Estimation of ZZ background in the dilepton+MET final state using Zgamma data

Abstract

In the search for Dark Matter (DM) at the LHC, Standard Model particles are produced in association with Dark Matter particles, which are invisible as they do not interact with the detector. Thus events with large imbalance in transverse momentum are of interest. One such signature is the presence of two leptons and large missing transverse momentum (dilepton+MET). The dominant background contributing to the search for Dark Matter in this channel is ZZ->llvv. Currently, this background is determined using Monte Carlo simulation, with an uncertainty of ≈ 10%. The goal of this study is to establish a data driven method to estimate this background, and reduce the uncertainty. Using Z+gamma->ll+gamma, which is a process with low backgrounds and has a high production cross-section, it is possible to estimate the ZZ->llvv contribution. In regions where pT(gamma) >> MZ, the two processes are kinematically similar. They have the same production mechanisms, but differ due to the couplings of the photon and Z boson to the quarks being different, as well as the difference in mass (photons are massless, while Z bosons are massive). Introducing a transfer factor R as the ratio of the ZZ cross section to the Z+gamma cross section, which is determined from simulation, the contribution of ZZ->llvv to the background can be estimated from Z+gamma->ll+gamma data. The uncertainty on the prediction of R due to theoretical factors is estimated in this work.