Search for Higgs boson decays to a Z boson and a photon in proton-proton collisions at $$ \sqrt{s} $$ = 13 TeV

Abstract

Results are presented from a search for the Higgs boson decay H → Zγ, where Z → ℓ+ℓ− with ℓ = e or μ. The search is performed using a sample of proton-proton (pp) collision data at a center-of-mass energy of 13 TeV, recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 138 fb−1. Events are assigned to mutually exclusive categories, which exploit differences in both event topology and kinematics of distinct Higgs production mechanisms to enhance signal sensitivity. The signal strength μ, defined as the product of the cross section and the branching fraction $$ \left[\sigma \left(\textrm{pp}\to \textrm{H}\right)\mathcal{B}\left(\textrm{H}\to \textrm{Z}\upgamma \right)\right] $$ σ pp → H B H → Zγ relative to the standard model prediction, is extracted from a simultaneous fit to the ℓ+ℓ−γ invariant mass distributions in all categories and is measured to be μ = 2.4 ± 0.9 for a Higgs boson mass of 125.38 GeV. The statistical significance of the observed excess of events is 2.7 standard deviations. This measurement corresponds to $$ \left[\sigma \left(\textrm{pp}\to \textrm{H}\right)\mathcal{B}\left(\textrm{H}\to \textrm{Z}\upgamma \right)\right]=0.21\pm 0.08 $$ σ pp → H B H → Zγ = 0.21 ± 0.08 pb. The observed (expected) upper limit at 95% confidence level on μ is 4.1 (1.8), where the expected limit is calculated under the background-only hypothesis. The ratio of branching fractions $$ \mathcal{B}\left(\textrm{H}\to \textrm{Z}\upgamma \right)/\mathcal{B}\left(\textrm{H}\to \upgamma \upgamma \right) $$ B H → Zγ / B H → γγ is measured to be $$ {1.5}_{-0.6}^{+0.7} $$ 1.5 − 0.6 + 0.7 , which agrees with the standard model prediction of 0.69 ± 0.04 at the 1.5 standard deviation level.

The profile of the Higgs boson: status and prospects

“…This channel has a very low branching ratio in the SM, but new physics might lead to a significant enhancement, such that it might become visible. In the analyses of the ATLAS and CMS collaborations, first indications of a small excess have been seen, corresponding to observed significances of 2.2σ [75] (ATLAS) and 2.7σ [76] (CMS). Within the large uncertainties, the observed excesses above the backgrounds are in agreement with the expectations for SM H → Zγ decays.…”

Analysis of final state lepton polarization-dependent observables in H → ℓ+ℓ−γ in the SM at loop level

“…So far the Higgs couplings to W , Z, τ , b, t and photons have been measured experimentally by the CMS [7][8][9][10][11][12][13] and ATLAS [14][15][16][17][18][19][20][21][22] collaborations. It is worthwhile to mention here that the recently analyzed data [1,2] at a luminosity of 150 fb −1 is not sufficient to measure spin asymmetries. However, HL-LHC, which plans to increase the luminosity to 450-3000 fb −1 , as well as the planned e + e − collider would allow better measurements of these asymmetries [23,24].…”

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Recently, the CMS and ATLAS collaborations have announced the results for H → Z[→ ℓ + ℓ − ]γ with ℓ = e or µ [1,2], where H → Zγ is a sub-process of H → ℓ + ℓ − γ. This semi-leptonic Higgs decay gets both resonant and non-resonant contributions from the loop-induced H → Z[→ ℓ + ℓ − ]γ.

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NNLO QCD predictions of the asymmetry probe of the Z γ pair-production process

“…The ATLAS and CMS Collaborations at the LHC have carried out several searches for new resonances and presented stringent constraints on various new-physics models [20 -24], where nonresonant Zγ decays stand for the dominant background process in these searches. In addition, the Zγ process constitutes major background in searches for the Higgs-boson decay with a very small branching fraction H → Zγ at the LHC [25][26][27][28][29][30]. The H → Zγ process is not only important for extraction of Higgs-boson properties but also for searches for new-physics models including supersymmetry and extended Higgs sectors, as elaborated such as in the [31][32][33][34][35].…”

“…respectively, in the lepton pseudorapidity acceptance |η l | < 2.47. Photon is required to have > g p T30 GeV in the acceptance |η γ | < 2.37. Photon and leptons must be separated within a cone of radius ΔR(l, γ) > 0.4.…”