Over 700 physicists coming from 50 countries met in Venice to discuss the newest results in their field at EPS 2017, one of the world’s most important conferences on particle physics. The conference was co-organized by the European Physical Society, the Istituto Nazionale di Fisica Nucleare (INFN) and by the Department of Physics and Astronomy of the University of Padova.

The conference is providing the opportunity for all of the LHC experiments to present many new or final results from the first run at the LHC. These include searches for dark matter, supersymmetric and other exotic particles, as well as new precision measurements of Standard Model processes.

The presentations covered results from the 2015 and 2016 runs of the (LHC), while a large fraction of the program was also devoted to neutrino physics as well as developments related to gravitational waves. The lack so far of direct signs of new physics at the LHC triggered many discussions while physicists think of novel experimental techniques and new ideas that could inform future searches. After all the upgrades of the LHC allow us to search for new phenomena at energies never reached before while the neutrino program at the US – where CERN has a strong contribution – opens a new window for exploring the tiniest scales of nature.

The ATLAS Experiment presented evidence for a first sighting of the Higgs boson decaying to a pair of b-quarks, with an observed significance of 3.6 σ. This result combined Run 1 and Run 2 data together with many years of intense scrutiny by ATLAS teams. This evidence for H→bb fills in one of the big missing pieces of our knowledge of the Higgs sector, suggesting that the Higgs mechanism is responsible for the masses of quarks. Its sighting is an important milestone for the ATLAS Collaboration. Further precise studies with higher precision are important to look for hints of new physics beyond our current theories.

Moreover, the ATLAS Experiment was able to re-observe the Higgs boson using only Run 2 data. The now-familiar Higgs “bump” was seen in decays into two photons (H→γγ) and four leptons (H→ZZ*→4l). New explorations of the H→γγ and H→ZZ*→4l decays have also yielded insight into the behaviour of the Higgs boson, including its coupling properties and measurements of production rates (differential cross sections). Combining both channels has allowed ATLAS to test Standard Model predictions with higher precision.

The ATLAS Collaboration also presented its latest results in the search for extensions to the Standard Model. Final states with jets and high missing transverse momentum (“mono-jets”) were examined for signs of Dark Matter-candidate particles; diboson events where at least one boson decays into a pair of quarks were searched for signs of heavy new particles; and, where decays led to two tau leptons, ATLAS looked for evidence of additional SUSY Higgs bosons. While no significant deviations from the Standard Model were found in these searches, each result has helped set new limits on current theories.

LHCb presented an update on tensions with lepton universality from various semileptonic decays, which are at the moment at the 2-3 σ level and have the potential of uncovering new physics. The collaboration also reported the measurement of the first doubly-charmed baryon. The so-called Ξcc⁺⁺, is a baryon containing two charm quarks and one up quark, resulting in a double positive charge. It is a doubly charm counterpart of the well-known lower mass Ξ0 baryon, which is composed of two strange quarks and an up quark. The Ξcc⁺⁺ baryon is identified via its decay into a Λc⁺ baryon and three lighter mesons K^-, π⁺ and π⁺.

The above figure (Figure 4) shows the Λc⁺ à K^-π⁺π⁺ invariant mass spectrum obtained with 1.7 fb^-1 of data collected by LHCb in 2016 at the LHC centre-of-mass energy of 13 TeV. The mass is measured to be about 3621 MeV/c² which is almost four times heavier than the most familiar baryon, the proton, a property that arises from its doubly charmed-quark content. The existence of doubly charmed baryons was already known to be a possibility in the 1970s, after the discovery of the charm quark. In the early 2000s the observation of a similar particle was reported by the SELEX collaboration. This observation was not confirmed by subsequent experiments and the measured properties of this particle are not compatible with those of the Ξcc⁺⁺ baryon discovered by LHCb. This discovery opens a new field of particle physics research. An entire family of doubly charmed baryons related to the Ξcc⁺⁺ is predicted, and will be searched for with added enthusiasm. Furthermore, other hadrons containing different configurations of two heavy quarks, for example two beauty quarks or a beauty and charm quark, are waiting to be discovered. Measurements of the properties of all these particles will allow for precise tests of QCD.

CMS now has the first observation of the Higgs to tau leptons decay. The combination of the 13 TeV results with the corresponding results previously obtained at 7 and 8 TeV leads to an observed significance of 5.9 standard deviations, equal to the expected significance. This is of paramount importance to establishing the coupling of the Higgs boson to leptons and represents an important step towards measuring its couplings to third generation fermions.

In addition, precision tests of the SM include a new measurement of the effective weak mixing angle, using the forward-backward lepton asymmetry in Drell-Yan production, based on data collected at 8 TeV. The measured value of  sinθ = 0.23101 ± 0.00052 is currently the best measurement of this important SM parameter from the LHC and is competitive with previous measurements performed at the Tevatron. The large data sample to be delivered by the LHC in Run 2 will allow us to further improve this measurement.

Moreover, CMS presented more than 10 direct searches for new physics using the full 2016 data sample. The CMS-TOTEM Precision Proton Spectrometer, which started its operation in 2016, has obtained the first evidence for the central semi-exclusive production of dimuon pairs, pp → p μ⁺μ⁻ p(∗), with mass m(μμ) > 110 GeV, from 10 fb^-1 of data collected during the regular high-intensity runs of the LHC. This process is sensitive to new-physics contributions and has never before been measured. The search for the production of a pair of Higgs bosons in the bbγγ final state, sensitive to the Higgs self-coupling, provides the most stringent constraint on the product of the production cross section and branching fraction. The high luminosity LHC will provide valuable data to finally probe this fundamental parameter of the Higgs mechanism. New results in searches for exotic phenomena include more stringent constraints on Dark Matter mediators, excited states of light- and heavy-flavor quarks, extra dimensions, heavy vector-like quarks, long-lived particles, and the direct production of tau sleptons, as well as the electroweak production of charginos and neutralinos.

The new ALICE results from pp, p-Pb and Pb-Pb collisions marked an important advancement in the understanding of identified particle production, collective effects and hard processes. The extensive campaign of measurements in pp and p-Pb collisions suggests the possibility that a small droplet of quark-gluon plasma can be created in these smaller systems. The similarity of the different colliding systems, with respect to effects that were considered to be hallmarks of heavy ion collisions, was confirmed. In particular, measurements of collective effects and strangeness enhancement were presented. These measurements provide a pathway to understand the microscopic emergence of the quark-gluon plasma. They also challenge some of the established models of soft QCD (for what concerns the universality of string fragmentation) and of the quark-gluon plasma (how close does the system get to thermal equilibrium?).

The results on strangeness enhancement in pp collisions, recently published in Nature Physics, were complemented with new measurements on strange particle production in Pb-Pb collisions at centre-of-mass energy ofNN =5.02 TeV. In central Pb-Pb collisions, particle abundances are known to reflect (with some tensions) a thermal distribution, whose origin is not yet understood. The smooth evolution of ratios across colliding systems and size represents an opportunity to clarify the dynamical origin of this apparent equilibrium. The recent Run-II Pb-Pb measurements confirm the behaviours and trends observed at √sNN = 2.76 TeV with increased statistics and data quality, paving the road for a precise characterization of the quark-gluon plasma.

Lambda/K0 ratio vs transverse momentum (pT) in several centrality classes (V0M) in Pb-Pb collisions at 5.02 TeV from LHC run 2. Error bars depict statistical uncertainties while boxes show systematic uncertainties. [Credits: ALICE/CERN]

The status of collective effects of light-flavour hadrons measured in large and small systems was discussed in various talks.  This provides the baseline for the study of heavy-flavour particles: Run II results on the v2 of D mesons and J/ Ψ, with a much-reduced uncertainty as compared to Run I, were presented. The v2 is now found to be significant at more than 5σ for both mesons. These important results indicate that charm quarks thermalise with the quark-gluon plasma. They pave the road for the detailed heavy-flavour collectivity studies that ALICE will complete following the upgrade (in future runs of the LHC), which will ultimately elucidate the process of thermalisation in the deconfined QCD medium.

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

“The conference has attracted considerable interest and we shall have the chance to discover many exciting new results presented by experiments from all around the world”, said Yves Sirois the Chair of the EPS HEPP Division responsible for the physics program.

All in all, the structure of the Universe is one of the most fascinating topics and thanks to scientific instruments like LHC and LIGO/VIRGO we are now getting a deeper understanding. The higher collision energies achieved at LHC and the by now breath-taking precision of results from cosmological research will hopefully yield more information about the search for dark matter and the origin of our Universe.

The article is based on results discussed in the ATLAS, CMS and LHCb public websites as well as in ALICE Matters.