Search for Higgs boson decays to a photon and a Z boson in pp collisions at sqrt(s)=7 and 8 TeV with the ATLAS detector

TL;DR

This ATLAS study searches for a neutral Higgs boson decaying to Zγ with Z → ℓℓ in pp collisions at √s = 7 and 8 TeV, focusing on 120–150 GeV with SM-like production. It employs a data-driven background estimation, refined m_llγ mass reconstruction, and a ten-category event classification to maximize sensitivity, using a profile likelihood and CLs method to set upper limits. No significant Higgs-like excess is observed; the combined result at m_H = 125.5 GeV yields an observed limit of 11× the SM prediction, with 8 TeV data providing 0.13–0.5 pb cross-section limits. The analysis constrains potential deviations in H → Zγ couplings and strengthens tests of loop-induced Higgs decays within and beyond the SM.

Abstract

A search is reported for a neutral Higgs boson in the decay channel H->Z gamma, Z->l+l- (l=e, mu), using 4.5 fb-1 of pp collisions at sqrt(s) = 7 TeV and 20.3 fb-1 of pp collisions at sqrt(s) = 8 TeV, recorded by the ATLAS detector at the CERN Large Hadron Collider. The observed distribution of the invariant mass of the three final-state particles, m(llgamma), is consistent with the Standard Model hypothesis in the investigated mass range of 120-150 GeV. For a Higgs boson with a mass of 125.5 GeV, the observed upper limit at the 95% confidence level is 11 times the Standard Model expectation. Upper limits are set on the cross section times branching ratio of a neutral Higgs boson with mass in the range 120-150 GeV between 0.13 and 0.5 pb for sqrt(s)=8 TeV at 95% confidence level.

Figures (3)

• Figure 1: Three-body invariant-mass distribution for $H\to Z\gamma$, $Z\to\mu\mu$ (top) or $Z\to ee$ (bottom) selected events in the 8 TeV, $m_H=125$ GeV gluon-fusion signal simulation, after applying all analysis cuts, before (filled circles) and after (open diamonds) the corrections described in Section \ref{['ss:resolution']}. The solid and dashed lines represent the fits of the points to the sum of a Crystal Ball and a Gaussian function.
• Figure 2: Distribution of the reconstructed $\ell\ell\gamma$ invariant mass in data, after combining all the event categories (points with error bars). The solid blue line shows the sum of background-only fits to the data performed in each category. The dashed histogram corresponds to the signal expectation for a Higgs boson mass of 125 GeV decaying to $Z\gamma$ at 50 times the SM-predicted rate.
• Figure 3: Observed 95% $CL$ limits (solid black line) on the production cross section of a SM Higgs boson decaying to $Z\gamma$ divided by the SM expectation. The limits are computed as a function of the Higgs boson mass. The median expected 95% $CL$ exclusion limits (dashed red line), in the case of no expected signal, are also shown. The green and yellow bands correspond to the $\pm 1\sigma$ and $\pm2\sigma$ intervals.