Abstract
Abstract:
A search for exotic decays of the Higgs boson into a pair of spin-zero particles, H → aa, where the
a-boson decays into b-quarks promptly or with a mean proper lifetime cτa up to 6 mm and has a mass
in the range of 20–60 GeV, is presented. The search is performed in events where the Higgs boson is
produced in association with a W or Z boson, giving rise to a signature of one or two charged
leptons (electrons or muons) and multiple jets from b-quark
decays. The analysis is based on the dataset of proton–proton collisions at √s = 13 TeV
recorded in 2015 and 2016 by the ATLAS detector at the CERN Large Hadron Collider, corresponding to
an integrated luminosity of 36.1 fb−1. No significant excess of events above the Standard Model
background prediction is observed, and 95% confidence-level upper limits are derived for the
production cross-sections for pp → W H, Z H and their combination, times the branching ratio of the
decay chain H → aa → 4b. For a-bosons which decay promptly, the upper limit on the combination of
cross-sections for W H and Z H times the branching ratio of H → aa → 4b ranges from 3.0 pb for ma =
20 GeV to 1.3 pb for ma = 60 GeV, assuming that the ratio of W H to Z H cross-sections follows the
Standard Model prediction. For a-bosons with longer proper lifetimes, the most stringent limits are
1.8 pb and 0.68 pb, respectively, at cτa ∼ 0.4 mm.