Following a week of discussions, the European Strategy Group has released a statement reporting convergence on recommendations to guide the future of high-energy physics in Europe. The 60-or-so delegates, among them scientific representatives from each of CERN’s member and associate-member states, directors and representatives of major European laboratories and organisations, and invitees from outside Europe, now return home. Their recommendations will be presented to the CERN Council in March and made public at an event in Budapest, Hungary, on 25 May.
Statement from the European Strategy Group after the Bad Honnef drafting meeting, 25 January
The drafting session of the European Strategy Group preparing the next European Particle Physics Strategy Update took place in Bad Honnef (Germany) between 21-25 January 2020. After a week of fruitful discussions involving senior figures of European and international particle physics, convergence was achieved on recommendations that will guide the future of the field.
The drafting session marks a key stage of the strategy update process. The attendees of the Bad Honnef drafting session successfully carried out their ambitious task of identifying a set of priorities and recommendations. They built on the impressive progress made since the last update of the European Strategy for Particle Physics, in 2013, and the rich input received from the entire particle physics community in the current update process.
The next step in this process will be to submit the document outlining the recommendations to the CERN Council. It will be discussed by the Council in March and submitted for final approval at an extraordinary Council Session on 25 May, in Budapest, Hungary. Once approved, it can be made public.
The proposed Compact Linear Collider (CLIC) offers the most flexible option for European particle physics in the post-LHC era, write the leaders of the CLIC study in a preprint posted on arXiv on 15 January. Responding to a preprint by 53 authors in late December which backed a Future Circular Collider (FCC) over CLIC, the CLIC team argues that moving forward quickly with a linear collider “would allow a vibrant high-energy frontier programme to be maintained over the coming decades, while pursuing in parallel the accelerator R&D required to open future options”.
Acknowledging the widespread consensus that the next major collider should be an electron–positron collider to explore the Higgs sector in detail, the authors argue that the discussion of what will be the most appropriate high-energy frontier machine afterwards “must be kept open” such that it can be guided by new physics results and new technology. The three-page long note states that an initial CLIC programme undertaken in parallel with strong accelerator R&D and HL-LHC, followed by the best possible high-energy frontier machine when technologies are mature, “thus provides the most flexible and appealing strategic option” for collider physics in Europe.
Although FCC-ee is unique in offering a very high-statistics Z physics programme, state the CLIC authors, the potential for Higgs-boson studies with a first-stage 380 GeV CLIC or 365 GeV FCC-ee is similar when assuming equivalent running times. They also say that both machines have a similar performance at the top-quark energy and the same accelerator performance risk. “Previous limits of both circular and linear electron–positron colliders have been understood and overcome thanks to vast efforts in hardware developments and large-scale system tests across the planet,” says coauthor Daniel Schulte of CERN. “Both colliders have ambitious parameters, but we are confident that they can be achieved, as confirmed in detailed reviews of both projects.”
Ultimately we all want what is best for our science
Aidan Robson
CLIC studies during the past few years have focused on energy consumption and construction costs, which CLIC project leader Steinar Stapnes of CERN says are now “very favourable” compared with FCC-ee. “Owing to CLIC’s compactness the construction is relatively fast, and we have also deliberately kept the 380 GeV baseline operation time relatively short at eight years,” he says. “We feel strongly that the possibilities for the subsequent step – whether a linear collider energy extension, or a proton or muon collider option – need to be kept on timescales that are not too far away.”
Priorities for European particle physics are under discussion this week at a meeting in Bad Honnef, Germany, as the update of the European strategy for particle physics enters its final stages.
“The European strategy is being developed in a complex environment where particle physics projects continue to become larger and longer-scale,” says Aidan Robson of the University of Glasgow, who is spokesperson of the CLIC detector & physics collaboration. “Ultimately we all want what is best for our science. CLIC at 380 GeV offers a rapid and exciting e+e– programme, and opens doors for R&D for several possible future colliders going much higher in energy. This provides the key elements that offer attractive and challenging opportunities for the young people who will drive the future of our field.”
Today, senior figures in European particle physics have gathered in the small town of Bad Honnef, Germany, for a week of intense discussions that will guide the future of fundamental exploration. The “strategy drafting session” marks the final stage of the update of the European strategy for particle physics. Convened by the European Strategy Group (ESG) — which includes a scientific delegate from each of CERN’s member and associate-member states, directors and representatives of major European laboratories and organisations and invitees from outside Europe – the 60 or so attendees are tasked with identifying a set of priorities and recommendations to the CERN Council.
The ESG, a special body set up by the CERN Council approximately every five years, was invited to formulate an update of the European strategy for particle physics in September 2017. A call for input in 2018 attracted 160 submissions, which were discussed at an open symposium in Granada, Spain, in May 2019. The ESG then published a 200-page briefing book which distilled the input into an objective scientific summary and will form the basis for discussions in Germany this week.
The start of a new project in the early 2040s is crucial to keep the community motivated and engaged
Fabiola Gianotti
The focus of the latest strategy update, the third since 2005, is which major project should follow the LHC once its high-luminosity phase comes to an end in the late 2030s. There is broad support for an electron—positron collider that will explore the Higgs sector in detail, as well as for a high-energy proton–proton collider at CERN. In Europe, the possible options are the Compact Linear Collider and the Future Circular Collider, while an International Linear Collider (ILC) in Japan and a large Circular Electron-Positron Collider in China are also contenders. The strategy update will also consider non-collider experiments, computing, instrumentation and other key aspects of growing importance to the field such as energy efficiency and communication.
The previous strategy update, which concluded in 2013, made several high-priority recommendations: the full exploitation of the LHC, including the high-luminosity upgrade of the machine and detectors; R&D and design studies for a future energy-frontier machine at CERN; establishing a neutrino programme at CERN for physicists to develop detectors for experiments at accelerator-based neutrino facilities around the world; and the welcoming of a proposal from Japan to discuss the possible participation of Europe in the ILC. The first three are well under way, while a decision on the ILC still rests with the Japanese government. Other conclusions of the 2013 update included the need for closer collaboration with the astroparticle and nuclear physics communities, which has been met for example via the recently launched centre for astroparticle physics theory (EuCAPT) and the new Joint ECFA-NuPECC-APPEC Seminar series, JENAS. There was also a call for greater scientific diversity, leading to the CERN-led Physics Beyond Colliders initiative, which will also form a central part of this week’s discussions.
The recommendations from the ESG are due to formally be approved by the CERN Council on 25 May at an event in Budapest, Hungary.
During her annual address to personnel on 14 January, CERN Director-General Fabiola Gianotti acknowledged the enormous efforts that have gone into the strategy update, and said that she hoped that a recommendation on CERN’s next major collider would be among the ESG’s priorities.
“The start of a new project in the early 2040s is crucial to keep the community motivated and engaged,” said Gianotti, noting that CERN and Europe should also be open to participate in projects at the forefront of particle physics elsewhere in the world. “The Higgs boson is a guaranteed deliverable. It is related to the most obscure and problematic sector of the Standard Model and carries special quantum numbers and a new type of interaction. It is therefore a unique door into new physics, and one that can only be studied at colliders.”
On 10 October CERN welcomed the Republic of Croatia as an Associate Member State, following receipt of official notification that Croatia has completed its internal approval procedures in respect of an agreement signed on 28 February.
“It is a great pleasure to welcome Croatia into the CERN family as an associate member. Croatian scientists have made important contributions to a large variety of experiments at CERN for almost four decades, and as an associate member, new opportunities open up for Croatia in scientific collaboration, technological development, education and training,” said CERN Director-General Fabiola Gianotti.
Researchers from Croatia have contributed to many experiments at CERN, and a cooperation agreement concluded in 2001 increased the country’s participation in CERN’s research and educational programmes. As an Associate Member State, Croatia will be represented at the CERN Council and be entitled to attend meetings of the finance committee and the scientific policy committee. Nationals of Croatia will be eligible to apply for limited-duration positions as staff members and fellows, while firms offering goods and services originating from Croatia will be entitled to bid for CERN contracts, creating opportunities for industrial collaboration in advanced technologies.
Croatia joins India, Lithuania, Pakistan, Turkey and Ukraine as Associate Member States, while Cyprus and Slovenia are Associate Member States in the pre-stage to membership.
The Nobel Prize in Physics for 2019 has recognised two independent bodies of work that have transformed our view of the universe and humanity’s place in it. One half of the SEK 9 million prize, announced on 8 October in Stockholm, was granted to James Peebles of Princeton University for theoretical discoveries in physical cosmology, while the other was shared between Michel Mayor of the University of Geneva and Didier Queloz of the universities of Geneva and Cambridge for the discovery of an exoplanet orbiting a Sun-like star.
Peebles was instrumental in turning cosmology into the precision science it is today, with its ever closer links to collider and particle physics in general. Following the unexpected discovery of the cosmic microwave background (CMB) in 1965, he and others at Princeton used it to support the idea that the universe began in a hot, dense state. While the idea of a “big bang” was already many years old, Peebles paired it with concrete physics processes such as nucleosynthesis and described the role of temperature and density in the formation of structure. With others, he arrived at a model accounting for the density fluctuations in the CMB showing a series of acoustic peaks, which would demonstrate that the universe is geometrically flat and that ordinary matter constitutes just 5% of its total matter and energy content. In the early 1980s, Peebles was the first to consider non-relativistic “cold” dark matter and its effect on structure formation, and he went on to reintroduce Einstein’s forsaken cosmological constant – work that underpins today’s Lambda Cold Dark Matter model of cosmology.
Mayor and Queloz’s discovery of an exoplanet orbiting a solar-type star in the Milky Way opened a new field of study. 51 Pegasi b lies 50 light years from Earth and takes just four days to complete its orbit. It was spotted by tracking how it and its star orbit around their common centre of gravity: a subtle wobbling seen from Earth whose speed can be measured from the starlight via the Doppler effect. The problem is that the radial velocities are extremely low. Mayor mounted his first spectrograph on a telescope at the Haute-Provence Observatory near Marseille in 1977, but it was only sensitive to velocities above 300 ms–1 – too high to see a planet pulling on its star. It took almost two decades of work by him and his group to strike success, with doctoral student Queloz tasked with developing new methods to increase the machine’s light sensitivity. Today, more than 4000 exoplanets with a vast variety of forms, sizes and orbits have been discovered in our galaxy using the radial-velocity method and the newer technique of transit photometry, challenging ideas about planetary formation.
The 2018 André Lagarrigue Prize has been awarded to Michel Spiro, research director emeritus at CEA, for the exemplary nature of his career, from both a scientific and managerial point of view. Spiro contributed, among other things, to the discovery of the W and Z bosons with the UA1 experiment, was the initiator and spokesperson of the EROS experiment, and played a major role in the GALLEX experiment. He has held several senior positions at CEA and CNRS and from 2010–2013 was president of the CERN Council.
Wesley Smith of the University of Wisconsin-Madison, and member of the CMS collaboration, has won the American Physical Society (APS) 2020 W K H Panofsky Prize “for the development of sophisticated trigger systems for particle-physics experiments, which enabled measuring the detailed partonic structure of the proton using the ZEUS experiment at HERA and led to the discovery of the Higgs boson and the completion of the Standard Model with the CMS experiment at the LHC”.
The 2020 J J Sakurai Prize for theoretical particle physics went to Pierre Sikivie of the University of Florida for seminal work recognising the potential visibility of the invisible axion, devising novel methods to detect it, and for theoretical investigations of its cosmological implications.
In the accelerator arena, the 2020 Robert R Wilson Prize was awarded to Bruce Carlsten of Los Alamos National Laboratory for the discovery and subsequent implementation of emittance compensation in photo-injectors “that has enabled the development of high-brightness, X-ray free electron lasers such as the Linac Coherent Light Source”.
Among several other prizes awarded in the particle, nuclear, astrophysics and related fields, the 2020 Henry Primakoff Award for Early-Career Particle Physics went to Matt Pyle of the University of California at Berkeley for his development of high-resolution ultra-low-threshold cryogenic detectors for dark-matter searches.
Andrzej Buras of the Technical University of Munich has been awarded the Max Planck Medal by the German Physical Society for his outstanding contributions to applied quantum field theory, especially in flavour physics and quantum chromodynamics.
Astroparticle physicist Nigel Smith has been appointed to a three-year extension as executive director of SNOLAB in Canada. Smith, who has been in the role since 2009, agreed to remain in position until 31 December 2022, with a search for a successor being revisited during 2020.
Luigi Radicati, one of the eminent Italian theoretical physicists of the past century, passed away on 23 August 2019 in his home in Pisa, about 50 days before his 100th birthday.
Born in Milan, Radicati received his laurea in physics from the University of Torino under the supervision of Enrico Persico in 1943, and became the assistant professor of Eligio Perucca at Torino Polytechnic in 1948. In between this, during the Second World War he was also a member of a partisan division fighting against German occupation.
The years 1951–1953, which Radicati spent as a research fellow at the University of Birmingham in the group of Rudolf Peierls, had a major impact on his training. Then, in 1953 Radicati became a professor of theoretical physics, first at the University of Naples and two years later at the University of Pisa. In 1962 Radicati was finally called to the Scuola Normale Superiore (SNS) in Pisa as one of two professors in the “Classe di scienze”, the other being the great mathematician Ennio De Giorgi. Radicati remained at SNS until 1996, acting as vice-director between 1962 and 1964, and director between 1987 and 1991.
Luigi Radicati can be remembered for two main reasons: the special role that he attributed to symmetries; and the broadness of his interests in physics, as in the relations between physics and other disciplines. His most important and well known physics results stem from the early 1960s. After working with Paolo Franzini to show evidence for SU(4) symmetry in the classification of nuclear states, introduced by Wigner in 1937, in 1964 Radicati proposed, together with Feza Gürsey, the enlargement of SU(4) to SU(6) as a useful symmetry of hadrons. Gell-Mann had introduced the SU(3) symmetry in 1962 and at the beginning of 1964 had proposed, simultaneously with George Zweig, the notion of quarks. The SU(6)-subgroup SU(3) × SU(2) puts together Gell-Mann’s SU(3) with the spin SU(2) symmetry, thus unifying in single multiplets the pseudo-scalar together with the vector mesons and the J = 1/2 together with the J = 3/2 baryons. At a deeper level, SU(6) gave momentum to view the quarks as real entities obeying peculiar statistics, preliminary to the introduction of colour.
In the latter part of the 1960s Radicati began turning his attention to astrophysics, gravity, plasma physics and statistical physics. Here it is worth mentioning the long-lasting collaboration with Emilio Picasso, which started in 1977 during a discussion in the CERN cafeteria: the use of a gravitational-wave detector consisting of a system of two radio-frequency cavities, coupled to create a two-level system with a tunable difference between their oscillation frequencies.
Radicati’s collaborations brought frequent visits of eminent physicists to Pisa, among them Freeman Dyson, Feza Gürsey, T D Lee, Louis Michel, Rudolf Peierls, David Speiser and John Wheeler. Most of all, Radicati played a prominent role in bringing from CERN to the SNS Gilberto Bernardini, who acted as SNS director from 1964 to 1977, and Emilio Picasso, who was SNS director from 1992 to 1996.
Radicati was a member of the Accademia Nazionale dei Lincei from 1966, named Chevalier de la Légion d’Honneur and Doctor Honoris Causa at the École Normale in Paris in 1994, and was awarded the honour of Cavaliere di Gran Croce of the Italian Republic in 2004. During his career, he also translated and introduced important physics books into Italy, including The Meaning of Relativity by Albert Einstein, A History of Science by William Dampier and Quantum Mechanics by Leonard Schiff.
Luigi Radicati is survived by his wife and four of his sons.
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