Image credit: C Checchia.
Major scientific gatherings such as the European Physical Society (EPS) biennial international conference on High Energy Physics offer a valuable opportunity to reflect on the immense work and progress taking place in our field, including the growing connections between particle physics and the universe at large. This year’s EPS conference, held in Venice, Italy, from 5–12 July, was also the first large conference where the results from the 2015 and 2016 runs of the Large Hadron Collider (LHC) at 13 TeV were presented.
Setting the bar just a day into the Venice event, LHCb announced the discovery of a new doubly charmed baryon from precision measurements of B decays, with heavy-flavour analyses continuing to offer a rich seam of understanding. LHCb also presented the intriguing anomalies being seen in the ratios of certain Standard Model decays that hint at deviations from lepton universality, with further data from LHC Run 2 hotly anticipated.
The LHC is firmly in the precision business these days. In the last two years, the machine has delivered large amounts of collision data to the experiments and striking progress has been made in analysis techniques. These have enabled measurements of rare electroweak processes such as the associated production of a top quark, a Z boson and a quark (tZq) by ATLAS, for example, and the definitive observation of WW scattering by CMS. Top physics is another booming topic, with new top-mass and single-top production measurements and many other results, including “legacy” measurements from the Tevatron experiments, on show.
At the core of the LHC’s analysis programme is the exploration of the Higgs boson, which now enters its sixth year. Particularly relevant is how the Higgs interacts with other particles, since this could be altered by physics beyond the Standard Model. While the Higgs was first spotted decaying into other bosons (W, Z, γ), ATLAS reported the first evidence for the decay of the Higgs boson to a pair of bottom quarks, with a significance of 3.6σ, while CMS presented the first observation by a single experiment of the decay to a pair of τ leptons, with a significance of 5.9σ. The Higgs mass is also narrowing to 125 GeV, while the fundamental scalar nature of the new particle continues to raise hope that it will lead to new insights.
The lack of direct signs of new physics at the LHC is an increasing topic of discussion, and underlies the importance of precision measurements. Direct searches are pushing the mass limits for new particles well into the TeV range, but new physics could be hiding in small and subtle effects. It is clear that there is physics beyond the Standard Model, just not what it is, and one issue is how to communicate this scientifically fascinating but non-headline-worthy aspect of today’s particle-physics landscape.
High precision is also being attained in studies of the strong interaction. ALICE, for example, reported an increase in strangeness production with charged multiplicity that seems to connect smoothly the regimes seen in pp, pPb and PbPb collisions. Overall, and increasingly with complementary results from the other LHC experiments, ALICE is closing in on the evolution of the quark–gluon plasma, and thus on understanding the very early universe.
Particle physics, astrophysics and cosmology are closer today than ever, as several sessions at the Venice event demonstrated. One clear area of interplay is dark matter: if dark matter interacts only through gravity, then finding it will be very difficult for accelerator-based studies, but if it has a residual interaction with some known particles, then accelerators will be leading the hunt for direct detection. Cosmology’s transformation to a precision science continues with the recent detection of gravitational waves, with LIGO’s results already placing the first limits on the mass of the graviton at less than 7.7 × 10–23 eV/c2. There were also updates from dark-energy studies, and about precision CMB explorers beyond Planck.
Neutrino physics is also an extremely vibrant field, with neutrino oscillations continuing to offer chances for discovery. The various neutrino-mixing angles are starting to be well measured and Nova and T2K are zooming in on the value of the CP-violating phase, which seems to be large, given tantalising hints from T2K. The hunt for sterile neutrinos continues, and for neutrinoless double beta decay, with several searches ongoing worldwide.
In summary, the 2017 EPS-HEP conference clearly demonstrated how we are progressing towards a full understanding both of the vastness of the universe and of the tiniest constituents of matter. There are many more results to look forward to, many of which will be ready for the next EPS-HEP event in Ghent, Belgium, in 2019. As summed up by the conference highlights: the field is advancing on all fronts – and it’s impressive.
