CERN Accelerating science

New LHC results presented at EPS-HEP 2017

by Panos Charitos

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.