The wonder and awe that we sense when we look at the starry skies is a major motivation to do science. Both Plato (Theaetetus 155d) and Aristotle (Metaphysics 982b12) wrote that philosophy starts in wonder. Plato went even further to declare that the eye’s primary purpose is none other than to see and study the stars (Timaeus 47c). But wonder and awe also play a wider role beyond science, and are fundamental to other endeavours of human civilisation, such as religion. In Wonderstruck: How Wonder and Awe Shape the Way We Think, Helen De Cruz (Saint Louis University) traces the relationship between wonder and awe and philosophy, religion, magic and science, and the development of these concepts throughout history.
Essential emotion
De Cruz’s book is rich in content, drawing from psychology, anthropology and literature. Aptly for particle physicists, she points out that it is not only the very largest scales that fill us with awe, but also the very smallest, as for example in Robert Hooke’s Micrographia, the first book to include illustrations of insects and plants as seen through a microscope. Everyday things may be sources of wonder, according to philosopher and rabbi Abraham J Heschel, who has written on religion as a response to the awe that we feel when we look at the cosmos. Even hard-nosed economists recognise the fundamental role of wonder, she observes: Adam Smith, the famous economist who wrote The Wealth of Nations, believed that wonder is an essential emotion that underlies the pursuit of science, as it prompts people to explore the unknown and seek knowledge about the world. Although particle physics is not mentioned explicitly in the book – the closest instance is a quote from Feynman’s Lectures on Physics – the implications are clear. And while the sources quoted are mostly Western, other traditions are not ignored, with references to Chinese and Japanese culture present, among others.
The book also motivates questions that it does not address, some of which are especially interesting for fundamental physics. For example, modern human beings who live and work in cities spend most of their lives in an environment that alienates them from nature, and nature-induced awe must compete with technology-driven amazement. One can maybe glimpse that in outreach, where curiosity about technology sometimes, though not always, eclipses interest about the fundamental questions of science. While the book discusses this topic in the context of climate change – a reality that reminds us that we cannot ignore nature – there is more one can do with respect to the effects of such an attitude in motivating fundamental science.
At a time when large scientific projects, such as CERN’s proposed Future Circular Collider, are being considered, generating a lot of discussions about cost and benefit, this book reminds us that the major motivation of a new telescope or collider is to push into the frontiers of the unknown – a process that starts and finishes with wonder and awe. As such, the book is very useful reading for scientists doing fundamental research, especially those who engage with the public.
Neutrino physics requires baselines both big and small, and neutrinos both artificial and astrophysical. One of the most prominent experiments of the past two decades is Tokai-to-Kamioka (T2K), which observes electron–neutrino appearance in an accelerator-produced muon–neutrino “superbeam” travelling coast to coast across Japan. To squeeze systematics in their hunt for leptonic CP violation, the collaboration recently brought online an upgraded near detector.
“The upgraded detectors are precision detectors for a precision-physics era,” says international co-spokesperson Kendall Mahn (Michigan State). “Our current systematic constraint is at the level of a few percent. To make progress we need to be able to probe regions we’ve not probed before.”
T2K studies the oscillations of 600 MeV neutrinos that have travelled 295 km from the J-PARC accelerator complex in Tokai to Super-Kamiokande – a 50 kton gadolinium-doped water-Cherenkov detector in Kamioka that has also been used to perform seminal measurements of atmospheric neutrino oscillations and constrain proton decay. Since the start of data taking in 2010, the collaboration made the first observation of the appearance of a neutrino flavour due to quantum-mechanical oscillations and the most precise measurement of the θ23 parameter in the neutrino mixing matrix. As well as placing limits on sterile-neutrino oscillation parameters, the collaboration has constrained a wide range of the parameters that describe neutrino interactions with matter. The uncertainties of such measurements typically limit the precision of fits to the fundamental parameters of the three-neutrino paradigm, and constraining neutrino-interaction systematics is the main purpose of near detectors in superbeam experiments such as T2K and NOvA, and the future ones Hyper-Kamiokande and DUNE.
T2K’s near-detector upgrade improves the acceptance and precision of particle reconstruction for neutrino interactions. A new fine-grained “SuperFGD” detector (see pink rectangle, left, on “New and improved” image) serves as the target for neutrino interactions in the new experimental phase. Comprised of two million 1 cm3 cubes of scintillator strung with optical fibres, SuperFGD lowers the detection threshold for protons ejected from nuclei to 300 MeV/c, improving the reconstruction of neutrino energy. Two new time-projection chambers flank it above and below to more closely mimic the isotropic reconstruction of Super-Kamiokande. Finally, six new scintillator planes suppress particle backgrounds from outside the detector by measuring time of flight.
Following construction and testing at CERN’s neutrino platform, the new detectors were successfully integrated in the experiment’s global DAQ and slow-control system. The first neutrino-beam data with the fully upgraded detector was collected in June, with the collaboration also benefitting from an upgraded neutrino beam with 50% greater intensity. Beam intensity is set to increase further in the coming years, in preparation for commissioning the new 260 kton Hyper-Kamiokande water Cherenkov detector. Cavern excavation is underway in Kamioka, with first data-taking planned for 2027.
But much can already be accomplished in the new phase of the T2K experiment, says the team. As well as improving precision on θ23 and another key mixing parameter Δm223, and refining the theoretical models used in neutrino generators, T2K will improve its fit to δCP, the fundamental parameter describing CP violation in the leptonic sector. Measuring its value could shed light on the question of why theuniverse is dominated by matter.
“T2K’s current best fit to δCP is –1.97,” says Mahn. “We expect to be able to observe leptonic CP violation at 3σ significance if the true value of δCP is –π/2.”
This textbook for advanced undergraduate and graduate students, written by experimental particle-physicist Pascal Paganini of Ecole Polytechnique, aims to teach Standard Model calculations of quantities that are relevant for modern experimental research. Each chapter ends with a collection of unsolved problems to help the student practice the discussed calculations. The level is similar to the well-known textbook Quarks and Leptons by F Halzen and A D Martin (Wiley, 1984), but with a broader introduction and including more up-to-date material. The notation is also similar, and shared with several other popular textbooks at the same level, making it easy for students to use it along with other resources.
Comprehensive
Fundamentals of Particle Physics starts with a general introduction that is around 50 pages long and includes information on detectors and statistics. It continues with a recap of relativistic kinematics, quantum mechanics of angular momentum and spin, phase–space calculations for cross sections and decays as well as symmetries. The main part of the book begins with a discussion of relativistic quantum mechanics, covering the equations of motion of spin 0, 1 and ½ particles along with a detailed description of Dirac spinors and their properties. Then, it addresses quantum electrodynamics (QED), including the QED Lagrangian, standard QED cross-section calculations and a section dedicated to magnetic moments (g-2). About 100 pages are devoted to hadronic physics: deep inelastic scattering, parton model, parton-distribution functions and quantum chromodynamics (QCD). Calculations in perturbative QCD are discussed in some detail and there is also an accessible section in non-perturbative QCD that can serve as a very nice introduction to beginner graduate students.
The book continues with weak interactions, covering the Fermi theory, W-boson exchange, CKM matrix, neutrinos, neutrino mixing and CP-violation. The following chapter presents the electroweak theory and introduces gauge-boson interactions. A dedicated chapter is reserved for the Higgs boson. This includes a nice section about the discovery of the particle and the measurements that are performed at the LHC, as well as some comments about the pre-history (LEP and Tevatron) and the future (HL-LHC and FCC). A clear discussion about naturalness and several other conceptual issues offers a light and useful read for students of any level. The final chapter goes through the Standard Model as a whole, including a very useful evaluation of its successes and weaknesses. In terms of beyond-Standard Model physics, only dark matter and neutrino masses are covered.
Although this is not a quantum field-theory textbook, some of its elements are introduced; in particular second quantisation, S-matrix, Dyson’s expansion and a few words about renormalisation are included. These are very useful in bridging the gap between practical calculations and their theoretical background, also serving as a quick reference.
There are several useful appendices, most notably a 30-page introduction to group theory that can serve as a guide for a short standalone course in the subject or as a quick reference. The book also includes elements of the Lagrangian formalism, which could have been a bit more expanded to include a more detailed presentation of Noether’s theorem, probably in an additional appendix.
Overall the book achieves a good balance between calculations and more conceptual discussions. All students in the field can benefit from the sections on the Higgs-boson discovery and the Standard Model. Being concise and not too long, Fundamentals of Particle Physics can easily be used as a primary or secondary textbook for a particle-physics course that introduces calculations using Feynman diagrams in the Standard Model to students.
Ilario Boscolo, who was one of the proponents of the AEgIS experiment at CERN, passed away on 16 April 2024 at the age of 84.
Ilario Boscolo was born in Codevigo, Italy in 1940 and graduated from the nearby University of Padua. In 1968 he joined the University of Lecce, where he initiated research in accelerator physics, high-intensity electron beams and free electron lasers (FELs), and far-infrared and CO2 lasers. Among his important scientific contributions at that time were the development of a prototype electrostatic accelerator, investigations on far-infrared lasers optically pumped in a cavity, and a much-cited theoretical proposal for a two-stage FEL for coherent harmonic amplification (an optical klystron). Ilario spent long periods of study in international research institutes, including the ENEA fusion energy centre in Frascati and the University of California Santa Barbara, where he collaborated with world-leading FEL researchers Luis Elias and William Colson.
In 1987 Ilario was called to the University of Milan, where he became full professor, to participate in the INFN project ELFA (electron laser facility for acceleration) and was responsible for the photocathode emission. His interest then turned to other topics, including efficient electron sources based on field emission from carbon nanotubes or ferroelectric ceramics and, within CERN, pulsed laser phase coding systems for new acceleration facilities. Within the INFN SPARC–SPARX initiative, started in 2003 and based in Frascati, he focused on laser applications for the development of pulsed, high-brightness UV and X-ray FEL sources. In particular, he showed that the high beam quality of the electron sources depends on suitable shaping of comb laser pulses, the study of which was realised in a dedicated laser laboratory at Milan founded by Ilario.
In 2007 Ilario was one of the proponents of the AEgIS experiment at the CERN Antiproton Decelerator, which aimed to investigate the properties of antimatter, in particular its gravitational interactions. This required the production of a low-energy beam of antihydrogen atoms, obtained by a charge-exchange process with positronium atoms laser-excited at Rydberg levels. Led by Ilario, the Milan laser laboratory was responsible for the laser system that was required to make this pairing possible. AEgIS demonstrated the first pulsed-production of antihydrogen atoms in 2018, enabling a series of antimatter studies that are ongoing.
In all his activities, Ilario showed great passion and enthusiasm for both science and its applications. This positive attitude was also widely displayed through his didactical activity in various courses at the University of Milan. He was responsible for a new physics laboratory for the biology programme and for the laser laboratory for the physics programme. In addition, his greatest success was the complete reconstruction of the general physics laboratory for first-year students. By encouraging students to practice and elaborate on their own, with only little guidance from the teacher, this laboratory left an indelible mark on their training as physicists.
Another strong passion of Ilario was civil commitment, reflected in his constant engagement with university governance and studies of politics and economics, to which he dedicated himself with his usual inexhaustible enthusiasm, particularly after his retirement.
Ilario is remembered by his collaborators and students as a person of great culture, of brilliant insights, of a willingness to discuss physics and politics with anyone, and as an exquisite friend. He was a true scientist, leaving a deep mark on physics and a bright memory for everyone who had the honour of knowing him.
Alec Hester, a former editor of CERN Courier and later physics subject specialist at the CERN library for nearly 30 years, passed away in Geneva on 9 March at the age of 96.
Born in Hatfield, to the north of London, in 1928, Alec graduated in physics from Imperial College London in 1949. He continued there for his PhD, building a Van de Graaff accelerator to study (p, alpha) reactions in light nuclei. Yes, in those days postgraduate students built their own accelerators! One of his older fellow students was Don Perkins, who passed away in 2022.
In 1952 Alec interrupted his studies to take a job in the publicity department of General Electric at its site in Kent, England. Nine years later he came to CERN to take over the editorship of CERN Courier from Roger Anthoine. The Courier was then just two years old, and it was during Alec’s period as editor that it began to move beyond its initial role as the house journal for CERN staff to one that communicated the work of CERN and other laboratories to a wider scientific and technical readership. Marking the end of Alec’s editorship in the December 1965 issue, Anthoine wrote: “The editing and production of our periodical, with limited means, requires not only very definite intellectual qualities, for collecting and processing information from all over the Laboratory, but also considerable physical and moral toughness to cope with the many dictates of production, which are the lot of every editor… It is mainly thanks to [Alec’s] drive that CERN Courier, which now has a circulation of 6000 copies (French and English versions combined), has risen from the rank of ‘internal information journal’ to that of ‘world spokesman for European sub-nuclear physics’.”
In 1966 Alec moved to the CERN scientific information service as the physics subject specialist, remaining there until his retirement in February 1993. His accurate and painstaking work developing the library’s bibliographic databases provided the nucleus for those searchable on the CERN Document Server today.
Alec leaves behind Annemarie, his wife for over 70 years, his daughters Barbara and Dagmar, and his four grandchildren.
Renowned experimental physicist and co-initiator of relativistic heavy-ion physics, Rudolf Bock, passed away on 9 April 2024 aged 96.
Rudolf Bock was born in Mannheim, Germany in May 1927 and obtained his diploma in physics from the University of Heidelberg in 1954. He conducted his doctoral thesis on deuteron-induced nuclear reactions at the cyclotron of the Max Planck Institute for Medical Research in Heidelberg and received his doctorate from Heidelberg University in 1958. He then investigated nuclear reactions at the newly founded MPI for Nuclear Physics (MPIK) at the tandem accelerators there, initially with light ions and from 1963 with heavier ions.
In 1967 he was appointed full professor at the University of Marburg and was involved in the development of a joint accelerator project for heavy-ion research, ultimately leading to the UNILAC accelerator project. On 17 December 1969, the research centre GSI (Gesellschaft für Schwerionenforschung) was founded in Darmstadt–Wixhausen. As one of its founding fathers and subsequently as a long-standing member of the GSI board of directors, Rudolf Bock played a decisive role in the development of nuclear physics with heavy ions. At the same time, he maintained his contacts with Heidelberg as an honorary professor and as an external scientific member of the MPIK. In 2000 he was awarded an honorary doctorate from Goethe University Frankfurt.
Research with relativistic heavy-ion beams soon led to great successes. From 1974 Rudolf Bock established a working group at GSI under the leadership of Hans Gutbrod and Reinhard Stock, who set up and successfully carried out two major experiments at the Berkeley Bevalac accelerator. These resulted in the discovery of compressed, hot nuclear matter with hydrodynamic flow behaviour and thus formed the basis for his later experiments on quark–gluon plasma at CERN.
From the mid-1980s, the heavy-ion synchrotron SIS18 was set up at GSI under the leadership of director Paul Kienle. Thanks to Rudolf Bock’s guidance and in cooperation with surrounding universities, three new experiments (FOPI, KAOS and TAPS) were created, which focused on the formation of compressed nuclear matter as well as on hadron production and in particular the formation of light atomic nuclei. Around the same time, he was working on plans for experiments at much higher energies, which could ultimately only be realised at the CERN
SPS accelerator, with decisive contributions from GSI and LBL Berkeley. This led to the development of today’s global programme in ultra-relativistic nuclear–nuclear collisions, which has been pursued since the 1990s at the AGS and SPS, from 2000 with four experiments at RHIC and, since 2010, has been led by ALICE at the LHC at the highest energies.
The cooperation between GSI and LBL Berkeley was not only the beginning of relativistic heavy-ion physics. Supported by Hermann Grunder, then head of the LBL accelerator department, Rudolf Bock started the inertial confinement fusion programme in Germany. He also laid an important foundation for ion-beam therapy by supporting the secondment of Gerhard Kraft from GSI to the cancer-therapy programme at LBL. After his retirement in December 1995, Rudolf Bock maintained his scientific activities at GSI, his primary interest being the development of experiments on plasma physics and inertial-confinement fusion with high-intensity ion and laser beams.
Throughout the course of his scientific career, Rudolf Bock established numerous new research collaborations with institutes in Germany and abroad. As he himself had taken part in the Second World War and had spent several years as a prisoner of war in Russia, the idea of international understanding and peacekeeping was an important concern for him. As early as 1969 he invited many Russian scientists to the nuclear-physics conference at MPIK, and from the 1970s he promoted many collaborations between GSI and Russian institutes. He also pushed for Russia to become the largest member state in the GSI/FAIR project. The Russian invasion of Ukraine in February 2022 was therefore a great disappointment for him and for all of us.
Rudolf Bock was regularly present at GSI until his last days and continued to take an interest in current research and developments on campus. His advice and foresight will be sorely missed.
Theoretical physicist Werner Rühl died on 31 December 2023 in Füssen, Germany at the age of 86.
He was born in 1937, at a time when theoretical physics in Germany was being destroyed by the Nazis. After the Second World War, the ongoing study of cosmic rays and the availability of higher energies from accelerators made particle physics the most interesting field for budding researchers like him. Part of the way ahead was obvious: learn from the US and profit from the new spirit of European unity embodied by the creation of CERN.
Rühl followed this path in the straightest possible way. He obtained his PhD in 1962 in Cologne and became a research associate at CERN in 1964. Two years later he took up a postdoc at Rockefeller University, New York, before returning to CERN as a staff member in 1967, and obtained a chair in 1970 at the newly founded University of Kaiserslautern.
A more difficult decision concerned mathematics, for which many experimentalists had little regard. Initially, Einstein had shared this attitude, but then he worked hard on Riemannian geometry to understand gravity. Heisenberg’s successes were based on deep mathematics, too, but he tried his best to limit its scope. SU(2) and the analogy between spin and isospin were fundamental, and the representation theory of SU(2) had been fully explored in the context of atomic physics. Dirac’s understanding of spinors and his introduction of the delta distribution opened the way for a thorough investigation of non-compact groups like SL(2,C). This allowed us to break the wall between maths and physics, which happened initially in the Soviet Union. Rühl was very aware of this fact and was deeply impressed by the work of Israel Moiseevich Gelfand. In winter 1967/1968 he gave a series of lectures on this topic for the academic training programme at CERN, which in 1970 became the core of his book The Lorentz group and harmonic analysis. A mathematical fruit was his elementary proof of the Plancherel theorem for classical groups, published in 1969.
Rühl’s appointment to a chair at Kaiserslautern was a happy choice for both sides. Internationally recognised professors like Rühl had adequate resources for students, visitors and conferences, and four theory colleagues were hired between 1970 and 1973. In 1983–1985 Rühl was chairman of the physics department and member of the university senate. He published good papers with his PhD students and supported the global development of science, in particular through his work with postdocs from Oran University. For many years he also worked as a mentor for gifted students from all faculties for the prestigious Studienstiftung des deutschen Volkes scholarship foundation.
Despite his dominating affinity for mathematics, Rühl maintained an interest in experimental physics and occasionally published related work. His understanding of Russia facilitated successful collaborations with outstanding colleagues who had moved to the West, with some of his most important contributions stemming from his collaborations with colleagues from Yerevan. After his retirement in 2004 he continued to publish as before. Eleven years later he moved to Füssen near the Alps. For five years he could enjoy his passion for skiing, before an accident impaired his health.
Werner Rühl always had an open mind for new developments. He had studied the large-N behaviour of theories with symmetries like O(N) and did respected work on lattice theories. From the 1980s, his citation rate increased more and more – a tendency that lasted way beyond his retirement. The original take on AdS/CFT duality in the context of O(N) sigma models and high-spin theories stands out the most. At the end of his life it must have been a great satisfaction for Werner Rühl to watch the ripening of these late fruits.
Atsuhiko Ochi, a brilliant, passionate detector and experimental physicist, passed away on 29 April 2024 at the untimely age of 54. A source of innovative ideas at the forefront of radiation detectors, he made outstanding contributions to the development of micropattern gaseous detectors (MPGDs) that are recognised worldwide. He was also a distinguished lecturer whose inexhaustible passion, dedication and remarkable character captivated the many students he mentored.
Atsuhiko began his research at the Tokyo Institute of Technology, initially focusing on large-area avalanche photodiodes as fast photon and soft X-ray detectors. In 1998 he defended his PhD thesis “Study of Micro Strip Gas Chamber as a Time-Resolved X-ray Area Detector”, earning the second High Energy Physics Young Researcher’s Award from the Japan Association of High Energy Physicists. In 2000, alongside Toru Tanimori, he introduced the micro pixel chamber (micro-PIC), a new gaseous detector for X-ray, gamma-ray and charged-particle imaging. It was fully developed using printed circuit board technology and free of floating structures like wires, mesh or foils, featuring a pin-shaped anode surrounded by a ring-shaped cathode.
In 2001 Atsuhiko moved to Kobe University, where he joined the ATLAS experiment and devoted his efforts to commissioning the ATLAS thin gap chambers (TGCs). He was also in charge of integrating the front-end electronics on the KEK TGC detectors and of detector quality assurance and control. Later, at CERN, he led the acceptance quality control of the ATLAS TGCs.
Atsuhiko could always merge his love for experiments with a passion for new ideas. “We need new ‘eyes’ to catch a glimpse of science’s frontier”, he once said. Along with his group in Kobe, while making significant contributions in ATLAS to the design and construction of the new large resistive micromegas for the Muon New Small Wheel, he conducted R&D on the use of sputtered layers of diamond-like carbon (DLC) as resistive elements to quench discharges and played a crucial role in connecting with Japanese industry. He was among the first to test the technology with micromegas, apply it to the micro-PIC detector, and pioneer its use as electrodes for the novel resistive plate chambers he proposed for the MEG II experiment. He supported the use of DLC in the final TPC micromegas of the near detectors of the T2K experiment while serving as a liaison person with BE-Sput in Kyoto. DLC is now the predominant approach in most new resistive MPGD detectors.
In his research, Atsuhiko always placed great emphasis on mentoring students and giving them access to a worldwide community of experts, facilities and experiments. He meticulously shared all relevant research conducted by Japanese colleagues, ensuring proper visibility and recognition for his community. This has been crucial in the international RD51 collaboration on MPGD technologies, within which he played a significant role in its formation and management. During the transition from the MPGD-based RD51 collaboration to the upcoming DRD1, which encompasses a broader scope of technologies and applications, Atsuhiko made a crucial contribution by maintaining strong ties with the Asian community.
Atsuhiko’s vibrant enthusiasm and infectious smile leave an irreplaceable void. His departure is a profound loss, leaving behind a loving wife and two children.
Within CERN circles, Armin Hermann is mainly known as one of the co-editors of the authoritative History of CERN volumes covering the period from the beginnings of the Organization up to 1965. But he did so much more in the field of the history of science.
Armin Hermann was born on 17 June 1933 in Vernon, British Columbia, Canada and grew up in Upper Bavaria in Germany. He studied physics at Ludwig Maximilian University in Munich and obtained his doctorate in theoretical physics in 1963 with a dissertation on the “Mott effect for elementary particles and nuclei of electromagnetic structure”. He worked for a few years at DESY and performed synchrotron-oscillation calculations with an IBM 650 computer. Subsequently, Hermann decided to change his focus from physics proper to its history, which had preoccupied him since his student days.
Hermann was the first to occupy a chair in the history of science and technology at the University of Stuttgart – a chair not situated either at a science or mathematics faculty but rather among general historians. During his 30 year-long tenure, he authored important monographs on quantum theory, quantum mechanics and elementary particle theory. He wrote books on the history of atomic physics titled Weltreich der Physik: Von Galilei bis Heisenberg, The New Physics: The Route into the Atomic Age, and How Science Lost its Innocence, alongside numerous biographies (including Planck, Heisenberg, Einstein and Wirtz) and historical studies on companies, notably on the German optics firm Carl Zeiss. All became very popular among the physics community.
Meanwhile at CERN, the attitude among physicists towards studies in the history of science was rather negative – the mantra was “We don’t care of history, we make history”. However, in 1980, the advisory committee for the CERN History Project examined a feasibility study conducted by Hermann and decided to establish a European study team to write the history of CERN from its early beginnings until at least 1963, with an overview of later years. The project was to be completed within five years and financed outside the CERN budget. Hermann was asked by CERN Council to assume responsibility for the project, and from 1982 to 1985 he was freed from teaching obligations in Stuttgart to conduct research at CERN. He became co-editor of first two volumes on the history of CERN: Launching the European Organization for Nuclear Research and Building and Running the Laboratory, 1954–1965. A third volume covering the story of the history of CERN from the mid-1960s to the late 1970s later appeared under the editorship of John Krige in 1996.
Armin passed away in February 2024 in his home in Oberstarz near Miesbach, nestled among the alpine hills, which he had always felt attached to and which was also the main reason why he declined several tempting calls to other renowned universities. His wife Steffi, his companion of many decades, was by his side to the very end. Many historians of physics, science and technology in Germany and abroad mourn the loss of this influential pioneer in the history of science.
The Standard Model – an inconspicuous name for one of the great human inventions. It describes all known elementary particles and their interactions, except for gravity. About 19 free parameters tune its behaviour. To the best of our knowledge, they could in principle take any value, and no underlying theory yet conceived can predict their values. They include particle masses, interaction strengths, important technical numbers such as mixing angles and phases, and the vacuum strength of the Higgs field, which theorists believe has alone among fundamental fields permeated every cubic attometre of the universe, since almost the beginning of time. Measuring these parameters is the most fundamental experimental task available to modern science.
The basic constituents of matter interact through forces which are mediated by virtual particles that ping back and forth, delivering momentum and quantum numbers. The gluon mediates the strong interaction, the photon mediates the electromagnetic interaction, and the W and Z bosons mediate the weak interaction. Although the electromagnetic and weak forces operate very differently to each other in everyday life, in the Standard Model they are two manifestations of the broken electroweak interaction – an interaction that broke when the Higgs field switched on throughout the universe, giving mass to matter particles, the W and Z bosons, and the Higgs boson itself, via the Brout–Englert–Higgs (BEH) mechanism. The electroweak theory has been extraordinarily successful in describing experimental results, but it remains mysterious – and the BEH mechanism is the origin of some of those free parameters. The best way to test the electroweak model is to over-constrain its free parameters using precision measurements and try to find a breaking point.
Ever since the late 1960s, when Steven Weinberg, Sheldon Glashow and Abdus Salam unified the electromagnetic and weak forces using the BEH mechanism, CERN has had an intimate experimental relationship with the electroweak theory. In 1973 the Z boson was indirectly discovered by observing “neutral current” events in the Gargamelle bubble chamber, using a neutrino beam from the Proton Synchrotron. The W boson was discovered in 1983 at the Super Proton Synchrotron collider, followed by the direct observation of the Z boson in the same machine soon after. The 1990s witnessed a decade of exquisite electroweak precision measurements at the Large Electron Positron (LEP) collider at CERN and the Stanford Linear Collider (SLC) at SLAC National Accelerator Laboratory in the US, before the crown jewel of the electroweak sector, the Higgs boson, was discovered by the ATLAS and CMS collaborations at the Large Hadron Collider (LHC) in 2012 – a remarkable success that delivered the last to be observed, and arguably most mysterious, missing piece of the Standard Model.
What was not expected, was that the ATLAS, CMS and LHCb experiments at the LHC would go on to make electroweak measurements that rival in precision those made at lepton colliders.
Discovery or precision?
Studying the electroweak interaction requires a supply of W and Z bosons. For that, you need a collider. Electrons and positrons are ideally suited for the task as they interact exclusively via the electroweak interaction. By precisely tuning the energy of electron–positron collisions, experiments at LEP and the SLC tested the electroweak sector with an unprecedented 0.1% accuracy at the energy scale of the Z-boson mass (mZ).
Hadron colliders like the LHC have different strengths and weaknesses. Equipped to copiously produce all known Standard Model particles – and perhaps also hypothetical new ones – they are the ultimate instruments for probing the high-energy frontier of our understanding of the microscopic world. The protons they collide are not elementary, but a haze of constituent quarks and gluons that bubble and fizz with quantum fluctuations. Each constituent “parton” carries an unpredictable fraction of the proton’s energy. This injects unavoidable uncertainty into studies of hadron collisions that physicists attempt to encode in probabilistic parton distribution functions. What’s more, when a pair of partons from the two opposing protons interact in an interesting way, the result is overlaid by numerous background particles originating from the remaining partons that were untouched by the original collision – a complexity that is exacerbated by the difficult-to-model strong force which governs the behaviour of quarks and gluons. As a result, hadron colliders have a reputation for being discovery machines with limited precision.
The LHC has collided protons at the energy frontier since 2010, delivering far more collisions than comparable previous machines such as the Tevatron at Fermilab in the US. This has enabled a comprehensive search and measurement programme. Following the discovery of the Higgs boson in 2012, measurements have so far verified its place in the electroweak sector of the Standard Model, although the relative precisions of many measurements are currently far lower than those achieved for the W and Z bosons at LEP. But in defiance of expectations, the capabilities of the LHC experiments and the ingenuity of analysts have also enabled many of the world’s most precise measurements of the electroweak interaction. Here, we highlight five.
1. Producing W and Z bosons
When two streams of objects meet, how many strike each other depends on their cross-sectional area. Though quarks and other partons are thought to be fundamental objects with zero extent, particle physicists borrow this logic for particle beams, and extend it by subdividing the metaphorical cross section according to the resulting interactions. The range of processes used to study W and Z bosons at the LHC spans a remarkable eight orders of magnitude in cross section.
The most common interaction is the production of single W and Z bosons through the annihilation of a quark and an antiquark in the colliding protons. Measurements with single W and Z boson events have now reached a precision well below 1% thanks to the excellent calibration of the detector performance. They are a prodigious tool for testing and improving the modelling of the underlying process, for example using parton distribution functions.
The second most common interaction is the simultaneous production of two bosons. Measurements of “diboson” processes now routinely reach a precision better than 5%. Since the start of the LHC operation, the accelerator has operated at several collision energies, allowing the experiments to map diboson cross sections as a function of energy. Measurements of the cross sections for creating WW, WZ and ZZ pairs exhibit remarkable agreement with state-of-the art Standard Model predictions (see “Diboson production” figure).
The large amount of collected data at the LHC has recently allowed us to move the frontier to the observation of extremely infrequent “triboson” processes with three W or Z bosons, or photons, produced simultaneously – the first step towards confirming the existence of the quartic self-interaction between the electroweak bosons.
2. The weak mixing angle
The Higgs potential is famously thought to resemble a Mexican hat. The Higgs field that permeates space could in principle exist with a strength corresponding to any point on its surface. Theorists believe it settled somewhere in the brim a picosecond or so after the Big Bang, breaking the perfect symmetry of the hat’s apex, where its value was zero. This switched the Higgs field on throughout the universe – and the massless gauge bosons of the unified electroweak theory mixed to form the photon and W and Z boson mass eigenstates that mediate the broken electroweak interaction today. The weak mixing angle θW is the free parameter of the Standard Model which defines that mixing.
The θW angle can be studied using a beautifully simple interaction: the annihilation of a quark and its antiquark to create an electron and a positron or a muon and an antimuon. When the pair has an invariant mass in the vicinity of mZ, there is a small preference for the negatively charged lepton to be produced in the same direction as the initial quark. This arises due to quantum interference between the Z boson’s vector and axial-vector couplings, whose relative strengths depend on θW.
The unique challenge at a proton–proton collider like the LHC is that the initial directions of the quark and the antiquark can only be inferred using our limited knowledge of parton distribution functions. These systematic uncertainties currently dominate the total uncertainty, although they can be reduced somewhat by using information on lepton pairs produced away from the Z resonance. The CMS and LHCb collaborations have recently released new measurements consistent with the Standard Model prediction with a precision comparable to that of the LEP and SLC experiments (see “Weak mixing angle” figure).
Quantum physics effects play an interesting role here. In practice, it is not possible to experimentally isolate “tree level” properties like θW, which describe the simplest interactions that can be drawn on a Feynman diagram. Measurements are in fact sensitive to the effective weak mixing angle, which includes the effect of quantum interference from higher-order diagrams.
A crucial prediction of electroweak theory is that the masses of the W and Z bosons are, at leading order, related by the electroweak mixing angle: sin2θW = 1–m2W/m2Z, where mW and mZ are the masses of the W and Z bosons. This relationship is modified by quantum loops involving the Higgs boson, the top quark and possibly new particles. Measuring the parameters of the electroweak theory precisely, therefore, allows us to test for any gaps in our understanding of nature.
Surprisingly, combining this relationship with the mZ measurement from LEP and the CMS measurement of θW also allows a competitive measurement of mW. A measurement of sin2θW with a precision of 0.0003 translates into a prediction of mW with 15 MeV precision, which is comparable to the best direct measurements.
3. The mass and width of the W boson
Precisely measuring the mass of the W boson is of paramount importance to efforts to further constrain the relationships between the parameters of the electroweak theory, and probe possible beyond-the-Standard Model contributions. Particle lifetimes also offer a sensitive test of the electroweak theory. Because of their large masses and numerous decay channels, the W and Z bosons have mean lifetimes of less than 10–24 s. Though this is an impossibly brief time interval to measure directly, Heisenberg’s uncertainty principle smudges a particle’s observed mass by a certain “width” when it is produced in a collider. This width can be measured by fitting the mass distribution of many virtual particles. It is reciprocally related to the particle’s lifetime.
While lepton-collider measurements of the properties of the Z boson were extensive and achieved remarkable precision, the same is not quite true for the W boson. The mass of the Z boson was measured with a precision of 0.002%, but the mass of the W boson was measured with a precision of only 0.04% – a factor 20 worse. The reason is that while single Z bosons were copiously produced at LEP and SLC, W bosons could not be produced singly, due to charge conservation. W+W– pairs were produced, though only at low rates at LEP energies.
In contrast to LEP, hadron colliders produce large quantities of single W bosons through quark–antiquark annihilation. The LHC produces more single W bosons in a minute than all the W-boson pairs produced in the entire lifetime of LEP. Even when only considering decays to electrons or muons and their respective neutrinos – the most precise measurements – the LHC experiments have recorded billions of W-boson events.
But there are obstacles to overcome. The neutrino in the final state escapes undetected. Its transverse momentum with respect to the beam direction can only be measured indirectly, by measuring all other products of the collision – a major experimental challenge in an environment with not just one, but up to 60 simultaneous proton–proton collisions. Its longitudinal momentum cannot be measured at all. And as the W bosons are not produced at rest, extensive theoretical calculations and ancillary measurements are needed to model their momenta, incurring uncertainties from parton distribution functions.
Despite these challenges, the latest measurement of the W boson’s mass by the ATLAS collaboration achieved a precision of roughly 0.02% (see “Mass and width” figure, top). The LHCb collaboration also recently produced its first measurement of the W-boson mass using W bosons produced close to the beam line with a precision at the 0.04% level, dominated for now by the size of the data sample. Owing to the complementary detector coverage of the LHCb experiment with respect to the ATLAS and CMS experiments, several uncertainties are reduced when these measurements are combined.
The Tevatron experiments CDF and D0 also made precise W-boson measurements using proton–antiproton collisions at a lower centre-of-mass energy. The single most precise mass measurement, at the 0.01% level, comes from CDF. It is in stark disagreement with the Standard Model prediction and disagrees with the combination of other measurements.
A highly anticipated measurement by the CMS collaboration may soon weigh in decisively in favour either of the CDF measurement or the Standard Model. The CMS measurement will combine innovative analysis techniques using the Z boson with a larger 13 TeV data set than the 7 TeV data used by the recent ATLAS measurement, enabling more powerful validation samples and thereby greater power to reduce systematic uncertainties.
Measurements of the W boson’s width are not yet sufficiently precise to constrain the Standard Model significantly, though the strongest constraint so far comes from the ATLAS collaboration (see “Mass and width” figure, bottom). Further measurements are a promising avenue to test the Standard Model. If the W boson decays into any hitherto undiscovered particles, its lifetime should be shorter than predicted, and its width greater, potentially indicating the presence of new physics.
4. Couplings of the W boson to leptons
Within the Standard Model, the W and Z bosons have equal couplings to leptons of each of the three generations – a property known as lepton flavour universality (LFU). Any experimental deviation from LFU would indicate new physics.
As with mass and width, lepton colliders’ precision was superior for the Z boson than the W boson. LEP confirmed LFU in leptonic Z-boson decays to about 0.3%. Comparing the three branching fractions of the W boson in the electron, muon and tau–lepton decay channels, the combination of the four LEP experiments reached a precision of only about 2%.
At the LHC, the large cross section for producing top quark–antiquark pairs that both decay into a W boson and a bottom quark offers a unique sample of W-boson pairs for high-precision studies of their decays. The resulting measurements are the most precise tests of LFU for all three possible comparisons of the coupling of the lepton flavours to the W boson (see “Couplings to leptons” figure).
Regarding the tau lepton to muon ratio, the ATLAS collaboration observed 0.992 ± 0.013 decays to a tau for every one decay to a muon. This result favours LFU and is twice as precise than the corresponding LEP result of 1.066 ± 0.025, which exhibits a deviation of 2.6 standard deviations from unity. Because of the relatively long tau lifetime, ATLAS was able to separate muons produced in the decay of tau leptons from those produced promptly by observing the tau decay length of the order of 2 mm.
The best tau to electron measurement is provided by a simultaneous CMS measurement of all the leptonic and hadronic decay branching fractions of the W boson. The analysis splits the top quark–antiquark pair events based on the multiplicity and flavour of reconstructed leptons, the number of jets, and the number of jets identified as originating from the hadronisation of b quarks. All CMS ratios are consistent with the LFU hypothesis and reduce tension with the Standard Model prediction.
Regarding the muon to electron ratio, measurements have been performed by several LHC and Tevatron experiments. The observed results are consistent with LFU, with the most precise measurement from the ATLAS experiment boasting a precision better than 0.5%.
5. The invisible width of the Z boson
A groundbreaking measurement at LEP deduced how often a particle that cannot be directly observed decays to particles that cannot be detected. The particle in question is the Z boson. By scanning the energy of electron–positron collisions and measuring the broadness of the “lineshape” of the smudged bump in interactions around the mass of the Z, LEP physicists precisely measured its width. As previously noted, a particle’s width is reciprocal to its lifetime and therefore proportional to its decay rate – something that can also be measured by directly accounting for the observed rate of decays to visible particles of all types. The difference between the two numbers is due to Z-boson decays to so-called invisible particles that cannot be reconstructed in the detector. A seminal measurement concluded that exactly three species of light neutrino couple to the Z boson.
The LEP experiments also measured the invisible width of the Z boson using an ingenious method that searched for solitary “recoils”. Here, the trick was to look for the rare occasion when the colliding electron or positron emitted a photon just before creating a virtual Z boson that decayed invisibly. Such events would yield nothing more than a single photon recoiling from an otherwise invisible Z-boson decay.
The ATLAS and CMS collaborations recently performed similar measurements, requiring the invisibly decaying Z boson to be produced alongside a highly energetic jet in place of a recoil photon. By taking the ratio with equivalent recoil decays to electrons and muons, they achieved remarkable uncertainties of around 2%, equivalent to LEP, despite the much more challenging environment (see “Invisible width” figure). The results are consistent with the Standard Model’s three generations of light neutrinos.
Future outlook
Building on these achievements, the LHC experiments are now readying themselves for a more than comparable experimental programme, which is yet to begin. Following the ongoing run of the LHC, a high-luminosity upgrade (HL-LHC) is scheduled to operate throughout the 2030s, delivering a total integrated luminosity of 3 ab–1 to both ATLAS and CMS. The LHCb experiment also foresees a major upgrade to collect an integrated luminosity of more than 300 fb–1 by the end of the LHC operations. A tenfold data set, upgraded detectors and experimental methods, and improvements to theoretical modelling will greatly extend both experimental precision and the reach of direct and indirect searches for new physics. Unprecedented energy scales will be probed and anomalies with respect to the Standard Model may become apparent.
Despite the significant challenges posed by systematic uncertainties, there are good prospects to further improve uncertainties in precision electroweak observables such as the mass of the W boson and the effective weak mixing angle, thanks to the larger angular acceptances of the new inner tracking devices currently under production by ATLAS and CMS. A possible programme of high-precision measurements in electron–proton collisions, the LHeC, could deliver crucial input to reduce uncertainties such as from parton distribution functions. The LHeC has been proposed to run concurrently with the HL-LHC by adding an electron beam to the LHC.
Beyond the HL-LHC programme, several proposals for future particle colliders have captured the imagination of the global particle-physics community – and not least the two phases of the Future Circular Collider (FCC) being studied at CERN. With a circumference three to four times greater than that of the LEP/LHC tunnel, electron–positron collisions could be delivered with very high luminosity and centre-of-mass energies from 90 to 365 GeV in the initial FCC-ee phase. The FCC-ee would facilitate an impressive leap in the precision of most electroweak observables. Projections estimate a factor of 10 improvement for Z-boson measurements and up to 100 for W-boson measurements. For the first time, the top quark could be produced in an environment where it is not colour-connected to initial hadrons, in some cases reducing uncertainties by a factor of 10 or more.
The LHC collaborations have made remarkable strides forward in probing the electroweak theory – a theory of great beauty and consequence for the universe. But its most fundamental workings are subtle and elusive. Our exploration is only just beginning.
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