On 10 September the International Committee for Future Accelerators (ICFA) announced the structure and members of a new organisational team to prepare a “pre-laboratory” for an International Linear Collider (ILC) in Japan. The ILC International Development Team (ILC-IDT), which consists of an executive board and three working groups governing the pre-lab setup, accelerator, and physics and detectors, aims to complete the preparatory phase for the pre-lab on a timescale of around 1.5 years.
We hope that the effort by our Japanese colleagues will result in a positive move by the Japanese government
Tatsuya Nakada
The aim of the pre-lab is to prepare the ILC project, should it be approved, for construction. It is based on a memoranda of understanding among participating national and regional laboratories, rather than intergovernmental agreements, explains chair of the ILC-IDT executive board Tatsuya Nakada of École Polytechnique Fédérale de Lausanne. “The ILC-IDT is preparing a proposal for the organisational and operational framework of the pre-lab, which will have a central office in Japan hosted by the KEK laboratory,” says Nakada. “In parallel to our activities, we hope that the effort by our Japanese colleagues will result in a positive move by the Japanese government that is equally essential for establishing the pre-laboratory.”
In June the Linear Collider Board and Linear Collider Collaboration, which were established by ICFA in 2013 to promote the case for an electron–positron linear collider and its detectors as a worldwide collaborative project, reached the end of their terms in view of ICFA’s decision to set up the ILC-IDT.
The ILC has been on the table for almost two decades. Shortly after the discovery of the Higgs boson in 2012, the Japanese high-energy physics community proposed to host the estimated $7 billion project, with Japan’s prime minister at that time, Yoshihiko Noda, stressing the importance of establishing an international framework. In 2018 ICFA backed the ILC as a Higgs factory operating at a centre-of-mass energy of 250 GeV – half the energy set out five years earlier in the ILC’s technical design report.
Higgs factory
An electron–positron Higgs factory is the highest-priority next collider, concluded the 2020 update of the European strategy for particle physics (ESPPU). The ESPPU recommended that Europe, together with its international partners, explore the feasibility of a future hadron collider at CERN at the energy frontier with an electron–positron Higgs factory as a possible first stage, noting that the timely realisation of the ILC in Japan “would be compatible with this strategy”. Two further proposals exist: the Compact Linear Collider at CERN and the Circular Electron–Positron Collider in China. While the ILC is the most technically ready Higgs-factory proposal (see p35), physicists are still awaiting a concrete decision about its future.
In March 2019 Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT) expressed “continued interest” in the ILC, but announced that it had “not yet reached declaration” for hosting the project, arguing that it required further discussion in formal academic decision-making processes. In February KEK submitted an application for the ILC project to be considered in the MEXT 2020 roadmap for large-scale research projects. KEK withdrew the application the following month, announcing the move in September following the establishment of the ILC-IDT.
The ministry will keep an eye on discussions by the international research community
Koichi Hagiuda
“The ministry will keep an eye on discussions by the international research community while exchanging opinions with government authorities in the US and Europe,” said Koichi Hagiuda, Japanese minister of education, culture, sports, science and technology, at a press conference on 11 September.
Steinar Stapnes of CERN, who is a member of the ILC-IDT executive board representing Europe, says that clear support from the Japanese government is needed for the ILC pre-lab. “The overall project size is much larger than the usual science projects being considered in these processes and it is difficult to see how it could be funded within the normal MEXT budget for large-scale science,” he says. “During the pre-lab phase, intergovernmental discussions and negotiation about the share of funding and responsibilities for the ILC construction need to take place and hopefully converge.”
“It is our vision for CERN to be a role model for environmentally responsible research,” writes CERN Director-General Fabiola Gianotti in her introduction to a landmark environmental report released by the laboratory on 9 September. While CERN has a longstanding framework in place for environmental protection, and has documented its environmental impact for decades, this is its first public report. Two years in the making, and prepared according to the Global Reporting Initiative Sustainability Reporting Standards, it details the status of CERN’s environmental footprint, along with objectives for the coming years.
Given the energy consumption of large particle accelerators, environmental impact is a topic of increasing importance for high-energy physics research worldwide. Among the recommendations of the 2020 update of the European strategy for particle physics was a strong emphasis on the need to continue with efforts to minimise the environmental impact of accelerator facilities and maximise the energy efficiency of future projects.
When the Large Hadron Collider (LHC) is operating, CERN uses an average of 4300 TJ of electricity every year (30–50% less when not in operation) – enough energy to power just under half of the 200,000 homes in the canton of Geneva. “This is an inescapable fact, and one that CERN has always taken into consideration when designing new facilities,” states Frédérick Bordry, director for accelerators and technology.
Action plan
An energy-management panel established at CERN in 2015 has already led to actions, including free cooling and air-flow optimisation, better optimised LHC cryogenics, and the implementation of SPS magnetic cycles and stand-by modes, which significantly reduce energy consumption. The LHC delivered twice as much data per Joule in its second run (2015–2018) compared to its first (2010–2013), states the new report. With the High-Luminosity LHC due to deliver a tenfold increase in luminosity towards the end of the decade, CERN has made it a priority to limit the increase in energy consumption to 5% up to the end of 2024, with longer-term objectives to be set in future reports.
CERN procures its electricity mainly from France, whose production capacity is 87.9% carbon-free. In terms of direct greenhouse-gas emissions, the 192,000 tonnes of carbon-dioxide equivalent emitted by CERN in 2018 is mainly due to fluorinated gases used in the LHC detectors for cooling, particle detection, air conditioning and electrical insulation. CERN has set a formal objective that, by 2024, direct greenhouse emissions will be reduced by 28% by replacing fluorinated gases – which were designed in the 1990s to be ozone-friendly – with carbon dioxide, which has a global-warming potential several thousand times lower.
CERN has set a formal objective that, by 2024, direct greenhouse emissions will be reduced
by 28%
Other areas of environmental significance studied in the report include radiation exposure, noise and waste. CERN commits to limit the emission of ionising radiation to no more than 0.3 mSv per year – less than a third of the annual dose limit for public exposure set by the European Council. The report states that the actual dose to any member of the public living in the immediate vicinity of CERN due to the laboratory’s activities is below 0.02 mSv per year, which is less than the exposure received from cosmic radiation during a transatlantic flight.
A 2018 measurement campaign showed that noise levels at CERN have not changed since the early 1990s, and are low by urban standards. Nevertheless, CERN has have invested 0.7 million CHF to reduce noise at its perimeters to below 70 dB during the day and 60 dB at night (which corresponds to the level of conversational speech). The organisation has also introduced approaches to preserve the local landscape and protect flora, including 15 species of orchid growing on CERN’s sites.
Waste not
Water consumption, mostly drawn from Lac Léman, has slowly decreased over the past 10 years, the report notes, and CERN commits to keeping the increase in water consumption below 5% to the end of 2024, despite a growing demand for cooling from upgraded facilities. CERN also eliminates 100% of its waste, states the report, and has a recycling rate of 56% for non-hazardous waste (which comprises 81% of the total). A major project under construction since last year will see waste hot water from the cooling system for LHC Point 8 (where the LHCb experiment is located) channeled to a heating network in the nearby town of Ferney-Voltaire from 2022, with LHC Points 2 and 5 being considered for similar projects.
CERN plans to release further environment reports every two years. “Today, more than ever, science’s flag-bearers need to demonstrate their relevance, their engagement, and their integration into society as a whole,” writes Gianotti. “This report underlines our strong commitment to environmental protection, both in terms of minimising our impact and applying CERN technologies for environmental protection.”
On 17 January 1957, a few months after Chien-Shiung Wu’s discovery of parity violation, Wolfgang Pauli wrote to Victor Weisskopf: “Ich glaube aber nicht, daß der Herrgott ein schwacher Linkshänder ist” (I cannot believe that God is a weak left-hander). But maximal parity violation is now well established within the Standard Model (SM). The weak interaction only couples to left-handed particles, as dramatically seen in the continuing absence of experimental evidence for right-handed neutrinos. In the same way, the polarisation of photons originating from transitions that involve the weak interaction is expected to be completely left-handed.
The LHCb collaboration recently tested the handedness of photons emitted in rare flavour-changing transitions from a b-quark to an s-quark. These are mediated by the bosons of the weak interaction according to the SM – but what if new virtual particles contribute too? Their presence could be clearly signalled by a right-handed contribution to the photon polarisation.
New virtual particles could be clearly signalled by a right-handed contribution to the photon polarisation
The b → sγ transition is rare. Fewer than one in a thousand b-quarks transform into an s-quark and a photon. This process has been studied for almost 30 years at particle colliders around the world. By precise measurements of its properties, physicists hope to detect hints of new heavy particles that current colliders are not powerful enough to produce.
The probability of this b-quark decay has been measured in previous experiments with a precision of about 5%, and found to agree with the SM prediction, which bears a similar theoretical uncertainty. A promising way to go further is to study the polarisation of the emitted photon. Measuring the b → sγ polarisation is not easy though. The emitted photons are too energetic to be analysed by a polarimeter and physicists must find innovative ways to probe them indirectly. For example, a right-handed polarisation contribution could induce a charge-parity asymmetry in the B0→ KSπ0γ or Bs0→ φγ decays. It could also contribute to the total rate of radiative b → sγ decays, containing any strange meson, B → Xsγ.
The LHCb collaboration has pioneered a new method to perform this measurement using virtual photons and the largest sample of the very rare B0→ K*0e+e– decay ever collected. First, the sub-sample of decays that come from B0→ K*0γ with a virtual photon that materialises in an electron–positron pair is isolated. The angular distributions of the B0→ K*0e+e– decay products are then used as a polarimeter to measure the handedness of the photon. The number of decays with a virtual photon is small compared to the decays with a real photon, but these latter decays cannot be used as the information on the polarisation is lost.
The size of the right-handed contribution to b → sγ is encoded in the magnitude of the complex parameter C′7/C7. This is a ratio of the right- and left-handed Wilson coefficients that are used in the effective description of b → s transitions. The new B0→ K*0e+e– analysis by the LHCb collaboration constrains the value of C′7/C7, and thus the photon polarisation, with unprecedented precision (figure 1). The measurement is compatible with the SM prediction.
This result showcases the exceptional capability of the LHCb experiment to study b → sγ transitions. The uncertainty is currently dominated by the data sample size, and thus more accurate studies are foreseen with the large data sample expected in Run 3 of the LHC. More precise measurements may yet unravel a small right-handed polarisation.
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.
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.
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.
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 (vn) in a Fourier decomposition of the azimuthal distribution of deuterons in Pb–Pb collisions at √sNN = 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 pT 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.
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, 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.
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.
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-pT gluon radiated from one of the incoming protons. This would provide a clear signature: a high-pT 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 pT > 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 pT > 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).
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.
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.
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.
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.
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”.
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