The Brout–Englert–Higgs mechanism solves the apparent theoretical impossibility of allowing weak vector bosons (the W and Z) to acquire mass. The discovery of the Higgs boson in 2012 via its decays into photons, Z and W pairs was therefore a triumph of the Standard Model (SM), which is built upon this mechanism. But the Higgs field is also predicted to provide mass to charged fermions (quarks and leptons) via “Yukawa couplings”, with interaction strengths proportional to the particle mass. The observation by ATLAS and CMS of the Higgs boson decaying into pairs of τ leptons provided the first direct evidence of this type of interaction and, since then, both experiments have confirmed the Yukawa coupling between the Higgs boson and the top quark.
Six years after the Higgs-boson discovery, ATLAS had observed about 30% of its decays predicted by the SM. However, the favoured decay of the Higgs boson into a pair of b quarks, which is predicted to account for almost 60% of all possible decays, had remained elusive up to now. Observing this decay mode and measuring its rate is mandatory to confirm (or not) the mass generation for fermions via Yukawa interactions, as predicted in the SM.
At the 2018 International Conference on High Energy Physics (ICHEP) held in Seoul on July 4–11, ATLAS reported for the first time the observation of the Higgs boson decaying into pairs of b quarks at a rate consistent with the SM prediction. Evidence of the H→bb decay was earlier provided at the Tevatron in 2012, and one year ago by the ATLAS and CMS collaborations, independently. Given the abundance of H→bb decays, why did it take so long to achieve this observation?
The main reason is that the most copious production process for the Higgs boson in proton–proton collisions leads to a pair of particle jets originating from the fragmentation of b quarks (b-jets), and these are almost indistinguishable from the overwhelming background of b-quark pairs produced via the strong interaction. To overcome this challenge, it was necessary to consider production processes that are less copious, but exhibit features not present in strong interactions. The most effective of these is the associated production of the Higgs boson with a W or Z boson. The leptonic decays W→lν, Z→ll and Z→νν (where l stands for an electron or a muon) allow for efficient triggering and a powerful reduction of strong-interaction backgrounds.
However, the Higgs-boson signal remains orders of magnitude smaller than the remaining backgrounds arising from top-quark or vector-boson production, which can lead to similar signatures. One way to discriminate the signal from such backgrounds is to select on the mass, mbb, of pairs of b-jets identified by sophisticated b-tagging algorithms. When all WH and ZH channels are combined and the backgrounds (apart from WZ and ZZ production) subtracted from the data, the mbb distribution (figure, left) exhibits a clear peak arising from Z-boson decays to b-quark pairs, which validates the analysis procedure. The shoulder on the upper side is consistent in shape and rate with the expectation from Higgs-boson production.
Since this is not yet statistically sufficient to constitute an observation, the mass of the b-jet pair is combined with other kinematic variables that show distinct differences between the signal and the various backgrounds. This combination of multiple variables is performed using boosted decision trees for which a combination of all channels, reordered in terms of signal-to-background ratio, is shown in the right figure. The signal closely follows the distribution predicted by the SM with the presence of H→bb decays.
The analysis of 13 TeV data collected by ATLAS during Run 2 of the LHC between 2015 and 2017 leads to a significance of 4.9σ. This result was combined with those from a similar analysis of Run 1 data and from other searches by ATLAS for the H→bb decay mode, namely where the Higgs boson is produced in association with a top quark pair or via vector boson fusion. The significance achieved by this combination is 5.4σ, qualifying for observation.
Furthermore, combining the present analysis with others that target Higgs-boson decays to pairs of photons and Z bosons measured at 13 TeV yields the observation at 5.3σ of associated ZH or WH production, in agreement with the SM prediction. ATLAS has now observed all four primary Higgs-boson production modes at hadron colliders: fusion of gluons to a Higgs boson; fusion of weak bosons to a Higgs boson; associated production of a Higgs boson with two top quarks; and associated production of a Higgs boson with a weak boson. With these observations, a new era of detailed measurements in the Higgs sector opens up, through which the SM will be further challenged.
ATLAS Collaboration 2018 ATLAS-CONF-2018-036.