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LHCP sees a host of new results

More than 1000 physicists took part in the ninth Large Hadron Collider Physics (LHCP) conference from 7 to 12 June. The in-person conference was to have been held in Paris: for the second year in a row, however, the organisers efficiently moved the meeting online, without a registration fee, thanks to the support of CERN and IUPAP. While the conference experience cannot be the same over a video link, the increased accessibility for people from all parts of the international community was evident, with LHCP21 participants hailing from institutes across 54 countries.

The LHCP format traditionally has plenary sessions in the mornings and late afternoons, with parallel sessions in the middle of the day. This “shape” was kept for the online meeting, with a shorter day to improve the practicality of joining from distant time zones. This resulted in a dense format with seven-fold parallel sessions, allowing all parts of the LHC programme, both experimental and theoretical, to be explored in detail. The overall vitality of the programme is illustrated by the raw statistics: a grand total of 238 talks and 122 posters were presented.

Last year saw a strong focus on the couplings to the second generation

Nine years on from the discovery of the 125 GeV Higgs boson, measurements have progressed to a new level of precision with the full Run-2 data. Both ATLAS and CMS presented new results on Higgs production, helping constrain the dynamics of the production mechanisms via differential and “simplified template” cross-section measurements. While the couplings of the Higgs to third-generation fermions are now established, last year saw a strong focus on the couplings to the second generation. After first evidence for Higgs decays to muons was reported from CMS and ATLAS results earlier in the year, ATLAS presented a new search with the full Run-2 data for Higgs decays to charm quarks using powerful new charm-tagging techniques. Both CMS and ATLAS showed updated searches for Higgs-pair production, with ATLAS being able to exclude a production rate more than 4.1 times the Standard Model (SM) prediction at 95% confidence. This is a process that should be observable with High-Luminosity LHC statistics, if it is as predicted in the SM. A host of searches were also reported, some using the Higgs as a tool to probe for new physics.

Puzzling hints

The most puzzling hints from the LHC Run 1 seem to strengthen in Run 2. LHCb presented analyses relating to the “flavour anomalies” found most notably in b→sµ⁺µ⁻ decays, updated to the full data statistics, in multiple channels. While no result yet passes a 5σ difference from SM expectations, the significances continue to creep upwards. Searches by ATLAS and CMS for potential new particles or effects at high masses that could indicate an associated new-physics mechanism continue to draw a blank, however. This remains a dilemma to be studied with more precision and data in Run 3. Other results in the flavour sector from LHCb included a new measurement of the lifetime of the Ω_c, four times longer than previous measurements (CERN Courier July/August 2021 p17) and the first observation of a mass difference between the mixed D⁰–D⁰ meson mass eigenstates (CERN Courier July/August 2021 p8).

A wealth of results was presented from heavy-ion collisions. Measurements with heavy quarks were prominent here as well. ALICE reported various studies of the differences in heavy-flavour hadron production in proton–proton and heavy-ion collisions, for example using D mesons. CMS reported the first observation of B_c meson production in heavy-ion collisions, and also first evidence for top-quark pair production in lead–lead collisions. ATLAS used heavy-flavour decays to muons to compare suppression of b- and c-hadron production in lead–lead and proton–proton collisions. Beyond the ions, ALICE also showed intriguing new results demonstrating that the relative rates of different types of c-hadron production differ in proton–proton collisions compared to earlier experiments using e⁺e⁻ and ep collisions at LEP and HERA.

Looking forward, the experiments reported on their preparations for the coming LHC Run 3, including substantial upgrades. While some work has been slowed by the pandemic, recommissioning of the detectors has begun in preparation for physics data taking in spring 2022, with the brighter beams expected from the upgraded CERN accelerator chain. One constant to rely on, however, is that LHCP will continue to showcase the fantastic panoply of physics at the LHC.

New charmed-baryon lifetime hierarchy cast in stone

**Fig. 1.** Measurements of charmed-baryon lifetimes from fixed-target experiments (PDG 2018) and from the LHCb experiment using charmed baryons produced from semileptonic b-decays (LHCb SL) and now from proton–proton collisions directly (LHCb Prompt), showing the progression in our understanding. Credit: CERN

Which charmed baryon decays first? The LHCb collaboration recently challenged the received wisdom of fixed-target experiments by almost quadrupling the measured lifetime of the doubly strange Ω_c⁰. Now, a follow-up measurement by the collaboration confirms the revised hierarchy, offering valuable input to theoretical models of the decays.

The situation changed dramatically in 2018

Ground-state baryons containing a charm quark (c), such as Λ_c⁺ (udc), Ξ_c⁺ (usc), Ξ_c⁰ (dsc) and Ω_c⁰ (ssc), decay via the weak interaction. The ordering of their lifetimes has long been thought to be τ(Ξ_c⁺) > τ(Λ_c⁺) > τ(Ξ_c⁰) > τ(Ω_c⁰), based on measurements from fixed-target experiments nearly 20 years ago. However, the situation changed dramatically in 2018 when LHCb joined the game using a sample of Ω_c⁰ baryons obtained from bottom- baryon semileptonic decays. That LHCb study measured the Ω_c⁰ lifetime to be nearly four times larger than previously measured, transforming the hierarchy into τ(Ξ_c⁺) > τ(Ω_c⁰) > τ(Λ_c⁺) > τ(Ξ_c⁰). One year later, LHCb significantly improved the precisions of the lifetimes of the other three charmed baryons using the same method, also finding the lifetime of the Ξ_c⁰ baryon to be larger than the world-average value by about 3σ (figure 1).

Theoretically challenging

The corresponding theoretical calculations are challenging. In the charm sector, an effective theory of heavy-quark expansion is taken to calculate lifetimes of charmed baryons through an expansion in powers of 1/m_c, where m_c is the constituent charm–quark mass. Calculations up to order 1/m_c³ imply a lifetime hierarchy consistent with the original fixed-target measurements, though only qualitatively. Attempts at higher-order calculations up to order 1/m_c⁴, however, cannot accommodate the old hierarchy, but can explain the new one if a suppression factor to the constructive Pauli-interference and semileptonic terms is written in. The origin of the suppression factor is still unknown, but probably due to even higher order effects. An independent measurement was therefore needed to confirm the experimental situation.

The charmed-baryon lifetime puzzle has now been resolved by a new measurement from LHCb using a much larger sample of Ω_c⁰ and Ξ_c⁰ baryons produced directly in proton–proton collisions. Both particles are detected in the final state pK^–K^–π⁺. The measurement is made relative to the lifetime distribution of the charmed meson D⁰ via D⁰ → K⁺K^–π⁺π^– decays, in order to control systematic uncertainties. Taking advantage of the performance and detailed understanding of the LHCb detector, the lifetimes of the Ω_c⁰ and Ξ_c⁰ baryons are found to be τ(Ω_c⁰) = 276.5 ± 13.4 (stat) ± 4.5 (syst) fs and τ(Ξ_c⁰) = 148.0 ± 2.3 (stat) ± 2.2 (syst) fs, respectively, where the precision of the Ω_c⁰ lifetime is improved by a factor of two compared to the semileptonic measurement. The new results are consistent with the previous LHCb measurements, and hence establish the new lifetime hierarchy. Combining this measurement with the previous LHCb results gives τ(Ω_c⁰) = 274.5 ± 12.4 fs and τ(Ξ_c⁰) = 152.0 ± 2.0 fs, the most precise charm-baryon lifetimes to date. The newly confirmed lifetime hierarchy will help improve our knowledge of QCD dynamics in charm hadrons, and provides a crucial input to calibrate theoretical calculations.

Four top quarks seen at once

**Four-top candidate** One top quark decays to a muon (red), one to an electron (green) and two decay hadronically. Missing transverse momentum is indicated by a dotted line and the blue cones are b-jets. Credit: ATLAS

The production of four top quarks is an extremely rare event at the LHC, with an expected cross section five orders of magnitude below the production of a top-quark pair. With the heaviest elementary particle in the Standard Model produced four times in the final state, it is also one of the most spectacular processes accessible at the LHC. By combining two analyses, the ATLAS collaboration has uncovered the first strong evidence to support the existence of this unique event topology with sensitivity to theories beyond the Standard Model (BSM).

This is the only process that could probe potentially anomalous effective four-heavy-fermion operators

**Fig. 1.** Distribution of the multivariate discriminant score for data events (black dots) together with the background and signal prediction (stacked histograms) for the analysis using events with two leptons with the same electric charge or three leptons. The lower panel shows the ratio of the data to the total prediction, where the band represents the total uncertainty. Credit: CERN

As a result of its large mass, the top quark plays a special role in numerous BSM theories, and many of these theories predict an increase in the four-top-quark production cross section. In particular, four-top-quark production is the only process that could probe potentially anomalous effective four-heavy-fermion operators. The cross section is also sensitive to the value of the top-quark Yukawa coupling, as a result of contributions mediated by Higgs bosons. However, until now, four-top-quark production has not been observed, in part because of its tiny production rate, and in part because the experimental signature of this process is very complex, requiring up to 12 particles to be reconstructed from the top-quark decays. The search is also affected by background sources in kinematic regions that are at the limit of the domain of validity of the simulations.

Despite these challenges, the ATLAS collaboration has recently released two studies of four-top-quark production using its full Run-2 data sample. The first study searches for events with two leptons (electrons or muons) with the same electric charge or with three leptons. This selection corresponds to only 13% of all possible four-top-quark final states, but is contaminated by only a small background, mainly from the production of a top-quark pair with a W, Z or Higgs boson and additional jets, or from events with one lepton with misidentified electric charge or a “fake” lepton that doesn’t correspond to a W or Z boson decay. Background processes were primarily simulated using the best available theoretical predictions; the rates of the most difficult ones were measured using control samples with similar properties to the signal events. The second study searches for events with one lepton or two oppositely-charged leptons. This selection retains 57% of the possible four-top-quark final states, but suffers from a large background from top-quark pairs produced in association with many jets, some of which are consistent with originating from b-quarks (b-jets). This background is difficult to model and was determined using data control samples. To better isolate the signal from the background, multivariate discriminants were trained in both analyses using distinct features of the signal, such as the number of b-jets and the kinematic properties of the reconstructed particles (see figure 1).

**Fig. 2.** Measurements of the four-top-quark cross section over its Standard Model prediction (μ) at 13 TeV, for the analysis using events with only one lepton or two oppositely-charged leptons (1L/2LOS), for the analysis with two leptons with the same electric charge or three leptons (2LSS/3L), and for the combination. Credit: CERN

Results from the two studies were combined, leading to a four-top-quark cross-section measurement at 13 TeV of 25⁺⁷_–6 fb, which is consistent with the Standard Model prediction of 12.0 ± 2.4 fb within 2.0σ (see figure 2). The statistical significance of the signal corresponds to 4.7σ, providing strong evidence for this process, close to the observation threshold of 5σ. LHC Run-3 data, possibly at a higher centre-of-mass energy, will allow ATLAS to verify whether the larger measured cross section relative to the prediction is confirmed or not.

Resistive Gaseous Detectors: Designs, Performance, and Perspectives

The first truly resistive gaseous detector was invented by Rinaldo Santonico and Roberto Cardarelli in 1981. A kind of parallel-plate detector with electrodes made of resistive materials such as Bakelite and thin-float glass, the design is sometimes also known as a resistive-plate chamber (RPC). Resistive gaseous detectors use electronegative gases and electric fields that typically exceed 10 kV/cm. When a charged particle is incident in the gas gap, the working operational gas is ionised, and primary electrons cause an avalanche as a result of the high electric field. The induced charge is then obtained on the readout pad as a signal. RPCs have several unique and important practical features, combining good spatial resolution with a time resolution comparable to that of scintillators. They are therefore well suited for fast spacetime particle tracking, as a cost-effective way to instrument large volumes of a detector, for example in muon systems at collider experiments.

Resistive gaseous detectors use electronegative gases and electric fields which typically exceed 10 kV/cm

Resistive Gaseous Detectors: Designs, Performance, and Perspectives, a new book by Marcello Abbrescia, Vladimir Peskov and Paulo Fonte, covers the basic principles of their operation, historical development, the latest achievements and their growing applications in various fields from hadron colliders to astrophysics. This book is not only a summary of numerous scientific publications on many different examples of RPCs, but also a detailed description of their design, operation and performance.

The book has nine chapters. The operational principle of gaseous detectors and some of their limitations, most notably the efficiency drop in a high-particle-rate environment, are described. This is followed by a history of parallel-plate detectors, the first classical Bakelite RPC, double-gap RPCs and glass-electrode multi-gap timing RPCs. A modern design of double-gap RPCs and examples for the muon systems like those at ATLAS and CMS at the LHC, the STAR detector at the Relativistic Heavy-Ion Collider at Brookhaven and the multi-gap timing RPC for the time-of-flight system of the HADES experiment at GSI are detailed. Advanced designs with new materials for electrodes for high-rate detectors are then introduced, and ageing and longevity are elaborated upon. A new generation of gaseous detectors with resistive electrodes that can be made with microelectronic technology is then introduced: these large-area electrodes can easily be manufactured while still achieving high spatial resolutions up to 12 microns.

Homeland security

The final chapter covers applications outside particle physics such as those in medicine exploiting positron-emission tomography. For homeland security, RPCs can be used in muon-scattering tomography with cosmic-ray muons to scan spent nuclear fuel containers without opening them, or to quickly scan incoming cargo trucks without disrupting the traffic of logistics. A key subject not covered in detail, however, is the need to search for environmentally friendly alternatives to gases with high global-warming potential, which are often needed in resistive gaseous detectors at present to achieve stable and sustained operation (CERN Courier July/August 2021 p20).

Abbrescia, Peskov and Fonte’s book will be useful to graduates specialising in high-energy physics, astronomy, astrophysics, medical physics and radiation measurements in general for undergraduate students and teachers.

What if scientists ruled the world?

A chemistry professor invents a novel way to produce chemical compounds, albeit with a small chance of toxicity. A paper is published. A quick chat with a science communicator leads to a hasty press release. But when the media picks up on it, the story is twisted.

“What if scientists ruled the world?” — a somewhat sensational but thought-provoking title for a play — is an interactive theatre production by the Australian Academy of Science in partnership with Falling Walls Engage. Staged on 8 May at the Shine Dome in Canberra, Australia, a hybrid performance explored the ramifications of an ill-considered press release, and provided a welcome opportunity for scientists to reflect on how best to communicate their research. The dynamic exchange of ideas between science experts and laypeople in the audience highlighted the power of words, and how they are used to inform, persuade, deceive or confuse.

After setting the scene, director Ali Clinch invited people participating remotely on Zoom and via a YouTube livestream to guide the actors’ actions, helping to advance and reframe the storyline with their ideas, questions and comments. Looking at the same story from different points of view invited the audience to think about the different stakeholders and their responsibility in communicating science. In the first part of the performance, for example, the science communicator talks excitedly about her job with students, but later has to face a crisis that the busy professor is unable or unwilling to deal with. At a critical point in the story, when a town-hall meeting is held to debate the future of a company that employs most of the people in the town, but which probably produced the same toxic chemical, everybody felt part of the performance. The audience could even take the place of an actor, or act in a new role.

The play highlighted the pleasures and tribulations of work at the interface between research and public engagement

The play highlighted the pleasures and tribulations of work at the interface between research and public engagement during euphoric discoveries and crisis moments alike, and has parallels both with the confusion encountered during the early stages of the COVID-19 pandemic and misguided early fears that the LHC could generate a black hole. In an age of fake news, sensationalism and misinformation, the performance adeptly highlighted the complexities and vested interests inherent in science communication today.

A relational take on quantum mechanics

It is often said that “nobody understands quantum mechanics” – a phrase usually attributed to Richard Feynman. This statement may, however, be misleading to the uninitiated. There is certainly a high level of understanding of quantum mechanics. The point, moreover, is that there is more than one way to understand the theory, and each of these ways requires us to make some disturbing concessions.

Carlo Rovelli’s Helgoland is therefore a welcome popular book – a well-written and easy-to-follow exploration of quantum mechanics and its interpretation. Rovelli is a theorist working mainly on quantum gravity and foundational aspects of physics. He is also a very successful popular author, distinguished by his erudition and his ability to illuminate the bigger picture. His latest book is no exception.

Helgoland is a barren German island of the North Sea where Heisenberg co-invented quantum mechanics in 1925 while on vacation. The extraordinary sequence of events between 1925 and 1926, when Heisenberg, Jordan, Born, Pauli, Dirac and Schrödinger formulated quantum mechanics, is the topic of the opening chapter of the book.

Rovelli only devotes a short chapter to discuss interpretations in general. This is certainly understandable, since the author’s main target is to discuss his own brainchild: relational quantum mechanics. This approach, however, does not do justice to popular ideas among experts, such as the many-worlds interpretation. The reader may be surprised not to find anything about the Copenhagen (or, more appropriately, Bohr’s) interpretation. This is for very good reason, however, since it is not generally considered to be a coherent interpretation. Having mostly historical significance, it has served as inspiration to approaches that keep the spirit of Bohr’s ideas, like consistent histories (not mentioned in the book at all), or Rovelli’s relational quantum mechanics.

Relational quantum mechanics was introduced by Rovelli in an original technical article in 1996 (Int. J. Theor. Phys. 35 1637). Helgoland presents a simplified version of these ideas, explained in more detail in Rovelli’s article, and in a way suitable for a more general audience. The original article, however, can serve as very nice complementary reading for those with some physics background. Relational quantum mechanics claims to be compatible with several of Bohr’s ideas. In some ways it goes back to the original ideas of Heisenberg by formulating the theory without a reference to a wavefunction. The properties of a system are defined only when the system interacts with another system. There is no distinction between observer and observed system. Rovelli meticulously embeds these ideas in a more general historical and philosophical context, which he presents in a captivating manner. He even speculates whether this way of thinking can help us understand topics that, in his opinion, are unrelated to quantum mechanics, such as consciousness.

Helgoland’s potential audience is very diverse and manages to transcend the fact that it is written for the general public. Professionals from both the sciences and the humanities will certainly learn something, especially if they are not acquainted with the nuances of the interpretations of modern physics. The book, however, as is explicitly stated by Rovelli, takes a partisan stance, aiming to promote relational quantum mechanics. As such, it may give a somewhat skewed view of the topic. In that respect, it would be a good idea to read it alongside books with different perspectives, such as Sean Carroll’s Something Deeply Hidden (2019) and Adam Becker’s What is Real? (2018).

Astroparticle theory in rude health

The EuCAPT census — **Unleashing potential** A visualisation of the “EuCAPT census” of theoretical astroparticle physicists and cosmologists who are affiliated to a research institution in a European country. Credit: N Sarcevic/R Nenad/M Rayner

The European Consortium for Astroparticle theory (EuCAPT) held its first annual symposium from 5 to 7 May. Hundreds of theoretical physicists from Europe and beyond met online to discuss the present and future of astroparticle physics and cosmology, in a dense and exciting meeting that featured 29 invited presentations, 42 lightning talks by young researchers, and two community-wide brainstorming sessions.

Participants discussed a wide array of topics at the interface between particle physics, astrophysics and cosmology, with particular emphasis on the challenges and opportunities for these fields in the next decade. Rather than focusing on experimental activities and the discoveries they might enable, the sessions were structured around thematic areas and explored the interdisciplinary multi-messenger aspects of each.

Two sessions were dedicated to cosmology, exploring the early and late universe. As stressed by Geraldine Servant (Hamburg), several unresolved puzzles of particle physics – such as the origin of dark matter, the baryon asymmetry, and inflation – are directly linked to the early universe, and new observational probes may soon shed new light on them.

Julien Lesgourgues (Aachen) showed how the very same puzzles are also linked to the late universe, and cautiously elaborated on a series of possible inconsistencies between physical quantities inferred from early- and late-universe probes, for example the Hubble constant. Those inconsistencies represent both a challenge and an extraordinary opportunity for cosmology, as they might “break” the standard Lambda–cold-dark-matter model of cosmology, and allow us to gain insights into the physics of dark matter, dark energy and gravity.

We are witnessing a proliferation of theoretically well-motivated models

New strategies to go beyond the standard models of particle physics and cosmology were also discussed by Marco Cirelli (LPTHE) and Manfred Lindner (Heidelberg), in the framework of dark-matter searches and neutrino physics, respectively. Progress in both fields is currently not limited by a lack of ideas – we are actually witnessing a proliferation of theoretically well-motivated models – but by the difficulty of identifying experimental strategies to conclusively validate or rule them out. Much of the discussion here concerned prospects for detecting new physics with dedicated experiments and multi-messenger observations.

Gravitational waves have added a new observational probe in astroparticle physics and cosmology. Alessandra Buonanno (Max Planck Institute for Gravitational Physics) illustrated the exciting prospects for this new field of research, whose potential for discovering new physics is attracting enormous interest from particle and astroparticle theorists. The connection between cosmic rays, gamma rays and high-energy neutrinos was explored in the final outlook by Elena Amato (Arcetri Astrophysical Observatory), who highlighted how progress in theory and observations is leading the community to reconsider some long-held beliefs – such as the idea that supernova remnants are the acceleration sites of cosmic rays up to the so-called “knee” – and stimulating new ideas.

In line with EuCAPT’s mission, the local organisers and the consortium’s steering committee organised a series of community-building activities. Participants stressed the importance of supporting diversity and inclusivity, a continuing high priority for EuCAPT, while a second brainstorming session was devoted to the discussion of the EuCAPT white paper currently being written, which should be published by September. Last but not least, Hannah Banks (Cambridge), Francesca Capel (TU Munich) and Charles Dalang (University of Geneva) received prizes for the best lightning talks, and Niko Sarcevic (Newcastle) was awarded an “outstanding contributor” prize for the help and support she provides for the analysis of the EuCAPT census (pictured).

The next symposium will take place in 2022, hopefully in person, at CERN.

Particle Detectors – Fundamentals and Applications

Throughout the history of nuclear, particle and astroparticle physics, novel detector concepts have paved the way to new insights and new particles, and will continue to do so in the future. To help train the next generation of innovators, noted experimental particle physicists Hermann Kolanoski (Humboldt University Berlin and DESY) and Norbert Wermes (University of Bonn) have written a comprehensive textbook on particle detectors. The authors use their broad experience in collider and underground particle-physics experiments, astroparticle physics experiments and medical-imaging applications to confidently cover the spectrum of experimental methods in impressive detail.

Particle Detectors – Fundamentals and Applications combines in a single volume the syllabus also found in two well-known textbooks covering slightly different aspects of detectors: Techniques for Nuclear and Particle Physics Experiments by W R Leo and Detectors for Particle Radiation by Konrad Kleinknecht. Kolanoski and Wermes’ book supersedes them both by being more up-to-date and comprehensive. It is more detailed than Particle Detectors by Claus Grupen and Boris Shwartz – another excellent and recently published textbook with a similar scope – and will probably attract a slightly more advanced population of physics students and researchers. This new text promises to become a particle-physics analogue of the legendary experimental-nuclear-physics textbook Radiation Detection and Measurement by Glenn Knoll.

The book begins with a comprehensive warm-up chapter on the interaction of charged particles and photons with matter, going well beyond a typical textbook level. This is followed by a very interesting discussion of the transport of charge carriers in media in magnetic and electric fields, and – a welcome novelty – signal formation, using the method of “weighting fields”. The main body of the book is devoted first to gaseous, semiconductor, Cherenkov and transition-radiation detectors, and then to detector systems for tracking, particle identification and calorimetry, and the detection of cosmic rays, neutrinos and exotic matter. Final chapters on electronics readout, triggering and data acquisition complete the picture.

Particle Detectors – Fundamentals and Applications is best considered a reference for lectures on experimental methods in particle and nuclear physics for postgraduate-level students. The book is easy to read, and conceptual discussions are well supported by numerous examples, plots and illustrations of excellent quality. Kolanoski and Wermes have undoubtedly written a gem of a book, with value for any experimental particle physicist, be they a master’s student, PhD student or accomplished researcher looking for detector details outside of their expertise.

Leadership in superconductors recognised

Amalia Ballarino of CERN has received the 2021 James Wong Award from the Institute of Electrical and Electronics Engineers (IEEE) for her significant and continuing contributions in the field of superconducting materials. The IEEE citation recognises her for: “leading successful R&D programs that establish a winning role for high temperature and MgB₂ superconductors in accelerator applications; piloting the development of MgB₂ wire suitable for cabling and its incorporation into a multi-kA power transmission system operating at 25 K, and directing the project to industrialise eight such systems for which over 1000 km of wire have been produced; promoting fruitful cooperation between research and industry; and launching R&D activity based on the use of superconductors (Nb-Ti, Nb₃Sn, MgB₂ and high-temperature superconductors) for future particle accelerators.

Ballarino was responsible for the several-thousand current leads that power the superconducting magnets of the LHC, including those based on the high-temperature superconductor BSCCO-2223, which have been the ﬁrst large-scale commercial application of high-temperature superconductors. She was awarded Superconducting Week’s “Superconductor Industry Person of the Year 2006” for the development. Following work on the commissioning of the LHC, Ballarino proposed cold-powering systems that use high-current MgB₂ transfer lines for feeding the new superconducting magnets of the High-Luminosity LHC (HL-LHC). She started a collaboration with industry to develop the conductor in the form of wire suitable for cabling. The wire has been successfully delivered to CERN in large quantities, while the cold-powering systems have been developed and qualified and they are now being industrialised.

CERN is home to more winners than any other institution

Ballarino joined CERN as PhD student. She is section leader in CERN’s magnets, superconductors and cryostats group and, as from January 2021, deputy group leader. The IEEE cited her service to the community as lecturer, member of program committees for international conferences, and technical editor and reviewer of papers for scientific journals. “In my opinion, this recognition has been a long time in coming,” says Bruce Strauss, past president and treasurer of the IEEE council on superconductivity.

The IEEE James Wong Award (formally named “Award for Continuing and Significant Contributions in the Field of Applied Superconductivity” until 2013) comes with a $5000 honorarium and a pure-niobium medal. It has been granted annually by the IEEE council on superconductivity since 2000, and CERN is home to more winners than any other institution, with Daniel Leroy, Lucio Rossi, Herman ten Kate, Robert Aymar, Arnaud Devred and Luca Bottura recognised in previous years.

Ballarino will receive the award during the MT27 International Conference on Magnet Technology in November.

FCC feasibility study comes into focus

This year’s Future Circular Collider (FCC) Week took place online from 28 June to 2 July, attracting 700 participants from all over the world to debate the next steps needed to produce a feasibility report in 2025/2026, in time for the next update to the European Strategy for Particle Physics in 2026/2027. The current strategy, agreed in 2020, sets an electron–positron Higgs factory as the highest priority facility after the LHC, along with the investigation of the technical and financial feasibility of such a Higgs factory, followed by a high-energy hadron collider placed in the same 100 km tunnel. The FCC feasibility study will focus on the first stage (tunnel and e⁺e^– collider) in the next five years.

Although the FCC is a long-term project with a horizon up to the 22nd century, its timescales are rather tight. A post-LHC collider should be operational around the 2040s, ensuring a smooth continuation from the High-Luminosity LHC, so construction would need to begin in the early 2030s. Placement studies to balance geological and territorial constraints with machine requirements and physics performance suggest that the most suitable scenarios are based on a 92 km-circumference tunnel with eight surface sites.

The next steps are subsurface investigations of high-risk areas, surface-site initial-state analysis and verification of in-principle feasibility with local authorities. A “Mining the Future” competition has been launched to solicit ideas for how to best use the nine million cubic metres of molasse that would be excavated from the tunnel.

The present situation in particle physics is reminiscent of the early days of superconductivity

A highlight of the week was the exploration of the physics case of a post-LHC collider. Matthew Reece (Harvard University) identified dark matter, the baryon asymmetry and the origin of primordial density perturbations as key experimental motivations, and the electroweak hierarchy problem, the strong CP problem and the mystery of flavour mixing patterns as key theoretical motivations. The present situation in particle physics is reminiscent of the early days of superconductivity, he noted, when we had a phenomenological description of symmetry breaking in superconductivity, but no microscopic picture. Constraining the shape of the Higgs potential could allow a similar breakthrough for electroweak symmetry breaking. Regarding recent anomalous measurements, such as those of the muon’s magnetic moment, Reece noted that while these measurements could give us the coefficients of one higher dimension operator in an effective-field-theory description of new physics, only colliders can systematically produce and characterise the nature of any new physics. FCC-ee and FCC-hh both have exciting and complementary roles to play.

A key technology for FCC-ee is the development of efficient superconducting radio-frequency (SRF) cavities to compensate for the 100 MW synchrotron radiation power loss in all modes of operation from the Z pole up to the top threshold at 365 GeV. A staged RF system is foreseen as the baseline scenario, with low-impedance single-cell 400 MHz Nb/Cu cavities for Z running replaced by four-cell Nb/Cu cavities for W and Higgs operation, and later augmented by five-cell 800 MHz bulk Nb cavities at the top threshold.

As well as investigations into the use of HIPIMS coating and the fabrication of copper substrates, an innovative slotted waveguide elliptical (SWELL) cavity design was presented that would operate at 600 or 650 MHz. SWELL cavities optimise the surface area, simplify the coating process and avoid the need for welding in critical areas, which could reduce the performance of the cavity. The design profits from previous work on CLIC, and may offer a simplified installation schedule while also finding applications outside of high-energy physics. A prototype will be tested later this year.

Several talks also pointed out synergies with the RF systems needed for the proposed electron–ion collider at Brookhaven and the powerful energy-recovery linac for experiments (PERLE) project at Orsay, and called for stronger collaboration between the projects.

Machine design

Another key aspect of the study regards the machine design. Since the conceptual design report last year, the pre-injector layout for FCC-ee has been simplified, and key FCC-ee concepts have been demonstrated at Japan’s SuperKEKB collider, including a new world-record luminosity of 3.12 × 10³⁴ cm^–2 s^–1 in June with a betatron function of β_γ* = 1 mm. Separate tests squeezed the beam to just β_γ* = 0.8 mm in both rings.

Other studies reported during FCC Week 2021 demonstrated that hosting four experiments is compatible with a new four-fold symmetric ring. This redundancy is thought to be essential for high-precision measurements, and different detector solutions will be invaluable in uncovering hidden systematic biases. The meeting also followed up on the proposal for energy-recovery linacs (ERLs) at FCC-ee, potentially extending the energy reach to 600 GeV if deemed necessary during the previous physics runs. First studies for the use of the FCC-ee booster as a photon source were also presented, potentially leading to applications in medicine and industry, precision QED studies and fundamental-symmetry tests.

Participants also tackled concepts for power reduction and power recycling, to ensure that FCC is sustainable and environmentally friendly. Ideas relating to FCC-ee include making the magnets superconducting rather than normal conducting, improving the klystron efficiency, using ERLs and other energy-storage devices, designing “twin” dipole and quadrupole magnets with a factor-two power saving, and coating SRF cavities with a high-temperature superconductor.

All in all, FCC Week 2021 saw tremendous progress across different areas of the study. The successful completion of the FCC Feasibility Study (2021–2025) will be a crucial milestone for the future of CERN and the field.

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