Standard Model stands strong at Moriond
The 66th Rencontres de Moriond, held in La Thuile, Italy, took place from 16 to 30 March, with the first week devoted to electroweak interactions and unified theories, and a second week to QCD and high-energy interactions. More than 200 physicists took part, presenting new results from precision Standard Model (SM) measurements to new exotic quark states, flavour physics and the dark sector.
A major theme of the electroweak session was flavour physics, and the star of the show was LHCb’s observation of CP violation in charm decays (see LHCb observes CP violation in charm decays). The collaboration showed several other new results concerning charm- and B-meson decays. One much anticipated result was an update on RK, the ratio of rare decays of a B+ to electrons and muons, using data taken at energies of 7, 8 and 13 TeV. These decays are predicted to occur at the same rate to within 1%; previous data collected are consistent with this prediction but favour a lower value, and the latest LHCb results continue to support this picture. Together with other measurements, these results paint an intriguing picture of possible new physics (p33) that was explored in several talks by theorists.
The LHC experiments presented many new results based on data collected during Run 2. ATLAS and CMS have measured most of the Higgs boson’s main production and decay modes with high statistical significance and carried out searches for new, additional Higgs bosons. From a combination of all Higgs-boson measurements, ATLAS obtained new constraints on the important Higgs self-coupling, while CMS presented updated results on the Higgs decay to two Z bosons and its coupling to top quarks.
Precision SM studies continued with first evidence from ATLAS for the simultaneous production of three W or Z bosons, and CMS presented first evidence for the production of two W bosons in two simultaneous interactions between colliding partons. The very large new dataset has also allowed ATLAS and CMS to expand their searches for new physics, setting stronger lower limits on the allowed mass ranges of supersymmetric and other hypothetical particles (see Boosting searches for fourth-generation quarks and Pushing the limits on supersymmetry). These also include new limits from CMS on the parameters describing slowly moving heavy particles, and constraints from both collaborations on the production rate of Z′ bosons. ATLAS, using the results of lead–ion collisions taken in 2018, also reported the observation of light-by-light scattering – a very rare process that is forbidden by classical electrodynamics.
New results and prospects in the neutrino sector were communicated, including Daya Bay and the reactor antineutrino flux anomaly, searches for neutrinoless double-beta decay, and the reach of T2K and NOvA in tackling the neutrino mass hierarchy and leptonic CP violation. Dark matter, axions and cosmology also featured prominently. New results from experiments such as XENON1T, ABRACADABRA, SuperCDMS and ATLAS and CMS illustrate the power of multi-prong dark-matter searches – not just for WIMPs but also very light or exotic candidates. Cosmologist Lisa Randall gave a broad-reaching talk about “post-modern cosmology”, in which she argued that – as in particle physics – the easy times are probably over and that astronomers need to look at more subtle effects to break the impasse.
Moriond electroweak also introduced a new session: “feeble interactions”, which was designed to reflect the growing interest in very weak processes at the LHC and future experiments.
LHCb continued to enjoy the limelight during Moriond’s QCD session, announcing the discovery of a new five-quark hadron, named Pc(4312)+, which decays to a proton and a J/ψ and is a lighter companion of the pentaquark structures revealed by LHCb in 2015 (p15). The result is expected to motivate deeper studies of the structure of these and other exotic hadrons. Another powerful way to delve into the depths of QCD, addressed during the second week of the conference, is via the Bc meson family. Following the observation of the Bc(2S) by ATLAS in 2014, CMS reported the existence of a two-peak feature in data corresponding to the Bc(2S) and the Bc*(2S) – supported by new results from LHCb based on its full 2011–2018 data sample. Independent measurements of CP violation in the Bs system reported by ATLAS and LHCb during the electroweak session were also combined to yield the most precise measurement yet, which is consistent with the small value predicted by the SM.
A charmed life
In the heavy-ion arena, ALICE highlighted its observation that baryons containing charm quarks are produced more often in proton–proton collisions than in electron–positron collisions. Initial measurements in lead–lead collisions suggest an even higher production rate for charmed baryons, similar to what has been observed for strange baryons. These results indicate that the presence of quarks in the colliding beams affects the hadron production rate. The collaboration also presented the first measurement of the triangle-shaped flow of J/ψ particles in lead–lead collisions, showing that even heavy quarks are affected by the quarks and gluons in the quark–gluon plasma and retain some memory of the collisions’ initial geometry.
The SM still stands strong after Moriond 2019, and the observation of CP violation in D mesons represents another victory, concluded Shahram Rahatlou of Sapienza University of Rome in the experimental summary. “But the flavour anomaly is still there to be pursued at low and high mass.”
Matthew Chalmers, CERN, with input from the LHC experiment collaborations.
Centennial conference honours Feynman
2018 marked the 100th anniversary of the birth of Richard Feynman. As one of several events worldwide celebrating this remarkable figure in physics, a memorial conference was held at the Institute of Advanced Studies at Nanyang Technological University in Singapore from 22 to 24 October, co-chaired by Lars Brink, KK Phua and Frank Wilzcek. The format was one-hour talks with 45 minute discussions.
Pierre Ramond began the conference with anecdotes from his time as Feynman’s next-door neighbour at Caltech. He discussed Feynman the MIT undergraduate, his first paper and his work at Princeton as a graduate student. There, Feynman learnt about Dirac’s idea of summing over histories from Herbert Jehle. Jehle asked Feynman about it a few days later. He said that he had understood it and had derived the Schrödinger equation from it. Feynman’s adviser was John Wheeler. Wheeler was toying with the idea of a single electron travelling back and forth in time – were you to look at a slice of time you would observe many electrons and positrons. After his spell at Los Alamos, this led Feynman to the idea of the propagator, which considers antiparticles propagating backwards in time as well as particles propagating forwards. These ideas would soon underpin the quantum description of electromagnetism – QED – for which Feynman shared the 1965 Nobel Prize in Physics with Tomonaga and Schwinger.
The propagator was the key to the eponymous diagrams Feynman then formulated to compute the Lamb shift and other quantities. At the Singapore conference, Lance Dixon exposed how Feynman diagrams revolutionised the calculation of scattering amplitudes. He offered as an example the calculation of the anomalous magnetic moment of the electron, which has now reached five-loop precision and includes 12,672 diagrams. Dixon also discussed the importance of Feynman’s parton picture for understanding deep-inelastic scattering, and the staggeringly complex calculations required to understand data at the LHC.
George Zweig, the most famous of Feynman’s students, and the inventor of “aces” as the fundamental constituents of matter, gave a vivid talk, recounting that it took a long time to convince a sceptical Feynman about them. He described life in the shadows of the great man as a graduate student at Caltech in the 1960s. At that time Feynman wanted to solve quantum gravity, and was giving a course on the subject of gravitation. He asked the students to suppose that Einstein had never lived: how would particle physicists discuss gravity? He quickly explained that there must be a spin-two particle mediating the force; by the second lecture he had computed the precession of the perihelion of Mercury, a juncture that other courses took months to arrive at. Zweig recounted that Feynman’s failure to invent a renormalisable theory of quantum gravity affected him for many years. Though he did not succeed, his insights continue to resound today. As Ramond earlier explained, Feynman’s contribution to a conference in Chapel Hill in 1957, his first public intervention on the subject, is now seen as the starting point for discussions on how to measure gravitational waves.
Cristiane Morais-Smith spoke on Feynman’s path integrals, comparing Hamiltonian and Lagrangian formulations, and showing their importance in perturbative QED. Michael Creutz, the son of one of Feynman’s colleagues at Princeton and Los Alamos, showed how the path integral is also necessary to be able to work on the inherently non-perturbative theory of quantum chromodynamics. Morais-Smith went on to illustrate how Feynman’s path integrals now have a plethora of applications outside particle physics, from graphene to quantum Brownian motion and dissipative quantum tunnelling. Indeed, the conference did not neglect Feynman’s famous interventions outside particle physics. Frank Wilczek recounted Feynman’s famous insight that there is plenty of room at the bottom, telling of his legendary after-dinner talk in 1959 that foreshadowed many developments in nanotechnology. Wilczek concluded that there is plenty of room left in Hilbert space, describing entanglement, quantum cryptography, quantum computation and quantum simulations. Quantum computing is the last subject that Feynman worked hard on. Artur Ekert described the famous conference at MIT in 1981 when Feynman first talked about the subject. His paper from this occasion “Simulating Physics with Computers” was the first paper on quantum computers and set the ground for the present developments.
Feynman was also interested in biology for a long time. Curtis Callan painted a picture of Feynman “hanging out” in Max Delbruck’s laboratory at Caltech, even taking a sabbatical at the beginning of the 1960s to work there, exploring the molecular workings of heredity. In 1969 he gave the famous Hughes Aerospace lectures, offering a grand overview of biology and chemistry – but this was also the time of the parton model and somehow that interest took over.
Robbert Dijkgraaf spoke about the interplay between art and science in Feynman’s life and thinking. He pointed out how important beauty is, not only in nature, but also in mathematics, for instance whether one uses a geometric or algebraic approach. Another moving moment of this wide-ranging celebration of Feynman’s life and physics was Michelle Feynman’s words about growing up with her father. She showed him both as a family man and also as a scientist, sharing his enthusiasm for so many things in life.
- Recordings of the presentations are available online.
Lars Brink, Chalmers University of Technology, Göteborg, Sweden.
Particle colliders: accelerating innovation
Around 100 researchers, academics and industry delegates joined a co-innovation workshop in Liverpool, UK, on 22 March to discuss the strategic R&D programme for a Future Circular Collider (FCC) and associated benefits for industry. Motivated by the FCC study, the aim of the event was to identify joint R&D opportunities across accelerator projects and disciplines.
New particle colliders provide industry with powerful test-beds with a high publicity factor. Well-controlled environments allow novel technologies and processes to be piloted, and SMEs are ideal partners to bring these technologies – which include superconducting magnets, cryogenics, civil engineering, detector development, energy efficiency and novel materials and material processing techniques – to maturity.
Short talks about FCC-related areas for innovation, examples of successful technology-transfer projects at CERN, as well as current and future funding opportunities stimulated interesting discussions. Several areas were identified as bases for co-innovation, including resource-efficient tunnelling, the transfer of bespoke machine-learning techniques from particle physics to industry, detector R&D, cooling and data handling. The notes from all the working groups will be used to establish joint funding bids between participants.
The co-innovation workshop was part of a bigger event, “Particle Colliders – Accelerating Innovation”, which was devoted to the benefits of fundamental science to society and industry, co-hosted by the University of Liverpool and CERN together with partners from the FCC and H2020 EuroCirCol projects, and supported by EU-funded MSCA training networks. Almost 1000 researchers and industrialists from across Europe, including university and high-school students, took part. An industry exhibition allowed more than 60 high-tech companies to showcase their latest products, also serving university students as a careers fair, and more than a dozen different outreach activities were available to younger students.
A separate event, held at CERN on 4 and 5 March, reviewed the FCC physics capabilities following the publication of the FCC conceptual design report in January (CERN Courier January/February 2019 p8). The FCC study envisages the construction of a new 100 km-circumference tunnel at CERN hosting an intensity-frontier lepton collider (FCC-ee) as a first step, followed by an energy-frontier hadron machine (FCC-hh). It offers substantial and model-independent studies of the Higgs boson by extending the range of measurable Higgs properties to its total width and self-coupling. Moreover, the combination of superior precision and energy reach allows a complementary mix of indirect and direct probes of new physics. For example, FCC-ee would enable the Higgs couplings to the Z boson to be measured with an accuracy better than 0.17%, while FCC-hh will determine model-independent ttH coupling to <1%.
Physics discussions were accompanied by a status report of the overall FCC project, reviewing the technological challenges for both accelerator and detectors, the project implementation strategy, and cost estimates. Construction of the more technologically ready FCC-ee could start by 2028, delivering first physics beams a decade later, right after the end of the HL-LHC programme. Another important aspect of the two-day meeting was the need for further improving theoretical predictions to match the huge step in experimental precision possible at the FCC.
Planning now for a 70-year-long programme may sound a remote goal. However, as Alain Blondel of the University of Geneva remarked in the concluding talk of the conference, the first report on the LHC dates back more than 40 years. “Progress in knowledge has no price,” he said. “The FCC sets ambitious but feasible goals for the global community resembling previous leaps in the long history of our field.”
Panos Charitos, CERN.
Upping the tempo on wakefield accelerators
Around 50 experts from around the world met at CERN from 26 to 29 March for the second ALEGRO workshop to discuss advanced linear-collider concepts at the energy frontier.
ALEGRO, the Advanced Linear Collider Study Group, was formed as an outcome of an ICFA workshop on advanced accelerators held at CERN in 2017 (CERN Courier December 2017 p31). Its purpose is to unite the accelerator community behind a > 10 TeV electron–positron collider based on advanced and novel accelerators (ANAs), which use wakefields driven by intense laser pulses or relativistic particle bunches in plasma, dielectric or metallic structures to reach gradients as high as 1 GeV/m. The proposed Advanced Linear International Collider – ALIC for short – would be shorter than linear colliders based on more conventional acceleration technologies such as CLIC and ILC, and would reach higher energies.
The main research topics ALEGRO identified are the preservation of beam quality, the development of stable and efficient drivers (in particular laser systems), wall-plug-to-beam-power efficiency, operation at high-repetition rates, tolerance studies, the staging of two structures and the development of suitable numerical tools to allow for the simulation of the accelerator as a whole.
The next ALEGRO workshop will be held in March 2020 in Germany.
Patric Muggli, Max Planck Institute for Physics, Munich, Germany.
Cross-fertilisation in detector development
More than 300 experts convened from 18-22 February for the 15th Vienna Conference on Instrumentation to discuss ongoing R&D efforts and set future roadmaps for collaboration. “In 1978 we discussed wire chambers as the first electronic detectors, and now we have a large number of very different detector types with performances unimaginable at that time,” said Manfred Krammer, head of CERN’s experimental physics department, recalling the first conference of the triennial series. “In the long history of the field we have seen the importance of cross-fertilisation as developments for one specific experiment can catalyse progress in many fronts.”
Following this strong tradition, the conference covered fundamental and technological issues associated with the most advanced detector technologies as well as the value of knowledge transfer to other domains. Over five days, participants covered topics ranging from sensor types and fast and efficient electronics to cooling technologies and their mechanical structures.
Contributors highlighted experiments proposed in laboratories around the world, spanning gravitational-wave detectors, colliders, fixed-target experiments, dark-matter searches, and neutrino and astroparticle experiments. A number of talks covered upgrade activities for the LHC experiments ahead of LHC Run 3 and for the high-luminosity LHC. An overview of LIGO called for serious planning to ensure that future ground-based gravitational-wave detectors can be operational in the 2030s. Drawing a comparison between the observation of gravitational waves and the discovery of the Higgs boson, Christian Joram of CERN noted “Progress in experimental physics often relies on breakthroughs in instrumentation that lead to substantial gains in measurement accuracy, efficiency and speed, or even open completely new approaches.”
Beyond innovative ideas and cross-disciplinary collaboration, the development of new detector technologies calls for good planning of resources and times. The R&D programme for the current LHC upgrades was set out in 2006, and it is already timely to start preparing for the third long shutdown in 2023 and the High-Luminosity LHC. Meanwhile, the CLIC and Future Circular Collider studies are developing clear ideas of the future experimental challenges in tackling the next exploration frontier.
Panos Charitos, CERN.
Neutrino connoisseurs talk stats at CERN
PHYSTAT-nu 2019 was held at CERN from 22 to 25 January. Counted among the 130 participants were LHC physicists and professional statisticians as well as neutrino physicists from across the globe. The inaugural meeting took place at CERN in 2000 and PHYSTAT has gone from strength to strength since, with meetings devoted to specific topics in data analysis in particle physics. The latest PHYSTAT-nu event is the third of the series to focus on statistical issues in neutrino experiments. The workshop focused on the statistical tools used in data analyses, rather than experimental details and results.
Modern neutrino physics is geared towards understanding the nature and mixing of the three neutrinos’ mass and flavour eigenstates. This mixing can be inferred by observing “oscillations” between flavours as neutrinos travel through space. Neutrino experiments come in many different types and scales, but they tend to have one calculation in common: whether the neutrinos are created in an accelerator, a nuclear reactor, or by any number of astrophysical sources, the number of events expected in the detector is the product of the neutrino flux and the interaction cross section. Given the ghostly nature of the neutrino, this calculation presents subtle statistical challenges. To cancel common systematics, many facilities have two or more detectors at different distances from the neutrino source. However, as was shown for the NOVA and T2K experiments, competitors to observe CP violation using an accelerator-neutrino beam, it is difficult to correlate the neutrino yields in the near and far detectors. A full cancellation of the systematic uncertainties is complicated by the different detector acceptances, possible variations in the detector technologies, and the compositions of different neutrino interaction modes. In the coming years these two experiments plan to combine their data in a global analysis to increase their discovery power – lessons can be learnt from the LHC experience.
The problem of modelling the interactions of neutrinos with nuclei – essentially the problem of calculating the cross section in the detector – forces researchers to face the thorny statistical challenge of producing distributions that are unadulterated by detector effects. Such “unfolding” corrects kinematic observables for the effects of detector acceptance and smearing, but correcting for these effects can cause huge uncertainties. To counter this, strong “regularisation” is often applied, biasing the results towards the smooth spectra of Monte Carlo simulations. The desirability of publishing unregularised results as well as unfolded measurements was agreed by PHYSTAT-nu attendees. “Response matrices” may also be released, allowing physicists outside of an experimental collaboration to smear their own models, and compare them to detector-level data. Another major issue in modeling neutrino–nuclear interactions is the “unknown unknowns”. As Kevin McFarland of the University of Rochester reflected in his summary talk, it is important not to estimate your uncertainty by a survey of theory models. “It’s like trying to measure the width of a valley from the variance of the position of sheep grazing on it. That has an obvious failure mode: sheep read each other’s papers.”
An important step for current and future neutrino experiments could be to set up a statistics committee, as at the Tevatron, and, more recently, the LHC experiments. This PHYSTAT-nu workshop could be the first real step towards this exciting scenario.
The next PHYSTAT workshop will be held at Stockholm University from 31 July to 2 August on the subject of statistical issues in direct-detection dark-matter experiments.
Olaf Behnke, DESY, Louis Lyons, University of Oxford and Davide Sgalaberna, CERN.