On 4 August 2024, the great physicist Tsung-Dao Lee (also known as T D Lee) passed away at his home in San Francisco, aged 97.

Born in 1926 to an intellectual family in Shanghai, Lee’s education was disrupted several times by the war against Japan. He neither completed high school nor graduated from university. In 1943, however, he took the national entrance exam and, with outstanding scores, was admitted to the chemical engineering department of Zhejiang University. He then transferred to the physics department of Southwest Associated University, a temporary setup during the war for Peking, Tsinghua and Nankai universities. In the autumn of 1946, under the recommendation of Ta-You Wu, Lee went to study at the University of Chicago under the supervision of Enrico Fermi, earning his PhD in June 1950.

From 1950 to 1953 Lee conducted research at the University of Chicago, the University of California, Berkeley and the Institute for Advanced Study, located in Princeton. During this period, he made significant contributions to particle physics, statistical mechanics, field theory, astrophysics, condensed-matter physics and turbulence theory, demonstrating a wide range of interests and deep insights in several frontiers of physics. In a 1952 paper on turbulence, for example, Lee pointed out the significant difference between fluid dynamics in two-dimensional and three-dimensional spaces, namely, there is no turbulence in two dimensions. This finding provided essential conditions for John von Neumann’s model, which used supercomputers to simulate weather.

Profound impact

During this period, Lee and Chen-Ning Yang collaborated on two foundational works in statistical physics concerning phase transitions, discovering the famous “unit circle theorem” on lattice gases, which had a profound impact on statistical mechanics and phase-transition theory.

Between 1952 and 1953, during a visit to the University of Illinois at Urbana-Champaign, Lee was inspired by discussions with John Bardeen (winner, with Leon Neil Cooper and John Robert Schrieffer, of the 1972 Nobel Prize in Physics for developing the first successful microscopic theory of superconductivity). Lee applied field-theory methods to study the motion of slow electrons in polar crystals, pioneering the use of field theory to investigate condensed matter systems. According to Schrieffer, Lee’s work directly influenced the development of their “BCS” theory of superconductivity.

In 1953, after taking an assistant professor position at Columbia University, Lee proposed a renormalisable field-theory model, widely known as the “Lee Model,” which had a substantial impact on the study of renormalisation in quantum field theory.

On 1 October 1956, Lee and Yang’s theory of parity non-conservation in weak interactions was published in Physical Review. It was quickly confirmed by the experiments of Chien-Shiung Wu and others, earning Lee and Yang the 1957 Nobel Prize in Physics – one of the fastest recognitions in the history of the Nobel Prize. The discovery of parity violation significantly challenged the established understanding of fundamental physical laws and directly led to the establishment of the universal V–A theory of weak interactions in 1958. It also laid the groundwork for the unified theory of weak and electromagnetic interactions developed a decade later.

In 1957, Lee, Oehme and Yang extended symmetry studies to combined charge–parity (CP) transformations. The CP non-conservation discovered in neutral K-meson decays in 1964 validated the importance of Lee and his colleagues’ theoretical work, as well as the later establishment of CP violation theories. The same year, Lee was appointed the Fermi Professor of Physics at Columbia.

In the 1970s, Lee published papers exploring the origins of CP violation, suggesting that it might stem from spontaneous symmetry breaking in the vacuum and predicting several significant phenomenological consequences. In 1974, Lee and G C Wick investigated whether spontaneously broken symmetries in the vacuum could be partially restored under certain conditions. They found that heavy-ion collisions could achieve this restoration and produce observable effects. This work pioneered the study of the quantum chromodynamics (QCD) vacuum, phase transitions and quark–gluon plasma. It also laid the theoretical and experimental foundation for relativistic heavy-ion collision physics.

From 1982, Lee devoted significant efforts to solving non-perturbative QCD using lattice-QCD methods. Together with Norman Christ and Fred Friedberg, he developed stochastic lattice field theory and promoted first-principle lattice simulations on supercomputers, greatly advancing lattice QCD research.

Immense respect

In 2011 Lee retired as a professor emeritus from Columbia at the age of 85. In China, he enjoyed immense respect, not only for being the first Chinese scientist (with Chen-Ning Yang) to win a Nobel Prize, but also for enhancing the level of science and education in China and promoting the Sino-American collaboration in high-energy physics. This led to the establishment and successful construction of China’s first major high-energy physics facility, the Beijing Electron–Positron Collider (BEPC). At the beginning of this century, Lee supported and personally helped the upgrade of BEPC, the Daya Bay reactor neutrino experiment and others. In addition, he initiated, promoted and executed the China–US Physics Examination and Application plan, the National Natural Science Foundation of China, and the postdoctoral system in China.

Tsung-Dao Lee’s contributions to an extraordinarily wide range of fields profoundly shaped humanity’s understanding of the basic laws of the universe.

Robert Aymar, CERN Director-General from January 2004 to December 2008, passed away on 23 September at the age of 88. An inspirational leader in big-science projects for several decades, including the International Thermonuclear Experimental Reactor (ITER), his term of office at CERN was marked by the completion of construction and the first commissioning of the Large Hadron Collider (LHC). His experience of complex industrial projects proved to be crucial, as the CERN teams had to overcome numerous challenges linked to the LHC’s innovative technologies and their industrial production.

Robert Aymar was educated at École Poly­technique in Paris. He started his career in plasma physics at the Commissariat à l’Énergie Atomique (CEA), since renamed the Commissariat à l’Énergie Atomique et aux Énergies Alternatives, at the time when thermonuclear fusion was declassified and research started on its application to energy production. After being involved in several studies at CEA, Aymar contributed to the design of the Joint European Torus, the European tokamak project based on conventional magnet technology, built in Culham, UK in the late 1970s. In the same period, CEA was considering a compact tokamak project based on superconducting magnet technology, for which Aymar decided to use pressurised superfluid helium cooling – a technology then recently developed by Gérard Claudet and his team at CEA Grenoble. Aymar was naturally appointed head of the Tore Supra tokamak project, built at CEA Cadarache from 1977 to 1988. The successful project served inter alia as an industrial-sized demonstrator of superfluid helium cryogenics, which became a key technology of the LHC.

As head of the Département des Sciences de la Matière at CEA from 1990 to 1994, Aymar set out to bring together the physics of the infinitely large and the infinitely small, as well as the associated instrumentation, in a department that has now become the Institut de Recherche sur les Lois Fondamentales de l’Univers. In that position, he actively supported CEA–CERN collaboration agreements on R&D for the LHC and served on many national and international committees. In 1993 he chaired the LHC external review committee, whose recommendation proved decisive in the project’s approval. From 1994 to 2003 he led the ITER engineering design activities under the auspices of the International Atomic Energy Agency, establishing the basic design and validity of the project that would be approved for construction in 2006. In 2001, the CERN Council called on his expertise once again by entrusting him to chair the external review committee for CERN’s activities.

When Robert Aymar took over as Director-General of CERN in 2004, the construction of the LHC was well under way. But there were many industrial and financial challenges, and a few production crises still to overcome. During his tenure, which saw the ramp-up, series production and installation of major components, the machine was completed and the first beams circulated. That first start-up in 2008 was followed by a major technical problem that led to a shutdown lasting several months. But the LHC had demonstrated that it could run, and in 2009 the machine was successfully restarted. Aymar’s term of office also saw a simplification of CERN’s structure and procedures, aimed at making the laboratory more efficient. He also set about reducing costs and secured additional funding to complete the construction and optimise the operation of the LHC. After retirement, he remained active as a scientific advisor to the head of the CEA, occasionally visiting CERN and the ITER construction site in Cadarache.

Robert Aymar was a dedicated and demanding leader, with a strong drive and search for pragmatic solutions in the activities he undertook or supervised. CERN and the LHC project owe much to his efforts. He was also a man of culture with a marked interest in history. It was a privilege to serve under his direction.

Theoretical physicist James D “BJ” Bjorken, whose work played a key role in revealing the existence of quarks, passed away on 6 August aged 90. Part of a wave of young physicists who came to Stanford in the mid-1950s, Bjorken also made important contributions to the design of experiments and the efficient operation of accelerators.

Born in Chicago on 22 June 1934, James Daniel Bjorken grew up in Park Ridge, Illinois, where he was drawn to mathematics and chemistry. His father, who had immigrated from Sweden in 1923, was an electrical engineer who repaired industrial motors and generators. After earning a bachelor’s degree at MIT, he went to Stanford University as a graduate student in 1956. He was one of half a dozen MIT physicists, including his adviser Sidney Drell and future director of the SLAC National Accelerator Laboratory Burton Richter, who were drawn by new facilities on the Stanford campus. This included an early linear accelerator that scattered electrons off targets to explore the nature of the neutron and proton.

Ten years later those experiments moved to SLAC, where the newly constructed Stanford Linear Collider would boost electrons to much higher energies. By that time, theorists had proposed that protons and neutrons contained fundamental particles. But no one knew much about their properties or how to go about proving they were there. Bjorken, who joined the Stanford faculty in 1961, wrote an influential 1969 paper in which he suggested that electrons were bouncing off point-like particles within the proton, a process known as deep inelastic scattering. He started lobbying experimentalists to test it with the SLAC accelerator.

Carrying out the experiments would require a new mathematical language and Bjorken contributed to its development, with simplifications and improvements from two of his students (John Kogut and Davison Soper) and Caltech physicist Richard Feynman. In the late 1960s and early 1970s, those experiments confirmed that the proton does indeed consist of fundamental particles – a discovery honoured with the 1990 Nobel Prize in Physics for SLAC’s Richard Taylor and MIT’s Henry Kendall and Jerome Friedman. Bjorken’s role was later recognised by the prestigious Wolf Prize in Physics and the 2015 High Energy and Particle Physics Prize of the European Physical Society.

While the invention of “Bjorken scaling” was his most famous scientific achievement, Bjorken was also known for identifying a wide variety of interesting problems and tackling them in novel ways. He was somewhat iconoclastic. He also had colourful and often distinctly visual ways of thinking about physics – for instance, describing physics concepts in terms of plumbing or a baked Alaska. He never sought recognition for himself and was very generous in recognising the contributions of others.

In 1979 Bjorken headed east to become associate director for physics at Fermilab. He returned to SLAC in 1989, where he continued to innovate. Over the course of his career, among other things, he invented ideas related to the existence of the charm quark and the circulation of protons in a storage ring. He helped popularise the unitarity triangle and, along with Drell, co-wrote the widely used graduate-level textbooks Relativistic Quantum Mechanics and Relativistic Quantum Fields. In 2009 Bjorken contributed to an influential paper by three younger theorists suggesting approaches for searching for “dark” photons, hypothetical carriers of a new fundamental force.

He was also awarded the American Physical Society’s Dannie Heineman Prize, the Department of Energy’s Ernest Orlando Lawrence Award, and the Dirac Medal from the International Center for Theoretical Physics. In 2017 he shared the Robert R Wilson Prize for Achievement in the Physics of Particle Accelerators for groundbreaking theoretical work he did at Fermilab that helped to sharpen the focus of particle beams in many types of accelerators.

Known for his warmth, generosity and collaborative spirit, Bjorken passionately pursued many interests outside physics, from mountain climbing, skiing, cycling and windsurfing to listening to classical music. He divided his time between homes in Woodside, California and Driggs, Idaho, and thought nothing of driving long distances to see an opera in Chicago or dropping in unannounced at the office of some fellow physicist for deep conversations about general relativity, dark matter or dark energy – once remarking: “I’ve found the most efficient way to test ideas and get hard criticism is one-on-one conversation with people who know more than I do.”

Experimental particle physicist Max Klein, whose exceptional career spanned theory, detectors, accelerators and data analysis, passed away on 23 August 2024.

Born in Berlin in 1951, Max earned his diploma in physics in 1973 from Humboldt University of Berlin (HUB, East-Germany, GDR) with a thesis on low-energy heavy-ion physics. He received his PhD in 1977 from the Institute for High Energy Physics (IHEP) of the Academy of Sciences of the GDR in Zeuthen (now part of DESY) on the subject of multiparticle production, and his habilitation degree in 1984 from HUB. From 1973 to 1991 he conducted research at IHEP Zeuthen, spending several years from 1977 at the Joint Institute for Nuclear Research in Dubna, and from the 1980s at DESY and CERN. For his role in determining the asymmetry of the interaction of polarised positive and negative muons with the NA4 muon spectrometer at CERN’s SPS M2 muon beam, he was awarded the Max von Laue Medal by the Academy of Sciences of the GDR in 1985.

Max worked as a scientist at DESY from 1992 to 2006. As a member of the H1 experiment at the lepton–proton collider HERA since 1985, his research focused on investigating the internal structure of protons using deep inelastic scattering. He served as spokesperson of the H1 collaboration from 2002 to 2006 for two mandates.

Max became a professor at the University of Liverpool in 2006, and the following year he joined the ATLAS collaboration. He served as chair of the ATLAS publication committee and as editorial-board chair of the ATLAS detector paper and other important works. Max made key contributions to data analysis, notably on the high-precision 7 TeV inclusive W and Z boson production cross sections and associated properties, and was a convener of the PDF forum in 2015–2016. From 2017 to 2019, Max was chair of the ATLAS collaboration board, during which he made invaluable contributions to the experiment and collaboration life. He led the Liverpool ATLAS team from 2009 to 2017. Under his guidance, the 30-strong group contributed to the maintenance of the SCT detector, as well as to ATLAS data preparation and physics analyses. The group also developed hybrids, mechanics and software for the new ITk pixel and strip detectors.

In recent years, Max’s scientific contributions extended well beyond ATLAS. He was a strong advocate for the development of an electron-beam upgrade of the LHC, the LHeC, and collaborated closely with the CERN accelerator group and international teams on the development of energy-recovery linacs. Here, he was influential in the development of the PERLE demonstrator accelerator at IJCLab, for which he acted as spokesperson until 2023.

A strong advocate for the responsibility of scientists toward their societies

In 2013 Max was awarded the Max Born Prize by the Deutsche Physikalische Gesellschaft and the UK Institute of Physics for his fundamental experimental contributions to the elucidation of the proton structure using deep-inelastic scattering. The prize citation stands as a testament to his scientific stature: “In the last 40 years, Max Klein has dedicated himself to the study of the innermost structure of the proton. In the 1990s he was a leading figure in the discovery that gluons form a surprisingly large component of proton structure. These gluons play an important role in the production of Higgs bosons in proton–proton collisions for which experiments at CERN have recently found promising candidates.”

Besides being a distinguished scientist, Max was a man of unwavering principles, grounded in his selfless interactions with others and his deep sense of humanity. Drawing from his experience as a bridge between East and West, he was a strong advocate for international scientific collaboration and the responsibility of scientists toward their societies. He had a strong desire and ability to mentor and support students, postdocs and early-career researchers, and an admirably wise and calm approach to problem solving.

Max Klein had a profound knowledge of physics and a tireless dedication to ATLAS and to experimental particle physics in general. His passing is a profound loss for the entire community, but his legacy will endure.

Experimental particle physicist Ian Shipsey, a remarkable leader and individual, passed away suddenly and unexpectedly in Oxford on 7 October.

Ian was educated at Queen Mary University of London and the University of Edinburgh, where he earned his PhD in 1986 for his work on the NA31 experiment at CERN. Moving to the US, he joined Syracuse as a post-doc and then became a faculty member at Purdue, where, in 2007, he was elected Julian Schwinger Distinguished Professor of Physics. In 2013 he was appointed the Henry Moseley Centenary Professor of Experimental Physics at the University of Oxford.

Ian was a central figure behind the success of the CLEO experiment at Cornell, which was for many years the world’s pre-eminent detector in flavour physics. He led many analyses, most notably in semi-leptonic decays, from which he measured four different CKM matrix elements, and oversaw the construction of the silicon vertex detector for the CLEO III phase of the experiment. He served as co-spokesperson between 2001 and 2004, and was one of the intellectual leaders that saw the opportunity to re-configure the detector and the CESR accelerator as a facility for making precise exploration of physics at the charm threshold. The resulting CLEO-c programme yielded many important measurements in the charm system and enabled critical experimental validations of lattice–QCD predictions.

Influential voice

At CMS, Ian played a leading role in the construction of the forward-pixel detector, exploiting the silicon laboratory he had established at Purdue. His contributions to CMS physics analy­ses were no less significant. These included the observation of upsilon suppression in heavy-ion collisions (a smoking gun for the production of quark–gluon plasma) and the discovery, reported in a joint Nature paper with the LHCb collaboration, of the ultra-rare decay B_s → μ⁺μ^–. He was also an influential voice as CMS collaboration board chair (2013–2014).

After moving to the University of Oxford and, in 2015, joining the ATLAS collaboration, Ian became Oxford’s ATLAS team leader and established state-of-the-art cleanrooms, which are used for the construction of the future inner tracker (ITk) pixel end-cap modules. Together with his students, he contributed to measurements of the Higgs boson mass and width, and to the search for its rare di-muon decay. Ian also led the UK’s involvement in LSST (now the Vera Rubin Observatory), where Oxford is providing deep expertise for the CCD cameras.

Following his tenure as the dynamic head of the particle physics sub-department, Ian was elected head of Oxford physics in 2018 and re-elected in 2023. Among his many successful initiatives, he played a leading role in establishing the £40 million UKRI “Quantum Technologies for Fundamental Physics” programme, which is advancing quantum-based applications across various areas of physics. With the support of this programme, he led the development of novel atom interferometers for light dark matter searches and gravitational-wave detection.

Ian took a central role in establishing roadmaps for detector R&D both in the US and (via ECFA) in Europe. He was one of the coordinators and driving force of the ECFA R&D roadmap panel, and co-chair of the US effort to define the basic research needs in this area. As chair of the ICFA instrumentation, innovation and development panel, he promoted R&D in instrumentation for particle physics and the recognition of excellence in this field.

Among his many prestigious honours, Ian was elected a Fellow of the Royal Society in 2022 and received the James Chadwick Medal and Prize from the Institute of Physics in 2019. He served on numerous collaboration boards, panels, and advisory and decision-making committees shaping national and international science strategies.

The success of Ian’s career is even more remarkable given that he lost his hearing in 1989. He received a cochlear implant, which restored limited auditory ability, and gave unforgettable talks on this subject, explaining the technology and its impact on his life.

Ian was an outstanding physicist and also a remarkable individual. His legacy is not only an extensive body of transformative scientific results, but also the impact that he had on all who met him. He was equally charming, whether speaking to graduate students or lab directors. Everyone felt better after talking to Ian. His success derived from a remarkable combination of optimism and limitless energy. Once he had identified the correct course of action, he would not allow himself to be dissuaded by cautious pessimists who worried about the challenges ahead. His colleagues and many graduate students will continue to benefit for many years from the projects he initiated. The example he set as a physicist, and the memories he leaves as friend, will endure still longer.