CMS-HIG-23-002 ; ATLAS-CONF-2023-025

CMS-HIG-23-002 ; ATLAS-CONF-2023-025 ; CERN-EP-2023-114
Evidence for the Higgs boson decay to a Z boson and a photon at the LHC
ATLAS and CMS Collaborations
7 September 2023
Phys. Rev. Lett. 132 (2024) 021803
Abstract: The first evidence for the Higgs boson decay to a Z boson and a photon is presented, 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 yield is 2.2 $\pm$ 0.7 times the Standard Model prediction, and agrees with the theoretical expectation within 1.9 standard deviations.
Links: e-print arXiv:2309.03501 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Examples of Feynman diagrams for H ${\rightarrow}$ Z $\gamma$ decay.
png pdf	Figure 2: The Z $\gamma$ invariant mass distribution. Events from all categories in the ATLAS and CMS analyses are shown. As different ranges in $m_{\mathrm{Z}\gamma}$ are used in the two analyses, only the common subrange is visualized here. The data (points 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. The $S$ and $B$ values are derived from the fit to data. The error bars are invisible because of their small values. The fitted signal-plus-background (background) probability density functions (pdfs) in each category are also weighted in the same way and summed, and represented by a red solid (blue dashed) line. The lower panel shows the background-subtracted results with the same data and pdfs.
png pdf	Figure 3: The negative profile log-likelihood test statistic, where $\Lambda$ represents the likelihood ratio, as a function of the signal strength $\mu$ derived from the ATLAS data (blue line), the CMS data (red line), and the combined result (black line). The different Higgs boson masses assumed by ATLAS and CMS have a negligible impact on the results.

Summary

In summary, a combined analysis of ATLAS and CMS searches for the Higgs boson decay to a Z boson and a photon, where the Z boson decays into an electron or muon pair, is presented. The results are based on the 13 TeV proton--proton collision data recorded by the ATLAS and CMS experiments at the CERN LHC, amounting to integrated luminosities of 139 fb

$^{-1}$ and 138 fb

$^{-1}$ respectively. Evidence for H

${\rightarrow}$ Z

$\gamma$ decay is established, with an observed significance of 3.4 standard deviations. The observed signal yield is 2.2

$\pm$ 0.7 times the Standard Model prediction. The measured H

${\rightarrow}$ Z

$\gamma$ branching fraction is ( 3.4

$\pm$ 1.1 )

$\times$ 10

$^{-3}$ . The result agrees with the Standard Model prediction within 1.9 standard deviations.

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