ATLAS focuses on Higgs-boson decays to vector bosons

Decays of the Higgs boson to vector bosons (WW, ZZ, γγ) provide precise measurements of the boson’s coupling strength to other Standard Model (SM) particles. In new analyses, ATLAS has measured these decays for different production modes using the full 2015 and 2016 LHC datasets recorded at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 36.1 fb^–1.

With a predicted branching fraction of 21%, the Higgs-boson decay to two W bosons (H → WW) is the second most common decay mode after its decay to two b quarks. The new analysis follows a similar strategy to the earlier ones carried out using the LHC datasets recorded at 7 and 8 TeV. It focuses on the gluon-gluon fusion (ggF) and vector-boson fusion (VBF) production modes, with the subsequent decay to an electron, a muon and two neutrinos (H → WW → eνμν). The main backgrounds come from SM production of W and top-quark pairs; other backgrounds involve Z → ττ with leptonic τ decays and single-W production with misidentified leptons from associated jets.

Events are classified according to the number of jets they contain: events with zero or one jet are used to probe ggF production, while events with two or more jets are used to target VBF production. Due to the spin-zero nature of the Higgs boson, the electron and muon are preferentially emitted in the same direction. The ggF analysis exploits this and other kinematic information via a sequence of selection requirements, while the VBF analysis combines lepton and jet variables in a boosted decision tree to separate the Higgs-boson signal from background processes.

The transverse mass of the selected events from the zero and one-jet signal regions is shown in the left figure, with red denoting the expectation from the Higgs boson and other colours representing background processes. These events are combined with those from the two-jet signal region to derive cross sections times branching fractions for ggF and VBF production of 12.3 ^+2.3_–2.1 pb and 0.50^+0.30_–0.29 pb, respectively, to be compared to the SM predictions of 10.4 ± 0.6 pb and 0.81 ± 0.02 pb.

ATLAS also performed a combination of inclusive and differential cross-section measurements using Higgs-boson decays to two photons and two Z bosons, where each Z decays to a pair of oppositely charged electrons or muons. The combination of the two channels allows the study of Higgs-boson production rates versus event properties with unprecedented precision. For example, the measurement of the Higgs-boson rapidity distribution can provide information about the underlying parton density functions. The transverse momentum distribution (figure) is sensitive to the coupling between the Higgs boson and light quarks at low transverse momentum, and to possible couplings to non-SM particles at high values. The measured cross sections are found to be consistent with SM predictions.