Abstract
A search for exotic decays of the Higgs boson into a pair of spin-zero particles, H → aa, where the a-boson decay sin to 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 sqrt{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 → WH, ZH 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 WH and ZH 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 WH to ZH 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.
Highlights
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 distributions of the discriminant for each of the analysis categories are combined in a profile likelihood fit to test for the presence of signal, while simultaneously determining the normalizations and constraining the differential distributions of the most important background components
The Poisson probability depends on the predicted number of events in each bin, which in turn is a function of the signal-strength parameter μ = σ ×B(H → aa → 4b), where σ are the pp → W H and ZH cross-sections
Summary
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. When applying the matrix method in the case of high jet and b-tagged jet multiplicities, the number of events in data satisfying the loose and tight lepton selections is significantly reduced, leading to large fluctuations in the background predictions. In the single-lepton signal regions, background ttevents can only satisfy the selection criteria if accompanied by additional b-tagged jets.
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