ICHEP day 2: Two years after Higgs boson announcement

Yesterday, CMS and ATLAS presented their most complete and most comprehensive measurements of the properties and decay modes of the Higgs boson

Yesterday, exactly two years after CERN announced the discovery of the Higgs boson, CMS and ATLAS presented their most complete and most comprehensive measurements of the properties and decay modes of the Higgs boson. These new measurements included expanded studies of the Higgs boson’s interactions with different types of fermions and results combining measurements from its various well-studied decay channels. All of these new combination measurements of the Higgs boson’s mass, spin and parity, production modes and decay patterns are in agreement with the Standard Model’s predictions.

CMS for instance presented the final combination of the measurements carried out on 16 different combinations of channels for the Standard Model Higgs boson: this is the culmination of many years of work where the analyses of the five main decay modes are combined together with analyses targeting the rarest production mode (ttH).

In addition, ATLAS presented new measurements of strong-interaction processes involving W and Z bosons. These measurements allow physicists to test the newest theoretical calculations of such processes, which promise not less than a break-through in terms of precision predictions. These tests are of paramount importance to improve the precision with which the experiments can probe the Higgs boson sector and search for new phenomena.

In another session, ATLAS announced the observation of a new particle state of mass and decay properties consistent with expectations for an excited state of the B_c meson. Mesons are composite particles made of one quark and one antiquark. A Bc meson contains one bottom quark and one charm antiquark. All mesons also come in excited versions where the quark and antiquark have a lot of angular momentum (i.e. they spin around each other) or in the case of this Bc meson, more distance between the two quarks. Having discovered this new state will enable theorists to test various models describing such objects. In principle, both the CMS and LHCb experiments can confirm this observation. The ATLAS experiment will need new data to better study this state and see if this is indeed the Bc(2S) excited state. 

CMS presented also very competitive new precision measurements of the top-quark mass, based on the full dataset recorded in the year 2012. The top quark is the heaviest known elementary particle to-date, and its mass is a fundamental parameter of the Standard Model. These results constitute a significant step in the effort to further improve the precision of the top quark mass, a fundamental constant of nature.

 

Dark matter remains in the dark

In the morning, both the CMS and ATLAS experiments also showed the status of their searches for dark matter, this elusive form of matter that has so far only been detected through its gravitational effects. To this day, there is no convincing proof of direct observation for dark matter, despite the on-going efforts of many and very diversified experiments. More recently, the LHC experiments have joined this effort with the hope that dark matter could be produced at the LHC. But even if this were the case, the dark matter particles would be invisible in the LHC detectors. 

Scientists from both ATLAS and CMS must therefore look for accompanying standard particles such as photons, quarks, leptons, W or Z bosons created along the invisible dark matter particles. The signature in the detectors would be striking: one of these particles created all by itself and nothing else in the event but missing energy carried away by the dark matter particles. 

Both groups reported on their detailed searches but nothing has been found so far. These measurements complement all the other direct searches. Until now, the LHC experiments were the only experiments sensitive to very low mass dark matter particles, a region not covered by the direct search experiments. But today, CRESST and CDMS-lite, two experiments operating underground, showed they have greatly improved their background rejection techniques, allowing them to also probe this low mass region. The two types of experiments will therefore complement each other and we should soon know if dark matter is made of particles with masses around a few GeV, that is, a few times the mass of a proton.

 

For more information, visit the experiments’ websites:

http://atlas.ch/

http://cern.ch/cms