Semi-visible higgs decay as a probe for new invisible particles

Abstract

We discuss the HL-LHC sensitivity to probe new invisible particles including scalars and fermions using semi-visible Higgs decays in the $pp\to ZH, Z\to jj\, (\ell^+\ell^-), ~H\to \ell^+\ell^- (jj) + ~\rm E{\!\!\!/}_T$ production mode. The kinematics of these decays allow new particle masses below $m\lesssim 50$ GeV. We carry out our analysis using both a cut-based approach and a multivariate method based on a boosted decision tree. We work within the dark-SMEFT framework with operators up to dimension six and a discrete $\mathbb{Z}_2$ symmetry under which the new particles are odd and the SM particles are even. We compare our results to those obtained from considering the invisible $Z$-width, as well as perturbative unitarity arguments. Finally, we outline kinematic strategies at the LHC to distinguish different operator structures of the postulated invisible particles.