Constraints on new phenomena via Higgs boson couplings and invisible decays with the ATLAS detector

Abstract

Abstract:The ATLAS experiment at the LHC has measured the Higgs boson couplings and mass, and searched for invisible Higgs boson decays, using multiple production and decay channels with up to 4.7 fb −1 of pp collision data at √ s = 7 TeV and 20.3 fb −1 at √ s = 8 TeV. In the current study, the measured production and decay rates of the observed Higgs boson in the γγ, ZZ, W W , Zγ, bb, τ τ , and µµ decay channels, along with results from the associated production of a Higgs boson with a top-quark pair, are used to probe the scaling of the couplings with mass. Limits are set on parameters in extensions of the Standard Model including a composite Higgs boson, an additional electroweak singlet, and two-Higgs-doublet models. Together with the measured mass of the scalar Higgs boson in the γγ and ZZ decay modes, a lower limit is set on the pseudoscalar Higgs boson mass of m A > 370 GeV in the "hMSSM" simplified Minimal Supersymmetric Standard Model. Results from direct searches for heavy Higgs bosons are also interpreted in the hMSSM. Direct searches for invisible Higgs boson decays in the vector-boson fusion and associated production of a Higgs boson with W/Z (Z → , W/Z → jj) modes are statistically combined to set an upper limit on the Higgs boson invisible branching ratio of 0.25. The use of the measured visible decay rates in a more general coupling fit improves the upper limit to 0.23, constraining a Higgs portal model of dark matter. Conclusions 26The ATLAS collaboration 36 IntroductionThe ATLAS and CMS Collaborations at the Large Hadron Collider (LHC) announced the discovery of a particle consistent with a Higgs boson in 2012 [1,2]. Since then, the collaborations have together measured the mass of the particle to be about 125 GeV [3][4][5]. Studies of its spin and parity in bosonic decays have found it to be compatible with a J P = 0 + state [6][7][8]. Combined coupling fits of the measured Higgs boson production and decay rates within the framework of the Standard Model (SM) have found no significant deviation from the SM expectations [4,9, 10]. These results strongly suggest that the newly discovered particle is indeed a Higgs boson and that a non-zero vacuum expectation value of a Higgs doublet is responsible for electroweak (EW) symmetry breaking [11][12][13].The observed CP-even Higgs boson is denoted as h throughout this paper. A crucial question is whether there is only one Higgs doublet, as postulated by the SM, or whether the Higgs sector is more complex, for example with a second doublet leading to more than one Higgs boson of which one has properties similar to those of the SM Higgs -1 - JHEP11(2015)206boson, as predicted in many theories beyond the Standard Model (BSM). 1 The "hierarchy problem" regarding the naturalness of the Higgs boson mass, the nature of dark matter, and other open questions that the SM is not able to answer also motivate the search for additional new particles and interactions. Astrophysical observations provide strong evidence of dark matter that could be explained by the e...

Probing light top partners with CP violation

“…The solid and dashed lines show the bounds for ξ ≡ v 2 /f 2 = 0.1 and ξ = 0.05 respectively, which roughly correspond to the present constraints on ξ coming from Higgs couplings measurements [73] and to the projected bounds for high-luminosity LHC [74][75][76]. The impact of ξ on the bounds is however quite mild.…”

“…tests [20] and from present Higgs couplings measurements [73]) and y L4 = 1. We also fix y R4 by requiring the top mass to have the correct value.…”

Monojet searches for momentum-dependent dark matter interactions

“…The momentum-dependence reduces to a constant, so that momentum-dependent interactions become indistinguishable from their momentum-independent counterparts. In this regime, bounds from monojet searches are found to be weaker than the constraints stemming from the Higgs invisible width results [17][18][19],…”

ALPs effective field theory and collider signatures

“…Current experimental searches by ATLAS [90,91] and CMS [92] constrain the branching ratio for Higgs decay into invisible states Br(h → inv) to [91] Br(h → inv) < 0.23 (95% CL).…”