Praxis Archives – Page 49 of 181

European strategy enters next phase

The European Strategy for Particle Physics got under way last year. Credit: CERN

Physicists in Europe have published a 250-page “briefing book” to help map out the next major paths in fundamental exploration. Compiled by an expert physics-preparatory group set up by the CERN Council, the document is the result of an intense effort to capture the status and prospects for experiment, theory, accelerators, computing and other vital machinery of high-energy physics.

Last year, the European Strategy Group (ESG) — which includes scientific delegates from CERN’s member and associate-member states, directors and representatives of major European laboratories and organisations and invitees from outside Europe — was tasked with formulating the next update of the European strategy for particle physics. Following a call for input in September 2018, which attracted 160 submissions, an open symposium was held in Granada, Spain, on 13-16 May at which more than 600 delegates discussed the potential merits and challenges of the proposed research programmes. The ESG briefing book distills input from the working groups and the Granada symposium to provide an objective scientific summary.

“This document is the result of months of work by hundreds of people, and every effort has been made to objectively analyse the submitted inputs,” says ESG chair Halina Abramowicz of Tel Aviv University. “It does not take a position on the strategy process itself, or on individual projects, but rather is intended to represent the forward thinking of the community and be the main input to the drafting session in Germany in January.”

Collider considerations
An important element of the European strategy update is to consider which major collider should follow the LHC. The Granada symposium revealed there is clear support for an electron–positron collider to study the Higgs boson in greater detail, but four possible options at different stages of maturity exist: an International Linear Collider (ILC) in Japan, a Compact Linear Collider (CLIC) or Future Circular Collider (FCC-ee) at CERN, and a Circular Electron Positron Collider (CEPC) in China. The briefing book states that, in a global context, CLIC and FCC-ee are competing with the ILC and with CEPC. As Higgs factories, however, the report finds all four to have similar reach, albeit with different time schedules and with differing potentials for the study of physics topics at other energies.

Also considered in depth are design studies in Europe for colliders that push the energy frontier, including a 3 TeV CLIC and a 100 TeV circular hadron collider (FCC-hh). The briefing book details the estimated timescales to develop some of these technologies, observing that the development of 16 T dipole magnets for FCC-hh will take a comparable time (about 20 years) to that projected for novel acceleration technologies such as plasma-wakefield techniques to reach conceptual designs.

“The Granada symposium and the briefing book mention the urgent need for intensifying accelerator R&D, including that for muon colliders,” says Lenny Rivkin of Paul Scherrer Institut, who was co-convener of the chapter on accelerator science and technology. “Another important aspect of the strategy update is to recognize the potential impact of the development of accelerator and associated technology on the progress in other branches of science, such as astroparticle physics, cosmology and nuclear physics.”

The bulk of the briefing book details the current physics landscape and prospects for progress, with chapters devoted to electroweak physics, strong interactions, flavour physics, neutrinos, cosmic messengers, physics beyond the Standard Model, and dark-sector exploration. A preceding chapter about theory emphasises the importance of keeping theoretical research in fundamental physics “free and diverse” and “not only limited to the goals of ongoing experimental projects”. It points to historical success stories such as Peter Higgs’ celebrated 1964 paper, which had the purely theoretical aim to show that Gilbert’s theorem is invalid for gauge theories at a time when applications to electroweak interactions were well beyond the horizon.

“While an amazing amount of progress has been made in the past seven years since the Higgs boson discovery, our knowledge of the couplings of the Higgs-boson to the W and Z and to third-generation charged fermions is quite imprecise, and the couplings of the Higgs boson to the other charged fermions and to itself are unmeasured,” says Beate Heinemann of DESY, who co-convened the report’s electroweak chapter. “The imperative to study this unique particle further derives from its special properties and the special role it might play in resolving some of the current puzzles of the universe, for example dark matter, the matter-antimatter asymmetry or the hierarchy problem.”

Readers are reminded that the discovery of neutrino oscillations constitutes a “laboratory” proof of physics beyond the Standard Model. The briefing book also notes the significant role played by Europe, via CERN, in neutrino-experiment R&D since the last strategy update concluded in 2013. Flavour physics too should remain at the forefront of the European strategy, it argues, noting that the search for flavour and CP violation in the quark and lepton sectors at different energy frontiers “has a great potential to lead to new physics at moderate cost”. An independent determination of the proton structure is needed if present and future hadron colliders are to be turned into precision machines, reports the chapter on strong interactions, and a diverse global programme based on fixed-target experiments as well as dedicated electron-proton colliders is in place.

Europe also has the opportunity to play a leading role in the searches for dark matter “by fully exploiting the opportunities offered by the CERN facilities, such as the SPS, the potential Beam Dump Facility, and the LHC itself, and by supporting the programme of searches for axions to be hosted at other European institutions”. The briefing book notes the strong complementarity between accelerator and astrophysical searches for dark matter, and the demand for deeper technology sharing between particle and astroparticle physics.

Scientific diversity
The diversity of the experimental physics programme is a strong feature of the strategy update. The briefing book lists outstanding puzzles that did not change in the post-Run 2 LHC era – such as the origin of electroweak symmetry breaking, the nature of the Higgs boson, the pattern of quark and lepton masses and the neutrino’s nature – that can also be investigated by smaller scale experiments at lower energies, as explored by CERN’s dedicated Physics Beyond Colliders initiative.

Finally, in addressing the vital roles of detector & accelerator development, computing and instrumentation, the report acknowledges both the growing importance of energy efficiency and the risks posed by “the limited amount of success in attracting, developing and retaining instrumentation and computing experts”, urging that such activities be recognized correctly as fundamental research activities. The strong support in computing and infrastructure is also key to the success of the high-luminosity LHC which, the report states, will see “a very dynamic programme occupying a large fraction of the community” during the next two decades – including a determination of the couplings between the Higgs boson and Standard Model particles “at the percent level”.

Following a drafting session to take place in Bad Honnef, Germany, on 20-24 January, the ESG is due to submit its recommendations for the approval of the CERN Council in May 2020 in Budapest, Hungary.

“Now comes the most challenging part of the strategy update process: how to turn the exciting and well-motivated scientific proposals of the community into a viable and coherent strategy which will ensure progress and a bright future for particle physics in Europe,” says Abramowicz. “Its importance cannot be overestimated, coming at a time when the field faces several crossroads and decisions about how best to maintain progress in fundamental exploration, potentially for generations to come.”

Black-hole snap scoops 2020 Breakthrough Prize in Fundamental Physics

The first direct image of a black hole, obtained by the Event Horizon Telescope (EHT) collaboration earlier this year, has been recognized by the 2020 Breakthrough Prize in Fundamental Physics. The $3 million prize will be shared equally between 347 researchers who were co-authors of the six papers published by the EHT collaboration on 10 April.

The EHT is a network of eight radio dishes in Antarctica, Chile, Mexico, Hawaii, Arizona and Spain that creates an Earth-sized interferometer. Its ultra-high angular resolution images of radio emission from a supermassive black hole at the heart of galaxy M87* opened a new window on black holes and other phenomena. Recently, a team at Brookhaven National Laboratory used the EHT image to disfavour “fuzzy” models of ultra-light boson dark matter.

Also announced were six New Horizons Prizes worth $100,000 each, which recognize early-career achievements in physics and mathematics. In physics, Jo Dunkley (Princeton); Samaya Nissanke (University of Amsterdam) and Kendrick Smith (Perimeter Institute) were awarded for the development of novel techniques to extract fundamental physics from astronomical data. Simon Caron-Huot (McGill University) and Pedro Vieira (Perimeter Institute) were recognized for their “profound contributions to the understanding of quantum field theory”.

The Breakthrough Prize was founded in 2012 by former physicist and entrepreneur Yuri Milner, with sponsors including Google’s Sergey Brin and Facebook’s Mark Zuckerberg. In August, a Special Breakthrough Prize in Fundamental physics was awarded to Sergio Ferrara, Daniel Freedman and Peter van Nieuwenhuizen for the discovery of supergravity.

All prize recipients, along winners in mathematics and biology, will receive their awards at a ceremony in California on 3 November.

Ghent event surveys future of the field

**Making connections** EPS-HEP participants size up the future of the field. Credit: D Perepelitsa

Almost 750 high-energy physicists met from 10–17 July in Ghent, Belgium, for the 2019 edition of EPS-HEP. The full scope of the field was put under a microscope by more than 500 parallel and plenary talks and a vibrant poster session. The ongoing update of the European Strategy for Particle Physics (ESPP) was a strong focus, and the conference began with a session jointly organised by the European Committee for Future Accelerators to seek further input from the community ahead of the publication of the ESPP briefing book in September.

The accepted view, explained ESPP secretary Halina Abramowicz, is that an electron–positron collider should succeed the Large Hadron Collider (LHC). The question is whether to build a linear collider that is extendable to higher energies, or a circular collider whose infrastructure could later be reused for a hadron collider. DESY’s Christophe Grojean weighed up the merits of a Large Electron Positron collider (LEP)-style Z-pole run at a high-luminosity circular machine – a “tera-Z factory” – against the advantages of the polarised beams proposed at linear facilities, and questioned the value of polarisation to measurements of the Higgs boson at energies above 250 GeV. Furthermore, he said, sensitivities should be evaluated in light of the expected performance of the high-luminosity LHC (HL-LHC).

Blue skies required

Presentations on accelerator and detector challenges emphasised the importance of sharing development between competing projects: while detector technology for an electron–positron machine could begin production within about five years, proposed hadron colliders require a technological leap in both radiation hardness and readout speed. CERN’s Ariella Cattai expressed concern for excessive utilitarianism in detector development, with only 5% of R&D being blue-sky despite the historical success of this approach in developing TPC, RICH and silicon strip detectors, among others. She also pointed out that although 80% of R&D specialists believe their work has potential social outcomes, less than a third feel adequately supported to engage in technology transfer. Delegates agreed on the need for more recognition for those who undertake this crucial work. CERN’s Graeme Stewart highlighted the similar plight of theorists developing event generators, whose work is often not adequately rewarded or supported. The field also needs to keep pace with computing developments outside the field, he said, by designing data models and code that are optimised for graphics-processing units rather than CPUs (central-processing units).

The accepted view is that an electron–positron collider should succeed the LHC

The beginning of the main EPS conference was dominated by impressive new results from ATLAS and CMS, as they begin to probe Higgs couplings to second-generation fermions, and as the experiments continue to search for new phenomena and rare processes. Several speakers noted that the LHC even has the potential to exceed LEP in precision electroweak physics: although the hadronic environment increases systematic uncertainties, deviations arising from beyond-Standard Model (SM) phenomena are expected to scale with the centre-of-mass energy squared. Giulia Zanderighi of the Max Planck Institute and Claude Duhr of CERN also highlighted the need to improve the precision of theoretical calculations if they are to match experimental precision by the end of the HL-LHC’s run, showcasing work to extend next-to-next-to-leading order (NNLO) calculations to two-to-three processes, and the latest moves to N³LO calculations.

The flavour-physics scene was updated with new SM-consistent constraints from Belle on the ratios R(D) and R(D*), somewhat lessening the suggestion of lepton-universality violation in B-meson decays. With the advent of Belle II, and the impending analysis of LHCb’s full Run 2 dataset, the flavour anomalies will surely soon be confirmed or resolved. LHCb also presented new measurements of the gamma angle of the unitarity triangle, which show a mild 2σ tension between the values obtained from B⁺ and B_s⁰ decays. Meanwhile, long-baseline neutrino-oscillation experiments provided tantalising information on leptonic CP violation, with T2K data excluding CP conservation at 2σ irrespective of the neutrino mass hierarchy, and NOVA disfavouring an inverted hierarchy of neutrino mass eigenstates at 1.9σ.

Background checks

A refrain common to both collider and non-collider searches for dark-matter candidates was the need to eliminate backgrounds. A succession of talks scaled the 90 orders of magnitude in mass that dark-matter candidates might occupy. CERN’s Kfir Blum explained that: “The problem with gravity is that it doesn’t matter if you’re a neutrino or a rhinoceros – if you sit on a geodesic you’re going to move in the same way,” making it difficult to infer the nature of dark matter with cosmological arguments. Nevertheless, he reported work on the recent black-hole image from the Event Horizon Telescope, which excludes some models of ultra-light dark matter. Above this, helioscopes such as CAST continue to encroach on the parameter space of QCD axions, while more novel haloscopes cut thin swathes down to low couplings in the 20 orders of magnitude of mass explored by searches for axion-like particles. Meanwhile, searches for WIMPs are sensitive to masses just beyond this, from 1 to 1000 GeV/c². Carlos de los Heros of Uppsala University explained that experiments such as XENON1t are pushing close to the so-called neutrino floor, and advocated for the development of directional detection methods that can distinguish solar neutrinos from WIMPs, and plunge into what is rather a neutrino “swamp”.

An exciting synergy between heavy-ion physics and gravitational waves was in evidence, with the two disparate approaches both now able to probe the equation of state of nuclear matter. Particular emphasis was placed on the need to marry the successful hydrodynamical and statistical description of ion–ion collisions with that used to describe proton–proton collisions, especially in the tricky proton-ion regime. These efforts are already bearing fruit in jet modelling. On the cosmological side, speakers reflected on the enduring success of the ΛCDM model to describe the universe in just six parameters, with François Bouchet of the Institut d’Astrophysique de Paris declaring that “the magic of the cosmic microwave background is not dead”, and explaining that Planck data have ruled out several models of inflation. Interdisciplinarity was also on display in reports on multi-messenger astronomy, with particular excitement reserved for the proposed European-led Einstein Telescope gravitational-wave observatory, which Marek Kowalski of DESY reported will most likely be built in either Italy or the Netherlands, and that will boast 10-times better sensitivity than current instruments.

This year’s EPS prize ceremony rewarded the CDF and D0 collaborations for the discovery of the top quark, and the WMAP and Planck collaborations for their outstanding contributions to astroparticle physics and cosmology. Today’s challenges are arguably even greater, and the spirit of EPS-HEP 2019 was to reject a false equivalence between physics being “new” and being beyond the SM. Participants’ hunger for the technological innovation required to answer the many remaining open questions was matched by an openness to reconsider theoretical thinking on fine tuning and naturalness, and how these principles inform the further exploration of the field.

EPS-HEP 2021 will take place in Hamburg from 21–28 July.

ICTP announces next director

Atish Dabholkar, a theorist from India, has been appointed the next director of the International Centre for Theoretical Physics (ICTP) in Trieste, Italy. Currently head of ICTP’s high-energy, cosmology and astroparticle physics section, Dabholkar will take up his new position in November. He will succeed Fernando Quevedo, who has led the centre since 2009. Dabholkar’s research has focused on string theory and quantum black holes, and his appointment comes at a time of expansion for ICTP. Over the past 10 years, the centre has hired more researchers and created new research initiatives in quantitative life sciences, high-performance computing, renewable energies and quantum technology. In addition, ICTP has increased its presence with the opening of four partner institutes in Brazil, China, Mexico and Rwanda. “Directing ICTP is a once in a lifetime opportunity due to its unique mission and its big impact in developing countries. I am glad that when I leave in November the institute will be in very good hands,” says Quevedo.

Guido Altarelli Award 2019

Jonathan Gaunt. Credit: Private collection

The fourth edition of the Guido Altarelli Award, which recognises exceptional achievements from young scientists in the field of deep inelastic scattering and related subjects, was awarded during the DIS2019 workshop in Torino, Italy, on 8 April. Jonathan Gaunt of CERN was recognised for his pioneering contributions to the theory and phenomenology of double and multiple parton scattering. Josh Bendavid, also CERN, and a member of the CMS collaboration, received the award for his innovative contributions with original tools to Higgs physics and proton parton density functions at the LHC. The brother of the late Guido Altarelli, Massimo Altarelli, was present at the ceremony and handed the certificates to the two winners.

2019 Dirac Medal and Prize

From left: Viatcheslav Mukhanov, Alexei Starobinsky and Rashid Sunyaev. Credits: Wikicommons; Gruber Foundation; A Korzhimanov

The International Centre for Theoretical Physics (ICTP) in Italy has awarded its 2019 Dirac Medal and Prize to three physicists whose research has made a profound impact on modern cosmology. Viatcheslav Mukhanov (Ludwig Maximilian University of Munich), Alexei Starobinsky (Landau Institute for Theoretical Physics) and Rashid Sunyaev (Max Planck Institute for Astrophysics) share the prize for “their outstanding contributions to the physics of the cosmic microwave background with experimentally tested implications that have helped to transform cosmology into a precision scientific discipline by combining microscopic physics with the large-scale structure of the universe”.

Julius Wess Award 2018

Sally Dawson of Brookhaven National Laboratory has been granted the 2018 Julius Wess Award by the KIT Elementary Particle and Astroparticle Physics Center of Karlsruhe Institute of Technology. She is recognised for her outstanding scientific contributions to the theoretical description and in-depth understanding of processes in hadron colliders, in particular her work relating to the physics of the Higgs boson and top quark. The Julius Wess Award is endowed with €10,000 and is granted annually to elementary particle and astroparticle physicists for exceptional experimental or theoretical scientific achievements.

Winners of 2019 Beamline for Schools competition

Students from the Praedinius Gymnasium in Groningen, Netherlands. Credit: M Mug

Two teams of high-school students, one from the Praedinius Gymnasium in Groningen, Netherlands (pictured), and one from the West High School in Salt Lake City, US, have won CERN’s 2019 Beamline for Schools competition. In October, the teams will travel to DESY in Germany to carry out their proposed experiments together with scientists from CERN and DESY. The Netherlands team “Particle Peers” will compare the properties of the particle showers originating from electrons with those created from positrons, while the “DESY Chain” team from the US will focus on the properties of scintillators for more efficient particle detectors. Since Beamline for Schools was launched in 2014, almost 10,000 students from 84 countries have participated. This year, 178 teams from 49 countries worldwide submitted a proposal for the sixth edition of the competition. Due to the current long shutdown of CERN’s accelerators for maintenance and upgrade, there is currently no beam at CERN, which has opened up opportunities to explore partnerships with DESY and other laboratories.

Building Balkan bridges in theory

Networking SEENET-MTP aims to strengthen fundamental physics in southeast Europe. Credit: SEENET-MTP

Twenty years ago, distinguished Austrian theorist and co-inventor of supersymmetric quantum field theory, Julius Wess, concluded that something must be done to revitalise science in former Yugoslavia. One of the 12 founding members of CERN, Yugoslavia was a middle-sized European country with corresponding moderate activities in high-energy physics. Its breakup resulted in a dramatic deterioration of conditions for science, the loss of connections and an overwhelming sense of isolation inside the region.

Wess strongly believed that science is a powerful means to influence the development of society. From 1999 to 2003, his initiative “Wissenschaftler in Global Verantwortung” (WIGV), which translates to “Scientists in Global Responsibility”, provided a platform to connect and support individual researchers, groups and institutions with a focus on former Yugoslavia. Much was achieved during this short time, such as the granting of scholarships in mathematics and theoretical physics, a revival of interrupted schools and conferences and the modernisation of intranet at several Serbian institutions. Funding, initially from Germany, provided an opportunity to researchers from former Yugoslavia to establish contacts and cooperation with many excellent researchers from all around the world.

**Goran Djordjevic** is a theorist at the University of Niš in Serbia and the founding member of SEENET-MTP. Credit: Goran Djordjevic

It was natural to expand the WIGV initiative to bridge the gap between southeastern and the rest of Europe. Countries to the east and south of Yugoslavia – such as Bulgaria, Greece, Romania and Turkey – have a reasonably strong presence in high-energy physics. On the other hand, they share some similar economic and scientific problems, with many research groups facing insufficient financing, isolation and lacking critical mass.

Therefore, the participants of the UNESCO-sponsored Balkan Workshop 2003 held in Serbia created the southeastern European Network in Mathematical and Theoretical Physics (SEENET-MTP). The network has since grown to include 16 full and seven associated member institutions from 11 countries, and more than 450 individual members. There are also 13 partner institutions worldwide. During its 15 years SEENET-MTP has undertaken: more than 18 projects, mostly concerning mobility and training; 30 conferences, workshops and schools; more than 300 researcher and student exchanges and fellowships; and more than 250 joint papers. Following initial support from CERN’s theory department, a formal collaboration agreement resulted in the joint CERN–SEENET-MTP PhD Training Program with at least 80 students taking part in the first cycle from 2015 to 2018. Vital support also came from the European Physical Society and ICTP Trieste.

In total, the investment provided for SEENET-MTP from international funds, its members, national funds and in-kind support amounts to almost €1 million. It is quite an achievement – if we consider that the results rely mostly on the efforts and good will of many individuals – but it is still much less than an average “EU project”. This raises important questions about maintaining SEENET-MTP’s efforts.

SEENET-MTP has “thermalised” the system – the network has made people in the region interact. Yet today, we find ourselves asking similar questions that its founders asked themselves 15 years ago. Is there something specific to southeast Europe that deserves special treatment? Is there something specific in high-energy theoretical physics that merits specific funding? Is the financing of high-energy physics primarily a responsibility of governments? And, if so, can Balkan countries do it properly?

Is there something specific to southeast Europe that deserves special treatment?

If the answers to the first three questions are “yes”, and to the last one “no”, a pressing issue concerns extra funding and the role of the European Union (EU). In the six or seven countries in the region that are not yet members of the EU (and which have a very unclear perspective about joining), we need to work out how to fund fundamental sciences in a similar way that Poland, Czech Republic, or “older” EU countries do. At the same time, it is important to consider the future roles of non-EU institutions such as CERN and the ICTP. The recent accession of Serbia to CERN as a full member state, and with Croatia and Slovenia in the process of joining, are promising signs towards closer European integration.

Networking is the most natural and promising auxiliary mechanism to preserve and build local capacity in fundamental physics in the region. The next SEENET Scientific-Advisory Committee and its Council meeting will take place at ICTP Trieste from 20 to 23 October. It will be the right place, if not the last possibility, to transfer the initial ideas and achieved results to an EU-supported project to bolster best practice in the Balkans.

www.seenet-mtp.info/bridges

The cutting edge of cancer research

**Cancer physics** Breast cancer cells attached to a surface rich in collagen. Credit: X Trepat/IBEC

Cancer is a heterogeneous phenomenon that is best viewed as a complex system of cells interacting in a changing micro-environment. Individual experiments may fail to capture this reality, given spatially and temporally limited scales of observation, however, in recent years, physicists have contributed insights into the interplay of phenomena at different scales: gene regulatory networks and communities of cells or organisms are two examples of systems whose properties emerge from the behaviour of individual components. Unfortunately, however, such research is usually confined to journals and specialised conferences, hindering the entry of interested physicists into the field. The publication of a new interdisciplinary textbook is therefore most welcome.

La Porta and Zapperi’s The Physics of Cancer, one of the few books devoted to this subject, brings 15 years of exciting and important results in cancer research to a wide audience. The book approaches the subject from the perspective of physics, chemistry, mathematics and computer science. As a result of the vastness of the subject and the brevity of the book, the discussion can occasionally feel superficial, but the main concepts are introduced in a manner accessible to physicists. The authors follow a logical thread within each argument, and furnish the reader with abundant references to the original literature.

The book begins by observing that the “hallmarks” of cancer are not only yet to be understood, but have increased in number. Published at the turn of the millennium, Douglas Hanahan and Robert Weinberg’s seminal paper identified six: sustaining proliferative signalling; evading growth suppressors; enabling replicative immortality; activating invasion and metastasis; inducing angiogenesis; and resisting cell death. Just 11 years later the same authors published an updated review adding four more hallmarks: avoiding immune destruction; promoting inflammation; genome instability and mutation; and deregulating cellular energetics. The amount of research that has been distilled into a handful of concepts is formidable. However, La Porta and Zapperi argue that a more abstract and unifying approach is now needed to gain a deeper understanding. They advocate studying cancer as a complex system with the tools of several disciplines, in particular subfields of physics such as biomechanics, soft-condensed-matter physics and statistical mechanics.

The book is structured in 10 self-contained chapters. The first two present essential notions of cell and cancer biology. The subsequent chapters deal with different features of cancer from an interdisciplinary perspective. A discussion on statistics and computational models of cancer growth is followed by a chapter exploring the generation of vascular networks in its biological, hydrodynamical and statistical aspects. Next comes a mathematical discussion of tumour growth by stem cells – the active and self-differentiating cells thought to drive the growth of cancers. A couple of chapters treat the biomechanics of cancer cells and their migration in the body, before La Porta and Zapperi turn to the dynamics of chromosomes and the origin of the genetic mutations that cause cancer. The final two chapters focus on how to fight tumours, from the perspectives of both the immune system and pharmacological agents.

La Porta and Zapperi’s book isn’t just light reading for curious physicists – it can also serve to guide interested researchers into a rich interdisciplinary area.

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