Jobs – Page 101 of 521

Spiralling into the femtoscale

Radio-frequency quadrupole — **On the plus side** Inside view of the radio-frequency quadrupole at the entrance to SPIRAL2’s superconducting linear accelerator. Credit: R Ferdinand

Nuclear physics is as wide-ranging and relevant today as ever before in the century-long history of the subject. Researchers study exotic systems from hydrogen-7 to the heaviest nuclides at the boundaries of the nuclear landscape. By constraining the nuclear equation of state using heavy-ion collisions, they peer inside stars in controlled laboratory tests. By studying weak nuclear processes such as beta decays, they can even probe the Standard Model of particle physics. And this is not to mention numerous applications in accelerator-based atomic and condensed-matter physics, radiobiology and industry. These nuclear-physics research areas are just a selection of the diverse work done at the Grand Accélérateur National d’Ions Lourds (GANIL), in Caen, France.

GANIL has been operating since 1983, initially using four cyclotrons, with a fifth Cyclotron pour Ions de Moyenne Energie (CIME) added in 2001. The latter is used to reaccelerate short-lived nuclei produced using beams from the other cyclotrons – the Système de Production d’Ions Radioactifs en Ligne (SPIRAL1) facility. The various beams produced by these cyclotrons drive eight beams with specialised instrumentation. Parallel operation allows the running of three experiments simultaneously, thereby optimising the available beam time. These facilities enable both high-intensity stable-ion beams, from carbon-12 to uranium-238, and lower intensity radioactive-ion beams of short-lived nuclei, with lifetimes from microseconds to milliseconds, such as helium-6, helium-8, silicon-42 and nickel-68. Coupled with advanced detectors, all these beams allow nuclei to be explored in terms of excitation energy, angular momentum and isospin.

The new SPIRAL2 facility, which is currently being commissioned, will take this work into the next decade and beyond. The most recent step forward is the beam commissioning of a new superconducting linac – a major upgrade to the existing infrastructure. Its maximum beam intensity of 5 mA, or 3 × 10¹⁶ particles per second, is more than two orders of magnitude higher than at the previous facility. The new beams and state-of-the-art detectors will allow physicists to explore phenomena at the femtoscale right up to the astrophysical scale.

Landmark facility

SPIRAL2 was approved in 2005. It now joins a roster of cutting-edge European nuclear-physics-research facilities which also features the Facility for Antiproton and Ion Research (FAIR), in Darmstadt, Germany, ISOLDE and nTOF at CERN, and the Joint Institute for Nuclear Research (JINR) in Russia. Due to their importance in the European nuclear-physics roadmap, SPIRAL2 and FAIR are both now recognised as European Strategy Forum on Research Infrastructures (ESFRI) Landmark projects, alongside 11 other facilities, including accelerator complexes such as the European X-Ray Free-Electron Laser, and telescopes such as the Square Kilometre Array.

Construction began in 2011. The project was planned in two phases: the construction of a linac for very-high-intensity stable beams, and the associated experimental halls (see “High intensity” figure); and infrastructure for the reacceleration of short-lived fission fragments, produced using deuteron beams on a uranium target through one of the GANIL cyclotrons. Though the second phase is currently on hold, SPIRAL2’s new superconducting linac is now in a first phase of commissioning.

Superconducting linac and experimental halls — **High intensity** SPIRAL2’s new superconducting linac and its experimental halls. Credit: GANIL

Most linacs are optimised for a beam with specific characteristics, which is supplied time and again by an injector. The particle species, velocity profile of the particles being accelerated and beam intensity all tend to be fixed. By tuning the phase of the electric fields in the accelerating structures, charged particles surf on the radio-frequency waves in the cavities with optimal efficiency in a single pass. Though this is the case for most large projects, such as Linac4 at CERN, the Spallation Neutron Source (SNS) in the US and the European Spallation Source in Sweden, SPIRAL2’s linac (see “Multitasking” figure) has been designed for a wide range of ions, energies and intensities.

The multifaceted physics criteria called for an original design featuring a compact multi-cryostat structure for the superconducting cavities, which was developed in collaboration with fellow French national organisations CEA and CNRS. Though the 19 cryomodules are comparable in number to the 23 employed by the larger and more powerful SNS accelerator, the new SPIRAL2 linac has far fewer accelerating gaps. On the other hand, compared to normal-conducting cavities such as those used by Linac4, the power consumption of the superconducting structures at SPIRAL2 is significantly lower, and the linac conforms to additional constraints on the cryostat’s design, operation and cleanliness. The choice of superconducting rather than room-temperature cavities is ultimately linked not only to the need for higher beam intensities and energies, but also to the potential for the larger apertures needed to reduce beam losses.

SPIRAL2 joins a roster of cutting-edge European nuclear-physics-research facilities

Beams are produced using two specialised ion sources. At 200 kW in continuous-wave (CW) mode, the beam power is high enough to make a hole in the vacuum chamber in less than 35 µs, placing additional severe restrictions on the beam dynamics. The operation of high beam intensities, up to 5 mA, also causes space-charge effects that need to be controlled to avoid a beam halo which could activate accelerator components and generate neutrons – a greater difficulty in the case of deuteron beams.

For human safety and ease of technical maintenance, beam losses need to be kept below 1 W/m. Here, the SPIRAL2 design has synergies with several other high-power accelerators, leading to improvements in the design of quarter-wave resonator cavities. These are used at heavy-ion accelerators such as the Facility for Rare Isotope Beams in the US and the Rare Isotope Science Project in Korea; for producing radioactive-ion beams and improving beam dynamics at intense-light particle accelerators worldwide; for producing neutrons at the International Fusion Materials Irradiation Facility, the ESS, the Myrrha Multi-purpose Hybrid Research Reactor for High-tech Applications, and the SNS; and for a large range of studies relating to materials properties and the generation of nuclear power.

Beam commissioning

Initial commissioning of the linac began by sending beams from the injector to a dedicated system with various diagnostic elements. The injector was successfully commissioned with a range of CW beams, including a 5 mA proton beam, a 2 mA alpha-particle beam, a 0.8 mA oxygen–ion beam and a 25 µA argon–ion beam. In each case, almost 100% transmission was achieved through the radio-frequency quadrupoles. Components of the linac were installed, the cryomodules cooled to liquid-helium temperatures (4.5 K), and the mechanical stability required to operate the 26 superconducting cavities at their design specifications demonstrated.

Superconducting cryomodules — **Multitasking** The 19 superconducting cryomodules of the SPIRAL2 linac. Credit: GANIL

As GANIL is a nuclear installation, the injection of beams into the linac required permission from the French nuclear-safety authority. Following a rigorous six-year authorisation process, commissioning through the linac began in July 2019. An additional prerequisite was that a large number of safety systems be validated and put into operation. The key commissioning step completed so far is the demonstration of the cavity performance at 8 MV/m – a competitive electric field well above the required 6.5 MV/m. The first beam was injected into the linac in late October 2019. The cavities were tuned and a low-intensity 200 µA beam of protons accelerated to the design value of 33 MeV and sent to a first test experiment in the neutrons for science (NFS) area. A team from the Nuclear Physics Institute in Prague irradiated copper and iron targets and the products formed in the reaction were transported by a fast-automatic system 40 m away, where their characteristic γ-decay was measured. Precise measurements of such cross-sections are important in order to benchmark safety codes required for the operation of nuclear reactors.

SPIRAL2 is now moving towards its design power by gradually increasing the proton beam current and subsequently the duty cycle of the beam – the ratio of pulse duration to the period of the waveform. A similar procedure with alpha particles and deuteron beams will then follow. Physics programmes will begin in autumn next year.

Future physics

With the new superconducting linac, SPIRAL2 will provide intense beams from protons to nickel – up to 14.5 MeV/A for heavy ions – and continuous and quasi-mono energetic beams of neutrons up to 40 MeV. With state-of-the-art instrumentation such as the Super Separator Spectrometer (S³), the charged particle beams will allow the study of very rare events in the intense background of the unreacted beam with a signal to background fraction of 1 in 10¹³. The charged particle beams will also characterise exotic nuclei with properties very different from those found in nature. This will address questions related to heavy and super-heavy element/isotope synthesis at the extreme boundaries of the periodic table, and the properties of nuclei such as tin-100, which have the same number of neutrons and protons – a far cry from naturally existing isotopes such as tin-112 and tin-124. Here, ground-state properties such as the mass of nuclei must be measured with a precision of one part in 10⁹ – a level of precision equivalent to observing the addition of a pea to the weight of an Airbus A380. SPIRAL2’s low-energy experimental hall for the disintegration, excitation and storage of radioactive ions (DESIR), which is currently under construction, will further facilitate detailed studies of the ground-state properties of exotic nuclei fed both by S³ and SPIRAL1, the existing upgraded reaccelerated exotic-beams facility. The commissioning of S³ is expected in 2023 and experiments in DESIR in 2025. In parallel, a continuous improvement in the SPIRAL2 facility will begin with the integration of a new injector to substantially increase the intensity of heavy-ion beams.

Properties must be measured with a level of precision equivalent to observing the addition of a pea to the weight of an Airbus A380

Thanks to its very high neutron flux – up to two orders of magnitude higher, in the energy range between 1 and 40 MeV, than at facilities like LANSCE at Los Alamos, nTOF at CERN and GELINA in Belgium – SPIRAL2 is also well suited for applications such as the transmutation of nuclear waste in accelerator-driven systems, the design of present and next-generation nuclear reactors, and the effect of neutrons on materials and biological systems. Light-ion beams from the linac, including alpha particles and lithium-6 and lithium-7 impinging on lead and bismuth targets, will also be used to investigate more efficient methods for the production of certain radioisotopes for cancer therapy.

Developments at SPIRAL2 are quickly moving forwards. In September, the control of the full emittance and space–charge effects was demonstrated – a crucial step to reach the design performance of the linac – and a first neutron beam was produced at NFS, using proton beams. The future looks bright. With the new SPIRAL2 superconducting linac now supplementing the existing cyclotrons, GANIL provides an intensity and variety of beams that is unmatched in a single laboratory, making it a uniquely multi-disciplinary facility in the world today.

Fragile light nuclei flow through freeze-out

**Fig. 1.** The elliptic flow of deuterons, compared with other species (left) and phenomenological models (middle), and their triangular flow (right), in Pb–Pb collisions at √s_NN = 5.02 TeV in the centrality class 20–40%. Credit: CERN

Ultra-relativistic heavy-ion collisions create a system of deconfined quarks and gluons known as the quark–gluon plasma (QGP). Among other particles, a large number of light nuclei such as the deuteron, triton, helium-3, helium-4 and their corresponding antinuclei are produced, and can be measured with very good precision by the ALICE experiment at the LHC thanks to its excellent tracking and particle-identification capabilities via specific energy loss and time-of-flight measurements. Considering that the binding energies of light (anti)nuclei do not exceed a few MeV, it is not clear how such fragile objects can survive the hadron gas phase created after the phase transition from the QGP to hadrons, where particles rescatter with a typical momentum transfer in excess of 100 MeV. The production mechanism of light (anti)nuclei in these collisions is still not understood and is under intense debate in the scientific community. Constraining models of light antinuclei production is also important for predicting the backgrounds to indirect dark-matter searches using cosmic rays, as performed by experiments in space and in hot-air balloons, for which light antinuclei are promising signals.

The measured elliptic flow of light nuclei is bracketed by the simple coalescence approach and the blast-wave model

Azimuthal anisotropies of light (anti)nuclei production with respect to the symmetry plane of the collision are key observables to study interactions in the hadron-gas phase, and can shed light on the production mechanism of these fragile objects. The ALICE collaboration has recently reported the measurements of two harmonic coefficients (v_n) in a Fourier decomposition of the azimuthal distribution of deuterons in Pb–Pb collisions at √s_NN = 5.02 TeV: their elliptic flow, v₂, and the first measurement of their triangular flow, v₃. A clear mass ordering is observed in the elliptic flow of non-central Pb–Pb collisions at low p_T when the deuteron results are compared with other particle species, as expected for an expanding hydrodynamic system (figure 1, left).

Blast wave is best

The results are often compared to three phenomenological models, namely the statistical hadronisation model, the coalescence model, and the blast-wave model. In the statistical hadronisation model, light (anti)nuclei are assumed to be emitted by a source of thermal and hydrochemical equilibrium, like other hadron species, and their abundances fixed at the chemical freeze-out – the time at which inelastic interactions cease. However, this model only describes their yields, and not their flow. On the other hand, the coalescence model predicts that light nuclei are formed by the coalescence of protons and neutrons that are close in phase space at the kinetic freeze-out – the time at which elastic interactions cease. The blast-wave model, which is based on a simplified version of relativistic hydrodynamics, describes their transverse momentum spectra with just a few parameters, such as the kinetic freeze-out temperatures and transverse velocity.

In the new ALICE results, the measured elliptic flow of light nuclei is bracketed by the simple coalescence approach and the blast-wave model, which describe the data in different multiplicity regimes (figure 1, middle). The deuteron triangular flow is consistent with the coalescence model predictions, but large uncertainties do not allow a conclusive statement (figure 1, right). This specific aspect will be addressed with the larger data sample that ALICE will record in Run 3, which will also allow measurement of the flow of heavier nuclei. These results will contribute to shed light on their production mechanism and to study the properties of the hadron gas phase.

Leptoquarks and the third generation

**Fig. 1.** 95% confidence limits on the mass and coupling of a spin-1 (vector) leptoquark (LQ_V), considering single (orange) and pair (purple) production mechanisms, and their combination (blue), compared to the region favoured by models seeking to explain the B-physics anomalies (yellow). The regions to the left are excluded. Credit: CERN

The Standard Model (SM) groups quarks and leptons separately to account for their rather different observed properties, but might they be unified through a new particle that couples to both and turns one into the other? Such “leptoquarks” emerge quite naturally in several theories that extend the SM. Searches for leptoquarks have been an important part of the LHC’s research programme since the beginning, and have received additional attention recently in the light of hints of deviations from the principle of lepton universality – the so-called flavour anomalies.

In a recent CMS analysis, where the events collected in pp collisions during Run 2 (137 fb^–1) are analysed, researchers have challenged the SM by investigating a previously unexplored leptoquark signature involving the third generation of fermions. The motivation for considering the third generation is to confront the principle of lepton universality, which asserts that the couplings of leptons with gauge bosons are flavour independent. This principle is built into the SM, but has recently been put under stress by a series of anomalies observed in precision measurements of certain B-meson decays by the LHCb, Belle and BaBar collaborations. A possible explanation for these anomalies, which are still under investigation and not yet confirmed, lies in the existence of leptoquarks that preferentially couple to the heaviest fermions.

These results are the most stringent limits to date on the presence of leptoquarks that couple preferentially to the third generation

The new CMS search looks for both single and pair production of leptoquarks. It considers leptoquarks that decay to a quark (top or bottom) and a lepton (tau or neutrino), targeting the signature with a top quark, a tau lepton, missing transverse momentum due to a neutrino, and, in the case of double production, an additional bottom-quark jet. This is the first search to simultaneously consider both production mechanisms by categorising events with one or two jets originating from a bottom quark. The analysis also includes a dedicated selection for the case of a large mass splitting between the leptoquark and the top quark, which would boost the top quark and could cause its decay products to be inseparable given the spatial resolution of jets.

The observations are found to be in agreement with the SM prediction, and exclusion limits are derived in the plane of the leptoquark–lepton–quark vertex coupling λ and the leptoquark mass. The results are derived separately for hypothetical spin-0 and spin-1 (figure 1) leptoquarks, reflecting the two types allowed by theoretical models. The analysis assumes that the leptoquark decays half the time to each of the possible quark–lepton flavour pairs, for example, in the case of a spin-1 leptoquark, to a top quark and a neutrino, or to a bottom quark and a tau lepton. CMS finds a range of lower limits on the leptoquark mass between 0.98 and 1.73 TeV, at 95% confidence, depending on λ and the spin.

These results are the most stringent limits to date on the presence of leptoquarks that couple preferentially to the third generation of fermions. They also probe the parameter space preferred by the B-physics anomalies in several models, excluding relevant portions. As theories predict leptoquark masses as high as many tens of TeV, the pursuit of this promising solution for the unification of quarks and leptons must continue. The CMS collaboration has a broad programme for further investigations that will exploit the larger data samples from Run 3 and the high-luminosity LHC under different hypotheses. If leptoquarks exist, they may well be revealed in the coming data.

Jacques Séguinot 1932–2020

Jacques Séguinot, a founding father of the ring-imaging Cherenkov detector, passed away on 12 October.

Born in 1932 in a small village in Vendée, Jacques studied electromechanical engineering at the University of Caen and received his PhD in physics in 1954. His solid engineering base was visible in every experiment that Jacques designed and built throughout his long career, which followed a classic French academic path – from a stagiaire de recherche in 1954 to a directeur de recherche in 1981, which he held until his official retirement in 1990.

His first studies saw him spend several months at the French cosmic-ray laboratory on the Col du Midi near Mont Blanc, after which he worked on accelerator-based experiments: first at Saturne (CEA Saclay), and from 1964 onwards at CERN’s Proton Synchrotron studying strong interactions with pion and kaon beams. At the end of the 1960s, Jacques began a long and fruitful collaboration with Tom Ypsilantis, leading to a seminal 1977 paper establishing a new particle identification technology that became known as the RICH (Ring Imaging Cherenkov Counter).

The idea was to use the recently introduced multiwire proportional chamber, filled with a photosensitive gas, to detect and localise ultraviolet photons emitted by fast charged particles in a radiating medium, and to use a suitable optical arrangement to create a ring pattern whose radius depends on the particle speed. Combined with magnetic analysis, the RICH made it possible to identify a particle’s mass in a wide range of energies. In further work, Séguinot and Ypsilantis developed algorithms to optimise the momentum resolution of the detectors, as well as adapting radiators to cover different momentum ranges where other technologies were ineffective.

The early RICH devices were successfully deployed at the fixed-target experiments OMEGA at CERN and E605 at Fermilab. The ability of the detector to extend over most of the solid angle around the target or colliding-beam intersections also made it particularly relevant for experiments at the newly commissioned LEP and SLD accelerators. The RICH detector at LEP’s DELPHI experiment came close to the original design, with nearly 4π angular coverage, and Jacques’ contribution to this detector was key.

In view of the growing interest in meson factories, Jacques and Tom worked on faster RICH devices with shorter photo-conversion lengths, and also on CsI solid photo-converters. This led to applications in the RICH for CLEO at the CESR storage ring, the CsI-based RICH detectors in CERN’s ALICE, COMPASS and other experiments. Another very ambitious R&D programme, which started in the mid-1990s, aimed at developing highly segmented photodetectors sensitive to visible light. Jacques saw the potential of such hybrid photo detectors (HPD) for applications in medical imaging, and proposed an innovative PET device in which matrices of long scintillation crystals are read from both sides by HPDs. In the meantime, SiPM photodetectors had become available, with a number of practical advantages over HPDs. In the AX–PET collaboration, Jacques and several others built a fully operational axial PET with SiPM readout.

The high-energy physics community has lost an excellent detector physicist with an extraordinary sense of engineering. His groundbreaking ideas live on, including in the most recent detectors such as Belle II in Japan. But we will also remember Jacques’ fine personality, patience and decency.

Cornering WIMPs with ATLAS

Dark matter is estimated to account for an unseen 85% of matter in the universe, but its nature is unknown. One possible explanation is weakly-interacting massive particles, or WIMPs, which could interact with ordinary matter through the exchange of a Higgs boson (“Higgs-portal” models) or a new mediator field yet to be discovered. The ATLAS collaboration has recently released two new investigations of WIMPs based on the full Run-2 data set.

**Fig. 1.** Monojet missing transverse momentum compared to SM predictions, and the distributions of illustrative supersymmetric (purple dashed line), WIMP (blue dashed line) and dark-energy (black dashed line) signals. Credit: ATLAS

At the LHC, a mediator may be produced and decay into a pair of stable WIMPs, which then escape the detector unseen – an undetectable process, unless the mediator is produced, for example, in association with a high-p_T gluon radiated from one of the incoming protons. This would provide a clear signature: a high-p_T jet and significant missing transverse momentum (MET). A first “monojet” analysis sought events with MET in excess of 200 GeV, recoiling against a jet with p_T > 150 GeV, with up to three additional jets and no leptons or photons. The leading background arises from events wherein a Z boson decays to neutrinos – a process experimentally indistinguishable from WIMP production. The predictions of this and other backgrounds benefitted from stateof- the-art theoretical calculations, detailed groundwork on particle reconstruction in ATLAS, and the use of data-control regions rich in W and Z boson decays. No significant excess was observed with respect to the Standard Model (SM) (figure 1).

As invisible Higgs-boson decays have a branching fraction of just 10^–3, any signal would indicate new physics

Dijet analysis

A second “dijet” WIMP analysis searches for invisible decays of Higgs bosons produced via vector-boson fusion. Though accounting for just 10% as many Higgs bosons as the dominant gluon-fusion process at the LHC, the topology’s clear signature, with two widely separated jets in pseudorapidity, lends itself to searching for MET, as the jets tend to be close together in the transverse plane when recoiling against a Higgs boson with p_T > 200 GeV. The art of this analysis is again in the precise modelling of SM backgrounds – a feat accomplished here with extrapolations from control regions and the use of jet kinematics to separate signal events from Z-boson decays to neutrinos, and W decays with an undetected charged lepton. As invisible Higgs-boson decays in the SM (chiefly H → ZZ* → 4ν) have a branching fraction of just 10^–3, any significant signal would indicate new physics. No deviation from the SM was observed, allowing a 95% confidence upper limit to be placed on the branching fraction for invisible Higgs-boson decays of 13% – a factor two improvement in sensitivity compared to the previous analysis, despite the increase in pileup – or 9% when combining with other ATLAS Higgs-boson measurements. The results are complementary to direct-detection experiments looking for relic WIMPs with deep underground detectors, as they plumb lower WIMP masses than direct-detection experiments can currently access (figure 2).

The elastic WIMP-neutron scattering cross section — **Fig. 2.** 90% confidence upper limits on the elastic WIMP-neutron scattering cross section as a function of the mass of a scalar (purple) or spin-½ Majorana (red) WIMP. Credit: ATLAS

These results also translate into limits on alternative dark-matter-related theories such as axion-like-particles (ALPs) and large extra-dimensions, and into model-independent limits on new phenomena. ATLAS will continue to explore the parameter space of dark-sector models such at ALPs, dark photons, dark scalars and heavy neutral leptons, complementing the results of dedicated smaller-scale experiments.

ALICE’s dark side

The nature of dark matter (DM) remains one of the most intriguing unsolved questions of modern physics. Astrophysical and cosmological observations suggest that DM accounts for roughly 27% of the mass-energy of the universe, with dark energy comprising 68% and ordinary baryonic matter as described by the Standard Model accounting for a paltry 5%. This massive hole in our understanding of the universe continues to drive multiple experimental searches for DM both in the laboratory and in space. No clear evidence for DM has yet been found, severely constraining the parameter space of the most popular “thermal” DM models.

Assuming DM is a material substance comprised of particles – not an illusion resulting from an imperfect understanding of gravity – there are three independent ways to search for it. One is to directly measure the production of DM particles in a high-energy collider such as the LHC. Another is to infer the presence of DM particles via their scattering off nuclei, as investigated by large underground detectors such as XENON1T and LUX. A third, similarly indirect, strategy is to search for the annihilation or decay of DM particles into ordinary (anti) particles such as positrons or antinuclei – as employed by the AMS experiment on board the International Space Station and in balloon-borne experiments such as GAPS. Low-energy light antinuclei, such as antideuterons and antihelium, are particularly promising signals for such indirect DM searches, since the background stemming from ordinary collisions between cosmic rays and the interstellar medium is expected to be low with respect to the DM signal.

ALICE is the only experiment at the LHC that is able to study the production and annihilation of low-energy antinuclei

The ability to interpret any future observation of antinuclei in our galaxy – especially when trying to identify their creation in exotic processes like DM annihilations – requires a quantitative understanding of light antinuclei production and annihilation mechanisms within the interstellar medium. However, the production of light antinuclei in hadronic collisions between cosmic rays and the interstellar medium is still not fully understood: different models compete to explain how these loosely bound objects can be formed in such high-energy collisions. Furthermore, the inelastic annihilation cross section of light antinuclei with matter is completely unknown in the kinematic region relevant for indirect DM searches, forcing current estimates to rely on extrapolations and modelling.

Fortunately, both the antinuclei production mechanism and the interactions between antinuclei and ordinary matter can be studied on Earth using large accelerators. The main contributions so far have come from the LHC at CERN and from the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. Thanks to its unique low-material-budget tracker, which provides excellent tracking and particle-identification performance for low-momentum particles, ALICE is the only experiment at the LHC that is able to study the production and annihilation of low-energy antinuclei.

Antinucleosynthesis in the lab
While antinuclei can also be produced at lower collision energies, only at the LHC are matter and antimatter generated in equal abundances in the region transverse to the beam direction. The most abundant non-trivial antinucleus produced is the antideuteron, which consists of an antineutron and an antiproton. At low momentum, deuterons and antideuterons can be clearly identified thanks to their high energy loss in the ALICE detector’s time-projection chamber. At larger momenta, a clean identification of antideuterons is possible using the ALICE time-of-flight detector. This information, combined with the measured track length and the particle momentum, provide a precise determination of the particle mass. Using these and other techniques, the ALICE collaboration has recently measured the production of (anti)deuterons in proton–proton collisions, as well as in other colliding systems, and set tight constraints on the production models of (anti)nuclei.

Ratio of anti-deuterons to protons — **Competing models** The ratio of (anti)deuterons to protons in p–p, p–Pb and Pb–Pb collisions as a function of the average number of charged particles produced. The purple line shows the expectation of the coalescence model while the solid and dashed black lines represent extreme parameter configurations for the statistical hadronisation model. Credit: *Eur*. Phys. J. C 80 889

There are two main ways to model the production mechanism of (anti)nuclei. Coalescence models assume that primary (anti)neutrons and (anti)protons can bind if they are close enough in phase space. Statistical hadronisation models, on the other hand, assume that hadrons and (anti) nuclei emerge when the collision system reaches thermodynamical equilibrium, making the temperature and the volume of the system the key parameters. Measurements of nuclei-to-proton ratios in various colliding systems have recently enabled the ALICE collaboration to compare the two model approaches in detail (see “Competing models” figure). As can be seen, the two models give different predictions for the evolution of the nuclei-to-proton ratio versus particle multiplicity, with the latest ALICE measurements slightly favouring the coalescence approach.

Similar conclusions about the two models can be drawn using heavier antinuclei, like ³He and ⁴He, which were already measured by ALICE in p–Pb and Pb–Pb collisions. The achievable precision of the measurement is limited by the available data: the antinuclei production rate in pp collisions goes down by a factor of about 1000 for every additional antinucleon in the antinucleus.

The precision of the measurements from proton–proton collisions places strong constraints on the production models, which can then be used to predict the antinuclei fluxes in space. Indeed, the ALICE measurements combined with different coalescence models have already been employed to estimate the antideuteron and antihelium flux from cosmic-ray interactions measurable by the AMS and GAPS experiments. These predictions will allow correct interpretations of the eventual antinuclei signal that might be observed in the future by the two collaborations.

Further helping clarify the results of indirect DM searches, ALICE has recently performed the first measurement of the antideuteron inelastic cross section in the momentum range 0.3 < p < 4 GeV/c – significantly extending our knowledge about this cross section from previous measurements at momenta of 13 and 25 GeV/c at the Serpukhov accelerator complex in Russia in the early 1970s. The collaboration took advantage of the ability of antideuterons produced at the LHC to interact inelastically with the detector material. To quantify this process, ALICE has employed a novel approach based on the antideuteron- to-deuteron ratio reconstructed in collisions of protons and heavy ions at a centre-of-mass energy per nucleon–nucleon pair of 5.02 TeV. Such a ratio depends on both of the inelastic cross sections of deuterons and antideuterons. The former has been measured in various previous experiments at different momenta; the latter can be constrained from the ALICE data by comparing the measured ratio with detailed Monte Carlo simulations.

**Interaction probability** Antideuteron inelastic interaction cross section measured in different ALICE subdetectors. The dashed red line represents the parameterisations used for antideuterons in the Geant4 toolkit, and the full grey line shows the parameterisations for deuterons. The full points correspond to the experimental measurements, whereas the purple and orange bands are the one and two sigma uncertainty intervals, respectively. Credit: arXiv:2005.11122

The resulting antideuteron inelastic cross section is shown (see “Interaction probability” figure), where the two panels correspond to the different sub-detectors employed in the analysis and therefore to different average material crossed by (anti)deuterons – corresponding to a difference of about a factor two in average mass number. The inelastic cross sections include all possible inelastic antideuteron processes such as break-up, annihilation or charge exchange, and the analysis procedure was validated by demonstrating consistency with existing antiproton results from traditional scattering experiments.

The momentum range covered is of particular importance to evaluate the signal predictions for indirect dark-matter searches

The momentum range covered in this latest analysis is of particular importance to evaluate the signal predictions for indirect dark-matter searches. Additionally, these measurements can help researchers to understand the low-energy antideuteron inelastic processes and to model better the inelastic antideuteron cross sections in widely-used toolkits such as Geant4. Together with the proper modelling of antinuclei formation, the obtained results will impact the antideuteron flux expectations at low momentum for ongoing and future satellite- and balloon-borne experiments.

The heavier, the better
ALICE is studying the full range of antinuclei physics with unprecedented precision. These results, which have started to emerge only since 2015, are contributing significantly to our understanding of antinuclei formation and annihilation processes, with important ramifications for DM searches. Both the statistical hadronisation and coalescence models can describe antideuteron production at the LHC, while the detector material can be used as an absorber to study the antinuclei inelastic cross section at low energies relevant for the astrophysical applications.

For the foreseeable future, ALICE will continue to provide an essential reference for the interpretation of astrophysics measurements of antinuclei in space. With the increased integrated luminosity that will be acquired by ALICE during LHC Run 3 from early 2022, it will be possible to extend the current analyses to heavier (anti)nuclei, such as ³He and ⁴He, with even better precision than the currently available measurements for (anti)deuterons. This will allow the collaboration to perform fundamental tests of the production and annihilation mechanisms with heavier, doubly-charged antinuclei, which are more easily identified by satellite-borne experiments and thus expected to provide an even clearer DM signature.

Pulsars hint at low-frequency gravitational waves

NANOGrav uses pulsate to detect potential distortions in space time — NANOGrav uses variations in the arrival time of the signals from an array of very stable, quickly rotating pulsars to detect potential distortions in space time caused by low-frequency gravitational waves. Credit: D Champion

The direct detection of a gravitational wave (GW) in 2015 by the LIGO and Virgo collaborations confirmed the existence of these long sought after events. However, these and other GW events detected so far constitute only a small fraction – in the kHz regime — of the vast GW spectrum. As a result, they only probe certain phenomena such as stellar mass black-hole and neutron-star mergers. On the opposite side of the spectrum to LIGO and Virgo are Pulsar Timing Array (PTA) experiments, which search for nHz frequency GWs. Such low-frequency signals can originate from supermassive black-hole binaries (SMBHBs), while in more exotic models they can be proof of cosmic strings, phase transitions or a primordial GW background. The NANOGrav (North American Nanohertz Observatory for Gravitational Waves) collaboration has now found possible first hints of low-frequency GWs.

To detect such rumblings of space—time, which also have minute amplitudes, researchers need to track subtle movements of measurement points spread out over the size of a galaxy. For this purpose, the NANOGrav collaboration uses millisecond pulsars, several tens of which have been detected in our galaxy. Pulsars are quickly rotating neutron stars which emit cones of electromagnetic emission from their poles. When a pole points towards Earth it is detected as a short pulse of electromagnetic radiation. Not only is the frequency of millisecond pulsars high, making it easier to detect small variations in arrival time, but it is very stable over periods of many years. Combined with their great distances from Earth, this makes millisecond-pulsar emissions sensitive to any small alterations in their travel path — for example, those introduced by distortions of space–time by low-frequency gravitational waves. Such waves would cause the pulses to arrive a few nanoseconds early during January and a few nanoseconds late in June, for instance. By observing the radio emission of these objects once a week throughout many years, researchers can search for such effects.

The new results show a clear sign of a common spectrum between the studied pulsars

The problem is that GWs are not the only things which can cause a change in the arrival time of the pulses. Changes in the Earth’s atmosphere already alter the arrival time, as do changes in the position of the pulsar itself (which is usually part of a quickly rotating binary system), and the movement of Earth with respect to the source. The complexity of the measurements lies mostly in correcting for all of these effects. The latest results from NANOGrav, for example, reduce systematics by incorporating unprecedented precision (of the order of tens of km) in the orbital parameters of Jupiter.

Whereas previous results by NANOGrav and other PTA collaborations only allowed upper limits to be set on the amplitude of the GW background travelling through our galaxy, the new results show a clear sign of a common spectrum between the studied pulsars. Based on 12.5 years of data and a total of 47 pulsars studied using the ultra-sensitive Arecibo Observatory and Green Bank Telescope, the spectrum of variations in the pulsar signal arrival time was found to agree with theoretical predictions of the GW background produced by SMBHBs. The uncertainties remain large, however, which admits alternative interpretations such as cosmic strings which predict only a slightly different spectral shape. Furthermore, a key ingredient is still missing: a spatial correlation between the pulsar variations, which would confirm the quadrupole nature of GWs and provide clear proof of the nature of the signal. Finding this “smoking gun” will require longer observation times, more pulsars and smaller systematic errors — something the NANOGrav team is now working towards.

While the NANOGrav collaboration remains cautious, several exotic interpretations have already been proposed. The final sentences of their preprint summarise the status of this exciting field well: “The LIGO–Virgo discovery of high-frequency, transient GWs from stellar black-hole binaries appeared meteorically, with incontrovertible statistical significance. By contrast, the PTA discovery of very-low-frequency GWs from SMBHBs will emerge from the gradual and not always monotonic accumulation of evidence and arguments. Still, our GW vista on the unseen universe continues to get brighter”.

Willem de Boer 1948–2020

Willem (“Wim”) de Boer passed away on 13 October, aged 72. Wim studied physics at the University of Delft and graduated in 1974 with a thesis on the dynamic orientation of nuclei at low temperatures, which laid the foundation of polarised targets in high-energy physics. Following a CERN fellowship, he joined the University of Michigan, Ann Arbor and worked on polarised proton–proton scattering at the ANL synchrotron, where he found an unexplained difference in the cross sections for parallel and antiparallel spins.

In 1975 Wim took up a position at the Max Planck Institute for Physics in Munich where he stayed, interrupted by a sabbatical at SLAC in 1987, for 14 years. In Munich he joined the team working on the CELLO experiment at DESY, where he took responsibility for the data-acquisition system. The CELLO years were instrumental for precision studies of QCD, out of which the triple-gluon coupling and the running of the strong coupling constant emerged – a subject Wim pursued ever after.

Following his appointment to a professorship at the University of Karlsruhe in 1989, Wim created research groups at LEP’s DELPHI experiment, the AMS-02 experiment on the International Space Station, and he coordinated a group at the LHC’s CMS experiment. Having studied the running of the coupling constants of the weak, electromagnetic and strong interactions, Wim found, together with Ugo Amaldi and Hermann Fürstenau, that these could only meet in a unified way at high energies if phenomena beyond the Standard Model, such as supersymmetry, existed. This was published in their seminal 1991 paper “Comparison of grand unified theories with electroweak and strong coupling constants measured at LEP”, which led to the expectation that a new energy domain would open up at the TeV scale with the lightest supersymmetric particle constituting dark matter. The paper has been cited almost 2000 times.

Wim contributed a multitude of ideas, studies and publications to each of the experiments he worked on, driven by the single question: where is supersymmetry? He looked for dark-matter signals at the lowest energies in our galaxy using earth-bound observatories, balloon experiments and satellites, at signals from direct production at LEP and the LHC, and in anomalous decay modes of bottom mesons using data from the Belle and BaBar experiments, among others.

It is our belief that Wim was most fascinated by AMS-02. Not only did he and his group contribute an electronic readout system to the detector, he also saw it take off from Cape Canaveral with the penultimate Space Shuttle flight in 2011, celebrated by the visit of the whole crew of astronauts to Karlsruhe later that year.

Wim’s career saw him work across detectors using gases, liquids, silicon and diamonds, and study their performance in magnetic fields and high-radiation backgrounds. He also investigated the use of detectors for medical and technical applications. His last R&D effort began only a few weeks before his death: the development of a novel cooling system for high-density batteries.

Our field has lost a great all-round physicist with unparalleled creativity and diligence, a warm collegiality and a very characteristic dry humour. Well aware of his rapid illness, his last words to his family were: “Hij gaat nog niet, want hij heeft nog zoveel ideeën!” (roughly “He’s not going yet, because he still has so many ideas!). He will be missed deeply.

American Physical Society announces 2021 awards

Henry Sobel (Top) and Edward Kearns (Bottom). Credit: UCI/Boston University

W.K.H. Panofsky Prize

The W K H Panofsky Prize in experimental particle physics has been awarded to Henry Sobel, professor emeritus of the University of California, Irvine and Edward Kearns of Boston University for pioneering and leadership contributions to large underground experiments for the discovery of neutrino oscillations and sensitive searches for baryon-number violation. As the US co-spokesperson, Sobel is heavily involved with Japan’s Super-Kamiokande experiment (Super-K), and is also involved in the next-generation neutrino experiments – DUNE, in the US and Hyper-K in Japan. Kearns is also involved in Super-K and DUNE, along with being a member of the Tokai-To-Kamioka (T2K) experiment and active in the search for dark matter using techniques based on cryogenic noble liquids.

J.J. Sakurai Prize

The J J Sakurai Prize for theoretical physics has been given to Vernon Barger of the University of Wisconsin-Madison for pioneering work in collider physics contributing to the discovery and characterisation of the W boson, top quark and Higgs boson, and for the development of incisive strategies to test theoretical ideas with experiments.

Robert R. Wilson Prize

In the field of accelerators, Yuri Fyodorovich Orlov, formerly of Cornell University, was awarded the Robert R Wilson Prize for his pioneering innovation in accelerator theory and practice. Orlov received the news shortly before his passing on 27 September.

Maria Goeppert Mayer Award

Phiala E Shanahan of the Massachusetts Institute of Technology has been granted the Mario Goeppert Mayer Award, which recognises an outstanding contribution to physics research by a women, “for key insights into the structure and interactions of hadrons and nuclei using numerical and analytical methods”.

Chanda Prescod-Weinstein Credit: University of New Hampshire

Edward A.Bouchet Award

The Edward A Bouchet Award, which promotes the participation of underrepresented minorities in physics, has been awarded to Chanda Prescod-Weinstein of the University of New Hampshire for her contributions to theoretical cosmology and particle physics and for co-creating the Particles for Justice movement.

Herman Feshbach Prize

The Herman Feshbach Prize in theoretical nuclear physics has been awarded to Berndt Mueller of Brookhaven National Laboratory for his contributions to the identification of quark-gluon plasma signatures.

Henry Primakoff Award

The 2021 Henry Primakoff Award for early-career particle physics has gone to Jaroslav Trnka of the University of California, Davis for seminal work on the computation of particle scattering amplitudes.

Dwight Nicholson Medal

The 2020 Dwight Nicholson Medal for Outreach has been given to Michael Barnett of Lawrence Berkeley National Laboratory “for a lifetime of innovations in outreach bringing the discoveries and searches of particle physicists and cosmologist to multitudes of students and lay people around the world.”

Yuri Orlov 1924–2020

Yuri Orlov, a world-renowned accelerator physicist and a leading figure in the worldwide campaign for human rights in Soviet Russia, passed away at the end of September at the age of 96.

Yuri was born in Moscow in 1924. He studied and worked there until 1956, when a critical pro-democracy speech he gave at the Institute for Theoretical and Experimental Physics resulted in him being fired and banned from scientific work. He then moved to the Yerevan Physics Institute in Armenia where he earned his first doctorate (“Nonlinear theory of betatron oscillations in the strong-focusing synchrotron”) in 1958, followed by the award of a second doctorate in 1963. While in Yerevan, he designed the 6 GeV electron synchrotron, became head of the electromagnetic interaction laboratory, and was elected to the Armenian Academy of Sciences.

In 1972 Yuri returned to Moscow and joined the influential dissident movement that included Andrei Sakharov and Aleksandr Solzhenitsyn. When the final documents of the Helsinki Conference on Security and Co-operation in Europe were signed in 1975, Yuri founded the Moscow Helsinki Group with the aim of having all human rights guaranteed in the Helsinki documents accorded to all citizens of the Soviet Union. As was to be expected, Yuri was arrested in 1977, tried in a political mock trial in 1978 and convicted to seven years in a labour camp in Perm.

As soon as Yuri Orlov’s ordeal became known in Europe and North America, physicists began to protest against the treatment of their colleague. At CERN, where several physicists had had personal contacts with Yuri, the Yuri Orlov Committee was founded with Georges Charpak as one of its founding members. The long-standing fruitful scientific collaboration with the Soviet Union was challenged and the support of eminent political leaders of the CERN member states was solicited.

Surviving a total of seven years of labour camp under extreme conditions, Yuri was deported to Siberia for a period of five years. Because of continuing international pressure, he was then deported to the US in 1986, where he was offered a position at Cornell University. Soon after his forced emigration, Yuri visited CERN and he spent a sabbatical there in 1988/1989 working in the accelerator division to develop the idea of ion “shaking”. He joined the muon g-2 experiment at Brookhaven National Laboratory and worked on Brookhaven proposals to measure the electric dipole moments of protons, electrons and deuterons. At Cornell he pursued this work as well as an alternative design for the proposed B-factory, and wrote on the foundations of quantum mechanics. In 2008 he was named a professor of physics and professor of government, and taught physics and human rights until his retirement in 2015.

Yuri authored or co-authored more than 240 scientific papers and technical reports, and wrote a memoir, Dangerous Thoughts: Memoirs of a Russian Life (William Morrow & Co, 1991). Among the many honours Yuri received are the American Physical Society’s 2006 Sakharov prize “For his distinction as a creative physicist and as a life-long, ardent leader in the defence and development of international human rights, justice and the freedom of expression for scientists”, and the APS 2021 Wilson Prize for outstanding achievements in the physics of particle accelerators, of which he was notified shortly before his death.

Yuri’s example as a scientist committed to the freedom of science, its cultural dimension in world affairs and his defence of the human right of expression of one’s convictions is an example and inspiration to all of us.

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