Jobs – Page 105 of 521

John Thompson: 1939-2020

John Thompson, a senior physicist at the UK’s Rutherford Appleton Laboratory (RAL), passed away on 20 August.

John obtained his PhD in nuclear physics for work on the Van de Graaff accelerator at the University of Liverpool in the early 1960s before moving to the University of Manitoba, Canada to work on particle physics. He then took a post at Daresbury Laboratory in the UK to work on experiments using the 5 GeV electron synchrotron, NINA. His first experiment involved a measurement of the total hadron photoproduction cross sections for energies from 1 to 4 GeV. These precise measurements have never been superseded and remain the definitive values documented by the Particle Data Group.

John was central to the formation of Daresbury’s LAMP group, which focused on a series of hadronic photoproduction experiments. He played a leading role in the development of the 480-element lead–glass array for studies of neutral particle production in the final phase of the LAMP experiment.

During this period, following the discovery of deep inelastic scattering at SLAC and DESY, John together with his colleagues became involved in the plans to study deep inelastic scattering at the higher energies afforded by the Super Proton Synchrotron (SPS) at CERN. He became a founding member of the European Muon Collaboration (EMC), which would go on to do experiments in the high-intensity muon beam at CERN.

John was a first rate, hands-on experimental physicist

As NINA came to the end of its life and Daresbury moved to host one of the first dedicated “light sources”, John moved with other colleagues in particle physics to RAL. Interested in the production of high-energy photons at the EMC, he organised the transfer of the LAMP lead–glass array from the UK to CERN to study the production of photons in the forward direction. In the final phase of the EMC activities he successfully led a team from RAL that implemented the change to a polarised target – a very difficult procedure that had to be done in just a few months.

In the 1980s John led the RAL group into the ALEPH experiment at CERN’s Large Electron Positron collider, LEP. The group undertook the task of building the end cap electromagnetic calorimeters, which operated successfully during the 11 years of ALEPH operations. John became heavily involved with early results for which the calorimeter performance was crucial, such as its use in counting the number of neutrino species from the radiative return reaction e⁺e^– → µµγ. During the LEP2 period in the late 1990s, John’s major contribution concerned the measurement of the W mass and width. This led to a highly productive collaboration between the Imperial College and RAL ALEPH teams, and saw John become instrumental in guiding the students. Following his retirement from RAL, he was appointed visiting professor at Imperial where he was lead author on the publication of the final ALEPH W mass measurements. He continued to advise and guide students, and took charge of graduate lectures until recently, when failing health made it difficult for him to do so.

John was a first rate, hands-on experimental physicist. He had a talent for understanding the difficulties of others and involving himself selflessly to help them progress. He always had a patient, calm, happy and relaxed manner, and will be sadly missed.

Terascale summer school goes global

In a joint venture by physicists at DESY, the first Terascale Summer School took place online from 23 July to 12 August, providing more than 160 undergraduate students from over 30 countries with an engaging introduction to the world of particle and astroparticle physics. Following a wide-ranging three weeks of teaching, an impromptu fortnight-long online tutorial, which only concluded yesterday, focused on strong interactions and Monte Carlo techniques, allowing students to deepen their knowledge through practical exercises.

Terascale chat — Terascale’s online format encouraged students to ask questions. Credit: I Melzer-Pellmann

As the school had been forced online due the ongoing pandemic, the organisers settled upon a reduced programme with just one or two 45 minutes lectures per day. Active moderation was key, with students typing questions in the chat box, and the moderator interrupting the lecturer when appropriate, to give the participants a chance to speak up. This format conferred upon less brash participants a more comfortable way to ask questions, several students noted. When one brave pioneer had broken the ice, queries flowed every few minutes – a resonance effect characterised by a lively, stimulating and relaxed atmosphere which boosted concentration levels.

With its global reach and breathing space for students to explore concepts independently, Terascale 2020’s compact online format may merit consideration during less extraordinary times too.

Neutrinos confirm rare solar fusion process

Despite being our closest star, much remains to be learned about the exact nature of the Sun and how it produces its energy. Two different fusion processes are thought to be at play in the majority of stars: the direct fusion of hydrogen into helium, which is thought to be responsible for approximately 99% of the Sun’s energy; and the fusion of hydrogen into helium via the six-stage carbon-nitrogen-oxygen (CNO) process (see diagram below). Although theorised in the 1930s, direct proof of this fusion process was missing. As a result, the amount of energy produced through the CNO cycle and the amount of elements such as carbon and nitrogen in the Sun’s core could only be estimated from models. Recently, the international Borexino collaboration directly detected neutrinos produced in the CNO cycle, providing the first direct proof of this process.

The Borexino detector was specifically developed to detect the extremely rare interactions between solar neutrinos and a highly pure liquid scintillator. It comprises 278 tonnes of scintillator held in a nylon balloon deep under the mountains at Gran Sasso National Laboratory in Italy. In 2012 the experiment detected neutrinos from the main solar fusion process. Now, one year before the end of its scheduled operations, the Borexino team has fully probed the solar energy production. The discovery of the CNO process was complicated both by the lower flux of neutrinos compared to that from the main fusion process, and by the large similarity between the signal and one of the main irreducible background processes taking place in the detector.

In the six-stage CNO cycle a proton is absorbed by a carbon nucleus, followed by a nuclear decay, followed by a second and third absorption of a proton, followed by another decay, the absorption of a fourth proton and finally a decay into a carbon nucleus, a helium nucleus and the release of around 25 MeV of energy. Source: Creative Commons/Borb.

Battling background
Despite minimising backgrounds from cosmic rays, trace amounts of radioactive nuclei which leak into the active volume of Borexino produce a background of the same magnitude as the sought-after signal. The most important background for the CNO analysis was ²¹⁰Bi, a product of ²¹⁰Pb of which trace amounts can diffuse into the scintillator from the nylon balloon surface. Since the energy spectrum of the beta-decay of ²¹⁰Bi resembles that induced by neutrinos produced in the CNO process, the key to detecting the CNO neutrinos was to directly measure the ²¹⁰Bi-induced background. This was made possible by delving into the fluid dynamics of the liquid scintillator.

The ²¹⁰Bi in Borexino’s scintillator produces ²¹⁰Po, which undergoes alpha decay with a half-life of 134 days. As the alpha decay is relatively easy to identify, the team used ²¹⁰Po decays to deduce the number of ²¹⁰Bi decays in the detector. However, as the different isotopes move around in the liquid it cannot be guaranteed that the ²¹⁰Bi distribution is equal to ²¹⁰Po unless the flow in the detector is well understood. To overcome this, the collaboration had to reduce the flow of the scintillator material by stabilising the temperature, both through insulation and direct temperature regulation. After the ²¹⁰Po decay distribution inside the detector was found to be stable over times exceeding its half-life, an area with low ²¹⁰Po activity was identified and used to measure the CNO neutrinos with a well-understood and relatively low background.

This was made possible by delving into the fluid dynamics of the liquid scintillator

The spectral measurements performed of the CNO cycle exclude a non-detection with a statistical significance of more than five sigma. The measured solar-neutrino flux (7.2^+3.0₋1.7 counts per day per 100 tonnes of target, at 68% confidence) furthermore agrees with models which predict that 1% of the energy produced in the Sun comes from the CNO process. Additionally, the results shine light on the density of elements other than hydrogen and helium — the metallicity — of the Sun’s core, which in recent years has been debated to potentially differ from that on the solar surface. The Borexino results indicate that the density is likely similar although more precise measurements with future detectors are required for precision measurements.

This groundbreaking study, which required not only some of the most precise techniques used in particle physics but also complex fluid-dynamics simulations, confirms predictions made almost a century ago. In doing so it provides a first probe into the processes at the core of the Sun and thus of other stars. Although it has now been proved that the CNO process is responsible for only a fraction of the Sun’s energy, for heavier and therefore hotter stars it is predicted to be the dominant fusion process, making future high-precision studies important to understand the evolution of the universe in general.

An intuitive approach to teaching

This elementary textbook, suitable for either advanced undergraduate or introductory postgraduate courses, is a gem. Its author, Andrew Larkoski, is a phenomenologist with expertise in QCD, and a visiting professor at Reed College. It is worth mentioning that Reed College is also home to David J. Griffiths, who is the author of several successful textbooks, including his well-known “Introduction to Elementary Particles” (Wiley, 2nd edition, 2008). Larkoski’s book has a similar scope to Griffiths’ and certainly lives up to its legacy.

Larkoski begins with an introduction to special relativity and the standard preliminaries to particle physics, such as the Dirac equation, Fermi’s Golden rule and a very accessible introduction to group theory. The book also features a superb 30-page chapter on experimental concepts and statistics — an excellent resource for any student starting a particle-physics project for the first time. The main menu follows: matrix element and cross-sections calculations for QED, QCD and weak interactions. The book includes a nice introduction to electroweak unification, the basics of flavour physics, neutrino oscillations, and an accessible discussion on parton evolution and jets. The latter will be particularly useful for students of LHC physics. The book closes with an insightful chapter on open problems in particle physics.

A very nice collection of unsolved exercises will serve as an invaluable resource for lecturers. Many refer to processes currently being studied at the LHC and other projects. The book’s modernity is also evident through mentions throughout the text on the latest results in dark matter and neutrino physics, and a discussion on how the Higgs boson discovery was made.

Analogies are drawn between Feynman diagrams and electrical circuits

A particularly attractive feature of Larkoski’s writing is his use of intuitive and conceptual discussions: dimensional analysis is used often in calculations to get an idea of what we expect; analogies are drawn between Feynman diagrams and electrical circuits; connections between space curvature and quantum chromodynamics are pointed out, just to mention some of the very many examples you can find in the book.

One point that the lecturers should be aware of is that Larkoski employs the Weyl basis of Dirac γ-matrices, whereas Griffiths, Thomson (Modern Particle Physics, Cambridge, 2013), Halzen & Martin (Quarks and Leptons, Wiley, 1984), and other popular textbooks which currently form the backbone of many university courses, use the Dirac basis. As a result, both equations and Feynman rules look different, and care will be required when multiple textbooks are used in the same course. In general, Larkoski is closer to Thomson and Griffiths, as it does not include the wide range of calculations of Halzen & Martin, which is slightly more advanced.

Larkoski’s new book will certainly find its way among the most popular particle physics textbooks. Its clear and intuitive presentation will doubtlessly deepen the understanding of students who read it, and inspire lecturers to a more conceptual approach to teaching.

Radiation rules of thumb

Don Cossairt and Matthew Quinn’s recently published book summarises both basic concepts of the propagation of particles through matter and fundamental aspects of protecting personnel and environments against prompt radiation and radioactivity. It constitutes a compact and comprehensive compendium for radiation-protection professionals working at accelerators. The book’s content originates in a course taught by Cossairt, a longstanding and recently retired radiation expert at Fermilab at numerous sessions of the US Particle Accelerator School (USPAS) since the early 1990s. It is also available as a Fermilab report which has stood the test of time as one of the standard health-physics handbooks for accelerator facilities for more than 20 years. Quinn, the book’s co-author, is the laboratory’s radiation-physics department manager.

Radiation-protection book cover — Credit: CRC Press

The book begins with a short overview of physical and radiological quantities relevant for radiation protection assessments, and briefly sketches the mechanisms for energy loss and scattering during particle transport in matter. The introductory part concludes with chapters on the Boltzmann equation, which in this context describes the transport of particles through matter, and its solution using Monte Carlo methods. The following chapters illustrate the radiation fields which are induced by the interactions of electron, hadron and ion beams with beamline components. The tools described in these chapters are parametrised equations, handy rules-of-thumb and graphs of representative particle spectra and yields which serve for back-of-the-envelope calculations and describe the fundamental characteristics of radiation fields.

Practical questions

The second half of the book deals with practical questions encountered in everyday radiation-protection assessments, such as the selection of the most efficient shielding material for a given radiation field, the energy spectra to be expected outside of shielding where personnel might be present, and lists of radiologically relevant nuclides which are typically produced around accelerators. It also provides a compact introduction to activation at accelerators. The final chapter gives a comprehensive overview of radiation-protection instrumentation traditionally used at accelerators, helping the reader to select the most appropriate detector for a given radiation field.

Nowadays, assessments are more readily and accurately obtained with Monte Carlo simulations

Some topics have evolved since the time when the material upon which the book is based was written. For example, the “rules-of-thumb” presented in the text are nowadays mostly used for cross-checking results obtained with much more powerful and user-friendly Monte Carlo transport programs. The reader will not, however, find information on the use and limitations of such codes. For example, the chapter on aspects of radiation dose attenuation through passage ways and ducts as well as environmental doses due to prompt radiation (“skyshine”) gives only analytical formulae, while assessments are nowadays more readily and accurately obtained with Monte Carlo simulations. There is risk, however, that such codes be treated as a “black box”, and their results blindly believed. In this regard, the book gives many tools necessary for obtaining rough but valuable estimates for setting up simulations and cross-checking results.

Max Zolotorev: 1941-2020

Max Samuilovich Zolotorev, a pioneer of experimental studies of atomic parity violation, passed away on 1 April in his home in Oregon, US.

Max was born in Petrovsk, a small town not far from the Russian city of Saratov, where his mother found herself evacuated from the advancing German army. Upon graduating from secondary school, despite showing unusual talent and ability from an early age, he was not admitted to an institute or even a vocational school because he was Jewish. After eventually securing a position with the Novosibirsk Electro Technical Institute in Siberia, where he demonstrated outstanding academic performance, he was able to transfer to the newly founded Novosibirsk State University. He graduated in 1966, before obtaining his first and second doctoral degrees in 1974 and 1979 at the Institute of Nuclear Physics in Novosibirsk Academgorodok.

Max started out by working on measurements of the hyperon magnetic moments. However, in the early 1970s he was drawn into studying fundamental physics using the methods of atomic, molecular and optical physics. Together with his mentor and colleague Lev Barkov, he was the first to discover parity violation in atoms by observing optical rotation of the plane of polarisation of light propagating through a bismuth vapour.

The 1978 measurement came at a crucial time in the development of the Standard Model. While observations of high-energy neutrino scattering on nuclei at CERN in 1973 provided evidence of neutral weak currents, there was no evidence that the neutral weak current violated parity as predicted by the Glashow–Weinberg–Salam (GWS) model. Furthermore, earlier atomic parity violation experiments had produced null results, in contradiction with theoretical predictions. The observation of parity violation in bismuth, followed later by measurements of parity violating electron scattering at SLAC, was crucial evidence that the GWS model was indeed the correct description of the weak interaction.

Max Zolotorev was an inspiring mentor and teacher who always set the highest expectations for his students

Max and his colleagues also established the foundation for some of today’s most sensitive magnetometers with their measurements in the late 1980s of nonlinear Faraday rotation, clearly identifying the crucial role of quantum coherences. In 1989 Max emigrated to the US and took up a research position at SLAC, later moving to Lawrence Berkeley National Laboratory, where he worked until his retirement in 2018. At SLAC, Max and colleagues proposed using lasers to cool hadrons in colliders as a variation on van der Meer’s stochastic cooling method. The “optical stochastic cooling” concept will soon be tested at Fermilab by a group led by a former student of Max’s. Another of his co-inventions is the so-called “slicing method” to produce ultrashort pulses of X-rays essential for time-resolved studies of the properties of condensed matter.

Max Zolotorev was an inspiring mentor and teacher who always set the highest expectations for his students. His ability to find “weak spots” in one’s scientific logic was legendary. One of Max’s great insights was that, as physicists, we should never design our experiments around what was sitting in our labs or in our heads. Instead, we should choose deep and important problems, think hard about them and develop the cleverest way to approach them that we can, learn new subjects, build new apparatus, and push our boundaries and limits. Max’s work exemplified the curiosity, creativity and rigour of physics at its best.

The LHC as a photon collider

ATLAS Forward Proton Spectrometer — **Future forward** Part of the ATLAS forward-proton spectrometer. Credit: ATLAS collaboration

Protons accelerated by the LHC generate a large flux of quasi-real high-energy photons that can interact to produce particles at the electroweak scale. Using the LHC as a photon collider, the ATLAS collaboration announced a set of landmark results at the 40th International Conference on High Energy Physics last week, among which is the first observation of the photo-production of W-boson pairs.

As it proceeds via trilinear and quartic gauge-boson vertices involving two W bosons and either one or two photons, the production of a pair of W bosons from two photons (ɣɣ → WW) tests a longstanding prediction of the Standard Model (SM). This process is extremely rare but predicted precisely by electroweak theory, such that any observed deviation would suggest that new physics is at play. The measurement relies on the large 139 fb^–1 dataset of proton–proton collisions recorded by ATLAS in LHC Run 2.

**Fig. 1.** To isolate a sample of ɣɣ → WW interactions, events with no additional reconstructed charged-particle tracks in the vicinity of the electron–muon pair (n_trk = 0) are selected. Credit: ATLAS collaboration

Protons usually remain intact or are excited into a higher energy state in photon collisions, with the products of any subsequent decay not reaching the innermost components of the ATLAS detector. In these cases, the electron and muon decaying from the W bosons – an event topology chosen to avoid the high background for same-flavour lepton pairs – are the only particles detected in the vicinity. However, if charged particles arise from nearby proton–proton collisions, the clean ɣɣ → WW signal can be missed. The main background is W-boson pairs produced in head-on proton–proton collisions where particles from the break-up of the protons are not detected due to imperfect detector coverage or reconstruction (figure 1). A total of 127 background events were predicted compared with 307 events observed in the data, corresponding to a signal excess of 8.4 standard deviations. This establishes the existence of light transforming into particles with weak-scale masses – a remarkable and previously unobserved phenomenon.

Innovation

Precisely testing SM predictions of photon collisions requires accurate knowledge of the rate protons remain intact relative to those that break apart. This is challenging to predict theoretically and probing these rates unambiguously requires directly detecting the intact protons. The ATLAS forward-proton spectrometer (AFP) is becoming increasingly indispensable for this task. Among the newest additions to the ATLAS experiment, and located a few millimetres from the beam 210 m either side of the collision point, the AFP can detect protons that have been scattered in photon–photon collisions but which have nevertheless been focused by the LHC’s magnets. Its pioneering results so far analyse a standard-candle process where a proton is scattered in photon collisions that produce electron or muon pairs (ɣɣ → ℓℓ). For these signals, the measured proton energy loss is equal to that predicted from the lepton pairs measured in the main ATLAS detector (figure 2). ATLAS reported 180 events with a proton having matched kinematics to the lepton pair with an expected background of about 20 events. This corresponds to a significance exceeding nine standard deviations for both lepton flavours, establishing the presence of the signal and the successful operation of the AFP spectrometer in high-luminosity data. The detectors were sufficiently well understood to measure the cross sections of these processes.

**Fig. 2.** A sample of ɣɣ → ℓℓ events can be isolated by observing a scattered proton in the AFP spectrometer. Here, the proton energy loss measured in the AFP installed either side (A and C) of the collision point (ξ_AFP, dimensionless) is shown to agree with that predicted from measurements of the lepton pair in the main detector (ξ_ℓℓ). Credit: ATLAS collaboration

Observing ɣɣ → WW and scattered protons in ɣɣ → ℓℓ interactions are long-awaited milestones in an emerging experimental programme studying photon collisions. These complement recent heavy-ion results where ATLAS measured muon pairs from photon collisions and the kinematic properties of light-by-light scattering – a very rare process predicted by quantum electrodynamics. Interestingly, the latter was also used to search for the axion-like particles predicted by certain extensions of the SM.

Observing ɣɣ → WW and scattered protons in ɣɣ → ℓℓ interactions are long-awaited milestones

The techniques developed to study ɣɣ → WW and ɣɣ → ℓℓ interactions lay the groundwork for future, more detailed tests of the SM. Further results using the AFP spectrometer can improve theoretical understanding of photon collisions that will also benefit future measurements of ɣɣ → WW production. These landmark experimental feats will only become more interesting with the increased dataset of Run 3 and the high-luminosity LHC.

Ulrich Becker 1938–2020

Ulrich J Becker, professor emeritus at MIT, passed away on 10 March at the age of 81. He was a major contributor to the L3 experiment, the Alpha Magnetic Spectrometer and the advancement of international collaborations in high-energy physics.

Becker was born in Dortmund, Germany, on 17 December 1938 – the day that nuclear fission was discovered in Berlin. As a young man, he was adept as an electrician, coal miner, and even in steel smelting, but he was more drawn to physics. He studied at the University of Marburg and obtained his PhD in Hamburg, focusing on the photo-production and leptonic decays of vector mesons.

In late 1965 Becker met Sam Ting, who admitted him to his group at DESY using the 6 GeV synchrotron to measure the size of the electron. It was a complementary match: Becker was a dogged researcher with detector and hardware acumen, and Ting was a master in scientific organization and politics. They presented their results at the XIIIth International Conference on High Energy Physics at Berkeley in 1966, showing that electrons have no measurable size, which contradicted earlier results.

In 1970 Becker joined the MIT faculty, where he found mentors including Victor Weisskopf and Martin Deutsch. He was promoted to associate professor in 1973, and the following year he began designing a precision spectrometer for Brookhaven National Laboratory. He joined a group led by Ting which used the spectrometer to search for heavy particles produced when protons were smashed into a fixed target of beryllium. Instead, the team recorded an unexpected bump in the data corresponding to the production of a heavy particle with a lifetime that was about a thousand times longer than predicted.

Meanwhile, MIT alumnus Burton Richter was reviewing data from Stanford Linear Accelerator Laboratory when he too found what looked like a long-lived heavy resonance. Ting flew to Stanford in November and he and Richter quickly organized a lab seminar. They presented their discovery of the J/Ψ particle, a bound state of a charm quark and antiquark, on 11 November 1974, sparking rapid changes in high-energy physics. One of Becker’s favorite stories was when he went to Munich in 1975 to share their finding, and Werner Heisenberg interrupted to comment: “Whenever they don’t know what it is, they invent a new quark.” To which Becker replied: “Look, Professor Heisenberg, I’m not arguing whether this is charm or not charm. I’m telling you it’s a particle which doesn’t go away.” A deadly silence followed before Heisenberg replied: “Accepted”. Ting and Richter shared the 1976 Nobel Prize in Physics for the J/Ψ discovery. If only one of the groups, MIT, had discovered it, it is likely that Becker would also have shared in the prize.

He enjoyed reviving broken and abandoned mechanical items.

Becker, who was made a full professor at MIT in 1977, developed several other major instruments which were the catalyst for discoveries. His large-area drift chamber would provide large acceptance coverage for experiments, and his drift tube enabled physicists to measure particles near the interaction point. Those developments led Becker to design and build the huge muon detectors for the MARK-J experiment at DESY, which resulted in the discovery of the three-jet pattern from gluon production. Becker then led hundreds of colleagues in designing the muon detector for the L3 experiment at LEP. He also made important contributions to advancing international collaboration in high-energy physics, for example involving China.

In 1993, Becker started to work with MIT’s team on building an Alpha Magnetic Spectrometer (AMS) — another Ting project which was born when he and Becker were on a coffee break while working on L3. The first AMS detector flew in the Space Shuttle in June 1998 and gathered about 100 hours of cosmic-ray data. Becker then went on to help design the transition radiation detector for AMS-02, which has so far collected more than 150 billion cosmic-ray events from its position on the International Space Station.

He enjoyed reviving broken and abandoned mechanical items. One of his biggest renovations was MIT’s cyclotron, which he converted into one of the biggest functioning magnets in the country, with a strength of up to 1 T. He used it to develop particle detectors for the International Linear Collider, and to characterise gas mixtures for the design of drift and other gas detectors in different magnetic and electric fields.

Becker was a mentor to many great physicists, and invested much to ensure his students received an excellent education. In 2013 he transitioned to emeritus status, but still he came in every day to mentor students. At the age of 81, he even picked up Python to continue his craft. His friendly approach and deep understanding of physics made him a superb teacher, even if his style was highly individual.

Our community has lost an excellent researcher and teacher, and a wonderful colleague and human being. Ulrich Becker is survived by his wife Gerda, his three children and two grandchildren.

Double digits for ultra-rare kaon decay

CERN’s NA62 collaboration has presented its latest progress in the search for K⁺→π⁺νν̄ – a “golden decay” with exceptional sensitivity to physics beyond the Standard Model. The new analysis, which includes the full dataset collected until 2018, provides the strongest evidence yet for the existence of this ultra-rare process, at 3.5σ significance. Presenting the result today during the penultimate plenary session of the 40th International Conference on High-Energy Physics, which is being virtually hosted from Prague, lead-analyst Giuseppe Ruggiero of Lancaster University described the result as a great achievement. “After several years of a very challenging analysis, battling ten orders of magnitude of background over the signal, we are proud to have achieved the first statistically significant evidence for a process which has great sensitivity to new physics,” he says.

An important virtue of K⁺→π⁺νν̄ is its clean theoretical character
Andrzej Buras

A flavour-changing, neutral-current process, K⁺→π⁺νν̄ is highly suppressed in the Standard Model, with contributions from Z-penguin and box diagrams with W, top quark and charm exchanges. The measured branching fraction of 110⁺⁴⁰_-35 per trillion K⁺→π⁺νν̄ decays is in agreement with the Standard Model prediction of 84 ± 10 per trillion (JHEP 11 033). “A particular and very important virtue of K⁺→π⁺νν̄ is its clean theoretical character, which can only be matched among meson decays by K_L→π⁰νν̄, and possibly B_s,d→μ⁺μ^–,” says Andrzej Buras of the Institute for Advanced Study in Garching, Germany. “This is related to the fact that the low-energy hadronic matrix elements are just those of the quark currents between the hadronic states, which can be extracted from the leading semileptonic decay K⁺→π⁰e⁺ν,” he explains, noting that higher-order QCD and electroweak corrections are already well known, and lattice QCD calculations should soon tackle the small, “long-distance” contributions to the amplitude.

Historical measurements and predictions of the branching fraction for K+→π+νν̄ — **Convergence** The precision of theoretical predictions for the K⁺→π⁺νν̄ branching fraction has improved dramatically since the Standard Model was inaugurated in the mid-1970s. In the same period, experimental limits have tumbled five orders of magnitude, to culminate in NA62’s recently announced 3.5σ sighting. Credit: NA62 collaboration

NA62 observes the 6% of positively charged kaons that are produced when 450 GeV protons from the Super Proton Synchrotron strike a beryllium target. The analysis is challenging because of the tiny branching fraction and the presence of a neutrino pair in the final state. Pioneering the technique of observing kaon decays in flight, the collaboration measures the kinematics of both the initial kaon and the final-state pion to isolate the kinematic signature of K⁺→π⁺νν̄, before then suppressing other decay modes by a further eight orders of magnitude using particle-identification techniques.

The collaboration’s new result adds a further 17 events to its previous analysis (arXiv:2007.08218, submitted to JHEP), wherein three events observed in 2016 and 2017 yielded an estimated branching fraction of 47 ⁺⁷²_-47 decays per trillion. The previous best measurement was by Brookhaven National Laboratory’s E787 and E949 experiments in the 2000s, which together inferred a branching fraction of 173 ⁺¹¹⁵_-105 per trillion (Phys. Rev. Lett. 101 191802).

Meanwhile in Japan

The NA62 result is expected soon to be complemented by a measurement of the related CP-violating K_L→π⁰νν̄ decay by the KOTO collaboration at the J-PARC research facility in Tokai, Japan. This even rarer process has a predicted Standard Model branching fraction of just 34 ± 6 per trillion. KOTO’s 2015 data yielded no event candidates and a 90% confidence upper limit on the branching fraction of 3.0 per billion (Phys. Rev. Lett. 122 021802). The collaboration is now finalising its results from the 2016–2018 run, and plans to improve its sensitivity to less than 0.1 per billion by increasing the beam intensity and upgrading the KOTO detectors.

As experimental uncertainties are expected to approach the theoretical precision in coming years, explains Buras, K⁺→π⁺νν̄ and K_L→π⁰νν̄ decays can probe scales as high as a few hundred TeV – beyond the reach of most B-meson decays. “K⁺→π⁺νν̄ is most sensitive to hypothetical Z′ gauge bosons, vector-like quark models, supersymmetry and some leptoquark models,” he says. “LHCb studies of K_S→ μ⁺μ^– and Belle II studies of B→ K(K*)νν̄ will also have a part to play, allowing a global analysis to test not only the concept of minimal flavour violation, but also probe new CP-violating phases and right-handed currents.”

Theorists expect to reach an accuracy of 5% on the predicted K⁺→π⁺νν̄ branching ratio towards the end of the decade. In the same period, the NA62 team is seeking to hone its resolution from the current 30% down to 10%. The collaboration will resume data taking in 2021, following upgrades to both beam and detector taking place during the ongoing second long shutdown of CERN’s accelerator complex.

Sensitivity to decay rates below the 10^–11 level is now in sight
Cristina Lazzeroni

“The horizon of a new-physics programme with a sensitivity to decay rates well below the 10^–11 level is now in sight,” says NA62 spokesperson Cristina Lazzeroni of the University of Birmingham, UK. “The instruments and techniques developed for the NA62 experiment will lead to the next generation of rare-kaon-decay experiments. For the longer term future, a high-intensity kaon-beam programme is starting to take shape at CERN, with prospects to measure the K⁺→π⁺νν̄ decay to a few per cent, address the analogous decay of the neutral kaon, and reach extreme sensitivities to a large variety of rare kaon decays that are complementary to investigations in the beauty-quark sector.”

Neutrino 2020 zooms into virtual reality

4,350 people from every continent, including Antarctica, participated from 22 June to 2 July in the XXIX International Conference on Neutrino Physics and Astrophysics, which was hosted online by Fermilab and the University of Minnesota. Originally planned as a five day, in-person June meeting at a large hotel in Chicago city centre, the organisers quickly pivoted in March, due to COVID-19, to an online programme with eight half days over two weeks, four poster sessions with both web-based and virtual-reality displays, and the use of the Slack platform for speaker questions and ongoing discussions.

A highlight of the conference was the first observation of solar CNO neutrinos

A highlight of the conference was the first observation of solar CNO neutrinos by the Borexino collaboration, which operates a 280-tonne liquid-scintillator detector in Italy’s Gran Sasso Laboratory. Dominant in stars more than 1.3 times the mass of the sun, the CNO cycle accounts for about 1% of the sun’s energy and generates a difficult-to-detect neutrino flux similar to backgrounds due to decays in the detector of ²¹⁰Bi and its daughter nucleus ²¹⁰Po. Gioacchino Ranucci (INFN, Milano) explained that the spectral fit to the observed data returns only the sum of CNO and ²¹⁰Bi neutrinos. “The quest for CNO is turned into the quest for ²¹⁰Bi through ²¹⁰Po,” he emphasised. “With this outcome, Borexino has completely unravelled the two processes powering the Sun—the pp chain and the CNO cycle.” The final data analysis yielded a 5.1σ statistic against a hypothesis of no CNO neutrinos, and a CNO flux at the Earth of 7.0_-1.9^+2.9 × 10⁸ cm^-2 s^-1.

Another highlight from Gran Sasso was the report from the Gerda collaboration on the search for neutrino-less double beta decay. If observed, this process would confirm the long-suspected Majorana rather than Dirac-fermion nature of neutrinos – a beyond the Standard Model feature with intriguing implications for why the neutrino mass is so small. Since Neutrino 2018, Gerda has nearly doubled its Phase 2 exposure and added a liquid-argon veto and a new detector string. The now complete Phase 2 result is a 90% confidence level half-life of >1.8 x 10²⁶ years according to a frequentist analysis, or >1.4 x 10²⁶ years, according to a Bayesian analysis with additional prior assumptions. Talks describing a half-dozen other double-beta-decay experiments displayed the high level of interest in this field.

Sterile neutrinos

Searches for additional “sterile” neutrinos with no Standard-Model gauge interactions were also featured. Takasumi Maruyama (KEK) described the liquid-scintillator JSNS² experiment as a direct test of the controversial LSND Experiment result, first reported about 25 years ago. JSNS² collected its first data during the three weeks before Neutrino 2020. Adrien Hourlier (MIT) reported on the now complete analysis of data from MiniBooNE that was collected during the past 17 years. Combining neutrino and anti-neutrino modes, MiniBooNE reports a 4.8σ excess. Hourlier presented soon-to-be published detailed distributions which the collaboration hopes “will guide theorists to explain our data”. Minerba Betancourt (Fermilab) then described the Fermilab Short-Baseline Neutrino (SBN) programme, which will use three detectors to obtain a definitive result on neutrino oscillations for an LSND and MiniBooNE-like ratio of oscillation distance to energy of ~1 m/MeV. The beam neutrino energy peaks at 700 MeV. A new liquid-argon near detector (SBND) will be placed 110 m from the target. The existing MicroBooNE is located at 470 m and the ICARUS Detector, moved from Gran Sasso, is installed at 600 m. Thomas Carroll (Wisconsin) reported on sterile-neutrino limits by muon disappearance determined by the now completed long-baseline MINOS/MINOS+ collaboration. These limits are in tension with the appearance data from both LSND and MiniBooNE when analysed as evidence for sterile neutrinos.

Two talks described the world’s two hundred-kilometre-scale neutrino-oscillation experiments, NOvA and T2K. The degeneracy of mass difference, mixing angle, hierarchy and possible CP violation make interpretation of these experiments’ results quite complex. Interestingly, there is mild tension, albeit only at the 1σ level, between the NOvA and T2K results regarding leptonic CP conservation and the neutrino mass hierarchy. The two collaborations are now working together on a combined analysis. Several talks discussed future initiatives. Lia Merminga (Fermilab) reported on LBNF and PIP-II, which will result in a new neutrino beam from Fermilab to the Sanford Laboratory in South Dakota for the DUNE experiment. Combined, these two projects will result in beam power of 2.4 MW, more than three times the intensity of the current NuMI beam. Michael Mooney (Colorado State) reported on the enormous progress of the DUNE project with two successful prototype detectors operating at CERN and pre-excavation work progressing at Sanford Laboratory. Complementary to the liquid-argon technology of DUNE is the recently approved Hyper-Kamiokande water-Cherenkov detector, which was described by Masaki Ishitsuka (Tokyo University of Science). Hyper-K will have a total mass of 260 kilo-tonne and 8.4 times the fiducial volume of the current Super-Kamiokande detector.

The VR feature attracted 3,409 conference participants

While much of Neutrino 2020 was modelled after the usual features of an in-person conference, the Virtual Reality (VR) poster presentation was novel and unique. Marco Del Tutto (Fermilab) created multiple virtual “rooms” for five posters each, along with additional rooms for topical discussions, sightseeing in Chicago and visiting Fermilab. The most enabling feature of the VR was that the software facilitated dialogue between participants whose avatars could move around the space and speak with one another. For example, if a group of avatars clustered around a poster, the participants could discuss the poster as a group. The VR feature attracted 3,409 participants. The VR was also supplemented by two-minute videos from presenters which enabled 5,800 YouTube views and 60,600 web displays.

In closing remarks, the organisers acknowledged the challenges of an online conference, but also emphasised the strengths of this novel approach. The exciting physics of Neutrino 2020 was made available to an extensive and diverse audience, including many scientists who would not have been able to attend an in-person conference because of funding, visas, family concerns or other issues. About 60% of participants were students or post-docs and the conference reached participants from 67 countries. The Slack discussions and posts on social media indicated wide-spread praise that the online format worked as well as it did. Some aspects of Neutrino 2020 may well affect the planning and organisation of future in-person and online conferences.

Topics

Reset your password

Registration complete

Practical questions

Innovation

Meanwhile in Japan

Sterile neutrinos