Jobs – Page 106 of 527

TESLA’s high-gradient march

Superconducting RF cavities — **Standing tall** Superconducting radio-frequency cavities at DESY. Credit: D Noelle/DESY

Energetic beams of charged particles are essential for high-energy physics research, as well as for studies of nuclear structure and dynamics, and deciphering complex molecular structures. In principle, generating such beams is simple: provide an electric field for acceleration and a magnetic field for bending particle trajectories. In practice, however, the task becomes increasingly challenging as the desired particle energy goes up. Very high electric fields are required to attain the highest energy beams within practical real-estate constraints.

The most efficient way to generate the very high electric fields in a vacuum environment required to transport a beam is to build up a resonant excitation of radio waves inside a metallic cavity. There is something of an art to shaping such cavities to “get the best bang for the buck” for a particular application. The radio-frequency (RF) fields are inherently time-varying, and bunches of charged particles need to arrive with the right timing if they are to see only forward-accelerating electric fields. Desirable very high resonant electric fields (e.g. 5–40 MV/m) require the existence of very high currents in the cavity walls. These currents are simply not sustainable for long durations using even the best normal-conducting materials, as they would melt from resistive heating.

Superconducting materials, on the other hand, can support sustainable high-accelerating gradients with an affordable electricity bill. Early pioneering work demonstrating the first beam-acceleration using superconducting radio-frequency (SRF) cavities took place in the late 1960s and early 1970s at Stanford, Caltech, the University of Wuppertal and Karlsruhe. The potential for real utility was clear, but techniques and material refinements were needed. Several individual laboratories began to take up the challenge for their own research needs. Solutions were developed for electron acceleration at CESR, HERA, TRISTAN, LEP II and CEBAF, while heavy-ion SRF acceleration solutions were developed at Stony Brook, ATLAS, ALPI and others. The community of SRF accelerator physicists was small but the lessons learned were consistently shared and documented. By the early 1990s, SRF technology had matured such that complex large-scale systems were credible and the variety of designs and applications began to blossom.

The TESLA springboard

In 2020, the TESLA Technology Collaboration (TTC) celebrates 30 years of collaborative efforts on SRF technologies. The TTC grew out of the first international TESLA (TeV Energy Superconducting Linear Accelerator) workshop, which was held at Cornell University in July 1990. Its aim was to define the parameters for a superconducting linear collider for high-energy physics operating in the TeV region and to explore how to increase the gradients and lower the costs of the accelerating structures. It was clear from the beginning that progress would require a large international collaboration, and the Cornell meeting set in motion a series of successes that are ongoing to this day – including FLASH and the European XFEL at DESY. The collaboration also led to proposals for several large SRF-based research facilities including SNS, LCLS-II, ESS, PIP-II and SHINE, as well as a growing number of smaller facilities around the world.

Accelerating gradients above 40 MV/m are now attainable with niobium

At the time of the first TESLA collaboration meeting, the state-of-the-art in accelerating gradients for electrons was around 5 MV/m in the operating SRF systems of TRISTAN at KEK, HERA at DESY, LEP-II at CERN and CEBAF at Jefferson Lab (JLab), which were then under construction. Many participants in this meeting agreed to push for a five-fold increase in the design accelerating gradient to 25 MV/m to meet the dream goal for TESLA at a centre-of-mass energy of 1 TeV. The initial focus of the collaboration was centred on the design, construction and commissioning of a technological demonstrator, the TESLA Test Facility (TTF) at DESY. In 2004, SRF was selected as the basis for an International Linear Collider (ILC) design and, shortly afterwards, the TESLA collaboration was re-formed as the TESLA Technology Collaboration with a scope beyond the original motivation of high-energy physics. The TTC, with its incredible worldwide collaboration spirit, has had a major role in the growth of the SRF community, facilitating numerous important contributions over the past 30 years.

30 years of gradient march

Conceptually, the objective of simply providing “nice clean” niobium surfaces on RF structures seems pretty straightforward. Important subtleties begin to emerge, however, as one considers that the high RF-surface currents required to support magnetic fields up to ~100 mT flow only in the top 100 nm of the niobium surface, which must offer routine surface resistances at the nano-ohm level over areas of around 1 m². Achieving blemish-free, contamination-free surfaces that present excellent crystal lattice structure even in this thin surface layer is far from easy.

The march of progress in cavity gradient for linacs and the many representative applications over the past 50 years (see figure “Gradient growth”) are due to breakthroughs in three main areas: material purity, fabrication and processing techniques. The TTC had a major impact on each of these areas.

**Gradient growth** SRF linac accelerating gradient achievements and application specifications since 1970. CW SRF Linacs – SCA: Stanford Superconducting Accelerator; MUSL: lllinois Microtron Using a Superconducting Linac; CEBAF: Continuous Electron Beam Accelerator Facility; JLab FEL: JLab Free Electron Laser; ELBE: HZDR Electron Linear accelerator with high Brillance and Low Emittance; ALICE: STFC Accelerators and Lasers In Combined Experiments; ARIEL: TRIUMF Advanced Rare IsotopE Laboratory; LCLS-II: Linac Coherence Light Source extension; SHINE: Shanghai High Brightness Photon Facility. Pulsed SRF Linacs – FAST: Fermilab Accelerator Science and Technology Facility; STF: KEK Superconducting RF Test Facility; E-XFEL: European X-Ray Free Electron Laser; ILC: International Linear Collider. Credit: Source: R Geng/JLab

With some notable exceptions, bulk niobium cavities fabricated from sheet stock material have been the standard, even though the required metallurgical processes present challenges. Cycles of electron-beam vacuum refining, rolling, and intermediate anneals are provided by only a few international vendors. Pushing up the purity of deliverable material required a concerted push, resulting in the avoidance of foreign material inclusions, which can be deadly to performance when uncovered in the final step of surface processing. The figure-of-merit for purity is the ratio of room-temperature to cryogenic normal-conducting resistivity – the residual resistance ratio, RRR. The common cavity-grade niobium material specification has thus come to be known as high-RRR grade.

Another later pursuit of pure niobium is the so-called “large grain” or “direct-from-ingot” material. Rather than insist on controlled ~30 µm grain-size distribution (grains being microcrystals in the structure), this material uses sheet slices cut directly from large ingots having much larger, but arbitrarily sized, grains. Although not yet widely used, this material has produced the highest gradient TESLA-style cavities to date – 45 MV/m with a quality factor Q₀ > 10¹⁰. Here again, though the topic was initiated at JLab, this fruitful work was accomplished via worldwide international collaborations.

As niobium is a refractory metal that promptly cloaks itself with about 4 nm of dielectric oxide, welding niobium components has to be performed by vacuum electron beam welding. Collaborative efforts in Europe, North America and Asia refined the parameters required to yield consistent niobium welds. The community gradually realised that extreme cleanliness is required in the surface-weld preparation, since even microscopic foreign material will be vaporised during the weld process, leaving behind small voids that become performance-limiting defects.

Having the best niobium is not sufficient, however. Superconductors have inherent critical magnetic field limitations, or equivalently local surface-current density limitations. Because the current flow is so shallow, local magnetic field enhancements induced by microscopic topography translate into gradient-limiting quench effects. Etching of fabricated surfaces has routinely required a combination of hydrofluoric and nitric acids, buffered with phosphoric acid. This exothermic etching process inherently yields step-edge faceting at grain boundaries, which in turn creates local, even nanoscopic, field enhancements, anomalous losses and quenches as the mean surface field is increased. A progression of international efforts at KEK, DESY, CEA-Saclay and JLab eliminated this problem through the development of electro-polishing techniques. Following a deeper understanding of the underlying electrochemistry, accelerating gradients above 40 MV/m are now attainable with niobium.

Another vexing problem that TTC member institutions helped to solve was the presence of “Q-drop” in the region of high surface magnetic field, for which present explanations point to subtle migration of near-surface oxygen deeper into the lattice, where it inhibits the subsequent formation of lossy nanohydrides on cool-down. Avoidance of nanohydrides, whose superconductivity by proximity effect breaks down in the Q-drop regime, is required to sustain accelerating gradients above 25 MV/m for some structures.

Cleaning up

TTC members have also shared analyses and best practices in cleaning and cleanroom techniques, which have evolved dramatically during the past 30 years. This has helped to beat down the most common challenge for developers and users of SRF accelerating cavities: particulate-induced field emission, whereby very high peak surface electric fields can turn even micron-scale foreign material into parasitic electron field emission sources, with resulting cryogenic and radiation burdens. Extended interior final rinsing with high-pressure ultra-pure water prior to cavity assembly has become standard practice, while preparation and assembly of all beamline vacuum hardware under ISO 4 cleanroom conditions is necessary to maintain these clean surfaces for accelerator operations.

**Vertical test** The first cavity of the ESS elliptical section developed by INFN, seen here on the left (next to a large-grain prototype), being prepared for a vertical test at DESY before shipment to CEA Saclay for assembly in the cryomodule. Credit: D Sertore/INFN

The most recent transformation has come with the recognition that interstitial doping of the niobium surface with nitrogen can reduce SRF surface resistance much more than was dreamed possible, reducing the cryogenic heat load to be cooled. While still the subject of material research, this new capability was rapidly adopted into the specification for LCLS-II cavities and is also being considered for an ILC. The effort started in the US and quickly propagated internationally via the TTC, for example in cavity tests at the European Spallation Source (see “Vertical test” image). Earlier this year, Q-values of 3–4 × 10¹⁰ at 2 K at 30 MV/m were reported in TESLA-style cavities – representing tremendous progress, but with much optimisation still to be carried out.

One of the main goals of the TTC has been to bridge the gap between state-of-the-art R&D on laboratory prototypes and actual accelerator components in operating facilities, with the clear long-term objective to enable superconducting technology for a TeV-scale linear collider. This objective demanded a staged approach and intense work on the development of all the many peripherals and subcomponents. The collaboration embraced a joint effort between the initial partners to develop the TTF at DESY, which aimed to demonstrate reliable operation of an electron superconducting linac at gradients above 15 MV/m in “vector sum” control – whereby many cavities are fed by a single high-power RF source to improve cost effectiveness. In 1993 the collaboration finalised a 1.3 GHz cavity design that is still the baseline of large projects like the European XFEL, LCLS-II and SHINE, and nearly all L-band-based facilities.

Towards a linear collider

An intense collaborative effort started for the development of all peripheral components, for example power couplers, high-order mode dampers, digital low-level RF systems and cryomodules with unprecedented heat load performances. Several of these components were designed by TTC partners in an open collaborative and competitive effort, and a number of them can be found in existing projects around the world. The tight requirements imposed by the scale of a linear collider required an integrated design of the accelerating modules, containing the cavities and their peripheral components, which led to the concept of the “TESLA style” cryomodules, variants of which provide the building blocks of the linacs in TTF, European XFEL, LCLS-II and SHINE.

Half-wave resonator string assembly — **Low-beta cavities** A half-wave resonator string assembly at Argonne National Laboratory for use in PIP-II at Fermilab. Credit: M Kelly/ANL

The success of the TTF, which delivered its first beam in 1997, led it to become the driver for a next-generation light source at DESY, the VUV-FEL, which produced first light in 2005 and which later became the FLASH facility. The European XFEL built on this strong heritage, its large scale demanding a new level of design consolidation and industrialisation. It is remarkable to note that the total number of such TESLA-style cavities installed or to be installed in presently approved accelerators is more than 1800. Were a 250 GeV ILC to go ahead in Japan, approximately 8000 such units would be required. (Note that an alternative proposal for a high-energy linear collider, the Compact Linear Collider, relies on a novel dual-beam acceleration scheme that does not require SRF cavities.)

Since the partners collaborating on the early TESLA goal of a linear collider were also involved in other national and international projects for a variety of applications and domains, the first decade of the 21st century saw the TTC broaden its reach. For example, we started including reports from other projects, most notably the US Spallation Neutron Source, and gradually opened to the community working on low-beta ion and proton superconducting cavities, such as the half-wave resonator string collaboratively developed at Argonne National Lab and now destined for use in PIP-II at Fermilab (see “Low-beta cavities” image). TTC meetings include topical sessions with industries to discuss how to shorten the path from development to production. Recently, the TTC has also begun to facilitate collaborative exchanges on alternative SRF materials to bulk niobium, such as Nb₃Sn and even hybrid multilayer films, for potential accelerator applications.

Sustaining success

The mission of the TTC is to advance SRF technology R&D and related accelerator studies across the broad diversity of scientific applications. It is to provide a bridge for open communication and sharing of ideas, development and testing across associated projects. The TTC supports and encourages the free and open exchange of scientific and technical knowledge, engineering designs and equipment. Furthermore, it is based on cooperative work on SRF accelerator technology by research groups at TTC member institution laboratories and test facilities. The current TTC membership consists of 60 laboratories and institutes in 12 countries across Europe, North America and Asia. Since progress in cavity performance and related SRF technologies is so rapid, the major TTC meetings have been frequent.

**Global view** Distribution of superconducting particle accelerators using SRF structures for electrons (orange), protons (purple) and heavy ions (pink). More than 30 SRF accelerators are in operation (circles), approximately 15 are presently under construction (triangles) and more than 10 future projects are under consideration (squares). Credit: CERN

Particle accelerators using SRF technologies have been applied widely, from small facilities for medical applications up to large-scale projects for particle physics, nuclear physics, neutron sources and free-electron lasers (see “Global view” figure). Five large-scale (> 100 cavities) SRF projects are currently under construction in three regions: ESS in Europe, FRIB and LCLS-II in the US, and SHINE (China) and RAON (Korea) in Asia. Close international collaboration will continue to support progress in these and future projects, including SRF thin-film technology relevant for a possible future circular electron–positron collider. Perhaps the next wave of SRF technology will be the maturation of economical small-scale applications with high multiplicity and international standards. As an ultimate huge future SRF project, realising an ILC will indeed require sustained broad international collaboration.

The open and free-exchange model that for 30 years has enabled the TTC to make broad progress in SRF technology is a major contribution to science diplomacy efforts on a worldwide scale. We celebrate the many creative and collaborative efforts that have served the international community well via the TESLA Technology Collaboration.

Black holes attract 2020 Nobel Prize

Penrose, Ghez and Genzel — **Space–time pioneers** From left: Nobel winners Roger Penrose, Andrea Ghez and Reinhard Genzel. Credit: Creative Commons

The 2020 Nobel Prize in Physics, announced on 6 October, has recognised seminal achievements in the theoretical and experimental understanding of black holes. One half of the SEK 10 million ($1.15 million) award went to Roger Penrose of the University of Oxford “for the discovery that black-hole formation is a robust prediction of the general theory of relativity”. The other half was awarded jointly to Andrea Ghez of the University of California, Los Angeles and Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics “for the discovery of a supermassive compact object at the centre of our galaxy”, after the pair led separate research teams during the 1990s to identify a black hole at the centre of the Milky Way.

You might ask where the greatest entropy is in the universe – by an absolutely enormous factor it is in black holes
Roger Penrose

As soon as Einstein had completed his theory of general relativity in 1915, it was clear that solutions in the vicinity of a spherically symmetric, non-rotating mass allow space–time to be “pinched” to a point, or singularity, where known physics ceases to apply. Few people, including Einstein himself, however, thought that black holes really exist. But 50 years later, Penrose invented a mathematical tool called a trapped surface to show that black holes are a natural consequence of general relativity, proving that they each hide a singularity. His groundbreaking article (Phys. Rev. Lett. 14 57) is heralded as the first post-Einsteinian result in general relativity.

Penrose is also known for the “Penrose process”, whereby a particle–antiparticle pair that forms close to the event horizon of a black hole can become separated, with one of the two particles falling into the black hole and the other one escaping and carrying away energy and angular momentum. He also proposed twistor theory, which has evolved into a rich branch of theoretical and mathematical physics with potential relevance to the unification of general relativity and quantum mechanics, among many other contributions.

“I really had to have a good idea of the space–time geometry. Not just 3D, you had to think of the whole 4D space–time… I do most of my thinking in visual terms, rather than writing down equations,” said Penrose in an interview with the Nobel Foundation following the award. “Black holes have become more and more important, also in ways that people don’t normally appreciate. They are the basis of the second law of thermodynamics… You might ask where the greatest entropy is in the universe – by an absolutely enormous factor it is in black holes.”

Preparatory ‘pre-lab’ proposed for ILC

ILC accelerating module — **Down the line** How an accelerating module for the International Linear Collider (ILC) might look. Credit: R Hori/KEK

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.”

CERN publishes first environmental report

Safety engineering and environment group members — **Greener pastures** Members of the CERN safety engineering and environment group at one of around 100 environment monitoring stations on and around the laboratory’s sites. Credit: CERN-PHOTO-201504-073-3

“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.”

In pursuit of right-handed photons

**Fig. 1.** 2σ constraints on the ratio of right- and left-handed Wilson coefficients, C^′₇/C₇, using the flavio software package. C₇ quantifies the coupling strength to photons, and is fixed to its SM value. Any deviation in C^′₇/C₇, from (0,0) would signify new physics. The new LHCb measurement is shown in red. Credit: CERN

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 → K_Sπ⁰γ or B_s⁰ → φγ decays. It could also contribute to the total rate of radiative b → sγ decays, containing any strange meson, B → X_sγ.

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.

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.

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