Search for dark matter and unparticles in events with a Z boson and missing transverse momentum in proton-proton collisions at sqrt(s) = 13 TeV

A search for evidence of dark matter (DM) and unparticle production at the LHC has been performed using events containing two charged leptons, consistent with the decay of a Z boson, and large missing transverse momentum. This study is based on data collected with the CMS detector corresponding to an integrated luminosity of 2.3 fb$^{-1}$ of pp collisions at the LHC at a center-of-mass energy of 13 TeV. No excess over the standard model expectation is observed. The results are interpreted in terms of a simplified model of DM production. For both vector and axial vector couplings between a mediator and DM particles, 95\% confidence level limits are set on the observed signal strength in the plane of mediator and DM particle mass. Additionally, 90\% confidence level limits are set on the DM-nucleon scattering cross section, as a function of the DM particle mass, for both spin-dependent and spin-independent coupling scenarios. In the context of an effective field theory, 95\% confidence level limits are set on the DM coupling parameters to $U(1)$ and $SU(2)$ gauge fields and on the scale of new physics. Additionally, 95\% confidence level limits are obtained on the unparticle model parameters.