Direct coupling of the Higgs boson to the bottom quark observed
Publication highlight: Prof. Martin Grunewald
Journal reference: Physical Review Letters 121, 121801 (2018)
An observation made by the CMS experiment at CERN, submitted for publication in Physical Review Letters today, represents yet another important milestone reached in the scrutiny of the Higgs boson and its interactions with Standard Model particles. Members of the School of Physics at UCD were responsible for the high-level trigger system to record the data analysed.
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.
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 [PRL 120 (2018) 231801] 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.
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.