Dark side of Higgs diphoton decays and muon g-2

Abstract

We propose that the LHC hints for a Higgs diphoton excess and the muon g - 2 (g(mu) - 2) discrepancy between theory and experiment may be related by vectorlike "leptons'' charged under both U(1)(Y) hypercharge and a "dark'' U(1)(d). Quantum loops of such leptons can enhance the Higgs diphoton rate and also generically lead to U(1)(Y)-U(1)(d) kinetic mixing. The induced coupling of a light U(1)(d) gauge boson Z(d) to electric charge can naturally explain the measured g(mu) - 2. We update Z(d) mass and coupling constraints based on comparison of the electron g - 2 experiment and theory, and find that explaining g(mu) - 2 while satisfying other constraints requires Z(d) to have a mass similar to 20-100 MeV. We predict new Higgs decay channels gamma Z(d) and Z(d)Z(d), with rates below the diphoton mode but potentially observable. The boosted Z(d) -> e(+)e(-) in these decays would mimic a promptly converted photon and could provide a fraction of the apparent diphoton excess. More statistics or a closer inspection of extant data may reveal such events.