On 8 September, Fermilab senior scientist and world-leading beam physicist Yuri I Alexahin died from a sudden stroke.
Yuri was born in 1948 in the Russian town of Vorkuta. After studying physics and graduating from Moscow State University, from 1971 to 1988 he worked at the Joint Institute for Nuclear Research in Dubna and, in 1980, received his PhD in physics from the Institute of High Temperatures of the USSR Academy of Sciences. In Dubna, Yuri developed an interest in the physics of accelerators and beams and, especially, of charged-particle colliders, which remained the focus of his work throughout his career. He generated brilliant ideas and made critical contributions to a number of facilities and projects. He proposed a new scheme for a tau-charm factory based on monochromatisation to reduce the collision energy spread, addressed a problem of limited dynamic aperture at high energies faced by CERN’s LEP collider, and recommended the low-emittance option for LEP operation at the W± production energies.
Yuri published pioneering works on the theory of coherent beam–beam oscillations and their stabilisation with Landau damping, laying the foundation for a parameter optimisation of the LHC. Among many other highlights, Yuri ingeniously predicted the loss of Landau damping for the two beams colliding in the LHC, derived analytical formulae describing the emittance growth in collision with transverse feedback and noise, and produced some of the most thought-provoking articles related to the LHC design.
In 2000, Yuri joined Fermilab’s accelerator division, where he made seminal contributions to the theory of nonlinear beam–beam compensation by electron lenses and was deeply engaged in Run II of the Tevatron, playing a critical role in the luminosity increases of what was then the world’s most powerful accelerator. Widely recognised is Yuri’s leading role in the design and implementation of optimal helical orbits to minimise Tevatron beam–beam effects at injection, acceleration and squeeze, and his optimisation of the beam lifetime at injection energy via reduction of the differential chromaticity.
From 2007 to 2018 Yuri led the accelerator theory group at Fermilab. These years were another extremely productive period, as he steered the interaction-region lattice development for an energy-frontier muon collider within the US Muon Accelerator Program. He also invented the so-called “helical FOFO-snake” muon ionisation cooling channel concept. Yuri was closely involved in the operation and upgrade of the existing Fermilab accelerator complex and in other intensity-frontier accelerators worldwide. Not only was he actively taking part in many experimental beam studies, but he also proposed new theoretical and numerical algorithms for space-charge dominated beams, for the Landau damping of beam instabilities provided by electron lenses and for novel space-charge compensation techniques.
Yuri will be remembered by his colleagues, friends and family as a highly intelligent, kind and soft-spoken person. An excellent mentor, he generously shared his knowledge with students and younger colleagues, and many world-renowned physicists are happy to call him their teacher. While being a workaholic, in his leisure time he loved to ski and was an avid sports fan.
Carlo Rubbia is one of three winners of the 2020 Global Energy Prize. The 39M Rouble ($0.5M) award, announced on 8 September in Kaluga, Russia by the Global Energy Association, cites the former CERN Director General for the promotion of sustainable nuclear energy use and natural-gas pyrolysis.
A renowned particle physicist, Carlo Rubbia is more widely known as the winner, alongside Simon van der Meer, of the 1984 Nobel Prize in Physics, for turning the Super Proton Synchrotron into a particle collider and using it to discover the W and Z bosons. He was appointed Director-General of CERN in 1989 in the crucial period leading up to the presentation of the Large Hadron Collider to the CERN Council in 1993.
The same year, Rubbia proposed the “energy amplifier”, which employs a particle accelerator to generate the neutrons needed to drive a nuclear reactor. Such technology promises the production of energy under sub-critical reactor conditions using thorium, with minimal if any long-lived nuclear waste compared to uranium fuels. In more recent years, he has been an advocate for using natural gas as the main source of energy worldwide, based on new CO2-free technologies.
“You have either energy from atoms or energy from nuclei,” said Rubbia on accepting the award via videoconference. “Energy from atoms is certainly the easiest thing to do… and natural gas is clean and can be used in such a way that the CO2 emissions are under control or eliminated. And you can go on until such a time you will develop an appropriate form of nuclear, which eventually will come, but will not be the nuclear of today.”
Rubbia won in the “conventional energy” category of the 2020 prize. Peidong Yang (University of California, Berkeley) topped the “non-conventional energy” category for his pioneering work in nanoparticle-based solar cell and artificial photosynthesis, and Nikolaos Hatziargyriou (University of Athens) won in the “new ways of energy application” category for using artificial intelligence to improve the stability of power grids.
There have been 42 winners of the annual prize, with 78 scientists from 20 countries put forward this year. Previous winners include another former CERN Director-General, Robert Aymar, who was recognised in 2006 for work to develop the scientific and engineering foundation of the ITER project, which seeks to demonstrate the feasibility of nuclear fusion as an energy source.
Understanding how the strong interaction binds the ingredients of atomic nuclei is the central quest of nuclear physics. Since the 1960s CERN’s ISOLDE facility has been at the forefront of this quest, producing the most extreme nuclear systems for examination of their basic characteristic properties.
A chemical element is defined by the number of protons in its nucleus, with the number of neutrons defining its isotopes. Apart from a few interesting exceptions, all elements in nature have at least one stable isotope. These form the so-called valley of stability in the nuclear chart of atomic number versus neutron number (see “Nuclear landscape” figure). Adding or removing neutrons disturbs the nuclear equilibrium and creates isotopes that are generally radioactive; the greater the proton–neutron imbalance, the faster the radioactive decay.
Most of the developments have been exported to other radioactive beam facilities around the world
The mass of a nucleus reveals its binding energy, which reflects the interplay of all forces at work within the nucleus from the strong, weak and electromagnetic interactions. Indications of sudden changes in the nuclear shape, when adding neutrons, are often revealed first indirectly as a sudden change in the mass, and can then be probed in detail by measurements of the charge radius and electromagnetic moments. Such diagnosis – performed by ion-trapping and laser-spectroscopy experiments on short-lived (from a few milliseconds upwards) isotopes – provides the first vital signs concerning the nature of nuclides with extreme proton-to-neutron ratios.
Recent mass-spectrometry measurements and high-precision measurements of nuclear moments and radii at ISOLDE demonstrate the rapid progress being made in understanding the stubborn mysteries of the nucleus. ISOLDE’s state-of-the-art laser-spectroscopy tools are also opening an era where molecular radioisotopes can be used as sensitive probes for physics beyond the Standard Model.
Tools of the trade
Progress in understanding the nucleus has gone hand in hand with the advancement of new techniques. Mass measurements of stable nuclei pioneered by Francis Aston nearly a century ago revealed a near-constant binding energy per nucleon. This pointed to a characteristic saturation of the nuclear force, which underlies the liquid-drop model and led to the semi-empirical mass formula for the nucleus developed by Bethe and von Weizsäcker. With the advent of particle accelerators in the 1930s, more isotopic mass data became available from reactions and decays, bringing new surprises. In particular, comparisons with the liquid drop revealed conspicuous peaks at certain so-called “magic” numbers (8, 20, 28, 50, 82, 126), analogous to the high atomic-ionisation potentials of the closed electron-shell noble-gas elements. These findings inspired the nuclear-shell model, developed by Maria Goeppert-Mayer and Hans Jensen, which is still used as an important benchmark today. The difference with the atomic system is that the force that governs the nuclear shells is poorly understood. This is because nucleons are themselves composite particles that interact through the complex interplay of three fundamental forces, rather than the single electromagnetic force governing atomic structure. The most important question in nuclear physics today is to describe these closed shells from fundamental principles (e.g. the strong interaction between quarks and gluons inside nucleons), to understand why shell structure erodes and how new shells arise far from stability.
A key to reaching a deeper understanding of nuclear structure is the ability to measure the size and shape of nuclei. This was made possible using the precision technique of laser spectroscopy, which was pioneered with tremendous success at ISOLDE in the late 1970s. While increased binding energy is a tell-tale sign of a deforming nucleus, it gives no specific information concerning nuclear size or shape. Closed-shell configurations tend to favour spherical nuclei, but since these are rather rare, a particularly important feature of nuclei is their deformation. Inspecting electromagnetic moments derived from the measured atomic hyperfine structure and the change in charge radii derived from its isotopic shift provides detailed information about nuclear shapes and deformation, beautifully complementing mass measurements.
During the past half-century, nuclear science at ISOLDE has expanded beyond fundamental studies to applications involving radioactive tracers in materials (including biomaterials) and the fabrication of isotopes for medicine (with the MEDICIS facility). But the bulk of the ISOLDE physics programme, around 70%, is still devoted to the elucidation of nuclear structure and the properties of fundamental interactions. These studies are carried out through nuclear reactions, by decay spectroscopy, or by measuring the basic global properties – mass and size – of the most exotic species possible.
Half a century of history
The fabrication of extreme nuclear systems requires a driver accelerator of considerable energy, and CERN’s expertise here has been instrumental. After many years receiving proton beams from a 600 MeV synchrocyclotron (the SC, now a museum piece at CERN), ISOLDE now lies just off the beam line to the Proton Synchrotron (PS), receiving 1.4 GeV beam pulses from the PS Booster (see “ISOLDE from above” figure). ISOLDE in fact receives typically 50% of the pulses in the so-called super-cycle that links the intricate complex of CERN’s injectors for the LHC.
The heart of ISOLDE is a cylindrical target that can contain various different materials. The stable nuclei in the target are dissociated by the proton impact and form exotic combinations of protons and neutrons. Heating the target (up to 2000 degrees) helps these fleeting nuclides to escape into an ionisation chamber, in which they form 1+ ions that are electrostatically accelerated to around 50 keV. Isotopes of one particular mass are selected using one of two available mass separators, and subsequently delivered to the experiments through more than a dozen beamlines. A similar number of permanent experimental setups are operated by several small international collaborations. Each year, more than 40 experiments are performed at ISOLDE by more than 500 users. More than 900 users from 26 European and 17 non-European countries around the world are registered as members of the ISOLDE collaboration.
A new era for fundamental physics research has opened up
ISOLDE sets the global standard for the production of exotic nuclear species at low energies, producing beams that are particularly amenable to study using precision lasers and traps developed for atomic physics. Hence, ISOLDE is complementary to higher energy, heavy-ion facilities such as the Radioactive Isotope Beam Factory (RIBF) at RIKEN in Japan, the future Facility for Rare Isotope Beams (FRIB) in the US, and the Facility for Antiproton and Ion Research (FAIR/GSI) in Europe. These installations produce even more exotic nuclides by fragmenting heavy GeV projectiles on a thin target, and are more suitable for studying high-energy reactions such as breakup and knock-out. Since 2001, ISOLDE has also driven low-energy nuclear-reaction studies by installing a post-accelerator that enables exotic nuclides to be delivered at MeV energies for the study of more subtle nuclear reactions, such as Coulomb excitation and transfer. Post-accelerated radioactive beams have superior optical quality compared to the GeV beams from fragment separators so that the radioactive beams accelerated in the REX and more recent HIE-ISOLDE superconducting linacs enable tailored reactions to reveal novel aspects of nuclear structure.
ISOLDE’s state-of-the art experimental facilities have evolved from more than 50 years of innovation from a dedicated and close-knit community, which is continuously expanding and also includes material scientists and biochemists. The pioneering experiments concerning binding energies, charge radii and moments were all performed at CERN during the 1970s. This work, spearheaded by the Orsay group of the late Robert Klapisch, saw the first use of on-line mass separation for the identification of many new exotic species, such as 31Na. This particular success led to the first precision mass measurements in 1975 that hinted at the surprising disappearance of the N = 20 shell closure, eight neutrons heavier than the stable nucleus 23Na. In collaboration with atomic physicists at Orsay, Klapisch’s team also performed the first laser spectroscopy of 31Na in 1978, revealing the unexpected large size of this exotic isotope. To reach heavier nuclides, a mass spectrometer with higher resolution was required, so the work naturally continued at the expanding ISOLDE facility in the early 1980s.
Meanwhile, another pioneering experiment was initiated by the group of the late Ernst-Wilhelm Otten. After having developed the use of optical pumping with spectral lamps in Mainz to measure charge radii, Otten’s group exploited ISOLDE’s first offerings of neutron-deficient Hg isotopes and discovered the unique feature of shape-staggering in 1972. Through continued technical improvements, the Mainz group established the collinear laser spectroscopy (COLLAPS) programme at ISOLDE in 1979, with results on barium and ytterbium isotopes. When tunable lasers and ion traps became available in the early 1980s, the era of high-precision measurements of radii and masses began. These atomic-physics inventions have revolutionised the study of isotopes far from stability and the initial experimental set-ups are still in use today thanks to continuous upgrades and the introduction of new measurement methods. Most of these developments have been exported to other radioactive beam facilities around the world.
Mass measurements with ISOLTRAP
ISOLTRAP is one of the longest established experiments at ISOLDE. Installed in 1985 by the group of Hans-Jürgen Kluge from Mainz, it was the first Penning trap on-line at a radioactive beam facility, spawning a new era of mass spectrometry. The mass is determined from the cyclotron frequency of the trapped ion, and bringing the technique on line required significant and continuous development, notably with buffer-gas cooling techniques for ion manipulation. Today, ISOLTRAP is composed of four ion traps, each of which has a specific function for preparing the ion of interest to be weighed.
Since the first results on caesium, published in 1987, ISOLTRAP has measured the masses of more than 500 species spanning the entire nuclear chart. The most recent results, published this year by Vladimir Manea (Paris-Saclay), Jonas Karthein (Heidelberg) and colleagues, concern the strength of the N = 82 shell closure below the magic (Z = 50) 132Sn from the masses of (Z = 48) 132,130Cd. The team found that the binding energy only two protons below the closed shell was much less than what was predicted by global microscopic models, stimulating new ab-initio calculations based on a nucleon–nucleon interaction derived from QCD through chiral effective-field theory. These calculations were previously available for lighter systems but are now, for the first time, feasible in the region just south-east of 132Sn, which is of particular interest for the rapid neutron-capture process creating elements in merging neutron stars.
The other iconic doubly magic nucleus 78Ni (Z = 28, N = 50) is not yet available at ISOLDE due to the refractory nature of nickel, which slows its release from the thick target so that it decays on the way out. However, the production of copper – just one proton above – is so good that CERN’s Andree Welker and his colleagues at ISOLTRAP were recently able to probe the N = 50 shell by measuring the mass of its nuclear neighbour 79Cu, finding it to be consistent with that of the doubly magic 78Ni nucleus. Masses from large-scale shell-model calculations were in excellent agreement with the observed copper masses, indicating the preservation of the N = 50 shell strength but with some deformation energy creeping in to help. Complementary observables from laser spectroscopy helped to tell the full story, with results on moments and radii from the COLLAPS and the more recent Collinear Resonance Ionization Spectroscopy (CRIS) experiments adding an interesting twist.
Laser spectroscopy with COLLAPS and CRIS
Quantum electrodynamics provides its predictions of atomic energy levels mostly by assuming the nucleus is point-like and infinitely heavy. However, the nucleus indeed has a finite mass as well as non-zero charge and current distributions, which impact the fine structure. Thus, complementary to the high-energy scattering experiments used to probe nuclear sizes, the energy levels of orbiting electrons offer a marvellous probe of the electric and magnetic properties of the nucleus. This fact is exploited by the elegant technique of laser spectroscopy, a fruitful marriage of atomic and nuclear physics realised by the COLLAPS collaboration since the late 1970s. COLLAPS uses tunable continuous-wave lasers for high-precision studies of exotic nuclear radii and moments, and similar setups are now running at other facilities, such as Jyvaskyla in Finland, TRIUMF in Canada and NSCL-MSU in the US.
A recent highlight from COLLAPS, obtained this year by Simon Kaufmann of TU Darmstadt and co-workers, is the measurement of the charge radius of the exotic, semi-magic isotope 68Ni. Such medium-mass exotic nuclei are now in reach of the modern ab-initio chiral effective-field theories, which reveal a strong correlation between the nuclear charge radius and its dipole polarisability. With both measured for 68Ni, the data provide a stringent benchmark for theory, and allow researchers to constrain the point-neutron radius and the neutron skin of 68Ni. The latter, in turn, is related to the nuclear equation-of-state, which plays a key role in supernova explosions and compact-object mergers, such as the recent neutron-star merger GW170817.
Building on pioneering work by COLLAPS, the collinear laser beamline, CRIS, was constructed at ISOLDE 10 years ago by a collaboration between the groups of Manchester and KU Leuven. In CRIS, a bunched atom beam is overlapped with two or three pulsed laser beams that are resonantly laser-ionised via a particular hyperfine transition. These ions are then deflected from the remaining background atoms and counted in quasi background-free conditions. CRIS has dramatically improved the sensitivity of the collinear laser spectroscopy method so that beams containing just a few tens of ions per second can now be studied with the same resolution as the optical technique of COLLAPS.
Ruben de Groote of KU Leuven and co-workers recently used CRIS to study the moments and charge radii of the copper isotopes up to 78Cu, providing critical information on the wave function and shape of these exotic neighbours, and insight on the doubly magic nature of 78Ni. Both the ISOLTRAP and CRIS results provide a consistent picture of fragile equilibrium in 78Ni, where the failing strength of the proton and neutron shell closures is shored up with binding energy brought by slight deformation.
These precision measurements in new regions of the nuclear chart bring complementary observables that must be coherently described by global theoretical approaches. They have stimulated and guided the development of new ab-initio results, which now allow the properties of extreme nuclear matter to be predicted. While ISOLDE cannot produce absolutely all nuclides on the chart (for example, the super-heavy elements), precision tests in other, key regions provide confidence in the global-model predictions in regions unreachable by experiment.
Searches for new physics
By combining the ISOLDE expertise in radioisotope production with the mass spectrometry feats of ISOLTRAP and the laser spectroscopy prowess from the CRIS and RILIS (Resonant Ionization Laser Ion Source) teams, a new era for fundamental physics research has opened up. It is centred on the ability of ISOLDE to produce short-lived radioactive molecules composed of heavy pear-shaped nuclei, in which a putative electric dipole moment (EDM) would be amplified to offer a sensitive test of time-reversal and other fundamental symmetries. Molecules of radium fluoride (RaF) are predicted to be the most sensitive probes for such precision studies: the heavy mass and octupole-deformed (pear shape) of some radium isotopes, immersed in the large electric field induced by the molecular RaF environment, makes these molecules very sensitive probes for symmetry-violation effects, such as the existence of an EDM. However, these precision studies require laser cooling of the RaF molecules, and since all isotopes of Ra are radioactive, the molecular spectroscopy of RaF was only known theoretically.
This year, for the very first time, an ISOLDE collaboration led by CRIS collaborator Ronald Garcia Ruiz at CERN was able to produce, identify and study the spectroscopy of RaF molecules, containing different long-lived radioisotopes of radium. Specific Ra isotopes were chosen because of their octupole nature, as revealed by experiments at the REX- and HIE-ISOLDE accelerators in 2013 and 2020. The measured molecular excitation spectral properties provide clear evidence for an efficient laser-cooling scheme, providing the first step towards precision studies.
Many interesting new-physics opportunities will open up using different kinds of radioactive molecules tuned for sensitivity to specific symmetry violation aspects to test the Standard Model, but also with potential impact in nuclear physics (for example, enhanced sensitivity to specific moments), chemistry and astrophysics. This will also require dedicated experimental set-ups, combining lasers with traps. The CRIS collaboration is preparing these new set-ups, and the ability to produce RaF and other radioactive molecules is also under investigation at other facilities, including TRIUMF and the low-energy branch at FRIB. More than 50 years after its breakthrough beginning, ISOLDE continues to forge new paths both in applied and fundamental research.
Just a few years after the discovery of the neutron by James Chadwick in 1932, investigations into the properties of neutrons by Fermi and others revealed the strong energy dependence of the neutron’s interactions with matter. This knowledge enabled the development of sustainable neutron production by fission, opening the era of atomic energy. The first nuclear-fission reactors in the 1940s were also equipped with the capacity for materials irradiation, and some provided low-energy (thermal) neutron beams of sufficient intensity for studies of atomic and molecular structure. Despite the high cost of investment in nuclear-research reactors, neutron science flourished to become a mainstay among large-scale facilities for materials research around the world.
The electrical neutrality of neutrons allows them to probe deep into matter in a non-destructive manner, where they scatter off atomic nuclei to reveal important information about atomic and molecular structure and dynamics. Neutrons also carry a magnetic moment. This property, combined with their absence of electric charge, make neutrons uniquely sensitive to magnetism at an atomic level. On the downside, the absence of electric charge means that neutron-scattering cross sections are much weaker than they are for X-rays and electrons, making neutron flux a limiting factor in the power of this method for scientific research.
Throughout the 1950s and 1960s, incremental advances in the power of nuclear-research reactors and improvements in moderator design provided increasing fluxes of thermal neutrons. In Europe these developments culminated in the construction of the 57 MW high-flux reactor (HFR) at the Institut Laue-Langevin (ILL) in Grenoble, France, with a compact core containing 9 kg of highly enriched uranium enabling neutron beams with energies from around 50 μeV to 500 meV. When the HFR came into operation in 1972, however, it was clear that nuclear-fission reactors were already approaching their limit in terms of steady-state neutron flux (roughly 1.5 × 1015 neutrons per cm2 per second).
Spallation has long been hailed as the method with the potential to push through to far greater neutron fluxes
In an effort to maintain pace with advances in other methods for materials research, such as synchrotron X-ray facilities and electron microscopy, accelerator-based neutron sources were established in the 1980s in the US (IPNS and LANSCE), Japan (KENS) and the UK (ISIS). Spallation has long been hailed as the method with the potential to push through to far greater neutron fluxes, and hence to provide a basis for continued growth of neutron science. However, after nearly 50 years of operation, and with 10 more modern medium- to high-flux neutron sources (including five spallation sources) in operation around the world, the HFR is still the benchmark source for neutron-beam research. Of the spallation sources, the most powerful (SNS at Oak Ridge National Laboratory in the US and J-PARC in Japan) have now been in operation for more than a decade. SNS has reached its design power of 1.4 MW, and J-PARC is planning for tests at 1 MW. At these power levels the sources are competitive with ILL for leading-edge research. It has long been known that the establishment of a new high-flux spallation neutron facility is needed if European science is to avoid a severe shortage in access to neutron science in the coming years (CERN Courier May/June 2020 p49).
Unprecedented performance
The European Spallation Source (ESS), with a budget of €1.8 billion (2013 figures), is a next-generation high-flux neutron source that is currently entering its final construction phase. Fed by a 5 MW proton linac, and fitted with the most compact neutron moderator and matched neutron transport systems, at full power the brightness of the ESS neutron beams is predicted to exceed the HFR by more than two orders of magnitude.
The idea for the ESS was advanced in the early 1990s. The decision in 2009 to locate it in Lund, Sweden, led to the establishment of an organisation to build and operate the facility (ESS AB) in 2010. Ground-breaking took place in 2014, and today construction is in full swing, with first science expected in 2023 and full user operation in 2026. The ESS is organised as a European Research Infrastructure Consortium (ERIC) and at present has 13 member states: Czech Republic, Denmark, Estonia, France, Germany, Hungary, Italy, Norway, Poland, Spain, Sweden, Switzerland and the UK. Sweden and Denmark are the host countries, providing nearly half of the budget for the construction phase. Around 70% of the funding from the non-host countries is in the form of in-kind contributions, meaning that the countries are delivering components, personnel or other support services to the facility rather than cash.
The unprecedented brightness of ESS neutrons will enable smaller samples, faster measurements and more complex experiments than what is possible at existing neutron sources. This will inevitably lead to discoveries across a wide range of scientific disciplines, from condensed-matter physics, solid-state chemistry and materials sciences, to life sciences, medicine and cultural heritage. A wide range of industrial applications in polymer science and engineering are also anticipated, while new avenues in fundamental physics will be opened (see “Fundamental physics at the ESS” panel).
Fundamental physics at the ESS
The ESS will offer a multitude of opportunities for fundamental physics with neutrons, neutrinos and potentially other secondary particles from additional target stations. While neutron brightness and pulse time structure are key parameters for neutron scattering (the main focus of ESS experiments), the total intensity is more important for many fundamental-physics experiments.
A cold neutron-beam facility for particle physics called ANNI is proposed to allow precision measurements of the beta decay, hadronic weak interactions and electromagnetic properties of the neutron. ANNI will improve the accuracy of measurements of neutron beta decay by an order of magnitude. Experiments will probe a broad range of new-physics models at mass scales from 1 to 100 TeV, far beyond the threshold of direct particle production at accelerators, and resolve the tiny effects of hadronic weak interactions, enabling quantitative tests of the non-perturbative limit of quantum chromodynamics.
Another collaboration is proposing a two-stage experiment at the ESS to search for baryon-number violation. The first stage, HIBEAM, will look for evidence for sterile neutrinos. As a second stage, NNBAR could be installed at the large beam port, with the purpose to search for oscillations between neutrons and anti-neutrons. Observing such a transition would show that the baryon number is violated by two units and that matter containing neutrons is unstable, potentially shedding light on the observed baryon asymmetry of the universe.
A design study, financed through the European Commission’s Horizon 2020 programme, is also under way for the ESS Neutrino Super Beam (ESSνSB) project. This ambitious project would see an accumulator ring and a separate neutrino target added to the ESS facility, with the aim of sending neutrinos to a large underground detector in mid-Sweden, 400–500 km from the ESS. Here, the neutrinos would be detected at their second oscillation maximum, giving the highest sensitivity for discovery and/or measurement of the leptonic CP-violating phase. An accumulator ring and the resulting short proton pulses needed by ESSνSB would open up for other kinds of fundamental physics as well as for new perspectives in neutron scattering, and muon storage rings.
Finally, a proposal has been submitted to ESS concerning coherent neutrino–nucleus scattering (CEνNS). The high proton beam power together with the 2 GeV proton energy will provide a 10 times higher neutrino flux from the spallation target than previously obtained for CEνNS. Measured for the first time by the COHERENT collaboration in 2017 at ORNL’s Spallation Neutron Source, CEνNS offers a new way to probe the properties of the neutrino including searches for sterile neutrinos and a neutrino magnetic moment, and could help reduce the mass of neutrino detectors.
From the start, the ESS has been driven by the neutron-scattering community, with strong involvement from all the leading neutron-science facilities around Europe. To maximise its scientific potential, a reference set of 22 instrument concepts was developed from which 15 instruments covering a wide range of applications were selected for construction. The suite includes three diffractometers for hard-matter structure determination, a diffractometer for macromolecular crystallography, two small-angle scattering instruments for the study of large-scale structures, two reflectometers for the study of surfaces and interfaces, five spectrometers for the study of atomic and molecular dynamics over an energy range from a few μeV to several hundred meV, a diffractometer for engineering studies and a neutron imaging station (see “ESS layout” figure). Given that the ESS target system has the capacity for two neutron moderators and that the beam extraction system allows viewing of each moderator by up to 42 beam ports, there is the potential for many more neutron instruments without major investment in the basic infrastructure. The ESS source also has a unique time structure, with far longer pulses than existing pulsed sources, and an innovative bi-spectral neutron moderator, which allows a high degree of flexibility in the choice of neutron energy.
Accelerator and target
Most of the existing spallation neutron sources use a linear accelerator to accelerate protons to high energies. The particles are stored in an accumulator ring and are then extracted in a short pulse (typically a few microseconds in length) to a heavy-metal spallation target such as tungsten or mercury, which have a high neutron yield. A notable exception is SINQ at PSI, which uses a cyclotron that produces a continuous beam.
ESS has a linear accelerator but no accumulator ring, and it will thus have far longer proton pulses of 2.86 ms. This characteristic, combined with the 14 Hz repetition rate of the ESS accelerator, is a key advantage of the ESS for studies of condensed matter, because it allows good energy resolution and broad dynamic range. The result is a source with unprecedented flexibility to be optimised for studies from condensed-matter physics and solid-state chemistry, to polymers and the biological sciences with applications to medical research, industrial materials and cultural heritage. The ESS concept is also of major benefit for experiments in fundamental physics, where the total integrated flux is a main figure of merit.
The high neutron flux at ESS is possible because it will be driven by the world’s most powerful particle accelerator, in terms of MW of beam on target. It will have a proton beam of 62.5 mA accelerated to 2 GeV, with most of the energy gain coming from superconducting radio-frequency cavities cooled to 2 K. Together with its long pulse structure, this gives 5 MW average power and 125 MW of peak power. For proton energies around a few GeV, the neutron production is nearly proportional to the beam power, so the ratio between beam current and beam energy is to a large extent the result of a cost optimisation, while the pulse structure is set by requirements from neutron science.
The neutrons are produced by spallation when the high-energy protons hit the rotating tungsten target. The 2.5 m-diameter target wheel consists of 36 sectors of tungsten blocks inside a stainless-steel disk. It is cooled by helium gas, and it rotates at approximately 0.4 Hz, such that successive beam pulses hit adjacent sectors to allow adequate heat dissipation and limiting radiation damage. The neutrons enter moderator–reflector systems above or below the target wheel. The unique ESS “butterfly” moderator design consists of interpenetrating vessels of water and parahydrogen, and allows viewing of either or both vessels from a 120° wide array of beam ports on either side. The moderator is only 3 cm high, ensuring the highest possible brightness. Thus each instrument is fed by an intense mix of thermal (room temperature) and cold (20 K) neutrons that is optimised to its scientific requirements. The neutrons are transported to the instruments through neutron-reflecting guides that are up to 165 m long. Neutron optics are quite challenging, due to the weak cross-sections, which makes the technology for transporting neutrons sophisticated. The guides consist of optically flat glass or metal channels coated with many thin alternating layers of nickel and titanium, in a sequence designed to enhance the critical angle for reflection. The optical properties of the guides allow for broad spectrum focusing to maximise intensity for varying sample sizes, typically in the range from a few mm3 to several cm3.
Under construction
Construction of the ESS has been growing in intensity since it began in 2014. The infrastructure part was organised differently compared to other scientific large-scale research facilities. A partnering collaboration agreement was set up with the main contractor (Skanska), with separate agreements for the design and target cost settled at the beginning of different stages of the construction to make it a shared interest to build the facility within budget and schedule.
Every year, up to 3000 researchers from all over the world are expected to carry out around 1000 experiments
Today, all the accelerator buildings have been handed over from the contractor to ESS. The ion source, where the protons are produced from hydrogen gas, was delivered from INFN in Catania at the end of 2017. After installation, testing and commissioning to nominal beam parameters, the ion source was inaugurated by the Swedish king and the Italian president in November 2018. Since then, the radio-frequency quadrupole and other accelerator components have been put into position in the accelerator tunnel, and the first prototype cryomodule has been cooled to 2 K. There is intense installation activity in the accelerator, where 5 km of radio-frequency waveguides are being mounted, 6000 welds of cooling-water pipes performed and 25,000 cables being pulled. The target building is under construction, and has reached its full height of 31 m. The large target vacuum vessel is due to arrive from in-kind partner ESS Bilbao in Spain later this year, and the target wheel in early 2021.
The handover of buildings for the neutron instruments started in September 2019, with the hall of the long instruments along with the buildings housing associated laboratories and workshops. While basic infrastructure such as the neutron bunker and radiation shielding for the neutron guides are provided by ESS in Lund, European partner laboratories are heavily involved in the design and construction of the neutron instruments and the sample-environment equipment. ESS has developed its own detector and chopper technologies for the neutron instruments, and these are being deployed for a number of the instruments currently under construction. In parallel, the ESS Data Management and Software Centre, located in Copenhagen, Denmark, is managing the development of instrument control, data management and visualisation and analysis systems. During full operation, the ESS will produce scientific data at a rate of around 10 PB per year, while the complexity of the data-handling requirements for the different instruments and the need for real-time visualisation and processing add additional challenges.
The major upcoming milestones for the ESS project are beam-on-target, when first neutrons are produced, and first-science, when the first neutron-scattering experiments take place. According to current schedules, these milestones will be reached in October 2022 and July 2023, respectively. Although beam power at the first-science milestone is expected to be around 100 kW, performance simulations indicate that the quality of results from first experiments will still have a high impact with the user community. The initiation of an open user programme, with three or more of the neutron instruments beginning operation, is expected in 2024, with further instruments becoming available for operation in 2025. When the construction phase ends in late 2025, ESS is expected to be operating at 2 MW, and all 15 neutron instruments will be in operation or ready for hot-commissioning.
The ESS has been funded to provide a service to the scientific community for leading-edge research into materials properties. Every year, up to 3000 researchers from all over the world are expected to carry out around 1000 experiments there. Innovation in the design of the accelerator, the target system and its moderators, and in the key neutron technologies of the neutron instruments (neutron guides, detectors and choppers), ensure that the ESS will establish itself at the vanguard of scientific discovery and development well into the 21st century. Furthermore, provision has been made for the expansion of the ESS to provide a platform for leading-edge research into fundamental physics and as yet unidentified fields of research.
On 19 June the prime minister of Estonia, Jüri Ratas, and CERN Director-General, Fabiola Gianotti, signed an agreement admitting Estonia as an associate member state in the pre-stage to membership of CERN. The agreement will enter into force once CERN has been informed by the Estonian authorities that all the necessary approval processes have been finalised.
“With Estonia becoming an associate member, Estonia and CERN will have the opportunity to expand their collaboration in, and increase their mutual benefit from, scientific and technological development as well as education and training activities,” said CERN Director-General Fabiola Gianotti. “We are looking forward to strengthening our ties further.”
Many important opportunities open up for Estonian entrepreneurs, scientists and researchers
Jüri Ratas
After joining the CMS experiment in 1997, Estonia became an active member of the CERN community. Between 2004 and 2016 new collaboration frameworks gradually boosted scientific and technical co-operation. Today, Estonia is represented by 25 scientists at CERN, comprising an active group of theorists, researchers involved in R&D for the Compact Linear Collider project, a CMS team involved in data analysis and the Worldwide LHC Computing Grid, and another team taking part in the TOTEM experiment.
CERN’s associate member states are entitled to participate in meetings of the CERN Council, Finance Committee and Scientific Policy Committee. Their nationals are eligible for staff positions and fellowships, and their industries are entitled to bid for CERN contracts.
“As an associate member, many important opportunities open up for Estonian entrepreneurs, scientists and researchers to work together on innovation and R&D, which will greatly benefit Estonia’s business sector and the economy as a whole,” said Jüri Ratas, Estonia’s prime minister, at the signing ceremony. “Becoming an associate member is the next big step for Estonia to deepen its co-operation with CERN before becoming a full member.”
The benefits of CERN membership go well beyond science and technology, confirms a study commissioned by the UK’s Science and Technology Facilities Council (STFC). The report “Evaluation of the benefits that the UK has derived from CERN”, published on 6 August, finds that around 500 UK firms have benefitted from supplying goods and services to CERN during the past decade, bringing in £183.3M in revenue. An additional £33.4M was awarded to UK firms for CERN experiments and from the CERN pension fund, while a further £1B in turnover and £110M in profit is estimated to have resulted from knock-on effects for UK companies after working with CERN.
Over the same 10-year period, 1000 or so individuals who have participated in CERN’s various employment schemes have received training estimated to be worth more than £4.9M. The knowledge and skills gained via working at CERN are deployed across sectors including IT and software, engineering, manufacturing, financial services and health, the report notes, with young UK researchers who have engaged with CERN estimated to earn 12% more across their careers (corresponding to an extra £489M in additional wages in the past 10 years).
Each year an average of 12,000 school students and other members of the public visit CERN in person; 220,000 visit CERN’s website; and 40,000 interact with its social media. More than 1000 teachers have attended CERN’s national teacher programme in the past decade, who go on to teach an estimated 175,000 school students within three months of their visit. A survey of 673 physics undergraduates in eight UK universities revealed that 95% were attracted to study science because of activities in particle physics, with more than 50% saying they were inspired by the discovery of the Higgs boson.
In terms of science diplomacy, the report acknowledges that CERN provides a platform for the UK to engage more widely in global initiatives and international networks, spilling over to favourable perceptions of its members and greater engagement in science, technology and beyond. “Fundamental research requires long-term engagement; international collaboration makes this essential pooling of efforts possible, and the report provides a promising testimony for the future of CERN membership,” said Charlotte Warakaulle, CERN director of international relations.
Being part of one of the biggest international scientific collaborations on the planet places the UK at the frontier of discovery science
Mark Thomson
Carried out by consulting firm Technopolis, the study also quantified the scientific benefits of CERN membership. Over the past decade, more than 20,000 scientific papers with a UK author have cited one of the 40,000 papers based directly on CERN research published in the past 20 years. The report estimates that the production of knowledge can be valued at more than £495M, before even considering the impact of the advances that this research may underpin. Bibliometric analyses also show that CERN research underpins many of the UK’s most influential physics papers.
The new report supports previous studies into the benefits of CERN membership. In particular, a recent study of the impact of the High Luminosity LHC conducted by economists at the University of Milan concluded that the quantifiable return to society is well in excess to the project’s costs (CERN Courier September 2018 p51).
The UK is one of CERN’s founding members, and currently contributes £144M per year to the CERN budget (representing 16% of Member State subscriptions) via the STFC. “Being part of one of the biggest international scientific collaborations on the planet places the UK at the frontier of discovery science, which in turn helps to inspire the next generation to study physics and other STEM subjects,” says STFC executive chair Mark Thomson. “This is of huge value to the UK – and for the first time this report goes some way to quantify this.”
A new international design study for a future muon collider began in July, following the recommendations of the 2020 update of the European strategy for particle physics (CERN Courier July/August 2020 p7). Initiated by the Large European Laboratory Directors Group, which exists to maximise co-operation in the planning, preparation and execution of future projects, the study will initially be hosted at CERN, and carried out in collaboration with international partners. Institutes can join by expressing their intent to collaborate via a Memorandum of Understanding. The goal of the study is to evaluate the feasibility of both the accelerator and its physics experiments (CERN Courier May/June 2020 p41). CERN’s Daniel Schulte has been appointed as interim project leader.
The discovery of the Higgs boson in 2012 was the culmination of almost five decades of research, beginning in 1964 with the theoretical proposal of the Brout–Englert–Higgs (BEH) mechanism. This discovery was monumental, but was itself just a beginning, and research into the properties of the Higgs boson and the BEH mechanism, which has unique significance for the dynamics of the Standard Model, stretches the horizons of even the most ambitious future-collider proposal. Despite this, the ATLAS and CMS collaborations have already made three major discoveries relating to the Higgs boson. These are the jewels in the crown of LHC research so far: an elementary spin-zero particle, the mechanism that makes the weak interaction short range, and the mechanism that gives the third-generation fermions their masses. They can be related to three distinct classes of measurements: the decay of the Higgs boson into two photons, and its production from and decays into the weak force carriers and third-generation fermions, respectively.
Until 2012, the list of elementary particles could be divided into just two broad classes: spin-1/2 matter particles (fermions) and spin-1 force carriers (vector bosons), with a spin-2 force carrier (the graviton) pencilled in by most theorists to mediate the gravitational force. The first jewel in the LHC’s crown is the discovery of an elementary spin-0 particle – the first and only particle of this type to have been discovered. The question of the spin of the Higgs boson is intrinsically linked to the dominant discovery mode in 2012: the decay into two photons. Conservation laws insist that only a spin-0 or spin-2 particle can decay into two photons.
To decide between the two spin options, a more complex study than just measuring decay rates was needed. The spin of the parent particle affects the angular distributions of the daughter particles of Higgs-boson decays. Studies began immediately within ATLAS and CMS, showing unambiguously that the newly discovered particle was spin-0. The ways in which this particle is produced and the ways in which it decays call for its identification with the only particle that was predicted by the Standard Model of particle physics that had not been observed by 2012 – the Higgs boson. The field related to this particle is the BEH field.
The next question was whether this new particle is elementary or composite. If the Higgs boson is actually a composite spin-0 particle, then there should be a whole series of new composite particles with different quantum numbers – in particular, spin-1 particles whose mass scale is roughly inversely proportional to the distance scale that characterises their internal structure.
One can test the question of whether the Higgs boson is elementary or composite in three ways. Firstly: indirectly. The virtual effects of these heavy spin-1 particles would modify the properties of the W and Z bosons. Part of the legacy of the LEP experiments, which operated at CERN between 1989 and 2000, and the SLD experiment, which operated in SLAC between 1992 and 1998, is a large class of precision measurements of these properties. The other two ways are pursued by the LHC experiments: the direct search for the new spin-1 particles, and precision measurements of properties of the Higgs boson itself, such as its couplings to electroweak vector-boson pairs, which would differ if it were composite. No such composite excitations have been discovered to date, and the Higgs boson shows no signs of internal structure down to a scale of 10–19 m – some four orders of magnitude smaller than the proton.
The electromagnetic and strong interactions are mediated by massless mediators – the photon and the gluon. Consequently, they are long-range, though colour confinement – the phenomenon that quarks and gluons cannot be isolated – renders the long-range effects of the strong interaction unobservable. By contrast, weak interactions are mediated by massive mediators – the W and Z bosons – with masses of the order of 100 times larger than that of the proton. As a result, the weak force is exponentially suppressed at distances larger than 10–18 m.
A common feature of the electromagnetic, strong and weak forces is that their mediators are all spin-1. This type of interaction is very special. By assuming that nature has certain gauge symmetries, our current quantum field theories can predict the existence of these types of interactions, and many of their features. There are numerous predictions stemming from these symmetries that have been successfully tested by experiments, such as the identical couplings between gluons and quarks of all flavours, the fact that photons don’t interact with each other, and the structure of higher-order corrections, for example the running of coupling constants and the anomalous magnetic moment of the electron and the muon. Yet, as the mass term in the Lagrangian isn’t invariant under gauge transformations, gauge symmetry predicts, at least naively, that the spin-1 force carriers should be massless. So, while the symmetries that predict the electromagnetic and strong interactions also explain why their force carriers are massless, the symmetry principle that predicts the weak interaction is challenged by the experimental fact that its force carriers are massive.
This conundrum has a possible solution if a symmetry is respected by the quantum field theory but not by the ground state of the universe (see “Broken symmetry” image). The theory’s predictions will then be different from those that would follow if the ground state were also symmetric. One way in which the symmetry can be broken is if there is a scalar field that does not vanish in the ground state. This is the case for the Higgs potential, which, unlike a purely parabolic potential, does not have rotational symmetry around its ground state. The weak-force carriers are affected by their interaction with the BEH field, and this interaction slows them down. Moving at speeds slower than the speed of light – the consequence of interacting with the BEH field in the ground state – is equivalent to having non-zero masses, making weak interactions short range. These insights also transformed our understanding of the early universe. Following the Glashow–Weinberg–Salam breakthrough shortly after the BEH proposal, the Standard Model presents a universe in which the ground state transitioned from zero to non-zero due to the spontaneous breaking of electroweak symmetry – a cosmological event that took place when the universe was about 10-11 seconds old.
A BEH field different from zero in the ground state of the universe has important observational and experimental consequences. For example, if the symmetry were unbroken, a process where a single Higgs particle decays into a pair of Z bosons would be forbidden. But, once the ground state of the universe breaks the symmetry – the BEH field is non-zero – this process is allowed to occur. (Strictly speaking, the Higgs boson cannot decay into two Z bosons because the sum of their masses is larger than the mass of the Higgs boson, however, the Higgs boson can decay into a real Z boson and a virtual one that produces a pair of fermions.) Similarly, the symmetry would not allow a single Higgs-boson production from Z-boson fusion. But, once the ground state of the universe breaks the symmetry, the latter process is also allowed to occur.
An asymmetric ground state costs the theory none of its predictive power. The strength of the interaction of the Z boson with the BEH field, measured by the mass it gains from this interaction, is closely related to the strength of the interaction of the Z boson with the Higgs particle, measured by the rate at which the Higgs boson decays into two Z bosons, or by the rate at which it is produced by Z-boson fusion. This relation is commonly expressed as the ratio μZZ* between the measured and the predicted rates: if the field related to the newly discovered spin-0 particle is indeed responsible for the mass of the Z boson, then μZZ* = 1.
ATLAS and CMS have established a new law of nature
The rate of the Higgs decay into two Z bosons was first measured with 5σ significance by the ATLAS and CMS experiments in 2016. Its current value is μZZ* ≈ 1.2 ± 0.1. The rate at which the Higgs boson decays into a pair of W bosons was measured in the same year. Its current value of μWW* ≈1.2 ± 0.1 also corresponds to the strength of interaction that would give the W boson its mass. Finally, the experiments measured the rate at which a single Higgs boson is produced in vector-boson fusion to be μVBF ≈ 1.2 ± 0.2. Thus, ATLAS and CMS have established a new law of nature: the force carriers of the weak interaction gain their masses via their interactions with the everywhere-present BEH field. The strength of this interaction is precisely the right size to limit the effects of the weak interaction to distances shorter than 10–18 metres.
Third generation, third jewel
The third jewel in the crown of the LHC is the explanation for how the tau-lepton and the top and bottom quarks – members of the third, heaviest fermion family – gain their masses. The same electroweak symmetry that predicts that the weak-force carriers should be massless also predicts that all 12 spin-1/2 matter particles known to us should also be massless. Experiments have shown, however, that all the matter particles are massive, with the one possible exception of the lightest neutrino. The fact that this symmetry is broken in the ground state of the universe also opens the door to the possibility that matter particles gain masses. But via what mechanism? For the ground state of the BEH field to slow down the fermions as well as the W and Z bosons, a new type of interaction has to exist: an interaction with a spin-0 mediator – the Higgs boson itself. Discovering a Higgs-boson decay into a pair of fermions would mean the discovery of this new type of spin-0 mediated interaction, which was first proposed in a different context by Hideki Yukawa in the 1930s.
Yukawa interactions are fundamentally different from the interactions through which the W and Z bosons get their mass because they are not deduced from a symmetry principle. Another difference, in contrast not only to weak, but also to strong and electromagnetic interactions, is that the interaction strength is not quantised. However, the strength of the interaction of a matter particle with the BEH field, measured by the mass it gains from this interaction, is still closely related to the strength of the Yukawa interaction of that matter particle with the Higgs boson, measured by the rate at which the Higgs boson decays into two such fermions. Once again, if the field that gives the matter particles their masses is indeed the one related to the newly discovered spin-0 particle, then the measured decay rate of the Higgs particle to fermion pairs should give a value of unity to the corresponding μ-ratio.
The three heaviest spin-1/2 particles – the top quark, the bottom quark and the tau lepton – are expected to have the strongest couplings to the Higgs boson, and consequently the largest rates of Yukawa interactions with it. The first Yukawa interaction to be measured, with the significance in both the ATLAS and CMS analyses rising to 5σ in 2015, concerned the decay of a Higgs boson into a tau lepton–antilepton pair. The current decay rate is μτ+τ– ≈ 1.15 ± 0.15, which, within present experimental accuracy, corresponds to the strength of interaction that would give the tau lepton its mass. The rate of Higgs-boson decays into the bottom quark–antiquark pair was measured by ATLAS and CMS three years later. The current value is μbb– ≈ 1.04 ± 0.13. Within present experimental accuracy, this corresponds to the strength of interaction that would give the bottom quark its mass.
The potential of the LHC to discover new facts about nature and the universe is far from saturated
In the case of the top quark, the Higgs boson has a vanishingly tiny decay rate into a top–antitop pair, because the mass of each is individually larger than that of a Higgs boson, and both would have to be produced virtually. To extract the strength of the Higgs–top interaction, experiments instead measure the rate at which this trio of particles is produced. The rate of the production of a Higgs boson together with a top quark–antiquark pair was measured by the ATLAS and CMS experiments in 2018. The current value is μtt–h ≈ 1.3 ± 0.2. Within present experimental accuracy, this value corresponds to the strength of interaction that would give the top quark its mass. (The remaining third-generation particle, a neutrino, is at least 12 orders of magnitude lighter than the top quark, and is suspected to derive its mass via a different mechanism, which is unlikely to be tested experimentally in the near future.)
ATLAS and CMS have therefore discovered a new fact about nature: the third-generation charged particles – the tau lepton, the bottom quark and the top quark – also gain their masses via their interaction with the everywhere-present BEH field. This is also the discovery of the new and rather special Yukawa interactions among elementary particles, which are mediated by a spin-0 force carrier, the Higgs boson.
The path forward
Answering questions about nature’s fundamental workings almost always leads to new questions. The discovery of the Higgs boson has already been the source of at least two. Firstly, the value of the Higgs boson’s mass suggests the possibility that our universe is likely in an unstable state. In the extremely distant future, a transition to an entirely different universe with a different ground state could occur. Should this remain true as precision improves, not only is there nothing special about Earth, nor the solar system, nor even Milky Way galaxy, but the fundamental structure of the universe is itself only temporary. What’s more, the lightness of the mass of the Higgs boson compared to both the Planck scale (above which quantum-gravity effects become significant) and the “seesaw scale” (below which new particles, beyond those of the Standard Model, are predicted to exist), poses a challenge to the basic framework that we use to formulate the laws of nature. In quantum field theory, cancellations between tree-level and higher order loop-diagram contributions to the mass of the Standard Model Higgs boson are huge, and require extreme fine-tuning, perhaps by as many as 32 orders of magnitude, between seemingly unrelated constants of nature. Various ideas of how to restore “naturalness”, such as supersymmetry and Higgs compositeness, have been suggested, but the LHC experiments have not uncovered any of the TeV-scale particles predicted by these models and are ruling out ever-increasing swathes of parameter space for the models.
The potential of the LHC to discover new facts about nature and the universe is far from saturated. There are at least two additional, big open questions that are guaranteed to be answered, at least in part, by the LHC experiments. First is the understanding of the mechanism that gives second-generation particles – in particular the muon and the charm quark – their masses. That may be the same mechanism as the one that has been shown to give the third-generation fermions masses, or it may be different (for the latest progress, see Turning the screw on H → μμ). Second is the question of what happened at the electroweak phase transition in the early universe? It may have been a smooth crossover, where the value of the BEH field changed from zero to its present value continuously and uniformly in space, as predicted by the combination of the Standard Model of particle physics and the Big Bang model, or it may have been a first-order phase transition, where bubbles with a finite value of the BEH field nucleated within the surrounding plasma. A first-order phase transition could open the door to a new mechanism to explain the matter–antimatter imbalance in the universe. These deep questions depend on a new chapter of Higgs research concerning the self-interaction of the Higgs boson, which will be carried forward by a future collider.
Beyond constituting amazing intellectual and technological achievements, the LHC experiments have already made a series of profound discoveries about nature. The existence of a spin-0 particle whose non-zero force field is responsible for both the short range of weak interactions and, in a distinct way, the masses of spin-1/2 particles, represents three major discoveries. That theorists have long speculated on these new laws of nature ideas must not diminish the significance of establishing them experimentally. These three jewels in the crown of LHC research, the first steps in the exploration of Higgs physics, begin a trek to some of the most significant open questions in particle physics and cosmology.
The existence of particles with fractional charges and fractional baryon numbers was a hard sell in 1964 when Gell-Mann and Zweig independently proposed the quark model. Physicists remained sceptical until the discovery of the J/ψ meson 10 years later. Heavier than anything previously seen and extremely narrow, with a width of just 0.1 MeV and a mass of 3097 MeV, the J/ψ pointed to the existence of a new quark with its own quantum number. This confirmed Glashow, Iliopoulos and Maiani’s 1970 hypothesis, which they cooked up to explain peculiarities in rare kaon decays. Any doubt as to the existence of a charm–anticharm system was eliminated by observing narrow excitations of the J/ψ, which lined up as expected in non-relativistic quantum mechanics. The spectrum of charmonium mesons soon became populated by states with widths up to hundreds of MeV as their masses surpassed the threshold for decaying to a pair of “open-charm” mesons with a single charm quark each.
Hadron spectroscopy continues to be a rich area of fundamental exploration today, with results from collider experiments over the past two decades revealing the existence of multi-quark states more exotic than the familiar mesons and baryons (CERN Courier April 2017 p31). The LHCb experiment at CERN is at the forefront of this work. Now, a structure in the J/ψ-pair mass spectrum consistent with a tetraquark state made up of two charm quarks and two charm antiquarks has been observed by the collaboration. With doubly hidden charm, the new cccc state is the most significant evidence so far for the existence of tightly bound tetraquarks composed of a pair of colour-charged “diquarks”, and sheds light on a difficult-to-model regime of quantum chromodynamics (QCD).
Multi-quark states
Gell-Mann and Zweig both acknowledged that the symmetries which led to the quark hypothesis allowed for more complicated quark configurations than just mesons (qq) and baryons (qqq). Tetraquarks (qqqq), pentaquarks (qqqqq) and hexaquarks (qqqqqq or qqqqqq) were all suggested. In the early 1970s, a deepening understanding of the dynamics of strong interactions brought about by QCD only furthered the motivation for seeking new multi-quark states. QCD not only predicted attractive forces between a quark and an antiquark, and between three quarks, but also between two quarks.
The attraction between two quarks can easily be proven when they are close together and the strong coupling constant is small enough to allow perturbative calculations. Similar interactions also likely occur in the non-perturbative regime. Such systems, known as diquarks, have the colour charge of an antiquark. (For example, red and blue combine to make an anti-green diquark.) As coloured objects, they can be confined in hadrons by partnering with other coloured constituents. A diquark can attract a quark to create a simple baryon. Alternatively, a diquark and an antidiquark can attract each other to create a tetraquark. As a result of their direct colour couplings, such compact tetraquarks can have binding energies of several hundreds of MeV.
Compact two-diquark tetraquarks stand in stark contrast to the alternative “molecular” model for tetraquarks, which was named by loose analogy with the exchange of electrons between atoms in molecules. In this picture, the tetraquark is arranged as a pair of mesons that attract each other by exchanging colour-neutral objects, such as light mesons and glueballs – an idea first proposed in 1935 by Hideki Yukawa, in the context of interactions between nucleons. Such exchanges only provide a binding energy of a few MeV per nucleon.
Molecular tetraquarks are therefore expected to be only loosely bound, with masses near the sum of the masses of their constituent mesons, however they could have rather narrow widths if their mass lies below the “fall-apart” threshold. As such states are most likely to be created without angular momentum between the mesons, the spin-parity combinations available to them are highly restricted. In contrast, a rich spectrum of radial and angular momentum excitations between the coloured constituents is predicted for diquark tetraquarks. The widths of these states could be large, as they can easily fall apart into lighter hadrons, with their binding energy transformed into a light quark–antiquark pair.
Unfortunately, it is difficult to rigorously apply QCD in the confining regime of multi-quark states. It is therefore up to experiments to discover which multi-quark states actually exist in nature. There have been some hints of tetraquark states built out of light quarks, though without definite proof. This is largely because additional light quark pairs can easily be created in the decay process of simple mesons and baryons, and the highly relativistic nature of such states makes model predictions for their excitations unreliable. Hidden charm states have proved helpful again, however, as the charmonium spectrum and the properties of such states are well predicted.
Experiments to the fore
Molecular tetraquark proposals were fuelled in 2003 by the unexpected discovery by the Belle collaboration, at the KEKB electron–positron collider in Tsukuba, Japan, of a new narrow state, right at the sum of the masses of a charmed-meson pair. Unlike other charmonium states near its mass, the state is surprisingly narrow, with a width of the order of just 1 MeV. Originally named X(3872), it is now conventionally referred to as χc1(3872), reflecting its nature as a possible triplet P-wave state with hidden charm and one unit of total angular momentum. Despite subsequent results from collider experiments around the world, there is no consensus about its exact nature, as it variously exhibits features of simple charmonium or a loosely bound molecule.
It is up to experiments to discover which multi-quark states actually exist in nature
Stronger evidence for the loose meson–meson binding of multi-quark states was provided by observations in 2013 of a hidden-charm tetraquark candidate Zc(3900) by the BES III collaboration at the BEPC II electron–positron collider in Beijing, China, and by Belle, and of the Zc(4020), also by BES III. Since they have electrically charged forms, they cannot be counted as charmonium states. They are both relatively narrow states near meson–meson thresholds for open charm, with widths of the order of tens of MeV. They are definitely tetraquarks, though it is still a moot point if they are genuinely bound states or merely manifestations of non-binding hadron–hadron forces that manifest in complicated forms. The molecular interpretation had also been reinforced in 2012 by Belle’s observations of the hidden-beauty Zb(10610) and Zb(10650) tetraquarks. These states also have relatively narrow widths of the order of tens of MeV and masses near the threshold for falling apart, in this case to “open-beauty” mesons.
Pentaquark observations have also weighed in on the debate. Last year’s observation of three narrow hidden-charm pentaquarks by the LHCb collaboration, with widths below tens of MeV and masses close to the charm meson-baryon threshold (CERN Courier May/June 2019 p15), also points to loose hadron–hadron binding, in this case between a meson and a baryon.
Bucking the trend
Yukawa-style bindings cannot, however, explain a large number of broader tetraquark-like structures with hidden charm, with widths of hundreds of MeV, which are not near any hadron–hadron threshold. Such states include the charged Zc(4430) observed by Belle in 2008 and later confirmed by LHCb in 2014, and a family of states that decay to a J/ψ φ final state, including X(4140) and X(4274), which were observed by the CDF collaboration at Fermilab in 2009 and later by CMS and LHCb at CERN. These states could be either manifestations of diquark interactions or kinematic effects near the fall-apart threshold. No single simple model can account for all of them.
Reaching states with hidden double charm (cccc) now promises new insights into multi-quark dynamics, as all the quarks are non-relativistic. Furthermore, there is no known mechanism for two charmonium mesons to be loosely bound, according to a molecular model, as no light valence quarks are available to be exchanged. Compact diquark-type tetraquarks have been predicted for such quark combinations, but it is not clear whether they might lead to experimentally detectable signatures – the tetraquarks could be too broad or their production rate too small. While collisions at the LHC provide enough energy to simultaneously produce pairs of charm–anticharm quark combinations, getting them close enough together to form diquarks is a tall order. Additionally, while observations of beauty-charm mesons such as Bc and doubly charmed baryons such as Ξcc showed that LHCb has reached the sensitivity to detect the interactions of two heavy quarks, it was unclear until recently if the interactions of diquark-model tetraquarks could be detected. The observation, reported in July, by LHCb, of a highly significant J/ψ-pair mass structure is therefore an exciting moment for the study of multi-quark dynamics.
Introducing the X(6900)
Exploiting the full data set collected from 2011 to 2018, LHCb investigated the J/ψ-pair invariant mass spectrum, where J/ψ meson candidates are reconstructed from the dimuon decay mode. A narrow peaking structure at 6900 MeV and a broader structure at approximately twice the J/ψ mass threshold was observed. The structure of X(6900) is consistent with the signature of a resonance (see figure), suggesting a four-charm-quark state.
While the peaking X(6900) structure is close to the χc0χc1 meson-pair threshold, its width, of the order of a hundred MeV, seems too large to fit into the loose-binding scheme, wherein decay modes other than the “fall-apart” topology are expected to be strongly suppressed, and in any case, there is no known loose binding mechanism between two charmonium states. Charmonium-pair re-scattering effects are also disfavoured due to the requirements of such interactions. This observation is therefore the most intriguing experimental indication so far for hadrons made out of diquarks.
It is less clear if the observed structure is made of one state, or several that may or may not interfere with each other. There is no information on the spin-parity of the observed structure. Neither do we yet know if mass structures also appear in the invariant mass spectra of other charmonium or doubly charmed baryon pairs.
This observation is the most intriguing experimental indication so far for hadrons made out of diquarks
The first LHCb upgrade is currently in progress and data taking will recommence at the beginning of LHC Run 3 in 2022, with a second upgrade phase planned to collect a much larger data set by 2030. The ATLAS and CMS experiments have highly performing muon detectors too, and could also make significant contributions to the study of the new X(6900) structure, with both existing and future data. A key contribution may also be made by Belle’s successor, Belle II, currently in its start-up phase, which observes electron–positron collisions at the SuperKEKB collider at energies above the observed J/ψ-pair mass structure. It is unclear, however, if the collision energy, luminosity and electromagnetic production cross sections will be high enough to achieve the required sensitivity.
Research is already moving forward quickly, with further evidence for diquark tetraquarks coming from an even more recent discovery by LHCb of two “X(2900)” states with widths between 57 and 110 MeV. As they decay to a D+K– final state, they are both openly charming and openly strange. Their most likely composition is that of a (cs)(ud) diquark tetraquark. While the X(2900) states decay strongly, similar heavy-light diquark systems, such as (cc)(ud), (bc)(ud) and (bb)(ud), have been studied theoretically, resulting in varying degrees of confidence that some may be stable with respect to strong interactions, and instead decay weakly, with measurable lifetimes. Hunting for such states is an exciting prospect for the upgraded LHCb experiment.
LHCb’s new tetraquark observations have once again thrown open the debate on the nature of multi-quark states. With the theory still mired in non-perturbative calculations, experimental observations will be decisive in leading the development of this subject. The community is waiting eagerly to see if other experiments confirm the LHCb observation, and shed light on its nature.
Steven Weinberg’s continuous leadership in particle physics, gravity and cosmology, has been recognised by a Special Breakthrough Prize in Fundamental Physics. While his contribution to the genesis of the Standard Model has undoubtedly been Weinberg’s greatest single achievement, states the selection committee for the $3M prize, he would be recognised as a leader in the field even if he had not made this particular contribution. “Steven Weinberg has developed many of the key theoretical tools that we use for the description of nature at a fundamental level,” said Juan Maldacena of the Institute for Advance Study in Princeton, chair of the selection committee.
Weinberg’s 1967 paper “A Model of Leptons” determined the direction of high-energy particle physics through the final decades of the 20th century and is one of the most cited in theoretical physics. The paper applied the notion of spontaneous symmetry breaking to the weak interaction, revealing that it is unified with the electromagnetic interaction and predicting the existence of the W, Z and Higgs bosons – all of which went on to be discovered at CERN. Weinberg also used spontaneous symmetry breaking to account for the masses of elementary fermions, which the LHC experiments are now probing. The electroweak theory won Weinberg, Abdus Salam and Sheldon Lee Glashow the 1979 Nobel Prize in Physics.
There was a special pleasure in being awarded the prize, because the selection committee is composed of a younger generation
Steven Weinberg
“Of course, nothing compares with the Nobel Prize in prestige, if only because of the long history of great scientists to whom it has been awarded in the past,” says Weinberg, when asked to compare the two awards, “but for me there was a special pleasure in being awarded the Breakthrough Prize, because the selection committee is composed of a younger generation of outstanding physicists who are today playing a leading role in research.”
The prize committee also cites Weinberg’s achievements in communicating science. His teaching and “meticulously written textbooks” have had a major influence on succeeding generations, they say, while also acknowledging Weinberg’s highly visible public role as a spokesman for science and rationality.
Weinberg is currently the Jack S Josey – Welch Foundation Chair in Science at the University of Texas at Austin.
Breakthrough Prize for Eöt-Wash group
On the same day, September 10th, the 2021 Breakthrough Prize in Fundamental Physics was announced. Also worth $3M, it is shared between Eric Adelberger, Jens H Gundlach and Blayne Heckel, the leaders of the Eöt-Wash group at the University of Washington, “for precision fundamental measurements that test our understanding of gravity, probe the nature of dark energy and establish limits on couplings to dark matter”. The trio have built equipment sensitive enough to measure the force of gravity on unprecedentedly low scales to test the inverse square law, with results earlier this year showing that the law holds true down to distances of 52mm.
Three New Horizons in Physics Prizes, each worth $100,000 and designed to recognise early-career researchers, were awarded to: Tracy Slatyer (MIT) “for major contributions to particle astrophysics, from models of dark matter to the discovery of the ‘Fermi Bubbles'”; Rouven Essig (Stony Brook University), Javier Tiffenberg (Fermilab), Tomer Volansky (Tel Aviv University) and Tien-Tien Yu (University of Oregon) “for advances in the detection of sub-GeV dark matter especially in regards to the SENSEI experiment”; and Ahmed Almheiri (IAS), Netta Engelhardt (MIT), Henry Maxfield (UC Santa Barbara) and Geoff Penington (UC Berkeley) “for calculating the quantum information content of a black hole and its radiation”.
The Breakthrough Prize in Fundamental Physics, which has taken place annually for the past nine years, was created “to recognize those individuals who have made profound contributions to human knowledge”, while the Special Breakthrough Prize in Fundamental Physics has only been handed out on six occasions and is not limited to recent discoveries. Last year, theorists Sergio Ferrara, Dan Freedman and Peter van Nieuwenhuizen received a Special Breakthrough Prize for their 1976 invention of supergravity. Other past winners include Steven Hawking (2013); the LIGO collaboration (2016); and seven CERN scientists (2013) for the discovery of the Higgs boson. The 2021 prize ceremony is due to take place in March.
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