Dark photons and resonant monophoton signatures in Higgs boson decays at the LHC

Motivated by possible massless dark photon solutions to long-standing cosmological and particle physics problems, we explore experimentally allowed scenarios where the Higgs boson coupling H is enhanced. Correspondingly, large rates are possible for the H ! decay, giving rise to one monochromatic photon with E ’ mH=2 (i.e., more than twice the photon energy in the rare standard-model decay H ! Z ! ), and similar amount of missing energy. We perform a model-independent study of this exotic resonant mono-photon signature at the LHC, featuring a distinctive E T peak around 60 GeV, and + = ET transverse invariant mass ruled by mH. At parton level, we nd a 5 sensitivity of the present LHC dataset for a H ! branching fraction of 0:5%. Such large branching fractions can be naturally obtained in dark U(1)F models explaining the origin and hierarchy of the standard model Yukawa couplings. We urge the LHC experiments to search for this new exotic resonance in the present dataset, and in future LHC runs.