La physique des particules à l’honneur à Chicago

Cet été, du 3 au 11 août, plus de 1 400 scientifiques, étudiants, enseignants et industriels du monde entier se sont réunis à Chicago (Illinois, États-Unis) pour la 38e Conférence internationale sur la physique des hautes énergies (ICHEP). Il s’agissait de la plus grande conférence jamais organisée sur le sujet, avec 51 pays d’Afrique, d’Asie, d’Océanie, d’Europe et d’Amérique du Nord et du Sud représentés. Les bosses mystérieuses, les bosons de Higgs redécouverts, les oscillations de neutrinos et la matière noire étaient parmi les principaux sujets scientifiques. Les technologies prometteuses ont également été un thème important, avec environ 400 résumés d’articles ou de présentations soumis ; les sessions portant sur des questions liées à la communication, à la diversité et à l’inclusion ont elles aussi attiré un large public.

This summer, the city of Chicago in Illinois was not only a vacation destination for North American tourists – it was also the preferred destination for more than 1400 scientists, students, educators and members of industry from around the world. Fifty-one countries from Africa, Asia, Australia, Europe, North America and South America were represented at the 38th International Conference on High Energy Physics (ICHEP), which is the largest such conference ever held.

Indeed, the unexpectedly large interest in the meeting caused some re-thinking of the conference agenda. A record 1600 abstracts were submitted, of which 600 were selected for parallel presentations and 500 for posters by 65 conveners. During three days of plenary sessions, 36 speakers from around the world overviewed results presented at the parallel and poster sessions.

One of the most popular parallel-session themes concerned enabling technologies, totalling around 400 abstract submissions, and rich collaborative opportunities were discussed in the new “technology applications and industrial opportunities” track. Another innovation at ICHEP 2016 concerned diversity and inclusion, which appeared as a separate parallel track. A number of new initiatives in communication, education and outreach were also piloted. These included lunchtime sessions aimed at increasing ICHEP participants’ skills in outreach and communication through news and social media, “art interventions” and a physics slam, where five scientists competed to earn audience applause through presentations of their research. The outreach programme was complemented by events at 30 public libraries in Chicago and a public lecture about gravitational waves.

While the public had an increasing number of ways to connect with the conference, however, the main attraction for attendees remained the same: new science results. And no result was more highly anticipated than the updates on the 750 GeV diphoton resonance hinted at in data from the ATLAS and CMS experiments recorded during 2015.

Exploring the unknown

The spectacular performance of the LHC during 2016, which saw about 20 fb–1 of 13 TeV proton–proton collisions delivered to ATLAS and CMS by the time of the conference, gave both experiments unprecedented sensitivity to new particles and interactions. The collaborations reported on dozens of different searches for new phenomena. In a dramatic parallel session, both ATLAS and CMS revealed that their 2016 data do not confirm the previous hints of a diphoton resonance at 750 GeV (figure 1); apparently, those hints were nothing more than tantalising statistical fluctuations. Disappointed theorists were happily distracted by other new results, however. As expected, these include interesting excesses worth keeping an eye on as more data become available. Still in the running for future big discoveries are the production of heavy particles predicted by supersymmetry and exotic theories, and the direct production at the LHC of dark-matter particles. So far, no signs of such particles have been seen at ATLAS or CMS.

Many other experiments reported on their own searches for new particles and interactions, including new LHCb results on the most sensitive search to date for CP violation in the decays of neutral D mesons which, if detected, would allow researchers to probe CP violation in the up-type quark sector. Final results from the MEG (Mu to E Gamma) experiment at the Paul Scherrer Institute in Switzerland revealed the most sensitive search to date for charged lepton-flavour violation, which would also be a clear signature of new physics. Using bottom and charm quarks to probe new physics, the Beijing Spectrometer (BES) at IHEP in China and the Belle experiment at KEK in Japan showcased a series of precision and rare-process results. While they have a few interesting discrepancies from Standard Model (SM) predictions, presently no signs of physics beyond the SM have emerged.

Meanwhile on the heavy-ion front, the ALICE experiment at the LHC joined ATLAS, CMS and LHCb in presenting new observations of the dramatic and mysterious properties of quark–gluon plasma. This was complemented by results from the STAR and PHENIX experiments at RHIC at the Brookhaven National Laboratory in the US.

Rediscovering the Higgs

Perhaps unsurprisingly, given that its discovery in 2012 was one of the biggest in particle physics for a generation, the Higgs boson was the subject of 30 parallel-session talks. New LHC measurements are a great indicator of how the Higgs boson is being used as a new tool for discovery. Already Run 2 of the LHC has produced more Higgs bosons than in Run 1, and the Higgs has been “rediscovered” in the new data with a significance of 10σ (figure 2). A major focus of the new analyses is to demonstrate the production of Higgs particles in association with a W or Z boson, or with a pair of top quarks and their decay patterns. These production and decay channels are important tests of Higgs properties, and so far the Higgs seems to behave just as the SM predicts.

About 20 new searches looking for heavier cousins of the Higgs were reported. These “heavy Higgs”, once produced, could decay in ways very similar to the Higgs itself, or might decay into a pair of Higgs bosons. Other searches covered the possibility that the Higgs boson itself has exotic decays: “invisible” decays into undetected particles, decays into exotic bosons or decays that violate the conservation of lepton flavour. No signals have emerged yet, but the LHC experiments are providing increasing sensitivity and coverage of the full menu of possibilities.

Neutrino mysteries

With neutrinos currently among the most interesting objects to study to look for signs of physics beyond the SM, ICHEP included reports from three powerful long-baseline neutrino experiments: T2K at J-PARC in Japan, and NOνA and MINOS at Fermilab in the US, which are addressing some of the fundamental questions about neutrinos such as CP violation, the ordering of their masses and their mixing behaviour. While not yet conclusive, the results presented at ICHEP show that neutrino physics is entering a new era of sensitivity and maturity. Data from T2K currently favour the idea of CP violation in the lepton sector, which is one of the conditions required for the observed dominance of matter over antimatter in the universe, while data from NOνA disfavour the idea that mixing of the second and third neutrino flavours is maximal, representing a test of a new symmetry that underlies maximal mixing (figure 3).

The long simmering issue of sterile neutrinos – hypothesised particles that do not interact via SM forces – also received new attention in Chicago. The 20 year-old signal from the LSND experiment at Los Alamos National Laboratory in the US, which indicates 4σ evidence for such a particle, was matched some years ago by anomalies from the MiniBooNE experiment at Fermilab. As reported at ICHEP, however, cosmological data and new results from IceCUBE in Antarctica and MINOS+ at Fermilab do not confirm the existence of sterile neutrinos. On the other hand, the Daya Bay experiment in China, Reno in South Korea and Double Chooz in France all confirm a reactor neutrino flux that is low compared with the latest modelling, which could arise from mixing with sterile neutrinos. However, all three of these experiments also confirm a “bump” in the neutrino spectrum at an energy of around 5 MeV that is not predicted, so there is certainly more work to be done in understanding the modelling.

Probing the dark sector

Dark matter dominates the universe, but its identity is still a mystery. Indeed, some theorists speculate about the existence of an entire “dark sector” made up of dark photons and multiple species of dark matter. Numerous approaches are being pursued to detect dark matter directly, and these are complemented by searches at the LHC, surveys of large-scale structure and attempts to observe high-energy particles from dark-matter annihilation or decay in or around our Galaxy. Regarding direct detection, experiments are advancing steadily in sensitivity: the latest examples reported at ICHEP came from LUX in the US and PandaX-II in China, and already they exclude a substantial fraction of the parameter space of supersymmetric dark-matter candidates (figure 4).

Dark energy – the name given to the entity thought to be driving the cosmic acceleration of today’s universe – is one of two provocative mysteries, the other concerning the primordial epoch of cosmic inflation. ICHEP sessions concerned both current and planned observations of such effects, using either optical surveys of large-scale structure or the cosmic microwave background. Both approaches together can probe the nature of dark energy by looking at the abundance of galaxy clusters as a function of redshift; as reported at the Chicago event, this is already happening via the Dark Energy Survey and the South Pole Telescope.

Progress in theory

Particle theory has been advancing rapidly along two main lines: new ideas and approaches for persistent mysteries such as dark matter and naturalness, and more precise calculations of SM processes that are relevant for ongoing experiments. As emphasised at ICHEP 2016, new ideas for the identity of dark matter have had implications for LHC searches and for attempts to observe astrophysical dark-matter annihilation, in addition to motivating a new experimental programme looking for dark photons. A balanced view of the naturalness problem, which concerns the extent to which fundamental parameters appear tuned for our existence, was presented at ICHEP. While supersymmetry is still the leading explanation, theorists are also studying alternatives such as the “relaxion”. This shifts attention to the dynamics of the early universe, with consequences that may be observable in future experiments.

There have also been tremendous developments in theoretical calculations with higher-order QCD and electroweak corrections, which are critical for understanding the SM backgrounds when searching for new physics – particularly at the LHC and, soon, at the SuperKEKB B factory in Japan. The LHC’s experimental precision on top-quark production is now reaching the point where theory requires next-to-next-to-next-to-leading-order corrections just to keep up, and this is starting to happen. In addition, recent lattice QCD calculations play a key role in extracting fundamental parameters such as the CKM mixing matrix, as well as squeezing down uncertainties to the point where effects of new phenomena may conclusively emerge.

Facilities focus

With particle physics being a global endeavour, the LHC at CERN serves as a shining example of a successful large international science project. At a session devoted to future facilities, leaders from major institutions presented the science case and current status of new projects that require international co-operation. These include the International Linear Collider (ILC) in Japan, the Circular Electron–Positron Collider (CEPC) in China, an energy upgrade of the LHC, the Compact Linear Collider (CLIC) and the Future Circular Collider (FCC) at CERN, the Long-Baseline Neutrino Facility (LBNF) and Deep Underground Neutrino Experiment (DUNE) in the US, and the Hyper-K neutrino experiment in Japan.

While the high-energy physics experiments of the future were a key focus, one of the well-attended sessions at ICHEP 2016 concerned professional issues critical to a successful future for the field of particle physics. Diversity and inclusion were the subject of four hours of parallel sessions, discussions and posters, with themes such as communication, inclusion and respect in international collaboration and how harassment and discrimination in scientific communities create barriers to access. The sessions were mostly standing-room only, with supportive but candid discussion of the deep divides, harassment, and biases – both explicit and implicit – that need to be overcome in the science community. Speakers described a number of positive initiatives, including the Early Career, Gender and Diversity office established by the LHCb collaboration, the Study Group on Diversity in the ATLAS collaboration, and the American Physical Society’s “Bridge Program” to increase the number of physics PhDs among students from under-represented backgrounds.

ICHEP 2016 clearly showed that there are a vast number of scientific opportunities on offer now and in the future with which to further explore the smallest and largest structures in the universe. The LHC is performing beyond expectations, and will soon enter a new era with its planned high-luminosity upgrade. Meanwhile, propelled by surprising discoveries from a series of pioneering experiments, neutrino physics has progressed dramatically, and its progress will continue with new and innovative experiments. Intense kaon and muon beams, and SuperKEKB, will provide excellent opportunities to search for new physics in different ways, and will help to inform future research directions. Diverse approaches to probe the nature of dark matter and dark energy are also on their way. While we cannot know what will be the headline results at the next ICHEP event – which will be held in 2018 in Seoul, South Korea – we can be certain that surprises are in store.