A search for heavy Higgs bosons decaying into vector bosons in same-sign two-lepton final states in pp collisions at $$ \sqrt{s} $$ = 13 TeV with the ATLAS detector

Published for SISSA by Springer Received: November 7, 2022 Accepted: March 8, 2023 Published: July 26, 2023 The ATLAS collaboration E-mail: Abstract: A search for heavy Higgs bosons produced in association with a vector boson and decaying into a pair of vector bosons is performed in final states with two leptons (electrons or muons) of the same electric charge, missing transverse momentum and jets. A data sample of proton–proton collisions at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider between 2015 and 2018 is used. The data correspond to a total integrated luminosity of 139 fb−1 . The observed data are in agreement with Standard Model background expectations. The results are interpreted using higherdimensional operators in an effective field theory. Upper limits on the production crosssection are calculated at 95% confidence level as a function of the heavy Higgs boson’s mass and coupling strengths to vector bosons. Limits are set in the Higgs boson mass range from 300 to 1500 GeV, and depend on the assumed couplings. The highest excluded mass for a heavy Higgs boson with the coupling combinations explored is 900 GeV. Limits on coupling strengths are also provided. Keywords: Higgs Physics, Hadron-Hadron Scattering, Proton-Proton Scattering ArXiv ePrint: 2211.02617 Open Access, Copyright CERN, for the benefit of the ATLAS Collaboration. Article funded by SCOAP3 . https://doi.org/10.1007/JHEP07(2023)200 JHEP07(2023)200 A search for heavy Higgs bosons decaying into vector bosons in same-sign two-lepton final states in pp √ collisions at s = 13 TeV with the ATLAS detector Contents 1 2 Phenomenology 2 3 ATLAS detector and data samples 4 4 Simulation of signal and background processes 5 5 Object reconstruction and identification 7 6 Event classification and selection 9 7 Background estimation 7.1 Electron charge-flip background 7.2 Non-prompt background 7.3 Photon conversion background 7.4 Validation of background estimates 11 11 12 13 14 8 Systematic uncertainties 8.1 Experimental uncertainties 8.2 Theoretical uncertainties 8.3 Data-driven background estimation uncertainties 14 14 16 17 9 Results 9.1 Statistical analysis 9.2 Data and background comparisons 9.3 Limits on the production of heavy Higgs bosons 17 17 18 20 10 Summary 23 The ATLAS collaboration 31 1 Introduction The discovery of the Higgs boson was a major success for the Standard Model (SM) and an important breakthrough in understanding electroweak symmetry breaking [1, 2]. It also opened new ways to search for physics beyond the Standard Model (BSM physics). Despite its success the SM is not without problems that may require extensions and new concepts. One natural extension common to many BSM physics models is an extended Higgs sector, which leads to the introduction of additional Higgs bosons. In such models, –1– JHEP07(2023)200 1 Introduction 2 Phenomenology In theories with multiple Higgs fields, the fields in the multi-Higgs potential interact and the mass eigenstates are formed from a mixture of the fields. In the simple case of a lightest (h) and next-to-lightest (H) neutral Higgs doublet, the couplings of the Higgs boson to the SM gauge bosons are scaled relative to SM gauge couplings because of the mixing. In addition to the leading-order dimension-four (dim-4) operators, dimension-six (dim-6) –2– JHEP07(2023)200 the lightest Higgs boson (h) often has properties similar to those of the observed SM Higgs boson. Additional Higgs bosons have been introduced to explain a very wide range of BSM phenomena, from the observed baryon asymmetry in the universe, and how to solve the strong CP problem with the help of axions, to the generation of non-zero neutrino masses [3–7]. Some models have incorporated recent potential deviations from the SM seen in muon g − 2 and W mass measurements [8]. Several searches for additional heavy Higgs bosons (H) have already been carried out with the ATLAS and CMS detectors at the Large Hadron Collider (LHC) [9–15]. These searches mainly relied on the gluon–gluon fusion (ggF) and vector-boson fusion (VBF) production mechanisms, which are the dominant production modes for the SM Higgs boson at the LHC. In ggF production, the gluons couple to the Higgs boson mainly via a topquark loop, so the non-observation of an additional Higgs boson in this channel could point to a reduced fermionic coupling. This analysis concentrates on the production of a heavy Higgs boson in association with a vector boson (V H, where V = W, Z) and H → V V decays. By utilising a general effective Lagrangian that includes dimension-six operators in an effective field theory (EFT), it can be shown that, relative to VBF production, the V H production mode benefits from having smaller SM backgrounds, and the production cross-section may be enhanced by higherorder contributions, especially at high Higgs boson mass and vector-boson momenta [16– 18]. The observed Higgs boson h, is assumed to have the production and decay modes as in the SM, with production through H → Zh negligible. Rather than focusing on any specific model, a generic search is performed for the same-sign dilepton signature (SS2L), targeting the W ± H → W ± W ± W ∓ → `± ν`± νjj decay channel. The corresponding Feynman diagram is shown in figure 1. In this article, the term ‘lepton’, unless stated otherwise, refers to either an electron or a muon. Electrons and muons from τ -lepton decays are also considered. The hadronically decaying W boson is reconstructed either as two small-radius jets or as a single large-radius jet for highermomentum W bosons. The presence of neutrinos prevents the full reconstruction of the heavy Higgs boson’s mass and is the main drawback of the W ± W ± W ∓ channel with leptonic decays of the W bosons. It is ameliorated with the help of the reconstruction methods described in section 6. Compared with other bosonic V H decay channels, the chosen channel has the highest signal sensitivity thanks to low SM backgrounds and a sizeable branching fraction for H → W W decay [18, 19]. SM processes produce same-sign lepton pairs at the LHC at a rate that is orders of magnitude below that of opposite-sign lepton-pair production in the SM. 𝑊∓ 𝑞 𝑊± ℓ± 𝑊± ഥ 𝑞′ ℓ± 𝜈 𝜈 Figure 1. Feynman diagram of the W ± H → W ± W ± W ∓ process. effective operators as described in refs. [16–18] are also considered. After electroweak symmetry breaking, the effective Lagrangian terms can be written as (4) LhW W = ρh gmW hW µ Wµ , (4) LhZZ = ρh gmW hZ µ Zµ , 2c2W (4) LHW W = ρH gmW HW µ Wµ , gmW HZ µ Zµ , 2c2W  fW  + −µ ν fW W + −µν (6) LHW W = ρH gmW 2 Wµν W ∂ H + h.c. − ρH gmW Wµν W H, 2Λ Λ2 c2 fW + s2W fB c4W fW W + s4W fBB (6) µ ν LHZZ = ρH gmW W Z Z ∂ H − ρ gm Zµν Z µν H, µν H W 2c2W Λ2 2c2W Λ2 (4) LHZZ = ρH where h, H, W and Z are the fields of the light and heavy Higgs bosons and the W and Z bosons, respectively; mW is the W boson mas (...truncated)