On 4 July 2012, two of the experiments at the CERN’s Large Hadron Collider (LHC), ATLAS and CMS, reported independently the discovery of the Higgs boson. The announcement created headlines worldwide: the discovery confirmed the existence of the last missing elementary particle of the Standard Model, half a century after the Higgs boson was predicted theoretically. At the same time the discovery marked also the beginning of an experimental programme aimed at  determining  the properties of the newly discovered particle. Reporting today at a seminar at CERN, the CMS collaboration announces yet another milestone in that programme, after a recent publication that announced the first observation of the direct Higgs boson coupling to the heaviest Standard Model particle, the top quark.

In the Standard Model, the Higgs boson can couple to fermions, with a coupling strength proportional to the fermion mass. The heaviest fermion that is lighter than half of the Higgs boson mass is the bottom quark, meaning that the Higgs boson can directly decay to a pair of bottom and anti-bottom quarks. The rate of such decays is related to the coupling strength squared, and it is this decay that now has been observed by the CMS collaboration, as well as by the ATLAS experiment that has submitted a similar result today.

While the direct decay of the Higgs boson to bottom quarks is actually the most frequent one of all possible Higgs decays, it has been a real experimental challenge to observe it. This is because there is an overwhelmingly large number of other Standard Model processes (called background) that can mimic the experimental signature characterized by the appearance of a bottom and an anti-bottom quark. Therefore, it was necessary to focus on particular signatures where a Higgs boson is produced in association with a vector boson (a W or Z particle, see the figure), producing a significant reduction in the background. Because this process  is quite rare, it was necessary to sift through a large number of collisions to find the signal. Fortunately, the LHC’s great performance in 2016 and 2017 made this possible.

“It was the ingenuity of CMS  scientists in deploying modern sophisticated analysis tools, including  machine learning techniques, and in combining the aforementioned signature with other sensitive Higgs boson processes, as well as the outstanding performance of the detector and the very large available data set, that made it possible to pass this milestone earlier than expected”, says CMS Spokesperson Joel Butler.

Some researcher from the Physics Department of the University of Trieste, together with colleagues from the Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, PhD students and post-docs, have been involved since many years in various sectors of the experiment, having contributed to the activities of the electromagnetic calorimeter and to the development and management of software and computing systems of the Collaboration. The group also participated in precision studies of the Standard Model processes that are important backgrounds for searches of new physics.

The researchers from Trieste contributed in particular to two crucial aspects in this measurement: the optimization of the performances of the electromagnetic calorimeter, fundamental for the detection of the decay products of the W and Z particles, and the measurement of the associated production of vector bosons and bottom quarks, one of the main backgrounds in this analysis.

With this observation of the Higgs boson coupling to the bottom quark, together with earlier observations of the Higgs coupling to the top quark and the tau lepton and therefore to all three of the heaviest known fermions, the CMS physics programme to characterise and more fully understand the Higgs boson has taken another important step. While the strength of the measured couplings is consistent with the Standard Model expectation, the precision of the measurements still leaves room for contributions from new physics. In the coming years, much more data will be collected and the precision will be improved, in order to see if the Higgs reveals the presence of physics beyond the Standard Model.

Figure caption: Candidate event showing the associated production of a Higgs boson and a Z boson, with the subsequent decay of the Higgs boson to a bottom quark and its antiparticle.

Link:
http://cms.cern/higgs-observed-decaying-b-quarks-submitted
http://cmsweb.ts.infn.it

prof. Giuseppe Della Ricca
dott. Vieri Candelise

Physics Department - University of Trieste and INFN, Trieste