Jobs – Page 87 of 527

2022 APS awards announced

2022 W.K.H Panofsky Prize in Experimental Particle Physics Winners — The 2022 Panofsky prizewinners (clockwise from top left): Regina Rameika, Byron G Lundberg, Vittorio Paolone and Kimio Niwa. Image credits: R Hahn; CERN; R Chapin-Paolone

Panofsky Prize for ν_τ discovery

The American Physical Society (APS) W K H Panofsky Prize in Experimental Particle Physics has been awarded to Byron G Lundberg and Regina Abby Rameika (Fermilab), Kimio Niwa (Nagoya University) and Vittorio Paolone (University of Pittsburgh) “for the first direct observation of the tau neutrino through its charged-current interactions in an emulsion detector”. Lundberg, Rameika Niwa and Paolone were leaders of the DONUT collaboration, which in July 2000 reported evidence of four tau neutrino interactions (with an estimated background of 0.34 events) in a sample of 203 neutrino-nucleus interactions, consistent with the Standard Model expectation. Although earlier experiments had produced convincing indirect evidence for the ν_τ existence, the DONUT/Fermilab result represented the first direct observation.

J J Sakurai Prize

The 2022 APS J J Sakurai Prize has been awarded to Nima Arkani-Hamed (Institute for Advanced Study) for the development of transformative new frameworks for physics beyond the Standard Model “with novel experimental signatures, including work on large extra dimensions, the Little Higgs, and more generally for new ideas connected to the origin of the electroweak scale”. One of the leading particle phenomenologists of his generation, Arkani-Hamed has argued that the extreme weakness of gravity relative to other forces of nature might be explained by the existence of extra spatial dimensions, and how the structure of comparatively low-energy physics is constrained within the context of string theory.

William G. Foster and Stephen D. Jones — William G Foster (left) and Stephen D Jones (right). (Images: W Foster and R Hahn)

Robert R Wilson Prize

In the field of particle accelerators, the Robert R Wilson Prize has been given to Fermilab’s William G Foster and Stephen D Holmes for leadership in developing the modern accelerator complex at Fermilab, enabling the success of the Tevatron program that supports rich programs in neutrino and precision physics. In 2008, three years before the Tevatron closed down, Foster was elected to the US Congress to represent the people of Illinois. Holmes was director of Fermilab’s PIP-II project when he retired in 2018 after 35 years at Fermilab .

Herman Feshbach Prize

The Herman Feshbach Prize was granted to David B Kaplan (University of Washington) for multiple foundational innovations in nuclear theory, including in lattice quantum chromodynamics, effective field theories, and nuclear strangeness, and for strategic leadership to broaden participation between nuclear theory and other fields. Kaplan is director of the Institute for Nuclear Theory at Washington with research interests also including quantum computing, cosmology, and physics beyond the Standard Model.

Henry Primakoff Award

The 2022 APS Henry Primakoff Award for Early-Career Particle Physics has been awarded to Benjamin Nachman of Lawrence Berkeley National Laboratory for innovative contributions to the search for new physics in collider data incorporating original machine learning algorithms, and for the effective communication of these new techniques to the broader physics community. A member of ATLAS, Nachman focuses on track reconstruction inside jets, and is involved in the design of a readout chip for the upgraded ATLAS pixel detector.

Sheldon Stone 1946-2021

Sheldon Stone, who passed away on 6 October, was one of the foremost physicists of his generation. In terms of creativity and productivity he had few equals in heavy-quark physics worldwide. His skills in leadership, physics analysis and instrumentation served our field well.

Sheldon had a central role in the success of the CLEO experiment at the Cornell Electron Storage Ring, which over a period of almost 30 years laid the foundations for our current understanding of heavy-flavour physics. He served as both CLEO analysis coordinator and co-spokesperson, and had a leading role in many important discoveries such as the observation of the B⁺, B⁰, and D_s mesons. In 2000 he was one of the intellectual leaders who proposed to convert CLEO into a charm factory, subsequently leading the measurement of the charm-decay constants f_D+ and f_Ds. These and other measurements demonstrated the applicability of lattice-QCD calculations of hadronic effects in the weak decays of hadrons with a heavy quark with precision of a few-percent, thereby enabling similar calculations to be used with confidence to interpret key measurements by other flavour-physics experiments worldwide.

His advocacy of the BTeV project at Fermilab was also vital in making the case for a forward flavour-physics detector at a hadron collider

In 2005 Sheldon became a member of the LHCb collaboration, where his and his group’s contribution to the physics exploitation of the experiment was second-to-none. Prominent examples include the first measurement of the beauty-production cross-section at the LHC, and a series of publications measuring CP-violating observables in time-dependent decays of B_smesons. In 2015 LHCb published the first observation of structures consistent with five-quark resonances – “pentaquarks” which were predicted at the dawn of the quark model but had evaded discovery for over 50 years until Sheldon and a small team of colleagues uncovered their existence in the LHCb dataset. This result has had an enormous impact on the field of hadron spectroscopy.

Sheldon also led the design and construction of novel and high-performance detectors underpinning CLEO’s outstanding physics output. These include a thallium-doped near-4π caesium-iodide calorimeter (the first application of a precision electromagnetic calorimeter to a general-purpose magnetic spectrometer) and a Ring-Imaging Cherenkov Counter providing four-sigma kaon-pion separation over the full accessible momentum range.

His advocacy of the BTeV project at Fermilab was also vital in making the case for a forward flavour-physics detector at a hadron collider. This led him to be heavily involved in shaping phase one of the LHCb upgrade project, serving as upgrade coordinator for three years during the preparation of the Letter of Intent, and recently in making innovative proposals for the phase-two upgrade. At the time of his passing, Sheldon was deputy project leader of the upstream tracker, a project that he and his group proposed and led for a decade, and is currently undergoing final assembly. This silicon-strip based detector will play an essential role in both the triggering and offline event reconstruction from Run 3.

Exceptionally effective at guiding others both junior and senior, Sheldon was a superb mentor to graduate students and a wonderful person to collaborate with. He was known to have a strong personality. He was direct and honest, and if you won his respect he was a tremendous friend.

Sheldon’s contributions to our field were at the highest level, recognised most recently with the American Physical Society Panofsky Prize in 2019. For over 30 years he also formed a formidable scientific and life partnership with physicist Marina Artuso who survives him.

Protons back with a splash

Upstream splash muons — **In business** Upstream “splash” muons in the ALICE Muon Forward Tracker recorded on 9 October, allowing the collaboration to check that its rejuvenated and recently closed detector is working as expected ahead of Run 3 next year. Credit: ALICE Collab.

After a three-year hiatus, protons are once again circulating in the LHC, as physicists make final preparations for the start of Run 3. At the beginning of October, a beam of 450 GeV protons made its way from the Super Proton Synchrotron (SPS) down the TI2 beamline towards Point 2, where it struck a dump block and sprayed secondary particles into the ALICE experiment (see image). Beam was also successfully sent down the TI8 transfer line, which meets the LHC near to where the LHCb experiment is located.

Today, counter-rotating protons were finally injected into the LHC, marking the latest milestone in the reawakening of CERN’s accelerator complex, which closed down at the end of 2018 for Long Shutdown 2 (LS2). Two weeks of beam tests are planned, along with first low-energy collisions in the experiments, before the machine is shut down for a 3-4 month maintenance period. Meanwhile, the experiments are continuing to ready themselves for more luminous Run-3 operations.

Final countdown

Beams have been back at CERN since the spring. After a comprehensive two-year overhaul, the Proton Synchrotron (PS) accelerated its first beams on 4 March and has recently started supplying experiments in the newly refurbished East Area and at the new ELENA ring at the Antimatter Factory. Connecting the brand-new Linac4 to the upgraded PS Booster (which also serves ISOLDE) was a major step in the upgrade programme. Together, they now provide the PS with a 2 GeV beam, 0.6 GeV up from before, for which the 60-year-old machine had to be fitted out with refurbished magnets, new beam-dump systems, instrumentation, and upgraded RF and cooling systems.

When the LHC comes back online for physics in May 2022, it will not only be more luminous, but it will also operate at higher energies

LS2 saw an even greater overhaul of the SPS, including the addition of a new beam-dump system, a refurbished RF system that now includes the use of solid-state amplifier technology, and a major overhaul of the control system. Combined with the LHC Injectors Upgrade project (the main focus of LS2), the accelerator complex is now primed for more intense beams, in particular for the High-Luminosity LHC (HL-LHC) later this decade.

The first bunch was injected from the PS into the SPS on 12 April, building up to “LHC-like” beams of up to 288 bunches a few weeks later. The SPS delivers beams to all of CERN’s North Area experiments, which include a new facility, NA65, approved in 2019 to investigate fast-neutron production for better understanding of the background in underground neutrino experiments. It also drives the AWAKE experiment, which performs R&D for plasma-wakefield acceleration and entered its second run in July with the goal of demonstrating acceleration gradients of 1 GV/m while preserving the beam quality. The restart of North Area experiments will also see pilot runs for new experiments such as AMBER (the successor of COMPASS) and NA64μ (NA64 running with muon beams).

Brighter and more powerful

When the LHC comes back online for physics in May 2022, it will not only be more luminous (with up to 1.8 × 10¹¹ protons per bunch compared to 1.3–1.4 × 10¹¹ during Run 2), but it will also operate at higher energies. This year, the majority of the LHC’s 1232 dipole magnets were trained to carry 6.8 TeV proton beams, compared to 6.5 TeV before, which involves operating with a current of 11.5 kA (with a margin of 0.1 kA). Following the beam tests this autumn, magnet training for the final two of the machine’s eight sectors will take place during a scheduled maintenance period from 1 November to 21 February. After that, the LHC tunnel and experiment areas will be closed for a two-week-long “cold checkout”, with beam commissioning commencing on 7 March and first stable beams expected during the first week of May.

Meanwhile, the LHC experiments are continuing to ready their detectors for the bumper Run-3 data harvest ahead: at least 160 fb^–1 (as for Run 2) to ATLAS and CMS; 25 fb^–1 to LHCb (compared to 6 fb^–1 in Run 2); and 7.5 nb^–1 of Pb–Pb collisions to ALICE (compared to 1.3 nb^–1 in Run 2). The higher integrated luminosities expected for ALICE and LHCb are largely possible thanks to the ability of their upgraded detectors to handle the Run-3 data rate, with LHCb teams currently working around the clock to ensure their brand-new sub-detectors are in place. New forward-experiments, FASER, FASERν and SND@LHC, which aim to make the first observations of collider neutrinos and open new searches for feebly interacting particles, are also gearing up to take first data when the LHC comes back to life.

“The injector performance reached in 2021 is just the start of squeezing out the potential they have been given during LS2, paving the way for the HL-LHC, but also benefiting the LHC’s performance during Run 3,” says Rende Steerenberg, head of the operations group. “Having beam back in the entire complex and routinely providing the experimental facilities with physics is testimony to the excellent and hard work of many people at CERN.”

LHCb tests lepton universality in new channels

Measurements of the ratios of muon to electron decays by LHCb (purple) and previous measurements from Belle (red). Source: *Phys. Rev. Lett.* **126** 161801 and arXiv:2110.09501

At a seminar at CERN today, the LHCb collaboration presented new tests of lepton universality in rare B-meson decays. While limited in statistical sensitivity, the results fit an intriguing pattern of recent results in the flavour sector, says the collaboration.

Since 2013, several measurements have hinted at deviations from lepton-flavour universality (LFU), a tenet of the Standard Model (SM) which treats charged leptons, ℓ, as identical apart from their masses. The measurements concern decay processes involving the transition between a bottom and a strange quark b→sℓ⁺ℓ^–, which are strongly suppressed by the SM because they involve quantum corrections at the one-loop level (leading to branching fractions of one part in 10⁶ or less). A powerful way to probe LFU is therefore to measure the ratio of B-meson decays to muons and electrons, for which the SM prediction, close-to-unity, is theoretically very clean.

In March this year, an LHCb measurement of R_K = BR(B⁺→K⁺μ⁺μ^–)/BR(B⁺→K⁺e⁺e^–) based on the full LHC Run 1 and 2 dataset showed a 3.1σ difference from the SM prediction. This followed departures at the level of 2.2—2.5σ in the ratio R_K*⁰ (which probes B⁰→K*⁰ℓ⁺ℓ^– decays) reported by LHCb in 2017. The collaboration has also seen slight deficits in the ratio R_pK, and departures from theory in measurements of the angular distribution of final-state particles and of branching fractions in neutral B-meson decays. None of the results is individually significant enough to constitute evidence of new physics. But taken together, say theorists, they point to a coherent pattern.

We are seeing a similar deficit of rare muon decays to rare electron decays that we have seen in other LFU tests
Harry Cliff

The latest LHCb analysis clocked the ratio of muons to electrons in the isospin-partner B-decays: B⁰→ K_S⁰ℓ⁺ℓ^– and B⁺→K*⁺ℓ⁺ℓ^–. As well as being a first at the LHC, it’s the first single-experiment observation of these decays, and the most precise measurement yet of their branching ratios. Being difficult to reconstruct due to the presence of a long-lived K_S⁰in the final state, however, the sensitivity of the results is lower than for previous “R_K” analyses. LHCb found R(K_S⁰) = 0.66+0.2/-0.15 (stat) +0.02/-0.04 (syst) and R(K*⁺) = 0.70+0.18/-0.13 (stat) +0.03/-0.04 (syst), which are consistent with the SM at the level of 1.5 and 1.4σ, respectively.

“What is interesting is that we are seeing a similar deficit of rare muon decays to rare electron decays that we have seen in other LFU tests,” said Harry Cliff of the University of Cambridge, who presented the result on behalf of LHCb (in parallel with a presentation at Rencontres de Blois by Cambridge PhD student John Smeaton). “With many other LFU tests in progress using Run 1 and 2 data, there will be more to come on this puzzle soon. Then we have Run 3, where we expect to really zoom in on the measurements and obtain a detailed understanding.”

The experimental and theoretical status of the flavour anomalies in b→sℓ⁺ℓ and semi-leptonic B-decays will be the focus of the Flavour Anomaly Workshop at CERN on Wednesday 20 October, at which ATLAS and CMS activities will also be discussed, along with perspectives from theorists.

Gauge–boson polarisation observed in WZ production

**Fig. 1.** Left: the measured WZ production cross section in various final states compared to predictions at different orders in perturbative QCD and electroweak theory (EWK). Right: measured polarisation fractions for the W boson in WZ production, compared with the Standard Model expectation (◊). Credit: CERN

At the collision energies of the LHC, diboson processes have relatively high production cross sections and often produce relatively clean final states with two or more charged leptons. Consequently, multilepton final states resulting from diboson processes are powerful signatures to study the properties of the electroweak sector of the Standard Model. In particular, WZ production is sensitive to the strength of the triple gauge coupling that characterises the WWZ vertex, which derives from the non-Abelian nature of the electroweak sector. Additionally, as the Higgs mechanism is responsible for the appearance of longitudinally polarised gauge bosons, studying W and Z boson polarisation indirectly probes the validity of the Higgs mechanism.

The results include the first observation at any experiment of longitudinally polarised W bosons in diboson production

A recent result from the CMS collaboration uses the full power of the data taken during Run 2 of the LHC to learn as much as possible from WZ production in the decay channels involving three charged leptons (electrons or muons). The results include the first observation at any experiment of longitudinally polarised W bosons in diboson production.

Reconstruction and event selection were optimised to reduce contributions from processes with “non-isolated” electrons and muons produced in hadron decays – traditionally one of the primary sources of experimental uncertainty in such measurements. The total production cross section for WZ production was measured with a simultaneous fit to the signal-enriched region and three different control regions. This elaborate fitting scheme paid off, as the final result has a relative uncertainty of 4%, down from the 6% obtained in past iterations of the measurement. The results are all consistent with state-of-the-art theoretical predictions (figure 1, left).

A highlight of the analysis is the study of the polarisation of both the W and the Z bosons in the helicity frame, using missing transverse energy as a proxy for the transverse momentum of the neutrino in the W decay. This choice, coupled with the precisely measured four-momenta of the three leptons and the requirement that W boson be on-shell, allows both the W and Z momenta to be fully reconstructed. The angle between the W (Z) boson and the (negatively) charged lepton originating from its decay is then computed. The resulting distributions are fitted to extract the polarisation fractions f_R, f_L, and f_o, which correspond to the proportion of bosons in the left, right and longitudinally polarised states in WZ production.

The measured polarisation fractions are consistent within 1σ with the Standard Model predictions (figure 1, right), in accordance with our knowledge of the electroweak spontaneous symmetry breaking mechanism. The significance for the presence of longitudinally polarised vector bosons is measured to be 5.6σ for the W boson and well beyond 5σ for Z-boson production. These new studies pave the way for future measurements of doubly polarised diboson cross sections, including the challenging doubly longitudinal polarisation mode in WW, WZ or ZZ production.

BICEP crunches primordial gravitational waves

The BICEP/Keck collaboration has published the strongest constraints to date on primordial gravitational waves, ruling out parameter space for models of inflation in the early universe (Phys. Rev. Lett. 2021 127 151301). A conjectured rapid expansion of the universe during the first fraction of a second of its existence, inflation was first proposed in the early 1980s to explain the surprising uniformity of the universe over scales which should not otherwise have been connected, and may have left an imprint in the polarisation of the cosmic-microwave background (CMB). Despite a high-profile false detection of gravitational-wave-induced “B-modes” by BICEP in 2014, which was soon explained as a mis-modelling of the galactic-dust foreground, the search for primordial gravitational waves remains one of the most promising avenues to study particle physics at extremely high energies, as inflation is thought to require a particle-physics explanation such as the scalar “inflaton” field proposed by Alan Guth.

Certain ‘standard’ types of inflation are now clearly disfavoured
Kai Schmitz

In its latest publication, the BICEP/Keck collaboration has managed to significantly improve the upper bound on the strength of gravitational waves produced during the epoch of inflation. “This is important for theorists because it further constrains the allowed range of viable models of inflation, and certain ‘standard’ types of models are now clearly disfavoured,” explains CERN theorist Kai Schmitz. “It’s also a great experimental achievement because it demonstrates that the sources of systematic uncertainties such as dust emission in our Milky Way are under good control. That’s a good sign for future observations.”

The BICEP/Keck collaboration searches for the imprint of gravitational waves in the polarisation pattern of the CMB, emitted 380,000 years after the Big Bang. Telescopes at the South Pole receive incoming CMB photons and focus them through plastic lenses onto detectors in the focal plane which are cooled to 300 mK, explains principal investigator Clem Pryke of the University of Minnesota. As the telescopes scan the sky they record the tiny changes in temperature due to the intensity of the incoming microwaves. The detectors are arranged in pairs with each half sensitive to one of two orthogonal linear polarisation components. The telescopes take their best data during the six-month long Antarctic night, during which intrepid “winter-overs” maintain the detectors and upload data via satellite to the US for further analysis.

“The big change since 2014 was to make measurements in multiple frequency bands to allow the removal of the galactic foreground,” says Pryke. “Back then we had data only at 150 GHz and were relying on models and projections of the galactic foreground – models which turned out to be optimistic as far as the dust is concerned. Now we have super-deep maps at 95, 150 and 220 GHz allowing us to accurately remove the dust component.”

The current analysis uses data recorded by BICEP2, the Keck Array and BICEP3 up to 2018. Since then, the collaboration has installed a new more capable telescope platform called the BICEP Array designed to increase sensitivity to primordial gravitational waves by a factor of three, in collaboration with a large-aperture telescope at the South Pole called SPT3G. With 21 telescopes at the South Pole and in the Chilean Atacama desert, the proposed CMB Stage-4 project plans to improve sensitivity by a further factor of six in the 2030s.

Miguel Virasoro 1940–2021

On 23 July, the Italian–Argentinian theorist Miguel Ángel Virasoro, one of the founders of string theory and an initiator of complexity studies, passed away. His scientific contributions were outstanding and stimulated an impressive number of subsequent developments. He was an extraordinarily intelligent visionary with a great sense of humour.

Born in Buenos Aires in 1940, Virasoro enrolled in physics at the University of Buenos Aires in 1958. However, in 1966 General Juan Carlos Onganía successfully led a coup d’état in Argentina, establishing a dictatorship that would last until 1973. The faculty of science at Buenos Aires became a centre of opposition: the police broke into the university, massacring the occupants. In the following months, some 300 professors emigrated abroad.

Virasoro finished his thesis working from home; at the end of 1966, as soon as he obtained his doctorate, he moved to the Weizmann Institute, Israel, invited by a newly appointed young Argentinian professor, Hector Rubinstein. A few months earlier, Gabriele Veneziano had also arrived as a graduate student. The three of them, together with Marco Ademollo, began a long series of investigations into the physics of strong interactions that eventually led to string theory. Although the first step towards string theory was Veneziano’s “open-string” model in 1968, those preliminary results established the conceptual framework in which Veneziano’s model could be conceived. A few months later, stimulated by Veneziano’s work, Virasoro extended it to a model describing closed strings.

The Virasoro condition

In the following years, first at the University of Wisconsin, then at Berkeley, Virasoro did brilliant work on string theory. In 1969 he made the fundamental observation that string theory could only be made free of pathologies by fixing a certain parameter. This “Virasoro condition” allowed for the existence of an infinite number of symmetries generated by an infinite set of operators obeying a “Virasoro algebra” – a tool at the basis of countless subsequent studies. The Virasoro condition proved to be a killer for string theory as a description of strong interactions, but it opened the way to the 1974 Scherk–Schwarz reinterpretation of it as a quantum theory of gravity, in which one particular closed string corresponds to the graviton.

In 1973 democracy was restored in Argentina; Virasoro returned to his own country and was elected, still very young, dean of the faculty of science in Buenos Aires, a politically exposed position. In 1975 he accepted an invitation to spend a year at Princeton. During his stay in the US, however, Videla’s 1976 coup d’état brought dictatorship back to Argentina, in a more cruel form than before: many professors and students were slaughtered at the university. Virasoro was not only fired, but he was told that, had he returned to Argentina, he would be arrested or worse.

Virasoro was convinced of the role that theoretical physics could have in building the capacity of developing countries

He moved to Europe, and after a year in Paris, arrived in Italy, first in Turin and then, from 1981, at La Sapienza in Rome, where he remained for 30 years as a full professor, taking Italian citizenship. Having started to investigate the relationship between the emerging theory of quarks and gluons (QCD) and string theory, in 1983 he changed direction. He began to work with Giorgio Parisi on the statistical mechanics of complex systems, first with other Parisian physicists (Marc Mézard, Nicolas Sourlas and Gerard Toulouse) and then with Mézard alone, who had moved to Rome for two years. The group obtained important results on which the bases of the physical theory of complexity rest and also wrote a book on these results. In 1988 Virasoro became passionate about studying how, starting from neural networks, we can understand the functioning of the brain.

From 1995 to 2002 he was called to direct the International Centre for Theoretical Physics (ICTP) in Trieste. Sharing the vision of its founder Abdus Salam, Virasoro was convinced of the role that theoretical physics could have in building the capacity of developing countries. He decided to enlarge and diversify ICTP’s scientific programme. Within the condensed-matter group, he established a strong subgroup in statistical mechanics and its applications, which was the beginning of quantitative biology. He established a joint project with the Beijer Institute and the Fondazione Eni Enrico Mattei in environmental and ecological economics, and founded an ICTP group devoted to the physics of weather and climate. He also succeeded in rendering compulsory the Italian contribution to the ICTP, and securing a significant increase in the contribution in 2000.

Back in Rome, in the last years before his 2011 retirement, he worked on applications of physical theories to finance, an activity that he continued in Argentina, where he returned, at the Universidad Nacional de General Sarmiento. In 2009 he received the Enrico Fermi Prize from the Italian Physical Society and in 2020 was awarded the ICTP Dirac medal for his work on string theory.

Miguel Virasoro cherished the ability to use knowledge learned in one field to make progress on a different one, opening up new vistas. He will be sorely missed.

MicroBooNE homes in on the sterile neutrino

Excitement is building in the search for sterile neutrinos – long-predicted particles which would constitute physics beyond the Standard Model. Although impervious to the electromagnetic, weak and strong interactions, such a fourth “right-handed” neutrino flavour could reveal itself by altering the rate of standard-neutrino oscillations – tantalising hints of which were reported by Fermilab’s MiniBooNE experiment in 2007. In a preprint published last week, sibling experiment MicroBooNE strongly disfavours a mundane explanation for such hints, with further scrutiny by the collaboration expected to be announced later this month.

“If the MiniBooNE effect is indeed a sterile neutrino, this of course would be a major discovery which would revolutionise particle physics, opening up a whole new sector to explore,” says MicroBooNE co-spokesperson Justin Evans of the University of Manchester.

The story of the sterile neutrino began in the 1990s, when the LSND experiment at Los Alamos reported seeing 88±23 (3.8σ) more electron antineutrinos than expected in a beam of accelerator-generated muon antineutrinos. This apparent short-baseline oscillation from muon to electron antineutrinos was incompatible with the oscillation rates established by Super-Kamiokande in 1998 and SNO in 2002, and would have to occur via an unknown intermediate neutrino flavour with a mass of about an electron-Volt. This hypothesised neutrino was dubbed sterile, as it would have to be insensitive to all interactions but gravity for it to have remained undiscovered this long.

The photon hypothesis

The plot thickened in 2007 when the MiniBooNE experiment at Fermilab tried to reproduce the LSND anomaly. The team also saw an excess of electron-like signals, though not quite at the energy corresponding to the LSND effect. The significance of the MiniBooNE anomaly grew to 4.5σ by the time the experiment finished running in November 2018. But a mundane possible explanation poured cold water on hopes for new physics: as a mineral-oil Cherenkov detector, MiniBooNE could not differentiate electrons from photons, and one particularly tricky-to-model background process might be contributing more photons than expected.

Many of us suspected that there could be something wrong with predictions for this background
Joachim Kopp

“High-energy single photons can be produced when a neutrino scatters on a nucleon via a neutral-current interaction and excites the nucleon to a Δ(1232) resonance,” explains CERN theorist Joachim Kopp. “Most of the time, the resonance decays to a pion and a nucleon, but there is a rare decay mode to a nucleon and a photon. The rate for this mode is very hard to predict, and many of us suspected that there could be something wrong with predictions for this background.”

Enter MicroBooNE, a liquid-argon time-projection-chamber sibling experiment to MiniBooNE which is capable of studying neutrino interactions in photographic detail, and differentiating the two signals. Having detected its first neutrino interactions in 2015, the MicroBooNE team has now set a limit on the neutral-current Δ→Nγ process is more than a factor of 50 better than existing constraints, explains Evans. “With this MicroBooNE result, we reject a Δ→Nγ model of the low-energy excess at 94.8% confidence, a strong indication that we must look elsewhere for the source of the excess.”

The electron hypothesis

Now that MicroBooNE has strongly disfavoured a leading-photon model for the MiniBooNE anomaly, attention shifts to the electron hypothesis – which would hint at the existence of a sterile neutrino, or something more exotic, if proven. And we don’t have long to wait. The MicroBooNE collaboration plans to release its search for an electron-like low-energy excess on 27 October, with results from three independent analyses looking at a range of inclusive and exclusive channels.

Beyond that, there is more to come, says Evans. “Our current round of results use only the first half of the total MicroBooNE data-set, and this is a programme that is only just beginning, with ICARUS and SBND within Fermilab’s short-baseline programme now coming online to turn this into a multi-baseline exploration of the richness of neutrino physics with unparalleled detail.”

The global picture is complex. In 2019, for example, the MINOS+ experiment failed to confirm the MiniBooNE signal (CERN Courier March/April 2019 p7). Were the sterile neutrino to exist, it should also have significant cosmological consequences which remain unobserved. But the anomalies are accumulating, says Kopp.

“LSND and MiniBooNE are quite consistent, and the short-baseline reactor experiments require parameters in the same region of parameter space, though these results are very much in flux and it’s not clear which ones are trustworthy, so it’s hard to make precise statements. The good news is that there’s realistic hope of resolving these puzzles over the next few years. ”

The inexplicable neutrino

Claustrophobia. South Dakota. A clattering elevator lowers a crew of hard-hat-clad physicists 1500 metres below the ground. 750,000 tonnes of rock are about to be excavated from this former gold mine at the Sanford Underground Research Facility (SURF) to accommodate the liquid-argon time projection chambers (TPCs) of the international Deep Underground Neutrino Experiment (DUNE). Towards the end of the decade, DUNE will track neutrinos that originate 1300 km away at Fermilab in Chicago, addressing leptonic CP violation as well as an ambitious research programme in astrophysics.

Having set the scene, director Geneva Guerin, co-founder of Canadian production company Cinécoop, cuts to a wide expanse: a climber scaling a rock face near the French–Swiss border. Francesca Stocker, the star of the film and then a PhD student at the University of Bern, narrates, relating the scientific method to rock climbing. Stocker and her fellow protagonists are engaging, and the film vividly captures the human spirit surrounding the birth of a modern particle-physics detector.

I don’t think it is possible to explain a neutrino for a general audience
Geneva Guerin

But the viewer is not allowed to settle for long in any one location. After zipping to CERN, and a tour through its corridors accompanied by eerie cello music, we meet Stocker in her home kitchen, explaining how she got interested in science as a child. Next, we hop to Federico Sánchez, spokesperson of the T2K experiment in Japan, explaining the basics of the Standard Model.

T2K, and its successor Hyper-Kamiokande, DUNE’s equal in ambition and scope, both feature in the one-hour-long film. But the focus is on the development of the prototype DUNE detector modules that have been designed, built and tested at the CERN Neutrino Platform – and here the film is at its best. Guerin had full access to protoDUNE activities, allowing her to immerse the viewer with the peculiar but oddly fitting accompaniment of a solo didgeridoo inside the protoDUNE cryostat. We gatecrash celebrations when the vessel was filled with liquid argon and the first test-beam tracks were recorded. The film focuses on detailed descriptions of the workings of TPCs and other parts of the apparatus rather than accessible explanations of the neutrino’s fascinating and mysterious nature. Unformatted plots and graphics are pulled from various sources. While authentic, this gives the film an unpolished, home-made feel.

Given the density of the exposition in some parts, beyond the most enthusiastic popular-science fans, Ghost Particle seems best tailored for physics students encountering experimental neutrino physics for the first time – a point that Guerin herself made during a live Q&A following the CineGlobe screening: “I was aiming at people like me – those who love science documentaries,” she told the capacity crowd. “Originally I envisaged a three-part series over a decade or more, but I realised that I don’t think it is possible to explain a neutrino for a general audience, so maybe it’s something for educational purposes, to help future generations get introduced to this exciting programme.”

The film ends as it began, with the rickety elevator continuing its 12-minute descent into the bowels of the Earth.

Exploding myths about antimatter

Antimatter captivates the popular imagination. Beatriz Gato-Rivera, a former CERN fellow in theoretical physics and now a researcher at the Spanish National Research Council, recently published a noteworthy book on the subject, entitled Antimatter: What It Is and Why It’s Important in Physics and Everyday Life. Substantially extending her text Antimateria, from the outreach collection “Qué Sabemos De”, this work will also be of interest to experts, thanks to well documented anecdotes of historical interest.

Gato-Rivera sets out with a detailed exploration of the differences between atoms and antiatoms, as well as of matter–antimatter annihilation, motivating the reader to delve into a fairly complete introduction to particle physics: the concepts that underpin the Standard Model, and some that lie beyond. She then focuses on diverse aspects of antimatter science, beginning with the differences between antimatter, dark matter and dark energy, and the different roles they play in the universe. This touches upon the observed accelerating expansion of the universe. In particular, Gato-Rivera discusses dark-matter and dark-energy candidates, attempts to detect dark matter and its relation to the fate of the universe. She also carefully explains the distinction between primordial and secondary antimatter, and their roles in cosmology.

Antimatter by Gato-Rivera — Credit: Springer

Next up, a historical chapter reviews the major landmarks of the discovery of antimatter particles, from elementary antiparticles to anti-hadrons, and anti-nuclei to antiatoms. In particular, the ground-breaking discovery of the first antiparticle, the positron, is described in excellent detail. In a separate appendix, Gato-Rivera passionately clears up a historical controversy about its discovery. The positron was first found in cosmic rays by Carl Anderson and later artificially produced en masse in particle accelerators. Gato-Rivera then turns to a detailed historical overview of cosmic-ray research, from balloon experiments to large-scale ground-based detectors, finally culminating in modern space-based detectors on board satellites and the ISS. The next chapter covers the production of antimatter by particle collisions in accelerators at high energies, including a brief history of the facilities at CERN.

The focus is then put on one of the most interesting and important conundrums in particle physics and astrophysics: the apparent huge asymmetry between matter and antimatter in the observed universe. This touches upon the processes of the primordial creation of matter and antimatter, and on the open question of whether anti-stars, or even anti-galaxies, could exist somewhere in the universe.

Gato-Rivera returns to Earth to discuss current experiments in particle physics such as those at CERN’s Antimatter Factory, asking whether antiatoms really have the same properties as atoms, at least as far as their excitation spectra and gravitational pull is concerned. The author doesn’t shy away from popular questions such as whether antimatter anti-gravitates and would float up away from Earth. While the answers to these questions are firmly predicted in theory, there could be surprises, like the discovery of CP violation in the 1950s, so it is important to actually test these fundamental properties.

Sceptical words dash hopes of using antimatter as an energy source

The book finishes by exploring practical uses of antimatter in everyday life, such as the use of PET scanners to detect positrons emitted from short-lived radioactive substances administered to patients. The same principle is also used in material analysis, for example to test the mechanical integrity of turbine blades. But sceptical words dash any hopes of using antimatter as an energy source: the effort of artificially producing a single gram of antimatter would be prohibitive.

Gato-Rivera’s semi-popular text is comprehensive and well structured, with a minimum of mathematical expressions and technicalities. It will be most profitable for a scientifically educated audience with an interest in particle physics, however, experienced researchers who are interested in the history of the subject will also enjoy reading it.

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Brighter and more powerful

The Virasoro condition

The photon hypothesis

The electron hypothesis