Search for Invisible Decays of a Higgs Boson Produced in Association with a Z Boson in ATLAS

Abstract

A search for evidence of invisible-particle decay modes of a Higgs boson produced in association with a Z boson at the Large Hadron Collider is presented. No deviation from the Standard Model expectation is observed in 4.5 fb$^{-1}$ (20.3 fb$^{-1}$) of 7 (8) TeV pp collision data collected by the ATLAS experiment. Assuming the Standard Model rate for ZH production, an upper limit of 75%, at the 95% confidence level is set on the branching ratio to invisible-particle decay modes of the Higgs boson at a mass of 125.5 GeV. The limit on the branching ratio is also interpreted in terms of an upper limit on the allowed dark matter--nucleon scattering cross section within a Higgs-portal dark matter scenario. Limits are also set on an additional neutral Higgs boson, in the mass range 110<mH<400 GeV, produced in association with a Z boson and decaying to invisible particles.

Figures (4)

• Figure 1: Distribution of for events with the invariant mass of the two leptons $76<m_{\ell\ell}<106 \gev$ in the 8 TeV data (dots). The stacked histograms represent the background predictions from simulation. The signal hypothesis is shown by a dotted line and assumes the SM $ZH$ production rate for a $m_H = 125.5 \gev$ Higgs boson with $\text{BR}(H\to \text{inv.})=1$. The inset at the bottom of the figure shows the ratio of the data to the combined background expectations as well as a band corresponding to the combined systematic uncertainties.
• Figure 2: Distribution of after the full selection in the 8 TeV data (dots). The filled stacked histograms represent the background expectations. The signal expectation for a Higgs boson with $m_H=125.5$ GeV, a SM $ZH$ production rate and $\text{BR}(H\to \text{inv.})=1$ is stacked on top of the background expectations. The inset at the bottom of the figure shows the ratio of the data to the combined background expectations. The hashed area shows the systematic uncertainty on the combined background expectation.
• Figure 3: Upper limits on $\sigma_{ZH}\times \text{BR}(H\to \text{inv.})$ at 95% CL for a Higgs boson with $110<m_H<400$ GeV, for the combined 7 and 8 TeV data. The full and dashed lines show the observed and expected limits, respectively.
• Figure 4: Limits on the DM--nucleon scattering cross section at 90% CL, extracted from the $\text{BR}(H\to \text{inv.})$ limit in a Higgs-portal scenario, compared to results from direct-search experiments Bernabei:2008yiAngle:2011Aprile:2012Angloher:2011uuAalseth:2011wpFox:2011pxAgnese:2013Akerib:2013. Cross-section limits and favored regions correspond to a 90% CL, unless stated otherwise in the legend. Favored regions for DAMA and CoGeNT are based on Ref. Fox:2011px. The results from the direct-search experiments do not depend on the assumptions of the Higgs-portal scenario.