An ATLAS researcher and a leading string theorist are among the winners of the 2021 Mustafa prize, which recognises researchers from the Islamic world. Yahya Tayalati (Mohammed V University, Rabat) was cited for contributions to searches for magnetic monopoles and his work on light-by-light scattering, which was first observed by ATLAS in 2019. Cumrun Vafa (Harvard) was recognised for developing F-theory. Among other laureates, M. Zahid Hasan (Princeton) was cited for his work on Weyl-fermion semimetals and topological insulators – materials which are insulators inside but conduct on their surfaces. Each wins $500,000.
Since its foundation in 2012, the Mustafa Prize has been announced every two years. This year is the first time that the prize has been awarded to researchers in fundamental science.
A 132 tonne superconducting magnet has set a new record for whole-body magnetic-resonance imaging (MRI), producing a field of 11.7 T inside a 0.9 m diameter and 5 m long volume. Four-times more powerful than typical hospital devices, the “Iseult” project at CEA-Paris-Saclay paves the way for imaging the brain in unprecedented detail for medical research.
Using a pumpkin as a suitably brain-like subject, the team released its first images on 7 October, validating the system and demonstrating an initial resolution of 400 microns in three dimensions. Other checks and approvals are necessary before the first imaging of human volunteers can begin.
This work will undoubtedly lead to major clinical applications
Stanislas Dehaene
“Thanks to this extraordinary MRI, our researchers are looking forward to studying the anatomical and structural organization of the brain in greater detail. This work will undoubtedly lead to major clinical applications,” said Stanislas Dehaene, director of NeuroSpin, the neuroimaging platform at CEA-Paris-Saclay.
The magnets that drive tens of thousands of MRI devices worldwide perform the vital task of aligning the magnetic moments of hydrogen atoms.Then, RF pulses are used to momentarily disturb this order in a specific region, after which the atoms are pulled back into equilibrium by the magnetic field, and radiate. The stronger the field, the higher the signal-to-noise ratio, and thus better image resolution.
Niobium-titanium
In addition to being the largest and most powerful MRI magnet ever built, claims the team, the Iseult solenoid (carrying a current of 1.5 kA) also sets a record for the highest ever field achieved using niobium-titanium conductor, the same as is used in the present LHC magnets. With various optimisations, and working with the European Union Aroma project on methodologies for optimal functioning of the new MRI device, a resolution approaching 100 to 200 microns is planned, around ten times higher than commercial 3T devices.
Designed and built over ten years, Iseult was jointly led by neuroscientists and magnet and MRI specialists at the CEA Institute of Research into the Fundamental Laws of the Universe (IRFU) and the Frédéric Joliot Institute for Life Sciences, along with several industry and academic partnerships in Germany. Although CERN was not directly involved, Iseult’s success is anchored in more than four decades of joined developments between CERN and the CEA, explains Anne-Isabelle Etienvre, head of CEA IRFU:
“It is thanks to the know-how developed for particle physics and fusion that MRI experts had the idea to ask us to design and build this unique and challenging magnet for MRI — in particular, CEA has played a major role, together with CERN and other partners, on LHC magnets, the ATLAS toroidal magnets and the CMS solenoid,” says Etienvre. “The collaboration between CEA and CERN is still very lively, in particular for advanced magnets for future accelerators.
The existence of an eV-scale sterile neutrino looks less likely today than at any time in the past 20 years. Such a particle has long been considered to be the simplest explanation for several related anomalies in neutrino physics, but results released yesterday by Fermilab’s MicroBooNE collaboration disfavour its existence relative to the Standard Model.
“MicroBooNE has made a very comprehensive exploration through multiple types of interactions, and multiple analysis and reconstruction techniques,” says co-spokesperson Bonnie Fleming of Yale. “They all tell us the same thing, and that gives us very high confidence in our results that we are not seeing a hint of a sterile neutrino.”
The collaboration says that the analyses favour the Standard Model over the anomalous signal seen by sibling-experiment MiniBooNE at more than 99% confidence, should its true origin be electrons from a neutrino oscillation via a hitherto-undetected sterile neutrino. “But that earlier data from MiniBooNE doesn’t lie,” says former co-spokesperson Sam Zeller of Fermilab. “There’s something really interesting happening that we still need to explain.”
There’s something really interesting happening that we still need to explain
Sam Zeller
Neutrinos suffer from an identity crisis regarding their mass. As a result, the three known flavours morph into each other as phase differences develop between three mass eigenstates. However, well before this model solidified around the turn of the millennium, a measurement by the LSND collaboration at Los Alamos in the US suggested the existence of an additional neutrino which had to be “sterile” with respect to the weak, electromagnetic and strong interactions, and much more massive, given how rapidly the oscillation developed. Since this first hint, the tale of the sterile neutrino has taken multiple twists and turns.
Twists and turns
In the mid-1990s, LSND reported seeing a 3.8σ excess of electron antineutrinos in a beam of accelerator-generated muon antineutrinos, but the KARMEN experiment at the Rutherford Appleton Laboratory in the UK failed to reproduce the effect. Evidence for an eV-scale sterile neutrino mounted with the observation of a deficit of electron neutrinos from 37Ar and 51Cr electron-capture decays at Gran Sasso in Italy and at the Baksan Neutrino Observatory in Russia (the gallium anomaly), and a reported deficit of electron antineutrinos from nuclear reactors (the reactor anomaly). Troublingly, however, long-baseline accelerator neutrino experiments such as MINOS+ do not observe the requisite “disappearance” of muon neutrinos required by the principle of unitarity, and the existence of such a sterile neutrino is also starkly incompatible with current models of cosmology. While the gallium anomaly should soon be probed definitively by the BEST experiment at Baksan (Phys. Rev. D 2018 97 073001), recent calculations of reactor fluxes may now be dissolving the reactor anomaly (see, for example, arXiv:2110.06820). But the most compelling single piece of evidence in favour of sterile neutrinos came when the MiniBooNE experiment at Fermilab tried to reproduce the LSND effect. In November 2018, the collaboration reported a 4.5σ excess of electron neutrinos and antineutrinos compared to Standard-Model expectations.
Few neutrino physicists foresaw that MicroBooNE would disfavour both hypotheses
Sibling experiment MicroBooNE has now released its first round of tests of the MiniBooNE anomaly. Equipped with a cutting-edge liquid-argon time-projection chamber, the collaboration observed neutrino interactions at the level of individual particle tracks – a key advantage compared to a Cherenkov detector such as MiniBooNE, which could not distinguish electrons from photons. The collaboration has now used half of its available data to probe which particle is the true origin of the anomaly. Earlier this month, MicroBooNE tested the hypothesis that MiniBooNE’s excess was actually due to an underestimated single-photon background, perhaps caused by a difficult-to-model rare decay of a Δ resonance. Now, MicroBooNE has tested the hypothesis that the MiniBooNE excess was caused by single electrons, most likely the result of neutrino oscillations via an eV-scale sterile neutrino. Few neutrino physicists foresaw that MicroBooNE would disfavour both hypotheses.
“Every time we look at neutrinos, we seem to find something new or unexpected,” says MicroBooNE co-spokesperson Justin Evans of the University of Manchester. “MicroBooNE’s results are taking us in a new direction, and our neutrino programme is going to get to the bottom of some of these mysteries.” The collaboration will now investigate whether more exotic topologies such as electron-positron pairs could be the source of the MiniBooNE anomaly. Such a final state might suggest the existence of heavier sterile neutrinos, say theorists.
“eV-scale sterile neutrinos no longer appear to be experimentally motivated, and never solved any outstanding problems in the Standard Model,” says theorist Mikhail Shaposhnikov of EPFL. “But GeV-to-keV-scale sterile neutrinos – so-called Majorana fermions – are well motivated theoretically and do not contradict any existing experiment. They can explain neutrino masses and oscillations, give a dark-matter candidate, and produce a baryon asymmetry in the universe: all the problems that the Standard Model is incapable of addressing. Experimental efforts at the intensity frontier should now be concentrated in this direction.”
Future scientific breakthroughs in high-energy physics will require unprecedented levels of international engagement, building on the successful model of the Large Hadron Collider at CERN. Joe Lykken, Fermilab deputy director for research, will describe how Fermilab is moving forward rapidly with CERN and other international partners to realise this vision.
The questions under scrutiny range from the nature of the Higgs field to the question of whether neutrinos play a role in the matter-antimatter asymmetry observed in the universe. PIP-II, an upgrade to the Fermilab accelerator complex that includes a leading-edge superconducting linear accelerator, is already under construction, with major “in-kind” contributions and expertise from partners in India, Italy, the UK, France and Poland. PIP-II will enable the world’s most intense beam of neutrinos for the Deep Underground Neutrino Experiment (DUNE), which will deploy 70,000 tonnes of liquid argon detectors in a deep underground site 1300 km from Fermilab. DUNE was formulated as an international project from the start, and now includes more than a thousand collaborators from 30 countries. Two large prototype detectors for DUNE have been successfully constructed and tested at the CERN Neutrino Platform. DUNE will have remarkable capabilities to determine how the properties of neutrinos have shaped our universe. At the same time, Fermilab has been developing and building next-generation superconducting magnets that will be deployed in the HL-LHC accelerator at CERN, and is the US lead for ambitious upgrades to the CMS experiment for the HL-LHC era. These technological capabilities will also make Fermilab an important partner for the proposed Future Circular Collider.
Joseph Lykken is Fermilab’s deputy director of research and leads the Fermilab Quantum Institute. A distinguished scientist at the laboratory, Lykken was a former member of the Theory Department, researching string theory and phenomenology, and is a member of the CMS experiment on the Large Hadron Collider at CERN. He received his PhD from the Massachusetts Institute of Technology and has previously worked for the Santa Cruz Institute for Particle Physics and the University of Chicago. Lykken began his tenure at Fermilab in 1989. He is a former member of the High Energy Physics Advisory Panel, which advises both the Department of Energy and the National Science Foundation, and served on the Particle Physics Project Prioritization Panel, developing a road map for the next 20 years of US particle physics. Lykken is a fellow of the American Physical Society and of the American Association for the Advancement of Science.
The Large Hadron Collider (LHC) complex is being upgraded to significantly extend its scientific reach. Following the ongoing 2019–2022 long shutdown, the LHC is expected to operate during Run 3 at close to its design of 7 TeV per beam and at luminosities more than double the original design. After the next shutdown, currently foreseen in 2025–2027, the High-Luminosity LHC (HL-LHC) will run at luminosities of 5–7 × 1034 cm–2s–1. This corresponds to 140–200 simultaneous interactions per LHC bunch crossing (“pileup”), which is three to four times the Run-3 expectation and up to eight times above the original LHC design value. The ATLAS experiment, like others at the LHC, is undergoing major upgrades for the new LHC era.
Coping with very high interaction rates while maintaining low transverse-momentum (pT) thresholds for triggering on electrons and muons from the targeted physics processes will be extremely challenging at the HL-LHC. Another issue for the ATLAS experiment is that the performance of its muon tracking chambers, particularly in the end-cap regions of the detector, degrades with increasing particle rates. If the original chambers were used for the HL-LHC, it would lead to a loss in the efficiency and resolution of muon reconstruction.
Muons are vital for efficiently triggering on, and thus precisely studying, processes in the electroweak sector such as Higgs, W and Z physics. It is therefore essential that the ATLAS detector cover as much volume as possible across the pseudorapidity distribution η = –ln tanθ/2, where θ is the angle with respect to the proton beam axis. In the central region of the detector, corresponding to a pseudorapidity |η| < 1, there is a good purity of muons originating from the proton collision point (see “Good muons” figure). In the end caps, |η| > 1.3, significant contributions, the so-called “fake” muon signals (see “Real or fake?” figure), arise from other sources. These include cavern backgrounds and muons produced in the halo of the LHC proton beams, both of which increase with larger instantaneous luminosities. Without modifications to the detector, the fake-muon trigger rates in the end caps would become unsustainable at the HL-LHC, requiring the muon pT thresholds in the Level-1 trigger to be raised substantially.
To resolve these issues, the ATLAS collaboration decided, as part of its major Phase-I upgrade, to replace the existing ATLAS muon small wheels with the “New Small Wheels” (NSW), capable of reconstructing muon track segments locally with 1 mrad resolution for both the Level-1 trigger and for offline reconstruction. The NSW will allow low-pT thresholds to be maintained for the end-cap muon triggers even at the ultimate HL-LHC luminosity.
The low-pT region for leptons is of critical importance to the ATLAS physics programme. As an example, Higgs-boson production via vector-boson fusion (VBF) is a powerful channel for precision Higgs studies, and low-pT end-cap lepton triggers are crucial for selecting H → ττ events used to study Higgs-boson Yukawa couplings. Within the current tracking detector acceptance of |η| < 2.5, the fraction VBF of H → ττ events with the leading muons having pT above 25 GeV (typical Run-2 threshold) is 60%, while this fraction drops to 28% for a pT threshold of 40 GeV (expected typical HL-LHC threshold if no changes to the detectors are made). Maintaining, or even reducing, the muon pT threshold is critical for extending the ATLAS physics programme in higher luminosity LHC operation.
Frontier technologies
The ATLAS NSW is a set of precision tracking and trigger detectors able to work at high rates with excellent spatial and time resolution using two innovative technologies: MicroMegas (MM) and small-strip thin-gap chambers (sTGC). These detectors will provide the muon Level-1 trigger system with online track segments with good angular resolution to confirm that they originate from the interaction point, reducing triggers from fake muons. They will also have timing resolutions below the 25 ns interbunch time, enabling bunch-crossing identification. With the NSW, ATLAS will keep the full acceptance of its muon tracking system at the HL-LHC while maintaining a low Level-1 pT threshold of around 20 GeV.
The ATLAS collaboration chose MM and sTGC technologies for the NSW after a detailed scrutiny of several available options. The idea was to build a robust and redundant system, using research-frontier and cost-effective technologies. Each NSW wheel has 16 sectors, with each sector containing four MM chambers and six sTGC chambers. Each sector, with a total surface area ranging from about 4 to 6 m2 , has eight sensitive planes of MM and eight of sTGC along the muon track direction. The 16 overall measurement planes allow for redundancy in the track reconstruction.
MM detectors were proposed in the 1990s in the framework of the Micro-Pattern Gaseous Detectors (MPGD) R&D programme including the RD51 project at CERN (see “Robust and redundant” figure, top). They profit from the development of photolithographic techniques for the design of high-granularity readout patterns and, in parallel, from the development of specialised front-end electronics with an increased number of channels. A dedicated R&D programme introduced, developed and realised the concept of resistive MM detectors. The main challenge for ATLAS was to scale the detectors from a few tens of cm in size to chambers of 2–3 m2 with a geometry under control at the level of tens of μm. This required additional R&D together with a very detailed mechanical design of the detectors. The resulting detectors represent the largest and most complex MPGD system ever built.
Thin-gap chambers have been used for triggering and to provide the azimuthal coordinate of muons in the ATLAS muon spectrometer end caps since the beginning of LHC operations, and were used previously in the OPAL experiment at LEP. The sTGC is an extension of established TGC technology to allow for precise online tracking that can be used both in the trigger and in offline muon tracking, with a strip pitch of 3.2 mm (see “Robust and redundant” figure, bottom).
A common readout front-end chip, named VMM, was developed for the readout of the MM strips and of the active elements of the sTGC (strips, pads and wires). This chip is a novel “amplifier-shaper-discriminator” front-end ASIC able to perform amplification and shaping, peak finding and digitisation of the detector signals. The overall system has about 2 million MM and 350,000 sTGC readout channels. The ATLAS trigger, using information from both detectors, will identify track segments pointing to the interaction region and share this information with the muon trigger.
International enterprise
The construction of the 128 MM and 192 sTGC chambers has been a truly international enterprise shared among several laboratories. The construction of the MM was shared among five construction consortia in France, Germany, Greece, Italy and Russia, with infrastructure and technical expertise inherited from the construction of the ATLAS Muon Spectrometer Monitored Drift Tube chambers. The construction of the sTGC was shared among five consortia located in Canada, Chile, China, Israel and Russia, including both institutes from the original TGC construction and new ones.
A key challenge in realising both technologies was the use of large-area circuit boards produced by industry. For the case of the MM, high-voltage instabilities observed since the construction of the first large-size prototypes were mostly due to the quality of the printed circuit boards. Two aspects in particular were investigated: the cleanliness of the surfaces, and the actual measured values of the board resistivity that were in many cases not large enough to prevent electrical discharges in the detector. For both problems, detailed mitigation protocols were developed and shared among the consortia: a cleaning protocol including polishing and washing of all the surfaces and a “passivation” procedure designed to mask detector regions with lower resistance where most of the discharges were observed to take place.
For the sTGC, the principal difficulty in the circuit-board production was maintaining mechanical tolerances and electrical integrity over the large areas. Considerable R&D and quality control were required before and during the board production, and when combined with X-ray measurements at CERN the sTGC layers are aligned to better than 100 μm.
Along with the chamber construction, several tests were carried out at the construction sites to evaluate the chamber quality. Some of the first full-size prototypes together with the first production chambers were exposed to test beams. All the sTGC chambers and a large fraction of the MM chambers were also tested at CERN’s GIF++ irradiation facility to evaluate their behaviour under a particle rate comparable to the one expected at the HL-LHC.
The integration of both MM and sTGC chambers to form the wheel sectors took place at CERN from 2018 to 2021. Four MM chambers form a double-wedge, assembled accounting for the severe alignment requirements, which is then equipped with all the necessary services and the final front-end electronics (see “Taking stock” image). The systems were fully tested in a dedicated cosmic-ray test stand to verify the functionality of the detector and to evaluate the detector efficiency. For the sTGCs, three chambers were glued to fibreglass frames using precision inserts on a granite table to form a wedge. After long-term high-voltage tests, the sTGC wedges were equipped with front-end electronics, cooling, and readout cables and fibres. All the sTGC chambers were tested with cosmic rays at the construction sites, and a few were also tested at CERN.
To form each sector, two sTGC wedges and one MM double-wedge were sandwiched together. The sectors were then precisely mounted on “spokes” installed on the large shielding disks that form the NSW wheels, along with a precision optical alignment system that allows the chamber positions to be tracked by ATLAS in real time (see “Revolutions” image). After completing final electrical, cooling and gas connections during 2020 and 2021, all sectors were commissioned and tested on the wheel. One unexpected problem encountered on the first sectors on wheel A was the presence of a noise level in the front-end electronics that was significantly higher than observed during integration. A large and ultimately successful effort was put in place to mitigate this new challenge, for example by improving the grounding and shielding, and adding filtering to the power supplies.
This final success follows more than a decade of research, design and construction by the ATLAS collaboration. The NSW initiative dates to early LHC operation, around 2010, and the technical design report was approved in 2013, with construction preparation starting soon afterwards. The impact of the COVID-19 pandemic on the NSW construction schedule was significant, mostly at the construction sites, where delays of up to a few months were accrued, but the project is now on schedule for completion during the current LHC shutdown.
The endgame
Prior to lowering the NSW into the ATLAS experimental cavern, other infrastructure was installed to prepare for detector operation. The service caverns were equipped with electronics racks, high-voltage and low-voltage power supplies, gas distribution systems, cooling infrastructure for electronics, as well as control and safety systems. Where possible, existing infrastructure from the previous ATLAS small wheels was repurposed for the NSW.
ATLAS is now close to the completion of its Phase-I upgrade goal of having both NSW-A and NSW-C installed for the start of Run 3
On 6 July, the first wheel, NSW-A, was shipped from Building 191 on the CERN site to LHC Point 1 and then, less than a week later, lowered into its position in ATLAS (see “In place” image). With the first NSW in its final position, the extensive campaign of connecting low voltage, high voltage, gas, readout fibres and electronics cooling was the next step. These connections were completed for NSW-A in July and August 2021, and an extensive commissioning programme is ongoing. In addition to powering both the chambers and the readout electronics, the integration of the NSW into the ATLAS controls and data-acquisition system is occurring at Point 1. NSW-A is planned to be fully integrated into ATLAS for the LHC pilot-beam run in October 2021, and then NSW-C will be lowered and installed.
Despite a tight schedule, ATLAS is now close to the completion of its Phase-I upgrade goal of having both NSW-A and NSW-C installed for the start of Run 3. The period up to February 2022 will be needed to complete commissioning and testing. Starting from March 2022, a very important “commissioning with beam” phase will be carried out to ensure stable collisions in Run 3. Even with the challenges of developing new technologies while working across a dozen countries during the COVID-19 pandemic, the ATLAS New Small Wheel upgrade will be ready for the exciting, new higher luminosities that will open up a novel era of LHC physics.
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 ντ existence, the DONUT/Fermilab result represented the first direct observation.
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.
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 .
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.
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, 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 Ds 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 fD+ and fDs. 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 Bs mesons. 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.
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.”
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 RK = 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 RK*0 (which probes B0→K*0ℓ+ℓ– decays) reported by LHCb in 2017. The collaboration has also seen slight deficits in the ratio RpK, 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→ KS0ℓ+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 KS0 in the final state, however, the sensitivity of the results is lower than for previous “RK” analyses. LHCb found R(KS0) = 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.
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 fR, fL, and fo, 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.
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Joseph Lykken is Fermilab’s deputy director of research and leads the Fermilab Quantum Institute. A distinguished scientist at the laboratory, Lykken was a former member of the Theory Department, researching string theory and phenomenology, and is a member of the CMS experiment on the Large Hadron Collider at CERN. He received his PhD from the Massachusetts Institute of Technology and has previously worked for the Santa Cruz Institute for Particle Physics and the University of Chicago. Lykken began his tenure at Fermilab in 1989. He is a former member of the High Energy Physics Advisory Panel, which advises both the Department of Energy and the National Science Foundation, and served on the Particle Physics Project Prioritization Panel, developing a road map for the next 20 years of US particle physics. Lykken is a fellow of the American Physical Society and of the American Association for the Advancement of Science.
