Praxis Archives – Page 28 of 179

Commemorating Bruno Touschek’s centenary

touschek_featured_img — Bruno Touschek, pictured here in the 1950s, was one of the first physicists in Europe who was
skilled in elementary particle theory and in functioning of accelerators. Credit: F Touschek

Bruno Touschek was born in Vienna on 3 February 1921. His mother came from a well-to-do Jewish family and his father was a major in the Austrian Army. Bruno witnessed the tragic consequences of racial discrimination that prevented him from both completing his high school and university studies in Austria. But he also experienced the hopes of the post-war era and played a role in the post-war reconstruction. With the help of his friends, he continued his studies in Hamburg, where he worked on the 15 MeV German betatron proposed by Rolf Widerøe and learnt about electron accelerators. After the war he obtained his PhD at the University of Glasgow in 1949 , where he was involved in theoretical studies and in the building of a 300 MeV electron synchrotron. Touschek emerged from the early-post war years as one of the first physicists in Europe endowed with a unique expertise in the theory and functioning of accelerators. His genius was nurtured by close exchanges with Arnold Sommerfeld, Werner Heisenberg, Max Born and Wolfgang Pauli, among others, and flourished in Italy, where he arrived in 1953 called by Edoardo Amaldi, his first biographer and first Secretary-General of CERN.

In 1960 he proposed and built the first electron-positron storage ring, Anello di Accumulazione (AdA), which started operating in Frascati in February 1961. The following year, in order to improve the injection efficiency, a Franco-Italian collaboration was born that brought AdA to Orsay. It was here that the “Touschek effect“, describing the loss and scattering of charged particles in storage rings, was discovered and the proof of collisions in an electron-positron ring was obtained.

AdA paved the way to the electron-positron colliders ADONE in Italy, ACO in France, VEPP-2 in the USSR and SPEAR in the US. Bruno spent the last year of his life at CERN, from where – already quite ill – he was brought to Innsbruck, Austria, where he passed away on 25 May 1978 aged just 57.

touschek_sketch — Bruno Touschek’s playful portrayal of T D Lee and parity violation. Credit: F Touschek

Bruno Touschek’s life and scientific contributions were celebrated at a memorial symposium from 2 to 4 December, held in the three institutions where Touschek has left a lasting legacy: Sapienza University of Rome, INFN Frascati National Laboratories and Accademia Nazionale dei Lincei. Contributions also came from the Irène Joliot-Curie Laboratoire, and sponsorship from the Austrian Embassy in Italy.

In addition to Touschek’s impact on the physics of particle colliders, the three-day symposium addressed the present-day landscape. Carlo Rubbia and Ugo Amaldi gave a comprehensive overview of the past and future of particle colliders, followed by talks about physics at ADONE and LEP, and future machines, such as a muon collider, the proposed Future Circular Collider at CERN and the Circular Electron Positron Collider in China, as well as new developments in accelerator techniques. ADONE’s construction challenges were remembered. Developments in particle physics since the 1960s – including the quark model, dual models and string theory, spontaneous symmetry breaking and statistical physics – were described in testimonies from the universities of Rome, Frascati, Nordita and Collège de France.

Touschek’s direct influence was captured in talks by his former students, from Rome and the Frascati theory group, which he founded in the mid 1960s. His famous lectures on statistical mechanics, given from 1959 to 1960, were remembered by many speakers. Giorgio Parisi, who graduated with Nicola Cabibbo, recollected the years in Frascati after the observation of a large hadron multiplicity in e⁺ e^– annihilations made by ADONE, and the ideas leading to QCD.

The final day of the symposium, which took place at the Accademia dei Lincei where Touschek had been a foreign member since 1972, turned to future strategies in high-energy physics, including neutrinos and other messengers from the universe. Also prominent were the many benefits brought to society by particle accelerators, reaffirming the intrinsic broader value of fundamental research.

Touschek’s life and scientific accomplishments have been graphically illustrated in the three locations of the symposium, including displays of his famous drawings on academic life in Roma and Frascati. LNF’s visitor center was dedicated to Touschek, in the presence of his son Francis Touschek.

Costas Kounnas 1952–2022

Renowned Cypriot–French theoretical physicist Costas Kounnas passed away suddenly on 21 January, two days before his 70th birthday. Born in Famagusta, Cyprus, Costas did his undergraduate studies at the National and Kapodistrian University of Athens before moving to Paris for his advanced degree. His studies were interrupted by military service during the events in Cyprus in 1974, after which he completed his PhD at the École polytechnique, carrying out important calculations of QCD effects in deep inelastic scattering and jets. He joined the CNRS in 1980, and later took up a postdoctoral fellowship at CERN, where he made seminal contributions to models of supersymmetry and supergravity. In particular, he helped develop supergravity models in which supersymmetry was broken spontaneously without generating any vacuum energy – a bugbear of globally supersymmetric theories. Working with Costas on these models was one of our most exhilarating collaborations.

Costas then moved to Berkeley where he became a world expert in the construction of string models, showing in particular how they could be formulated directly in four dimensions, without invoking the compactification of extra dimensions. In 1987 he took up a position at the École normale supérieure in Paris, where he remained for the rest of his career, apart from a CERN staff position between 1993 and 1998. Many of his best-known papers during these periods concerned cosmological aspects of string models, loop corrections and the breaking of supersymmetry – topics in which he was a world leader. He was also director of the theoretical physics group at the École normale supérieure between 2009 and 2013.

Costas showed how string models could be formulated directly in four dimensions

Among his accolades, Costas was awarded the Paul Langevin Prize of the French Physical Society in 1995 and the Gay-Lussac Humboldt Prize in 2013 for outstanding scientific contributions, especially to cooperation between Germany and France. In addition, he received a prestigious Research Award from the Adolf von Humboldt Foundation in 2014.

His many friends mourn the passing, not just of a distinguished theoretical physicist, but also of a warm colleague with a great heart that he was not shy of wearing on his sleeve. Costas enjoyed participating exuberantly in scientific discussions, always with the overriding aim of uncovering the truth. We remember a joyful and energetic friend who was passionate about many other aspects of life beyond science, including his many friendships and his home island of Cyprus. He was active in efforts to develop its relations with CERN, where it is now an Associate Member on its way towards full membership.

Ronald Shellard 1948–2021

2021-12-07-1169 Shellard — Ronald Shellard was a long-time proponent of Brazil joining CERN as an Associate Member State. Credit: LIP

Ronald Shellard began his journey in physics in the 1970s at the University of São Paulo, where he took his undergraduate degree, and at the Institute of Theoretical Physics of São Paulo State University, where he completed his master’s in 1973. He received his doctorate, titled “Particle physics field theory, dynamical symmetry breakdown at the two loop and beyond”, from the University of California, Los Angeles in 1978 after also spending time at the University of California, Santa Barbara.

After a period working in theoretical particle physics, Shellard devoted himself to experimental and astroparticle physics. He joined the DELPHI collaboration at the former LEP collider at CERN in 1989, and in 1995 he joined the Pierre Auger Observatory, where he made an outstanding contribution both as a researcher and as an articulator of Brazilian collaboration. Remaining in the astroparticle field, during the past decade he was also involved in the Cherenkov Telescope Array, the Large Array Telescope for Tracking Energetic Sources and the Southern Wide Field Gamma-Ray Observatory.

Shellard played a key role in efforts to make Brazil an official member of CERN

From 2009 to 2013, Shellard was vice president of the Brazilian Physical Society. He participated tirelessly on various initiatives to promote Brazilian physics, such as the establishment of the exchange programme with the American Physical Society, the strengthening of the Brazilian physics Olympiad, the in-depth study of physics and national development, the establishment of the internship programme of high-school teachers at CERN, and the initiative to create a professional master’s degree in physics teaching. He was a member of the Brazilian Academy of Science since 2017, director of the Centro Brasileiro de Pesquisas Físicas since 2015 and president of the Brazilian network of high-energy physics since 2019. He played a key role in efforts to make Brazil an official member of CERN – a process that appears to be reaching a successful conclusion, with the CERN Council voting in September 2021 to grant to Brazil the status of Associate Member State, pending the signature of the corresponding agreement and its ratification by Brazilian authorities. Active until a few days before he passed away on 7 December, Ron was very excited about this news and was making plans for the next steps of the accession procedure.

Ron Shellard had an innovative and sensibly optimistic spirit, with a comprehensive and progressive vision of the crucial role of physics, and science in general, for the progress of Brazilian society. He exerted a great influence on the formation of the research community in high-energy physics. He was the advisor of several graduate students and had a permanent commitment to the training of new scientists and the dissemination and popularisation of science in the country.

Connecting CERN and South Asia

The decision by CERN in 2010 to introduce a policy of geographical enlargement to attract new Member States and Associate Member States, including from outside Europe, marked a prominent step towards the globalisation of high-energy physics. It aimed to strengthen relations with countries that can bring scientific and technological expertise to CERN and, in return, allow countries with developing particle-physics communities to build capacity. From South Asia, researchers have made significant contributions to pioneering activities of CERN over the past decades, including the construction of the LHC.

The first CERN South Asian High Energy Physics Instrumentation (SAHEPI) workshop, held in Kathmandu, Nepal, in 2017, came into place shortly after Pakistan (July 2015) and India (January 2017) became CERN Associate Member States and follows similar regional approaches in Latin America and South-east Asia. Also, within the South Asia region, CERN has signed bilateral international cooperation agreements with Bangladesh (2014), Nepal (2017) and Sri Lanka (2017). The second workshop took place in Colombo, Sri Lanka, in 2019. SAHEPI’s third edition took place virtually on 21 October 2021, hosted by the University of Mauritius in collaboration with CERN. Its aim was to consolidate the dialogue from the first two workshops while strengthening the scientific cooperation between CERN and the South Asia region.

“SAHEPI has been very successful in strengthening the scientific cooperation between CERN and the South Asia region and reinforcing intra-regional links,” said Emmanuel Tsesmelis, head of relations with Associate Members and non-Member States at CERN. “SAHEPI provides the opportunity for countries to enhance their existing contacts and to establish new connections within the region, with the objective of initiating new intra-regional collaborations in particle physics and related technologies, including the promotion of exchange of researchers and students within the region and also with CERN.”

Rising participation

Despite its virtual mode, SAHEPI-3 witnessed the largest participation yet, with 210 registrants. Representatives from Afghanistan, Bangladesh, Bhutan, India, Maldives, Mauritius, Nepal, Pakistan, and Sri Lanka attended, with at least one senior scientist and one student from each country. The workshop brought together physicists and policy makers from the South Asia region and neighbouring countries, together with representatives from CERN. Societal applications of technologies developed for particle physics were key highlights of SAHEPI-3, explained Archana Sharma, senior advisor for relations with international organisations at CERN:

“In this decade, disruptive innovation underpinning the importance of science and technology is making a huge impact towards the United Nations Sustainable Development Goals. CERN plays its role at the forefront, whether it is advances in science and technology or dissemination of that knowledge with an emphasis on inclusive engagement. We see the percolation of this initiative with increasing engagement from the region in CERN programmes.”

Participants reviewed the status and operation of present facilities in particle physics, and the scientific experimental programme, including the LHC and its high-luminosity upgrade at CERN, while John Ellis captivated participants with his talk “Answering the Big Question: From the Higgs boson to the dark side of the Universe”. Sanjaye Ramgoolam topped off the workshop with a public lecture on “the simple and the complex” in elementary particle physics.

SAHEPI has been very successful in strengthening the scientific cooperation between CERN and the South Asia region and reinforcing intra-regional links
Emmanuel Tsesmelis

Country representatives presented several highlights of the ongoing experimental programmes in collaboration with CERN and other international projects. India’s contributions across the ALICE experiment (such as the development of the photon multiplicity detector), its plans to join the IPPOG outreach group, its activities for the Worldwide LHC computing grid, industrial involvement and contributions to CMS – where it is the seventh-largest country in terms of the number of members – were presented. For Afghanistan, representatives described the participation of the country’s first student in the CERN Summer Student School (2019) and the completion of master’s degrees by two faculty members based on measurements at ATLAS. The country hopes to team up with particle physicists outside Afghanistan to teach online courses at the physics faculty at Kabul University, provide postgraduate scholarships to students and involve more female faculty members at ICTP – the International Centre for Theoretical Physics.

Pakistan shared its contributions to the LHC experiments as well as accelerator projects such as CLIC/CTF3 and Linac4 and its role in the tracker alignment of CMS and Resistive Plate Chambers. Nepal representatives described the development of supercomputers at Kathmandu University (KU) and acknowledged the donation agreement between KU and CERN receiving servers and related hardware to set up a high-performance computing facility. In Sri Lanka, delegates highlighted a rising popularity of the CERN Summer Student Programme among university physics students following honours degrees. The country also mentioned its initiative of an island-wide online teacher training programme to promote particle physics. The representative from Bangladesh reported on the country’s long tradition in theoretical particle physics and plans for developing the experimental particle physics community in partnership with CERN. Maldives and Bhutan continue to be growing members from South Asia at CERN, with Bhutan preparing to host the second South Asia science education programme in a hybrid-mode this year.

Strengthening relations
Chief guest Leela Devi Dookun-Luchoomun, the Vice-Prime Minister and Minister of Education, Tertiary Education, Science and Technology of Mauritius, informed the audience about the formation of a research and development unit in her ministry and gave her strong support to a partnership between CERN and Mauritius. The Vice-Chancellor of the University of Mauritius, Dhanjay Jhurry, expressed his deep appreciation of SAHEPI and indicated his support for future initiatives via a partnership between CERN and the University of Mauritius.

The workshop and the initiative to reinforce particle-physics capacity in the region also form part of broader efforts for CERN to emphasise the role of fundamental research in development, notably to advance the United Nations Sustainable Development Goals agenda. In this regard, discussions took place for a follow-up on the first-of-its-kind professional development programme for high-school teachers of STEM subjects from South Asia, held in New Delhi in 2019, with Bhutan volunteering to host the next event in 2023 pandemic permitting. A poster competition engaged students from South Asia, and three prizes were announced to encourage further participation in big-science projects and towards capacity building in the local regions.

The motivation and enthusiasm of SAHEPI-participants was notable, and the efforts in support of research and education across the region were clear. Proceedings of the workshop will be presented to representatives of the governments from the participating countries to raise awareness at the highest political level of the growth of the community in the region and its value for broader societal development.

Discussions will follow in 2023 at SAHEPI-4, helping CERN continue to engage further with particle physics research and education across South Asia for the benefit of the field as a whole.

Roadmaps set a path to post-LHC facilities

The AWAKE plasma-wakefield experiment at CERN is one of a handful of technologies recommended for further study and optimisation. Credit: CERN-PHOTO-201910-349-7

In setting out a vision for the post-LHC era, the 2020 update of the European strategy for particle physics (ESPPU) emphasised the need to ramp up detector and accelerator R&D in the near and long term. To this end, the European Committee for Future Accelerators (ECFA) was asked to develop a global detector R&D roadmap, while the CERN Council invited the European Laboratory Directors Group (LDG) to oversee the development of a complementary accelerator R&D roadmap.

After more than a year of efforts involving hundreds of people, and comprising more than 500 pages between them, both roadmaps were completed in December. In addition to putting flesh on the bones of the ESPPU vision, they provide a rich and detailed snapshot of the global state-of-the-art in detector and accelerator technologies.

Future-proof detectors

Beyond the successful completion of the high-luminosity LHC, the ESPPU identified an e⁺e^– Higgs factory as the highest priority future collider, and tasked CERN to undertake a feasibility study for a hadron collider operating at the highest possible energies with a Higgs factory as a possible first stage. The ESPPU also acknowledged that construction of the next generations of colliders and experiments will be challenging, especially for machines beyond a Higgs factory.

The development of cost-effective detectors that match the precision-physics potential of a Higgs factory is one of four key challenges in implementing the ESPPU vision, states the ECFA roadmap report. The second is to push the limitations in radiation tolerance, rate capabilities and pile-up rejection power to meet the unprecedented requirements of future hadron-collider and fixed-target experiments, while a third is to enhance the sensitivity and affordably expand the scales of both accelerator and non-accelerator experiments searching for rare phenomena. The fourth challenge identified by ECFA is to vigorously expand the technological basis of detectors, maintain a nourishing environment for new ideas and concepts, and attract and train the next generation of instrumentation scientists.

To address these challenges, ECFA set up a roadmap panel, chaired by Phil Allport of the University of Birmingham, and defined six task forces spanning different instrumentation topics (gaseous, liquid, solid state, particle-identification and photon, quantum, calorimetry) and three cross-cutting task forces (electronics, integration, training), with the most crucial R&D themes identified for each. Tasks are mapped to concrete time scales ranging from the present to beyond 2045, driven by the earliest technically achievable experiment or facility start-dates. The resulting picture reveals the potential synergies between concurrent projects pursued by separate communities, as well as between consecutive projects, which was one of the goals of the exercise, explains ECFA chair Karl Jakobs of the University of Freiburg: “It shows the role of earlier projects as a stepping stone for later ones, opening the possibility to evaluate and to organise R&D efforts in a much broader strategic context and on longer timescales, and allowing us to suggest greater coordination,” he says.

Attracting R&D experts and recognising and sustaining their careers is one of 10 general strategic recommendations made by the report. Others include support for infrastructure and facilities, industrial partnerships, software, open science, blue-sky research, and recommendations relating to international coordination and strategic funding programmes. Guided by this roadmap, concludes the report, concerted and “resource-loaded” R&D programmes in innovative instrumentation will transform the ability of present and future generations of researchers to explore and observe nature beyond current limits.

“Ensuring the goals of future collider and non-collider experiments are not compromised by detector readiness calls now for an R&D collaboration programme, similar to that initiated in 1990 to better manage the activities then already underway for the LHC,” adds Allport. “These should be focused on addressing their unmet technology requirements through common research projects, exploiting where appropriate developments in industry and synergies with neighbouring disciplines.”

Accelerating physics

Although accelerator R&D is necessarily a long-term endeavour, the LDG roadmap focuses on the shorter but crucial timescale of the next five-to-ten years. It concentrates on the five key objectives identified in the ESPPU: further development of high-ﬁeld superconducting magnets; advanced technologies for superconducting and normal-conducting radio-frequency (RF) structures; development and exploitation of laser/plasma-acceleration techniques; studies and developments towards future bright muon beams and muon colliders; and the advancement and exploitation of energy-recovery linear accelerator technology. Expert panels were convened to examine each area, which are at different stages of maturity, and to identify the key R&D objectives.

The high-field-magnets panel supports continued and accelerated progress on both niobium-tin and high-temperature superconductor technology, placing strong emphasis on its inclusion into practical accelerator magnets and warning that final designs may have to reﬂect a compromise between performance and practicality. The panel for high-gradient RF structures and systems also identified work needed on basic materials and construction techniques, noting signiﬁcant challenges to improve efﬁciency. Longer term, it flags a need for automated test, tuning and diagnostic techniques, particularly where large-scale series-production is needed.

Energy consumption and sustainability are key considerations in defining R&D priorities and in the design of new machines

In the area of advanced plasma and laser acceleration, the panel focused on rapidly evolving plasma-wakeﬁeld and dielectric acceleration technologies. Further developments require reduced emittance and improved efﬁciency, the ability to accelerate positrons and the combination of accelerating stages in a realistic future collider, the panel concludes, with the goal to produce a statement about the basic feasibility of such a machine by 2026. The panel exploring muon beams and colliders also sets a date of 2026 to demonstrate that further investment is justiﬁed, focusing on a 10 TeV collider with a 3 TeV intermediate-scale facility targeted for the 2040s. Finally, having considered several medium-scale projects under way worldwide, the energy-recovery linacs panel identifies reaching the 10 MW power level as the next practical step, and states that future sustainability rests on developing 4.4 K superconducting RF technology for a next-generation e⁺e^– collider.

In addition to the technical challenges, states the report, new investment will be needed to support R&D and test facilities. Energy consumption and sustainability are explicitly identified as key considerations in defining R&D priorities and in the design of new machines. Having identified objectives, each panel set out a detailed work plan covering the period to the next ESPPU, with options for a number of different levels of investment. The aim is to allow the R&D to be pushed as rapidly as needed, but in balance with other priorities for the field.

Like its detector R&D counterpart, the report concludes with 10 concrete recommendations. These include the attraction, training and career management of researchers, observations on the implementation and governance of the programme, environmental sustainability, cooperation between European and international laboratories, and continuity of funding.

“The accelerator R&D roadmap represents the collective view of the accelerator and particle-physics communities on the route to machines beyond the Higgs factories,” says Dave Newbold, LDG chair and director of particle physics for STFC in the UK. “We now need to move swiftly forwards with an ambitious, cooperative and international R&D programme – the potential for future scientific discoveries depends on it.”

Bernhard Spaan 1960–2021

Bernhard Spaan, an exceptional particle physicist and a wonderful colleague, unexpectedly passed away on 9 December, much too early at the age of 61.

Bernhard studied physics at the University of Dortmund, obtaining his diploma thesis in 1985 working on the ARGUS experiment at DESY’s electron–positron collider DORIS. Together with CLEO at Cornell, ARGUS was the first experiment dedicated to heavy-flavour physics, which became the central theme of Bernhard’s research work for the following 36 years. Progressing from ARGUS and CLEO to the higher-statistics experiments BaBar and ultimately LHCb, for which he made early contributions, he was one of the pioneering leaders in the next generation of heavy-flavour experiments at both electron–positron and hadron colliders.

While working on tau–lepton decays at ARGUS for his doctorate, Bernhard led a study of tau decays to five charged pions and a tau neutrino, which resulted in the world’s best upper limit for the tau-neutrino mass at the time. He also pioneered a new method of reconstructing the pseudo mass of the tau lepton by approximating the tau direction with the direction of the hadronic system. This method led to a new tau-lepton mass, which was an important ingredient to resolve the long-standing deviation from lepton universality as derived from the measurements of the tau lifetime, mass and leptonic branching fraction.

In 1993 Bernhard joined McGill University in Montreal, where he contributed to CLEO operation, data-taking and analysis, and was brought into contact with the formative stages of an asymmetric electron–positron B-factory at SLAC. He was an author of the BaBar letter of intent in 1994 and remained a leading member of the collaboration for the two following decades.

Bernhard saw the unique potential of a dedicated B experiment at the LHC and joined the LHCb collaboration

In 1996 Bernhard started a professorship at Dresden where, together with Klaus Schubert, he built a strong German BaBar participation including involvement in the construction and operation of the calorimeter. At that time, BaBar was pioneering the use of distributed computing resources for data-processing. As one of the proponents of this approach, Bernhard played a crucial role in the German contribution via the computing centre at Karlsruhe, later “GridKa”. Building on the success of the electron–positron B-factories, Bernhard saw the unique potential of a dedicated B experiment at the LHC and joined the LHCb collaboration in 1998.

Bernhard’s scientific journey came full circle when he accepted a professorship at Dortmund University in 2004, which he used to significantly grow his LHCb participation. The Dortmund group is one of LHCb’s largest, with a long list of graduate students and main research topics including the determination of the CKM angles β and γ governing CP violation in rare B decays. In parallel with LHC Run 1 and 2 data-taking, Bernhard investigated the possibility of using scintillating fibres for a novel tracking detector capable of operating at much larger luminosities. In all phases of the “SciFi” detector, which was recently installed ahead of LHC Run 3, he supported the project with his ideas, his energy and the commitment of his group.

Bernhard was an outstanding experimental physicist whose many contributions shaped the field of experimental heavy-flavour physics. He was also a great communicator. His ability to resolve conflicts and to find compromises brought many additional tasks to Bernhard, whether as dean of the Dortmund faculty, chair of the national committee for particle physics, member of R-ECFA or chair of the LHCb collaboration board. When help was needed, Bernhard never said “no”.

We have lost a tremendous colleague and a dear friend who will be sorely missed not only by us, but the wider field.

One day in September: Copenhagen

**One day in September** The ghosts of Niels Bohr, Werner Heisenberg and Margrethe Bohr discuss the infamous 1941 meeting in Copenhagen. Credit: Winchester Today

“But why?” asks Margrethe Bohr. Her husband, Niels, replies “Does it matter my love now that we’re all three of us dead and gone?” Alongside Werner Heisenberg, the trio look like spirits meeting in an atemporal dimension, maybe the afterlife, under an eerie ring of light. Dominating an almost empty stage, they try to revive what happened on one day in September 1941, when Heisenberg, a prominent figure in Hitler’s Uranverein (Uranium Club), travelled to Nazi-occupied Denmark to visit his former mentor, Niels Bohr.

Why did Heisenberg go to meet Bohr that day? Did he seek an agreement not to develop the bomb in Germany? Was he searching for intelligence on Allied progress? To convince Bohr that there was no German programme? Or to pick Bohr’s brain on atomic physics? Or, according to Margrethe, to show off? Perhaps his motives were a superposition of all of these. No one knows what was said. This puzzle has intrigued historians ever since.

Eighty years after that meeting, and 23 since Michael Frayn’s masterwork Copenhagen premiered at the National Theatre in London, award-winning director Polly Findlay and Emma Howlett in her professional directorial debut have revived a play that contains little action but much physics and food for thought.

The three actors orbit like electrons in an atom

Frayn’s nonlinear script is based on three possible versions of the same meeting in Copenhagen in 1941, which can be construed as three different scenarios playing out in the many-worlds interpretation of quantum mechanics. He describes it as the process of rewriting a draft of a paper again and again, trying to unlock more secrets. In the afterlife, the trio’s dialogue jumps back and forth in time, adding confusing memories and contradicting hypotheses. Delivered at pace, the narrative explores historical information and their personal stories.

The three characters reflect on how German scientists failed to build the bomb, even though they had the best start; Otto Hahn, Lise Meitner and Fritz Strassmann having discovered nuclear fission in 1939. But Frayn highlights how Hitler’s Deutsche Physik was hostile to so-called Jewish physics and key Jewish physicists, including Bohr, who later fled to Los Alamos in the US. Frayn’s Heisenberg reveals the disbelief he felt when he learnt about the destruction of Hiroshima on the radio. At the time he was detained in Farm Hall, not far from this theatre in Cambridge in the UK, together with other members of the Uranium Club. Called Operation Epsilon, the bugged hall was used by the Allied forces to try to uncover the state of Nazi scientific progress.

The three actors orbit like electrons in an atom, while the theatre’s revolving stage itself spins. Superb acting by Philip Arditti and Malcolm Sinclair elucidates an extraordinary student–mentor relationship between Heisenberg and Bohr. The sceptical Mrs Bohr (Haydn Gwynne) steers the conversation and questions their friendship, cajoling Bohr to speak in plain language. Nevertheless, the use of scientific jargon could leave some non-experts in the audience behind.

Although Heisenberg wrote in his autobiography that “it would be better to stop disturbing the spirits of the past,” the private conversation between the two physicists has stirred the interest of the public, journalists and historians for years. In 1956 the journalist Robert Jungk wrote in his debated book, Brighter than a Thousand Suns, that Heisenberg wanted to prevent the development of an atomic bomb. This book was also an inspiration for Frayn’s play. More recently, in 2001, Bohr’s family released some letters that Bohr wrote and never sent to Heisenberg. According to these letters, Bohr was convinced that Heisenberg was building the bomb in Germany.

To this day, the reason for Heisenberg’s visit to Copenhagen remains uncertain, or unknowable, like the properties of a quantum particle that’s not observed. The audience can only imagine what really happened, while considering all philosophical interpretations of the fragility of the human species.

Witten reflects

**Trendsetter** Edward Witten, Charles Simonyi professor at the Institute for Advanced Study, Princeton, on the occasion of the award of the 2012 Breakthrough Prize in Fundamental Physics. Credit: B Lacombe/Breakthrough Prize

How has the discovery of a Standard Model-like Higgs boson changed your view of nature?

The discovery of a Standard Model-like Higgs boson was a great triumph for renormalisable field theory, and really for simplicity. By the time the LHC was operating, attempts to make the Standard Model (SM) work without an elementary Higgs field – using a dynamical mechanism instead – had become rather convoluted. It turned out that, as far as one can judge from what we have learned so far, the original idea of an elementary Higgs particle was correct. This also means that nature takes advantage of all the possible building blocks of renormalisable field theory – fields of spin 0, 1/2 and 1 – and the flexibility that that allows.

The other key fact is that the Higgs particle has appeared by itself, and without any sign of a mechanism that would account for the smallness of the energy scale of weak interactions compared to the much larger presumed energy scales of gravity, grand unification and cosmic inflation. From the perspective that my generation of particle physicists grew up with (and not only my generation, I would say), this is quite a shock. Of course, we lived through a somewhat similar shock a little over 20 years ago with the discovery that the expansion of the universe is accelerating – something that is most simply interpreted in terms of a very small but positive cosmological constant, the energy density of the vacuum. It seems that the ideas of naturalness that we grew up with are failing us in at least these two cases.

What about new approaches to the fine-tuning problem such as the relaxion or “Nnaturalness”?

Unfortunately, it has been very hard to find a conventional natural explanation of the dark energy and hierarchy problems. Reluctantly, I think we have to take seriously the anthropic alternative, according to which we live in a universe that has a “landscape”of possibilities, which are realised in different regions of space or maybe in different portions of the quantum mechanical wavefunction, and we inevitably live where we can. I have no idea if this interpretation is correct, but it provides a yardstick against which to measure other proposals. Twenty years ago, I used to find the anthropic interpretation of the universe upsetting, in part because of the difficulty it might present in understanding physics. Over the years I have mellowed. I suppose I reluctantly came to accept that the universe was not created for our convenience in understanding it.

Which experimental paths should physicists prioritise at this time?

It is extremely important to probe the twin mysteries of the cosmic acceleration and the smallness of the electroweak scale as thoroughly as possible, in order to determine whether we are interpreting the facts correctly and possibly to discover a new layer of structure. In the case of the cosmic acceleration, this means measuring as precisely as we can the parameter w (the ratio of pressure and energy), which equals –1 if the acceleration of the expansion is governed by a simple cosmological constant, but would be greater than –1 in most alternative models. In particle physics, we would like to probe for further structure as precisely as we can both indirectly, for example with precision studies of the Higgs particle, and hopefully directly by going to higher energies than are available at the LHC.

What might be lurking at energies beyond the LHC?

If it is eventually possible to go to higher energies, I can imagine several possible outcomes. It might become rather clear that the traditional idea of naturalness is not the whole story and that we have on our hands a “bare” Higgs particle, without a mechanism that would account for its mass scale. Alternatively, we might find out that the apparent failure of naturalness was an illusion and that additional particles and forces that provide an explanation for the electroweak scale are just beyond our current experimental reach. There is also an intermediate possibility that I find fascinating. This is that the electroweak scale is not natural in the customary sense, but additional particles and forces that would help us understand what is going on exist at an energy not too much above LHC energies. A fascinating theory of this type is the “split supersymmetry” that has been proposed by Nima Arkani-Hamed and others.

It seems that the ideas of naturalness that we grew up with are now failing us

There is an obvious catch, however. It is easy enough to say “such-and-such will happen at an energy not too much above LHC energies”. But for practical purposes, it makes a world of difference whether this means three times LHC energies, six times LHC energies, 25 times LHC energies, or more. In theories such as split supersymmetry, the clues that we have are not sufficient to enable a real answer. A dream would be to get a concrete clue from experiment about what is the energy scale for new physics beyond the Higgs particle.

Could the flavour anomalies be one such clue?

There are multiple places that new clues could come from. The possible anomalies in b physics observed at CERN are extremely significant if they hold up. The search for an electric dipole moment of the electron or neutron is also very important and could possibly give a signal of something new happening at energies close to those that we have already probed. Another possibility is the slight reported discrepancy between the magnetic moment of the muon and the SM prediction. Here, I think it is very important to improve the lattice gauge theory estimates of the hadronic contribution to the muon moment, in order to clarify whether the fantastically precise measurements that are now available are really in disagreement with the SM. Of course, there are multiple other places that experiment could pinpoint the next energy scale at which the SM needs to be revised, ranging from precision studies of the Higgs particle to searches for muon decay modes that are absent in the SM.

Which current developments in theory are you most excited about?

The new ideas about gravity and quantum mechanics that go under the rough title “It from qubit” are really exciting. Black-hole thermodynamics was discovered in the 1970s through the work of Jacob Bekenstein, Stephen Hawking and others. These results were fascinating, but for several decades it seemed to me – rightly or wrongly – that this field was evolving only slowly compared to other areas of theoretical physics. In the past decade or so, that is clearly no longer the case. In large part the change has come from thinking about “entropy” as microscopic or fine-grained von Neumann entropy, as opposed to the thermodynamic entropy that Bekenstein and others considered. A formulation in terms of fine-grained entropy has made possible new statements and more general statements which reduce to the traditional ones when thermodynamics is valid. All this has been accelerated by the insights that come from holographic duality between gravity and gauge theory.

How different does the field look today compared to when you entered it?

It is really hard to exaggerate how the field has changed. I started graduate school at Princeton in September 1973. Asymptotic freedom of non-abelian gauge theory had just been discovered a few months earlier by David Gross, Frank Wilzcek and David Politzer. This was the last key ingredient that was needed to make possible the SM as we know it today. Since then there has been a revolution in our experimental knowledge of the SM. Several key ingredients (new quarks, leptons and the Higgs particle) were unknown in 1973. Jets in hadronic processes were still in the future, even as an idea, let alone an experimental reality, and almost nothing was known about CP violation or about scaling violations in high-energy hadronic processes, just to mention two areas that developed later in an impressive way.

**Higher plane** 6D Calabi–Yau manifolds, on which the extra dimensions of string theory are conjectured to be “compactified”, exhibit powerful symmetries that Witten helped uncover. Credit: A J Hanson

Not only is our experimental knowledge of the SM so much richer than it was in 1973, but the same is really true of our theoretical understanding as well. Quantum field theory is understood much better today than was the case in 1973. There really is no comparison.

Perhaps equally dramatic has been the change in our understanding of cosmology. In 1973, the state of cosmological knowledge could be summarised fairly well in a couple of numbers – notably the cosmic-microwave temperature and the Hubble constant – and of these only the first was measured with any reasonable precision. In the intervening years, cosmology became a precision science and also a much more ambitious science, as cosmologists have learned to grapple with the complex processes of the formation of structure in the universe. In the inhomogeneities of the microwave background, we have observed what appear to be the seeds of structure formation. And the theory of cosmic inflation, which developed starting around 1980, seems to be a real advance over the framework in which cosmology was understood in 1973, though it is certainly still incomplete.

Exploring the string-theory framework has led to a remarkable series of discoveries

Finally, 50 years ago the gulf between particle physics and gravity seemed unbridgeably wide. There is still a wide gap today. But the emergence in string theory of a sensible framework to study gravity unified with particle forces has changed the picture. This framework has turned out to be very powerful, even if one is not motivated by gravity and one is just searching for new understanding of ordinary quantum field theory. We do not understand today in detail how to unify the forces and obtain the particles and interactions that we see in the real world. But we certainly do have a general idea of how it can work, and this is quite a change from where we were in 1973. Exploring the string-theory framework has led to a remarkable series of discoveries. This well has not run dry, and that is one of the reasons that I am optimistic about the future.

Which of the numerous contributions you have made to particle and mathematical physics are you most proud of?

I am most satisfied with the work that I did in 1994 with Nathan Seiberg on electric-magnetic duality in quantum field theory, and also the work that I did the following year in helping to develop an analogous picture for string theory.

Who knows, maybe I will have the good fortune to do something equally significant again in the future.

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Multidisciplinary CERN forum tackles AI

Machine-learning expert Anima Anandkumar of Caltech and NVIDIA speaking in the Globe at CERN on 20 September. Credit: CERN-PHOTO-202109-132-52

The inaugural Sparks! Serendipity Forum attracted 49 leading computer scientists, policymakers and related experts to CERN from 17 to 18 September for a multidisciplinary science-innovation forum. In this first edition, participants discussed a range of ethical and technical issues related to artificial intelligence (AI), which has deep and developing importance for high-energy physics and its societal applications. The structure of the discussions was designed to stimulate interactions between AI specialists, scientists, philosophers, ethicists and other professionals with an interest in the subject, leading to new insights, dialogue and collaboration between participants.

World-leading cognitive psychologist Daniel Kahneman opened the public part of the event by discussing errors in human decision making, and their impact on AI. He explained that human decision making will always have bias, and therefore be “noisy” in his definition, and asked whether AI could be the solution, pointing out that AI algorithms might not be able to cope with the complexity of decisions that humans have to make. Others speculated as to whether AI could ever achieve the reproducibility of human cognition – and if the focus should shift from searching for a “missing link” to considering how AI research is actually conducted by making the process more regulated and transparent.

Introspective AI

Participants discussed both the advantages and challenges associated with designing introspective AI, which is capable of examining its own processes and could be beneficial in making predictions about the future. Participants also questioned, however, whether we should be trying to make AI more self-aware and human-like. Neuroscientist Ed Boyden explored introspection through the lens of neural pathways, and asked whether we can design introspective AI before we understand introspection in brains. Following the introspection theme, philosopher Luisa Damiano addressed the reality versus fiction of “social-embodied” AI – the idea of robots interacting with us in our physical world – arguing that such a possibility would require careful ethical considerations.

AI is already a powerful, and growing, tool for particle physics

Many participants advocated developing so-called “strong” AI technology that can solve problems it has not come across before, in line with specific and targeted goals. Computer scientist Max Welling explored the potential for AI to exceed human intelligence, and suggested that AI can potentially be as creative as humans, although further research is required.

On the subject of ethics, Anja Kaspersen (former director of the UN Office for Disarmament Affairs) asked: who makes the rules? Linking to military, humanitarian and technological affairs, she considered how our experience in dealing with nuclear weapons could help us deal with the development of AI. She said that AI is prone to ethics washing: the process of creating an illusory sense that ethical issues are being appropriately addressed when they are not. Participants agreed that we should seek to avoid polarising the community when considering risks associated with current and future AI, and suggested a more open approach to deal with the challenges faced by AI today and tomorrow. Skype co-founder Jann Tallin identified AI as one of the most worrying existential risks facing society today; the fact that machines do not consider whether their decisions are unethical demands that we consider the constraints of the AI design space within the realm of decision making.

Fruits of labour

The initial outcomes of the Sparks! Serendipity Forum are being written up as a CERN Yellow Report, and at least one paper will be submitted to the journal Machine Learning Science and Technology. Time will tell what other fruits of the serendipitous interactions at Sparks! will bring. One thing is certain, however, AI is already a powerful, and growing, tool for particle physics. Without it, the LHC experiments’ analyses would have been much more tortuous, as discussed by Jennifer Ngadiuba and Maurizio Pierini (CERN Courier September/October 2021 p31)

Future editions of the Sparks! Serendipity Forum will tackle different themes in science and innovation that are relevant to CERN’s research. The 2022 event will be built around future health technologies, including the many accelerator, detector and simulation technologies that are offshoots of high-energy-physics research.

Training future experts in the fight against cancer

**Hands on** The “matRad” user interface showing a carbon-therapy treatment plan for a prostate tumour. Credit: DKFZ

The leading role of CERN in fundamental research is complemented by its contribution to applications for the benefit of society. A strong example is the Heavy Ion Therapy Masterclass (HITM) school, which took place from 17 to 21 May 2021. Attracting more than 1000 participants from around the world, many of whom were young students and early-stage researchers, the school demonstrated the enormous potential to train the next generation of experts in this vital application. It was the first event of the European Union project HITRIplus (Heavy Ion Therapy Research Integration), in which CERN is a strategic partner along with other research infrastructures, universities, industry partners, the four European heavy-ion therapy centres and the South East European International Institute for Sustainable Technologies (SEEIIST). As part of a broader “hands-on training” project supported by the CERN and Society Foundation with emphasis on capacity building in Southeast Europe, the event was originally planned to be hosted in Sarajevo but was held online due to the pandemic.

The school’s scientific programme highlighted the importance of developments in fundamental research for cancer diagnostics and treatment. Focusing on treatment planning, it covered everything needed to deliver a beam to a tumour target, including the biological response of cancerous and healthy tissues. The Next Ion Medical Machine Study (NIMMS) group delivered many presentations from experts and young researchers, starting from basic concepts to discussions of open points and plans for upgrades. Expert-guided practical sessions were based on the matRad open-source professional toolkit, developed by the German cancer research centre DKFZ for training and research. Several elements of the course were inspired by the International Particle Therapy Masterclasses.

Virtual visits to European heavy-ion therapy centres and research infrastructures were ranked by participants among the most exciting components of the course. There were also plenty of opportunities for participants to interact with experts in dedicated sessions, including a popular session on entrepreneurship by the CERN Knowledge Transfer group. This interactive approach had a big impact on participants, several of which were motivated to pursue careers in related fields and to get actively involved at their home institutes. This future expert workforce will become the backbone for building and operating future heavy-ion therapy and research facilities that are needed to fight cancer worldwide (see Linacs to narrow radiotherapy gap).

Further support is planned at upcoming HITRIplus schools on clinical and medical aspects, as well as HITRIplus internships, to optimally access existing European heavy-ion therapy centres and contribute to relevant research projects.

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