An observation made by the CMS experiment at CERN, submitted for publication in Physical Review Letters on August 28th, represents yet another important milestone reached in the scrutiny of the Higgs boson and its interactions with Standard Model particles.
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 discovery marked the beginning of an experimental programme aimed at determining the properties of the newly discovered particle. Reporting on Aug 28 at a seminar at CERN, the CMS collaboration announces yet another milestone in that programme, following a recent publication that announced the first observation of the direct Higgs boson coupling to the heaviest Standard Model particle, the top quark.
Although the direct decay of the Higgs boson to bottom quarks (also called beauty quark) 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, in order to allow a significant reduction of the background, it was necessary on one side to focus on particular signal-enriched signatures where a Higgs boson is produced in association with a vector boson (a W or Z particle, as seen in the feature image); and, on the other side, to adopt advanced analysis techniques exploiting deep neural networks and other machine learning tools. Being the process 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 elected Spokesperson Roberto Carlin, full professor at Padova University and researcher of INFN Padova.
Once more, this measurement confirms the predictions of the Standard Model. The present uncertainty on the decay rate is however still large, leaving wide possibilities for the improvements expected in the next decade during the so called “Phase 2” of LHC operations.
In the feature image the illustration of a 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.