.5106) that fits well with the US National Science Foundation’s recent identification of multi-wavelength astronomy as one of six future priorities, and a formal proposal will be completed in the next few years.
Physics in order
PINGU will build on the success of DeepCore in measuring atmospheric neutrino-oscillation parameters. It consists of a dense infill array in the centre of DeepCore with a threshold of a few GeV, allowing the ordering of the neutrino masses to be determined by matter-induced oscillations of the atmospheric neutrino flux. By precisely measuring the oscillation probability as a function of neutrino energy and zenith angle, PINGU will be able to determine which neutrino is lightest.
Like the present IceTop (a surface air-shower array that covers IceCube’s surface), an expanded surface array will tag and veto downgoing atmospheric neutrinos that are accompanied by cosmic-ray air showers. Current Gen2 designs envision a 75 km2 surface array that would allow IceCube to collect a clean sample of astrophysical neutrinos over a much larger solid angle, including the galactic centre. It will also result in much improved cosmic-ray studies and more sensitive searches for PeV photons from galactic sources. To study the highest-energy (above typically 1017 eV) neutrinos, Gen2 may also include an array of radio detectors to observe the coherent radio Cherenkov emission from neutrino-induced showers. Radio detection is now pursued by the ARA (the Askaryan Radio Array at the South Pole) and ARIANNA (located on Antarctica’s Ross Ice Shelf) experiments, but coincident observations with IceCube Gen2 would be preferable.
Of course, IceCube is not the only neutrino telescope in town. ANTARES has been taking data in the Mediterranean Sea since 2008 and will be followed by KM3NeT (CERN Courier March 2016 p12), while the Gigaton Volume Detector (Baikal-GVD) is currently being built in Lake Baikal, Russia (CERN Courier July/August 2015 p23). Seawater, lake water and Antarctic ice present different challenges and advantages to cosmic-neutrino observatories, and sites in the Northern Hemisphere benefit because the galactic centre is below the horizon. While we all benefit from friendly competition and from sharing R&D resources, size has undeniable advantages. IceCube-Gen2, should the project go ahead, will be larger than any of the proposed alternatives, and is therefore well placed to write the next chapter in neutrino astronomy.
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