Background to Higgs-boson Searches from Internal Conversions of Off-shell Photons Associated with $Z/\gamma ^$-boson Production at the LHC

This paper presents the studies of the background contribution to the H → 4l searches originating from the processes of off-shell (virtual) photon emissions and their conversions into lepton pairs accompanying the production of Z∕γ∗-bosons at the LHC. They extend the analyses of the irreducible background presented in the ATLAS and CMS Higgs papers [Phys. Lett. B716, 1 (2012); Phys. Lett. B726, 88 (2013); Phys. Rev. D90, 052004 (2014); CERN-PH-EP-2014-170, to appear in Phys. Rev. D; Phys. Lett. B716, 30 (2012); Phys. Rev. D89, 92007 (2014)] by taking into account the emissions of off-shell photons by parton showers. Including these effects does not change significantly the Higgs-searches background level, provided that the transverse momentum of each of the final-state leptons is restricted to the range of pT,l > 7 GeV. In the kinematical region extended towards lower lepton transverse momenta, the parton-shower contribution becomes important. A measurement method for pinning down the parton-shower effects has been proposed.