Search for a Higgs boson decaying into a Z and a photon in pp collisions at sqrt(s) = 7 and 8 TeV

TL;DR

This CMS study searches for Higgs boson decays to a Z boson and a photon in the 120–160 GeV mass window using 7 and 8 TeV data, focusing on the dilepton+photon final state. It employs a data-driven, unbinned background modeling approach across multiple event classes, including a VBF-like dijet-tagged class to boost sensitivity. No excess over the Standard Model is found, leading to 95% CL upper limits on σ(pp→H)×B(H→Zγ) that exclude large enhancements relative to SM predictions. The work provides the first LHC limits on this decay channel and constrains new physics that would elevate the Zγ decay rate of the Higgs boson.

Abstract

A search for a Higgs boson decaying into a Z boson and a photon is described. The analysis is performed using proton-proton collision datasets recorded by the CMS detector at the LHC. Events were collected at center-of-mass energies of 7 TeV and 8 TeV, corresponding to integrated luminosities of 5.0 and 19.6 inverse femtobarns, respectively. The selected events are required to have opposite-sign electron or muon pairs. No excess above standard model predictions has been found in the 120-160 GeV mass range and the first limits on the Higgs boson production cross section times the H to Z gamma branching fraction at the LHC have been derived. The observed at 95% confidence level limits are between about 4 and 25 times the standard model cross section times the branching fraction. For a standard model Higgs boson mass of 125 GeV the expected limit at the 95% confidence level is 10 and the observed limit is 9.5. Models predicting the Higgs boson production cross section times the Higgs to Z gamma branching fraction to be larger than one order of magnitude of the standard model prediction are excluded for most of the 125-157 GeV mass range.

Figures (6)

• Figure 1: Diagrams contributing to $\Gamma_{{Z}\xspace\gamma}$.
• Figure 2: The $m_{\ell\ell\gamma}$ spectrum in the electron and the muon channels for the 7 and 8$\,\text{Te\spaceV}$ data combined, without weighting by the expected signal to background ratio of the individual data samples. Also shown is the expected signal due to a 125$\,\text{Ge\spaceV}$ standard model Higgs boson, scaled by 75, and the sum of the individual fits made to the data for each channel and event class described in section \ref{['sec:classes']}. The uncertainty band reflects the statistical uncertainty from the fits to the data.
• Figure 3: The background model fit to the $m_{{\mathrm{e}\mathrm{e}}\gamma}$ distributions for event classes 1--4 for the two data samples. The statistical uncertainty bands shown are computed from a fit to data. Also shown is the expected signal due to a 125$\,\text{Ge\spaceV}$ standard model Higgs boson, scaled by 75.
• Figure 4: The background model fit to the $m_{\mu\mu\gamma}$ distributions for event classes 1--4 for the two data samples. The statistical uncertainty bands shown are computed from the data fit. Also shown is the expected signal due to a 125$\,\text{Ge\spaceV}$ standard model Higgs boson, scaled by 75.
• Figure 5: The $m_{\ell\ell\gamma}$ spectrum in the electron and the muon channels combined (separately) for the 7 (8)$\,\text{Te\spaceV}$ data for the dijet-tagged event class. The expected signal from a 125$\,\text{Ge\spaceV}$ standard model Higgs boson has been scaled by a factor of 10.