Jobs – Page 46 of 505

A(nother) day to remember

“I am an opportunist, in one way an extremely successful one. Weinberg and I were working along similar lines with similar attitudes. I wish you well for your celebrations and regret that I can’t be with you in person.”

Peter Higgs winner of the 2013 Nobel Prize in Physics.

“It was an overwhelming time for us. It took time to understand what had happened. I especially remember the excitement among the young researchers.”

Rolf Heuer former CERN Director-General.

“It took 14 years to build the LHC. At one point we had 1000 dipoles, each costing a million Swiss francs, stored on the surface, throughout rain and snow.”

Lyn Evans former LHC project director.

“The first two years of measuring Standard Model physics were essential to give us confidence in the readiness of the two experiments to search for new physics.”

Peter Jenni founding ATLAS spokesperson.

“A key question for CMS was: can tracking be done in a congested environment with just a few points, albeit precise ones? It was a huge achievement requiring more than 200 m² of active silicon.”

Michel Della Negra founding CMS spokesperson.

“I remember on 4 July 2012 a magnificent presentation of a historical discovery. I would also like to celebrate the life of Robert Brout, a great physicist and important man.”

François Englert winner of the 2013 Nobel Prize in Physics.

“The gist of the theory behind the Higgs boson would easily compete with the most far-fetched conspiracy theory, yet it seems nature chose it.”

Eliezer Rabinovici president of the CERN Council.

“The structure of the vacuum is intimately connected to how the Higgs boson interacts with itself. To probe this phenomenon at the LHC we can study the production of Higgs-boson pairs.”

André David CMS experimentalist (CERN).

“Collaboration between experiment and theory is even more necessary now to find any hints for BSM physics.”

Reisaburo Tanaka ATLAS experimentalist (Université Paris-Saclay).

“Precision Higgs physics is a telescope to high-scale physics, so I’m looking forward to the next 10 years of discovery.”

Sally Dawson theorist (BNL).

“Theory accuracy will be even more important to make the best of the HL-LHC data, especially in the case in which no evidence of new physics will show up… This is also crucial for the Monte Carlo tools used in the analyses.”

Massimiliano Grazzini theorist (University of Zurich).

“After 10 years we’ve measured the five main production and five major decay mechanisms of the Higgs boson.”

Kerstin Tackmann ATLAS experimentalist (DESY).

“What we know so far – Mass: known to 0.11%. Width: closing in on SM value of 3.2^+2.5_–1.7 MeV (plus evidence of off-shell Higgs production). Spin 0: spin 1 & 2 excluded at 99.9% CL. CP structure: in accordance with SM CP-even hypotheses.”

Marco Delmastro ATLAS experimentalist (CNRS/IN2P3 LAPP).

“We have learned much about the 125 GeV Higgs boson since its discovery. The LHC Run 3 starts tomorrow: ready for the next decade of Higgs-boson exploration!”

Adinda de Wit CMS experimentalist (University of Zurich).

“The Higgs boson is linked to profound structural problems in the Standard Model. It is therefore an extraordinary discovery tool that calls for a broad experimental programme at the LHC and beyond.”

Fabiola Gianotti CERN Director-General.

“Elusive non-resonant pairs of Higgs bosons are the prime experimental signature of the Higgs-boson self-coupling. We are all eager to analyse Run 3 data to further probe HH events!”

Arnaud Ferrari ATLAS experimentalist (Uppsala University).

“New physics can affect differently the different fermion generations. We have to precisely measure the couplings if we want to understand the Higgs boson’s nature.”

Andrea Marini CMS experimentalist (CERN).

“From its potential invisible, forbidden, and exotic decays to the possible existence of scalar siblings, the Higgs boson plays a fundamental role in searches for physics beyond the Standard Model.”

Roberto Salerno CMS experimentalist (CNRS/IN2P3 – LLR & École polytechnique).

“An incredible collaborative effort has brought us this far. But there is much more to come, especially during Long Shutdown 3, with HL-LHC paving the way from Run 3 to ultimate performance. Interesting times ahead to say the least!”

Mike Lamont CERN director for accelerators and technology.

“The hard work and creativity in reconstruction and analysis techniques are already evident since the last round of projections. Imagine what we can do in the next 20 years!”

Elizabeth Brost ATLAS experimentalist (BNL).

“The Higgs is the first really new elementary particle we’ve seen. We need to study it to death!”

Nima Arkani-Hamed theorist (IAS).

One hundred years ago, Otto Stern and Walther Gerlach performed their ground-breaking experiment shooting silver atoms through an inhomogeneous magnetic field, separating them according to their spatially quantised angular momentum. It was a clear victory of quantum theory over the still widely used classical picture of the atom. The results also paved the way to the introduction of the concept of spin, an intrinsic angular momentum, as an inherent property of subatomic particles.

The idea of spin was met with plenty of scepticism. Abraham Pais noted in his book George Uhlenbeck and the Discovery of Electron Spin that Ralph Kronig finishing his PhD at Columbia University in 1925 and travelling through Europe, introduced the idea to Heisenberg and Pauli, who dryly commented that “it is indeed very clever but of course has nothing to do with reality”. Feeling ridiculed, Kronig dropped the idea. A few months later, still against strong resistance by established experts but this time with sufficient backing by their mentor Paul Ehrenfest, Leiden graduate-students George Uhlenbeck and Samuel Goudsmit published their seminal Nature paper on the “spinning” electron. “In the future I shall trust my own judgement more and that of others less,” wrote Kronig in a letter to Hendrik Kramers in March 1926.

Spin crisis

Spin quickly became a cornerstone of 20th-century physics. Related works of paramount importance were Pauli’s exclusion principle and Dirac’s description of relativistic spin-1/2 particles, as well as the spin-statistics theorems (namely the Fermi–Dirac and Bose–Einstein distributions for identical half-integer–spin and integer–spin particles, respectively). But more than half a century after its introduction, spin re-emerged as a puzzle. By then, a rather robust theoretical framework, the Standard Model, had been established within which many precision calculations became a comfortable standard. It could have been all that simple: since the proton consists of two valence-up and one valence-down quarks, with spin up and down (i.e. parallel and antiparallel to the proton’s spin, respectively), the origin of its spin is easily explained. The problem dubbed “spin crisis” arose in the late 1980s, when the European Muon Collaboration at CERN found that the contribution of quarks to the proton spin was consistent with zero, within the then still-large uncertainties, and that the so-called Ellis–Jaffe sum rule – ultimately not fundamental but model-dependent – was badly violated. What had been missed?

Today, after decades of intense experimental and theoretical work, our picture of the proton and its spin emerging from high-energy interactions has changed substantially. The role of gluons both in unpolarised and polarised protons is non-trivial. More importantly, transverse degrees of freedom, both in position and momentum space, and the corresponding role of orbital angular momentum, have become essential ingredients in the modern description of the proton structure. This description goes beyond the picture of collinearly moving partons encapsulated by the fraction of the parent proton’s momentum and the scale at which they are probed; numerous effects, unexplainable in the simple picture, have now become theoretically accessible.

Understanding the mysteries

The HERMES experiment at DESY, which operated between 1995 and 2007, has been a pioneer in unravelling the mysteries of the proton spin, and the experiment is the protagonist in a new book by Richard Milner and Erhard Steffens, two veterans in this field as well as the driving forces behind HERMES. The subtitle and preface clarify that this is a personal account and recollection of the history of HERMES, from an emergent idea on both sides of the Atlantic to a nascent collaboration and experiment, and finally as an extremely successful addition to the physics programme of HERA (the world’s only lepton–proton collider, which started running at DESY 30 years ago for one and a half decades).

Milner and Steffens are both experts on polarised gas targets, with complementary backgrounds leading to rather different perspectives. Indeed, HERMES was independently developed within a North American initiative, in which Milner was the driving force, and a European initiative around the Heidelberg MPI-K led by Klaus Rith, with Erhard Steffens as a long-time senior group member. In 1988 two independent letters of intent submitted to DESY triggered sufficient interest in the idea of a fixed-target experiment with a polarised gas target internal to the HERA lepton ring; the proponents were subsequently urged to collaborate in submitting a common proposal. In the meantime, HERMES’ feasibility needed to be demonstrated. A sufficiently high lepton-polarisation had to be established, as well as smooth running of a polarised gas target in the harsh HERA environment without disturbing the machine and the main HERA experiments H1 and Zeus.

By summer 1993, HERMES was fully approved, and in 1995 the data taking started with polarised ³He. The subsequently used target of polarised hydrogen or deuterium employed the same concepts that Stern and Gerlach had already used in their famous experiment. The next decade saw several upgrades and additions to the physics programme, and data taking continued until summer 2007. In all those years, the backbone of HERMES was an intense and polarised lepton beam that traversed a target of pure gas in a storage cell, highly polarised or unpolarised, avoiding extensive and in parts model-dependent corrections. This constellation was combined with a detector that, from the very beginning, was designed to not only detect the scattered leptons but also the “spray” produced in coincidence. These features allowed a diverse set of processes to be studied, leading to numerous pioneering measurements and insights that motivated, and continue to motivate, new experimental programmes around the world, including some at CERN.

Richard Milner and Erhard Steffens provide extensive insights, in particular into the historic aspects of HERMES, which are difficult to obtain elsewhere. The book gives an insightful discussion of the installation of the experiment and of the outstanding efforts of a group of highly motivated and dedicated individuals who worked too often in complete ignorance of (or in defiance of) standard working hours. Their account enthrals the reader with vivid anecdotes, surprising twists and personal stories, all told in a colloquial style. While clearly not meant as a textbook – indeed, one might notice small mistakes and inconsistencies in a few places – this book makes for worthwhile and enjoyable reading, not only for people familiar with the subject but equally for outsiders. In particular, younger generations of physicists working in large-scale collaborations might be surprised to learn that it needs only a small group and little time to start an experiment that goes on to have a tremendous impact on our understanding of nature’s basic constituents.

Parallels

Released in March 2022 on Disney+, Parallels merges two of the most popular concepts in science fiction: time travel and the multiverse. The series, in French, created by Quoc Dan Trang and directed by Benjamin Rocher and Jean-Baptiste Saurel, is set in a village in the mountains of the French–Swiss border where a particle-physics laboratory called “ERN” and a collider strongly resembling the LHC have a major role.

The story begins with a group of four friends who recently graduated from middle school celebrating one of their birthdays near an area where, 10 years earlier, a kid called Hugo disappeared. At the same time, ERN is performing an experiment with its particle accelerator. However, something goes wrong. The lights go out in the village, while a strange space–time phenomenon unfolds, transporting the teenagers to different timelines once the lights are restored. Does this have anything to do with the particle accelerator? Where, or rather “when” are they? Each of the teenagers tries to unravel their temporal confusion in an attempt to return to their original timeline.

Parallels offers a chance to go beyond fiction and explore the often even more incredible ideas explored for real in particle physics

Although the age of the main characters targets younger audiences, Parallels addresses topics such as depression, regret and family issues, which, combined with some humour, make it relevant to other age groups. The visual effects and music create a suspenseful atmosphere and the compact nature of the series (six episodes of around 35 minutes each) draws the viewer into watching it in a single session.

CERN’s experiments and locations are referenced several times throughout, ranging from visual details in the ERN buildings to mentions of ATLAS, CMS and the Antiproton Decelerator – going so far as to reference an “FCC scheduled for operations in October 2025”. The Globe of Science and Innovation and the CMS silicon tracker are also represented.

Many of the concepts introduced, especially those related to the LHC experiments, are not scientifically accurate. The clear depiction of CERN in all but name may also make some physicists feel uncomfortable, given that the plot plays on YouTube-based conspiracy theories about what CERN’s experiments are capable of. For young science-fiction lovers, however, and especially for those who love to unravel temporal paradoxes, as in the popular Netflix series Stranger Things, Parallels is worth a look. For the more inquisitive and open-minded viewer, it also offers a chance to go beyond fiction and explore the often even more incredible ideas explored for real in particle physics.

Ben Roy Mottelson 1926–2022

Ben R Mottelson passed away on 13 May aged 95. He will be remembered as an outstanding physicist who played a decisive role in the understanding of atomic nuclei and as an inspiring and warm human being with an engaging and outgoing personality.

Ben Mottelson was born in Chicago in 1926 into a family where his father held a university engineering degree. He finished high school in 1944 and was drafted into the navy, which rapidly recognised the young man’s potential and sent him to Purdue University to train as a naval officer. He completed his bachelor degree there in 1947 and subsequently obtained his PhD from Harvard University in 1950 with Julian Schwinger as his supervisor. He won a Sheldon travel fellowship and chose in 1950 to go to the Niels Bohr Institute in Copenhagen, where he was to remain the rest of his life, becoming a Danish citizen in 1971.

After a number of temporary positions, Ben became a permanent member of CERN’s theoretical study group, which was temporarily established in Copenhagen in 1953–1957 while the Geneva site was being completed. He became a tenured professor at Nordita, then the Nordic Institute for Theoretical (Atomic) Physics, at the Niels Bohr Institute in 1957 and headed Nordita from 1981 to 1983.

In Copenhagen, he established a close scientific collaboration and friendship with Aage Bohr (1922–2009), the son of Niels Bohr. The pair worked on understanding the structure of atomic nuclei based on an inter-play between collective and single-particle degrees of freedom which, as first pointed out by James Rainwater, might not all be spherical. A consequence of deformation would be the existence of rotational bands, as for molecules, which were discovered experimentally early in the 1950s using Coulomb excitation with the cyclotron at the Niels Bohr Institute. A central question was why the effective moment of inertia of a deformed atomic nucleus is smaller than for a rigid rotor. This was understood by Aage, Ben and David Pines in 1958 as a consequence of the pairing of nucleons leading to an energy gap, in analogy with the pair correlations between electrons in a superconductor.

In subsequent decades Aage and Ben refined the theoretical description of nuclei with a unified nuclear model that accounted for the variety of nuclear excitations in a coherent fashion, establishing a lively collaboration with experimentalists from all over the world. In 1975 Aage, Ben and James Rainwater were awarded the Nobel Prize in Physics for their work. Ben also received the Atoms for Peace award in 1969.

Ben had a close scientific collaboration and friendship with Aage Bohr, the son of Niels Bohr

The partnership between Aage and Ben was fruitful in spite of their different personalities, Aage being the more reserved and Ben the more outgoing personality. The author of this obituary fondly remembers the pair attending the weekly experimental group meetings and attentively questioning all the speakers, sharing insights and always providing kind inspiration to both young and old. Later, Ben turned his attention to other manifestations of shell structure in mesoscopic systems of atomic clusters and to the properties of cold atomic Bose–Einstein gases. From 1993–1997 he was director of the ECT* theory centre, which he helped establish in Trento, Italy.

Ben Mottelson was an unpretentious, open and engaging family man. Until close to the end he continued to come regularly to the Niels Bohr Institute, attending seminars and scientific events, often to be seen on his bicycle. He will be sorely missed.

Bernard Bigot 1950–2022

Director-general of the ITER Organization, Bernard Bigot, passed away on 14 May, aged 72. An inspirational leader for more than four decades across multiple fields of science and energy, his personal dedication and commitment to ITER over the past seven years shaped every aspect of the project. While his untimely passing will be felt as a tragic blow to the global fusion community, Bigot’s careful design and preparation of the ITER senior management team in recent years gives reassurance for the project’s continued success.

Bigot took the helm at ITER in March 2015 at a critical point in the project’s history, when it was experiencing significant difficulties reflecting the managerial challenges inherent in both its complex engineering and its multinational approach to design, manufacturing and construction. He accepted these challenges with humility and unwavering resolve, proposing a multifaceted plan that transformed the project’s culture. Today, ITER is more than 75% complete and stands as a monumental example of scientific and engineering prowess, and a testimony to the merits of international collaboration.

Trained as a physical chemist at the École normale supérieure, with a PhD in chemistry, Bigot had a deep understanding of the challenges that went with mastering hydrogen fusion. He was a high-ranking university professor at the École normale supérieure de Lyon, which he helped to establish and then directed for several years. The author of more than 70 publications in theoretical chemistry, Bigot was also in charge of research at the École normale supérieure, director of the Institut de recherche sur la catalyse (a CNRS laboratory specialising in catalysis research) and president of the Maison de la Chimie foundation.

The experience he acquired at the highest levels of the scientific and research establishment – as private secretary to ministers, high commissioner for atomic energy, chairman and CEO of the CEA, and as such the principal interface between France and ITER between 2008 and 2015 – had prepared him for the daunting task of leading a 35-nation, long-term endeavour, as unique in its goals as it is in its organisation and governance. However, the uniqueness of ITER required more than experience in science, the management of large institutions and the oversight of complex construction projects. ITER – particularly at the time Bigot took over – also demanded political finesse and diplomatic subtlety, qualities that he had in abundance. Always ready to exchange with media representatives, politicians, economists, VIPs or general visitors, he knew how to make complex subjects understandable and meaningful.

He received numerous awards, including his status as a Commander in the French Order of the Legion of Honour, a Commander in the Royal Swedish Order of the Polar Star, an Officer of the French Order of the National Merit, the holder of the Gold and Silver Star in the Japanese Order of the Rising Sun, and the recipient of the China Friendship Award. Beyond these achievements and accolades, he will be remembered as a visionary leader, intensely focused on the enhancement of global society and the desire to leave the world a better place. The greatest honour we can pay is to continue delivering the ITER project with the same unwavering commitment and dedication that he demonstrated to all of us.

Bernard Bigot was a man of duty and service, who placed loyalty above all virtues, a deeply human leader, as demanding of others as he was of himself. He will be deeply missed.

Boris Lazarevich Ioffe 1926–2022

Leading Soviet particle physicist Boris Ioffe passed away in Moscow on 18 July at the age of 96.

Boris Ioffe was born in Moscow in 1926 into a Jewish family. In the late 1940s he passed Landau’s famous “theoretical minimum” entry exam and in 1949 he graduated frоm Moscow State University with a diploma in theoretical physics. He started his research work under the supervision of Isaak Pomeranchuk. Between 1950 and 1955 Ioffe participated in the original Soviet nuclear-bomb project, at its later stage devoted to the hydrogen bomb. Until 18 July he was the only participant of this project still alive.

In 1960–1980 Ioffe was one of the leading Soviet particle physicists. He was a pioneer of parity (non-)conservation (with Okun and Rudik, 1957). His work with E Shabalin (1967) provided an impetus for the creation of the Glashow–Iliopoulos–Maiani mechanism. Ioffe’s work on deep inelastic scattering (1969) helped establish the Bjorken scaling and the parton model of Feynman–Bjorken.

During the later stages of his career, Ioffe focused on quantum chromodynamics and its consequences for the theory of hadrons. His contributions to the theory of baryons are well-known and appreciated worldwide.

Ioffe never abandoned his early research in nuclear physics. In fact, he was an expert nuclear physicist and in the early 1970s was in charge of the physics design of the first commercial nuclear power plant in former Czechoslovakia. Since 1977 he was the head of the ITEP Laboratory for Theoretical physics, where he had a number of PhD students.

All his life Ioffe was a devoted mountaineer. It is hard to name a mountain peak that he had not conquered. His life journey was long, adventurous and misadventurous simultaneously. Ioffe’s memoirs are available both in Russian and English.

High-energy interactions in Bologna

**Welcome back** Discussions at ICHEP showed the field to be in a fascinating period. Credit: ICHEP

Involving around 1500 participants, 17 parallel sessions, 900 talks and 250 posters, ICHEP2022 (which took place in Bologna from 6 to 13 July) was a remarkable week of physics, technology and praxis. The energy and enthusiasm among the more than 1200 delegates who were able to attend in person was palpable. As the largest gathering of the community since the beginning of the pandemic – buoyed by the start of LHC Run 3 and the 10th anniversary of the Higgs-boson discovery – ICHEP2022 served as a powerful reminder of the importance of non-digital interactions.

Roberto Tenchini’s (INFN Pisa) heroic conference summary began with a reminder: it is 10 years since ICHEP included a session titled “Standard Model”, the theory being so successful that it now permeates most sessions. As an example, he highlighted cross-section predictions tested over 14 orders of magnitude at the LHC. Building on the Higgs@10 symposium at CERN on 4 July, the immense progress in understanding the properties and interactions of the Higgs boson (including legacy results with full Run 2 statistics in two papers by ATLAS and CMS published in Nature on 4 July) was centre stage. CERN Director-General Fabiola Gianotti gave a sweeping tour of the path to discovery and emphasised the connections between the Higgs boson and profound structural problems in the SM. Many speakers highlighted the concomitant role of the Higgs boson in exploring new physics, dashing notions that future precision measurements are “business as usual”. Chiara Mariotti (INFN Torino) pointed out that only 3% of the total Higgs data expected at the LHC has been analysed so far.

Hot topics

Another hot electroweak topic was CDF’s recent measurement of the mass of the W boson, as physicists try to understand what could cause it to lie so far from its prediction and from previous measurements. Andrea Rizzi (Pisa) confirmed that CMS is working hard on a W-mass analysis that will bring crucial information, on a time-scale to be decided. Patience is king with such a complex analysis, he said: “we are really trying to do the measurement the way we want to do it.”

CMS presented a total of 85 parallel talks and 28 posters, including new searches related to b-anomalies with taus, and the most precise measurement of B_s → μ⁺μ^–. Among new results presented by ATLAS in 71 parallel talks and 59 posters were the observation of a four charm–quark state consistent with one seen by LHCb, joint-polarisation measurements of the W and Z bosons, and measurements of the total proton–proton cross section and the ratio of the real vs imaginary parts of the elastic-scattering amplitude. ATLAS and CMS also updated participants on many searches for new particles, in particular leptoquarks. Among highlights were searches by ATLAS for events with displaced vertices, which could be caused by long-lived particles, and by CMS for resonances decaying to Higgs bosons and pairs of either photons or b quarks, which show interesting excesses. “Se son rose fioriranno!” said Tenchini.

The sigmas are rather higher for exotic hadrons. LHCb presented the discovery of a new strange pentaquark (with a minimum quark content ccuds) and two tetraquarks (one corresponding to the first doubly charged open-charm tetraquark with csud), taking the number of hadrons discovered at the LHC so far to well over 60, and introducing a new exotic-hadron naming scheme for “particle zoo 2.0” (Exotic hadrons brought into order by LHCb). LHCb also reported the first evidence for direct CP violation in the charm system (LHCb digs deeper in CP-violating charm decays) and a new precise measurement of the CKM angle γ. Vladimir Gligorov (LPNHE) described how, in addition to the flavour factories LHCb and Belle II, experiments including ATLAS, CMS, BESIII, NA62 and KOTO will be crucial to enable the next level of understanding in quark mixing. Despite no significant new results having been presented, the status of tests of lepton flavour universality (LFU) in B decays by LHCb generated lively discussions, while Toshinori Mori (Tokyo) described exciting prospects for LFU tests in charged-lepton flavour experiments, in particular MEG-II, which has just started operations at PSI, and the upcoming Mu2e and MUonE experiments.

ICHEP2022 served as a powerful reminder of the importance of non-digital interactions

Moving to leptons that are known to mix, neutrinos continue to play very important roles in understanding the smallest and largest scales, said Takaaki Kajita (Tokyo) via a link from the IUPAP Centennial Symposium taking place in parallel at ICTP Trieste. Status reports on DUNE, Hyper-K, JUNO, KM3NeT and SNB showed how these detectors will help constrain the still poorly-known PNMS matrix that describes leptonic mixing, while new results from NOvA and STEREO further reveal anomalous behaviour. Among the major open questions in neutrino physics summed-up by theorist Joachim Kopp (Mainz and CERN) were: how do neutrinos interact? What explains the oscillation anomalies? And how do supernova neutrinos oscillate?

Several plenary presentations showcased the increasing complementarity with astroparticle physics and cosmology, with the release of the first-science images from the James Webb Space Telescope on 12 July adding spice (Webb opens new era in observational astrophysics). Multiband gravitational-wave astronomy across 12 or more orders of magnitude in frequency will bloom in the next decade, predicted Giovanni Andrea Prodi (Trento), while larger datasets and synchronisation of experiments offer a bright future in all messengers, said Gwenhael De Wasseige (Louvain): “We are just at the beginning of the story.” The first results from the Lux–Zeplin experiment were presented, setting the tightest limits on spin-independent WIMP–nucleon cross-sections for WIMP masses above 9 GeV (CERN Courier September/October 2022 p13), while the increasingly crowded plot showing limits from direct searches for axions illustrate the vibrancy and shifting focus of dark-matter research. Indeed, among several sessions devoted to the exploration of high-energy QCD in heavy-ion, proton–lead and proton–proton collisions, Andrea Dainese (INFN Padova) described how the LHC is not only a collider of nuclei but an (anti-)nuclei factory relevant for dark-matter searches.

The unique ability of theorists to put numerous results and experiments in perspective was on full display. We should all renew the enthusiasm that built the LHC, and be a lot more outspoken about the profound ideas we explore, urged Veronica Sanz (Sussex); after all, she said, “we are searching for something that we know should be somewhere.” A timely talk by Gavin Salam (Oxford) summarised the latest understanding of QCD effects relevant to the muon g-2 and W-mass anomalies and also to future Higgs-boson measurements, concluding that, as we approach high precision, we should expect to be confronted by conceptual problems that we could, so far, ignore.

The unique ability of theorists to put numerous results and experiments in perspective was on full display

Accelerators (including a fast-paced summary of the HL-LHC niobium-tin magnet programme from Lucio Rossi), detectors (68 talks and posters revealing an increasingly holistic approach to detector design), computing (highlighting a period of rapid evolution thanks to optimisation, modernisation, machine-learning algorithms and increasing hardware diversity), industry, diversity and outreach were addressed in detail. A highly acclaimed outreach event in Bologna’s Piazza Maggiore on the evening of 12 July saw thousands of people listen to Fabiola Gianotti, Guido Tonelli, Gian Giudice and Antonio Zoccoli discuss the implications of the Higgs-boson discovery.

Only the narrowest snapshot of proceedings is possible in such a short report. What was abundantly clear from ICHEP2022 is that, following the discovery of the Higgs boson and as-yet no new particles beyond the SM, the field is in a fascinating and challenging period where confusion is more than matched by confidence that new physics must exist. The strategic direction of the field was addressed in two wide-ranging round-table discussions where laboratory directors and senior physicists answered questions submitted by participants. Much discussion concerned future colliders, and addressed a perceived worry in some quarters that the field is entering a period of decline. For anyone following the presentations at ICHEP2022, nothing could be further from the truth.

Your guide to becoming a CERN guide

**Going underground** Bryan Pérez Tapia in the ATLAS cavern in December 2021. Credit: B Pérez Tapia

Do you remember the first time you heard about CERN? The first time someone told you about that magical place where bright minds from all over the world work together towards a common goal? Perhaps you saw a picture in a book, or had the chance to visit in person as a student? It is experiences like these that motivate many people to pursue a career in science, whether in particle physics or beyond.

In 2016 I had the pleasure of visiting CERN on a school trip. We toured the Synchrocyclotron and the SM18 magnet test facility. I was hooked. The tour guides talked with passion about the laboratory, the film presenting CERN’s first particle accelerator and the laboratory’s mission, and all those big magnets being tested in SM18. It was this experience that motivated me to study physics at university and to try to come back as soon as I could.

Accreditation

That chance arrived in September 2021 when I started a one-year technical studentship as editorial assistant on the Courier. From the first day I was eager to see as much as I could. During the final months of Long Shutdown 2, my supervisor and I visited the ATLAS cavern. The experience motivated me to ask one of my newly made friends, also a technical student who had recently become a tour guide, how to apply. The process was positive and efficient. After completing all the required courses from the learning hub and shadowing experienced guides, I became a certified ATLAS underground guide in November 2021 and gave my first tour soon after. I was nervous and struggled with the iris scanner when accessing the cavern, but all ended well, and further tours were scheduled. Then, in mid-December, all in-person tours were cancelled due to COVID-19 restrictions. I needn’t have worried, as CERN was fully geared up to provide virtual visits. Among my first virtual audience members were students from the high school that brought me to CERN five years earlier and from my university, Nottingham Trent in the UK.

The most satisfying thing is people’s enthusiasm and their desire to learn more about CERN and its mission

The virtual visits were quite challenging at first. It was harder to connect with the audience than during an in-person visit. But managing these difficulties helped me to improve my communication skills and to develop self-confidence. During this period, I conducted more than 10 virtual visits for different institutes, universities, family and friends, in both English and Spanish.

At the beginning of March 2022, CERN moved into “level yellow” and in-person visits were resumed. Although only possible for a short period, I had the chance to guide visitors underground and had the honour of guiding the last in-person visit into the ATLAS cavern on 23 March before preparations for LHC Run 3 got under way. With the ATLAS cavern then off-limits, I signed up to present at as many CERN visit points as possible. At the time of writing, I am a guide for the Synchrocyclotron, the ATLAS Visitor Centre, Antimatter Factory, Data Centre, Low Energy Ion Ring and CERN Control Centre.

Get involved

The CERN visits service always welcomes new guides and is working towards opening new visit points. Anyone working at CERN or registered as a user can take part by signing up for visit-point training on the tour-guide website: guides.web.cern.ch. General training for new guides is also available. All you need to show CERN to the public is passion and enthusiasm, and you can sign up for as many or as few as your day job allows. Diversity is encouraged and those who are multilingual are also highly valued.

Today, visits are handled by a dedicated section in the Education, Communications and Outreach group. The number of visitors has gradually increased over recent years, with 152,000 annual visitors before the pandemic started, excluding special events such as the CERN Open Days. The profile of visitors ranges from school pupils and university students to common-interest groups such as engineers and scientists, politicians and VIPs, and people with a wide range of interests and educational levels.

The benefits of becoming a CERN guide are immense. It gives you access to areas that would otherwise not be possible, the chance to experience important events in-person and to see your work at CERN, whatever it involves, from a fresh perspective. My personal highlight was watching test collisions at 13.6 TeV before the official start of Run 3 while showing Portuguese high-school students the ATLAS control room. The most satisfying thing is people’s enthusiasm and their desire to learn more about CERN and its mission. I particularly remember how a small child asked me a question about the matter–antimatter asymmetry of the universe, and how another young visitor ran from Entrance B at the end of a tour just to tell me how much she loved the visit.

The visits service makes it as easy as possible to get involved, and exciting times for guides lie ahead with the opening of the CERN Science Gateway next year, which will enable CERN to welcome even more visitors. If a technical student based at CERN for just one year can get involved, so can you!

Jet-energy corrections blaze a trail

**Fig. 1.** The measurement of particle-flow jet to particle-jet momentum ratio (or response) with multiple different final states, combining two complementary techniques (DB+MPF) to explicitly account for biases from initial- and final-state radiation. The ratio between data and simulation is shown after accounting for systematic biases in a global fit. Source: CERN-CMS-DP-2021-033

Understanding hadronic final states is key to a successful physics programme at the LHC. The quarks and gluons flying out from proton–proton collisions instantly hadronise into sprays of particles called jets. Each jet has a unique composition that makes their flavour identification and energy calibration challenging. While the performance of jet-classification schemes has been increased by the fast-paced evolution of machine-learning algorithms, another, more subtle, revolution is ongoing in terms of precision jet-energy corrections.

CMS physicists have taken advantage of the data collected during LHC Run 2 to observe jets in many different final states and systematically understand their differences in detail. The main differences originate from the varying fractions of gluons making up the jets and the different amounts of final-state radiation (FSR) in the events, causing an imbalance between the leading jet and its companions. The gluon uncertainty was constrained by splitting the Z+jet sample by flavour, using a combination of quark–gluon likelihood and b/c-quark tagging, while FSR was constrained by combining the missing-E_T projection fraction (MPF) and direct balance (DB) methods. The MPF and DB methods have been well established at the LHC since Run 1: while in the DB method the jet response is evaluated by comparing the reconstructed jet momentum directly to the momentum of the reference object, the MPF method considers the response of the whole hadronic activity in the event, recoiling versus the reference object. Figure 1 shows the agreement achieved with the Run 2 data after carefully accounting for these biases for samples with different jet-flavour compositions.

Precise jet-energy corrections are critical for some of the recent high-profile measurements by CMS, such as an intriguing double dijet excess at high mass, a recent exceptionally accurate top-quark mass measurement, and the most precise extraction of the strong coupling constant at hadron colliders using inclusive jets.

The expected increase of pileup in Run 3 and at the High-Luminosity LHC will pose additional challenges in the derivation of precise jet-energy corrections, but CMS physicists are well prepared: CMS will adopt the next-generation particle-flow algorithm (PUPPI, for PileUp Per Particle Id) as the default reconstruction algorithm to tackle pileup effects within jets at the single-particle level.

Jets can be used to address some of the most intriguing puzzles of the Standard Model (SM), in particular: is the SM vacuum metastable, or do some new particles and fields stabilise it? The top-quark mass and strong-coupling-constant measurements address the former question via their interplay with the Higgs-boson mass, while dijet-resonance searches tackle the latter.

Underlying these studies are the jet-energy corrections and the awareness that each jet flavour is unique.

J/ψ photoproduction in hadronic PbPb collisions

ALICE figure 1 — **Fig. 1.** J/ψ photoproduction-cross section as a function of the mean number of nucleons participating in hadronic PbPb interactions, <N_part>, at a centre-of-mass energy of 5.02 TeV.
The results are compared with theory predictions from a UPC-like calculation accounting for the nuclear overlap. Source: arXiv:2204.10684

Photon-induced reactions are regularly studied in ultra-peripheral nucleus–nucleus collisions (UPCs) at the LHC. In these collisions, the accelerated ions, which carry a strong electromagnetic field, pass by each other with an impact parameter (the distance between their centres) larger than the sum of their nuclear radii. Hadronic interactions between nuclei are therefore strongly suppressed. At LHC energies, the photoproduction of charmonium (a bound state of charm and anti-charm quarks) in UPCs is sensitive to the gluon distributions in nuclei over a wide low Bjorken-x range. In particular, in coherent interactions, the photon emitted by one of the nuclei couples to the other nucleus as a whole, leaving it intact, while a J/ψ meson is emitted with a characteristic low transverse momentum (p_T) of about 60 MeV, which is roughly of the order of the inverse of the nuclear radius.

Surprisingly, in 2016 ALICE measured an unexpectedly large yield of J/ψ mesons at very low p_T in peripheral, not ultra-peripheral, PbPb collisions at a centre-of-mass energy of 2.76 TeV. The excess with respect to expectations from hadronic J/ψ-meson production was interpreted as the first indication of coherent photoproduction of J/ψ mesons in PbPb collisions with nuclear overlap. This effect comes with many theoretical challenges. For instance, how can the coherence condition survive in the photon–nucleus interaction if the latter is broken up during the hadronic collision? Do only the non-interacting spectator nucleons participate in the coherent process? Can the photoproduced J/ψ meson be affected by interactions with the formed and fast-expanding quark–gluon plasma (QGP) created in nucleus–nucleus collisions? Recent theoretical developments on the subject are based on calculations for UPCs in which the J/ψ meson photoproduction-cross section is computed as the product of an effective photon flux and an effective photonuclear cross section for the process γPb → J/ψPb, with both terms usually modified to account for the nuclear overlap.

The ALICE experiment has recently measured the coherently photoproduced J/ψ mesons in PbPb collisions at a centre-of-mass energy of 5.02 TeV, using the full Run 2 data sample. The measurement is performed at forward rapidity (2.5 < y < 4) in the dimuon decay channel. For the first time, a significant (> 5σ) coherently photoproduced J/ψ-meson signal is observed even in semi-central PbPb collisions. In figure 1, the coherently photoproduced J/ψ cross section is shown as a function of the mean number of nucleons participating in the hadronic interaction (<N_part>). In this representation, the most central head-on PbPb collisions correspond to large <N_part> values close to 400. The photoproduced J/ψ cross section does not exhibit a strong dependence on collision centrality (i.e. on the amount of nuclear overlap) within the current experimental precision. A UPC-like model (the red line in figure 1) reproduces the semi-central to central PbPb data if a modified photon flux and photonuclear cross section to account for the nuclear overlap are included.

To clarify the theory behind this experimental observation of coherent J/ψ photoproduction, the upcoming Run 3 data will be crucial in several aspects. ALICE expects to collect a much larger data sample, thereby measuring a statistically significant signal in most central collisions. At midrapidity, the larger data sample and the excellent momentum resolution of the detector will allow for p_T-differential cross-section measurements, which will shed light on the role of spectator nucleons for the coherence condition. By extending the coherently photo-produced J/ψ cross-section measurement towards most central PbPb collisions, ALICE will study the possible interaction of these charmonia with the QGP. Photoproduced J/ψ mesons could therefore turn out to be a completely new probe of the charmonium dissociation in the QGP.

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