CMS-PAS-HIG-23-002

CMS-PAS-HIG-23-002
Evidence for the Higgs boson decay to a Z boson and a photon at the LHC
ATLAS and CMS Collaborations
25 May 2023

Abstract: The first evidence is presented for the Higgs boson decay to a Z boson and a photon, with a statistical significance of 3.4 standard deviations. The result is derived from a combined analysis of the searches performed by the ATLAS and CMS Collaborations with proton-proton collision data sets collected at the CERN Large Hadron Collider (LHC) from 2015 to 2018. These correspond to integrated luminosities of around 140 fb$^{-1} $ for each experiment, at a center-of-mass energy of 13 TeV. The measured signal rate, relative to the Standard Model prediction, is 2.2 $ \pm $ 0.7, and agrees with the theoretical expectation within 1.9 standard deviations.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRL 132 (2024) 021803. The superseded preliminary plots can be found here.

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The Z$\gamma$ invariant mass distribution of events from all ATLAS and CMS analysis categories. The data events (dots with error bars) in each category are weighted by $ \ln(1 + S/B) $, where $ S $ and $ B $ are the observed signal and background yields in that category, in the 120-130 GeV interval, derived from the fit to data. The fitted signal-plus-background (background) pdfs in each category are also weighted in the same way and summed, and represented by a red solid (blue dashed) line. In the lower panel, the same data and the two models are compared after subtraction of the estimated background.
png pdf	Figure 2: Negative profile log-likelihood scan of the signal strength $ \mu $ from the analysis of ATLAS data (blue line), CMS data (red line), or the combined result (black line). The results are nearly identical for the two different Higgs boson mass hypotheses described in the text.

Summary

In summary, a combination of the ATLAS and CMS searches of the Higgs boson decay to a Z boson and a photon, where the Z boson decays to electron or muon pairs, is presented. The analysis uses the CERN LHC proton–proton collision data recorded by the ATLAS and CMS experiments between 2015 and 2018, corresponding to integrated luminosities of 139 fb$ ^{-1} $ (ATLAS) and 138 fb$ ^{-1} $ (CMS) at a center-of-mass energy of 13 TeV. Evidence for the $ H{\rightarrow}Z\gamma $ decay is established, with a significance of 3.4 standard deviations. The observed signal yield is 2.2 $ \pm $ 0.7 times the Standard Model (SM) prediction. The $ H{\rightarrow}Z\gamma $ branching fraction is measured to be (3.4 $ \pm $ 1.1) $\times$ 10$^{-3} $, in agreement within 1.9 standard deviations with the SM prediction.

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