Search for dark matter in events with a $Z$ boson and missing transverse momentum at ATLAS

A search is presented for production of dark matter particles recoiling against a leptonically decaying $Z$ boson in 20.3 fb$^−1$ of pp collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector at the Large Hadron Collider. Events with large missing transverse momentum and two oppositely-charged electrons or muons consistent with the decay of a $Z$ boson are analyzed. No excess above the Standard Model prediction is observed. Limits are set on the mass scale of the contact interaction as a function of the dark matter particle mass using an effective field theory description of the interaction of dark matter with quarks or with $Z$ bosons. In addition, we introduce a simplified model of dark matter, in which the mediating particle is accessible at the LHC, to address the possible problem of EFT validity. The interaction is mediated by a coloured scalar particle exchanged in the $t$-channel, and we set limits on the coupling strength and mediator mass.