Search for charged Higgs bosons decaying via H± → τ±ντ in the τ+jets and τ+lepton final states with 36 fb−1 of pp collision data recorded at $$ \sqrt{s}=13 $$ TeV with the ATLAS experiment

Abstract

Search for charged Higgs bosons decaying viaH ± → τ ± ν τ in the τ+jets and τ+lepton final states with 36 fb −1 of p p collision data recorded at √ s = 13 TeV with the ATLAS experimentThe ATLAS Collaboration Charged Higgs bosons produced either in top-quark decays or in association with a topquark, subsequently decaying via H ± → τ ± ν τ , are searched for in 36.1 fb −1 of proton-proton collision data at √ s = 13 TeV recorded with the ATLAS detector. Depending on whether the top-quark produced together with H ± decays hadronically or leptonically, the search targets τ+jets and τ+lepton final states, in both cases with a hadronically decaying τ-lepton. No evidence of a charged Higgs boson is found. For the mass range of m H ± = 90-2000 GeV, upper limits at the 95% confidence level are set on the production cross-section of the charged Higgs boson times the branching fraction B(H ± → τ ± ν τ ) in the range 4.2-0.0025 pb. In the mass range 90-160 GeV, assuming the Standard Model cross-section for tt production, this corresponds to upper limits between 0.25% and 0.031% for the branching fraction B(t → bH ± ) × B(H ± → τ ± ν τ ). Data and samples of simulated eventsThe ATLAS experiment [27] is a multipurpose detector with a forward-backward symmetric cylindrical geometry with respect to the LHC beam-axis.2 The innermost layers of ATLAS consist of tracking detectors in the pseudorapidity range |η| < 2.5, including the insertable B-layer [28, 29] installed for 2 The ATLAS experiment uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the z-axis. The pseudorapidity is defined in terms of the polar angle θ as η = − ln[tan(θ/2)]. Transverse momenta are computed from the three-momenta ì p as p T = | ì p| sin θ. The distance in the η-φ space is commonly referred to as ∆R = (∆η) 2 + (∆φ) 2 . Analysis strategyThis paper describes a search for a charged Higgs boson decaying via H + → τν in topologies where it is produced either in top-quark decays or in association with a top-quark. Depending on whether the top-quark produced with the H + decays hadronically or semileptonically, two channels are targeted: τ had-vis +jets or τ had-vis +lepton, respectively. The corresponding signal regions are described below.6

Charged Higgs bosons in naturally aligned two-Higgs-doublet models at the LHC

“…Charged Higgs bosons H ± may also decay through their couplings to fermions, with a preference for heavier fermions when kinematically allowed. Numerous searches have been performed at LEP [30], Tevatron [31][32][33], and the LHC [34][35][36][37][38] for the decays of H + to τ + ν τ , cs, and for sufficiently heavy H + , to tb. Charged Higgs bosons can, in principal, also decay to W ± -bosons and any of the neutral Higgs bosons, h, H and A.…”

A low-scale flavon model with a ℤN symmetry

“…That of the charged Higgs H ± is a top quark and a bottom quark. In other words, the branching fractions to the leptonic modes, which are more strongly constrained [135][136][137][138], are suppressed owing to a small tan β. There are substantial limits from the current searches for heavy Higgs bosons decaying to top quark at the LHC only if tan β 1 and the top Yukawa coupling is enhanced [139][140][141].…”

On lepton flavor universality in top quark decays

“…, it is clear that once the NP degrees of freedom cannot be produced on-shell, current measurements of top decays become ineffective in constraining violations of LFU or respectively the related NP parameters at any appreciable level. In that regime, one can do direct searches for the NP state produced in association with third generation quarks [38][39][40][41][42], but the limits will be model dependent. For example, the most recent ATLAS searches for charged bosons produced in association with top and b-quarks, and decaying to tb [42] or τ ν [41] are in principle sensitive to our simplified models, especially in the low mass range around 200 GeV.…”

“…In that regime, one can do direct searches for the NP state produced in association with third generation quarks [38][39][40][41][42], but the limits will be model dependent. For example, the most recent ATLAS searches for charged bosons produced in association with top and b-quarks, and decaying to tb [42] or τ ν [41] are in principle sensitive to our simplified models, especially in the low mass range around 200 GeV. However, such bounds may be avoided in more complete models, if for example the on-shell mediators predominantly decay to pairs of lighter (e.g.…”