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 $4.5{\mathrm{fb}}^{-1}$ ($20.3{\mathrm{fb}}^{-1}$) 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. Within the constraints of such a scenario, the results presented in this Letter provide the strongest available limits for low-mass dark matter candidates. Limits are also set on an additional neutral Higgs boson, in the mass range $110<m_H<400\mathrm{GeV}$, produced in association with a $Z$ boson and decaying to invisible particles.

• Received 13 February 2014

DOI:https://doi.org/10.1103/PhysRevLett.112.201802

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© 2014 CERN, for the ATLAS Collaboration