Search for the Higgs boson produced in association with a vector boson and decaying into two spin-zero particles in the H → aa → 4b channel in pp collisions at $$ \sqrt{s}=13 $$ TeV with the ATLAS detector

Abstract

Search for the Higgs boson produced in association with a vector boson and decaying into two spin-zeroparticles in the H → aa → 4b channel in p p collisions at √ s = 13 TeV with the ATLAS detectorThe ATLAS Collaboration 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 cτ a 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 m a = 20 GeV to 1.3 pb for m a = 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 cτ a ∼ 0.4 mm.The discovery of the Higgs boson by the ATLAS and CMS collaborations [1, 2] at the Large Hadron Collider (LHC) has been a major achievement for the Standard Model (SM). A comprehensive programme to explore the properties of this particle is underway, including measurements of the branching ratios to SM particles and searches for decays into "exotic" or non-SM particles. Exotic Higgs boson decays are a powerful probe for physics beyond the SM (BSM). The Higgs boson has a very narrow decay width, so even a small coupling to a non-SM particle could open up a sizeable decay mode. Measurements at the LHC are in agreement with SM predictions, constraining the non-SM branching ratio of the Higgs boson to less than approximately 30% at 95% confidence level (CL) using the 7 and 8 TeV datasets [3][4][5].Despite this experimental triumph, there is still ample room for exotic Higgs boson decays compatible with observations to date.The Higgs boson has been proposed as a possible "portal" for hidden-sector particles to interact with SM particles [6][7][8]. Exotic decays in particular are predicted by many BSM theories [9], including those with an extended Higgs sector such as the Next-to-Minimal Supersymmetric Standard Model (NMSSM) [10][11][12][13][14], models with a first-order electroweak phase transition [15,16], models with neutral naturalness [17][18][19] and models of dark matter [20][21][22][23][24].The decay of the Higgs boson into a pair of sp...

Two-real-scalar-singlet extension of the SM: LHC phenomenology and benchmark scenarios

“…Figure 4 (left) illustrates that experimental searches in this channel are beginning to directly constrain the TRSM for resonance masses between around 380 GeV and 550 GeV. In contrast, LHC searches [67][68][69][70][71][72][73] for the inverted signature of single-production of h 125 which then decays into a pair of light h a (a ∈ {1, 2}) are not yet sensitive, as the indirect constraints from Higgs signal rates on the possible new decay modes, BR(h 125 → NP) ≤ 7.3% (see Fig. 2), are much stronger than the direct limits from these searches.…”

“…In general, decays directly to SM particle final states as well as to two lighter scalar states ("Higgs-to-Higgs decays") are possible. While some of the former decays are already searched for by the LHC experiments, current searches for Higgs-to-Higgs decays focus almost exclusively on the signatures h S → h 125 h 125 (where h S denotes the new Higgs state with mass above 250 GeV) [55][56][57][58][59][60][61][62][63][64][65][66], or h 125 → h S h S (with the h S mass below 62.5 GeV) [67][68][69][70][71][72][73]. The model, however, features also Higgs decays to unidentical scalar bosons ("asymmetric decays"), Higgs decays involving only non-SMlike Higgs bosons, as well as the possibility of successive Higgs-to-Higgs cascade decays.…”

Detector-size upper bounds on dark hadron lifetime from cosmology

“…In these studies we assume the energy cuts required in real searches do not change the boost distribution significantly. This is a valid assumption when considering existing searches from Higgs decays at ATLAS/CMS [29,57,58] and the LLP searches at the LHCb [19,52,54], which do not require high p T final states in the searches. We also do not consider scenarios that allow long-lived particles to be trapped inside the detector and decay after a long time [60] and assume dark hadrons simply fly away from the production point.…”

“…The tracking information from the VELO detector is essential to reconstruct the decay location and veto hadronic backgrounds, and this is why most LHCb LLP searches have their best sensitivity for a O(1) cm scale decay length [28,53,54]. Having inner detector information in ATLAS/CMS searches helps to reconstruct displaced decay signals [30,31,[55][56][57][58]. The number of charged tracks from a hadronic LLP decay is also useful to distinguish the signal from QCD backgrounds.…”

Investigating extended scalar sectors at current and future colliders

“…While searches for new physics states that decay into the SM scalar, as well as the decay of the SM scalar into two lighter particles have already been investigated (see e.g. [69][70][71][72] for recent searches), single scalar production following by asymmetric scalar decays or symmetric decays, where all scalar masses differ from the SM-like scalar mass at 125 GeV, have not yet been fully explored. Simple counting reveals that for such scenarios at least three physical scalar states need to be present in the model, out of which one takes the role of the state already discovered by the LHC experiments.…”