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 vector-like "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 ~ 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.

Figures (4)

• Figure 1: New fermion ($F$) loop contribution to $H \to \gamma\gamma$ decay (left) and $\gamma-Z_d$ kinetic mixing (right).
• Figure 2: Exclusion region in $m_{Z_d} - \varepsilon^2$ space, updated from Refs. McKeown:2011yjDavoudiasl:2012qa to include recent Hanneke:2008tmBouchendira:2010esAoyama:2012wj$a_e$ results ("new $a_e$"); for comparison we also present the old bound ("old $a_e$").
• Figure 3: New fermion ($F$) loop contribution to $H \to \gamma Z_d$ decay (left) and $H \to Z_d Z_d$ decay (right). Both charged and neutral fermions should be included in the loop on the right.
• Figure 4: $r_{\gamma Z_d}$ ($r_{Z_d Z_d}$) bounded by blue solid (red dashed) curves versus $g_d / e$, for $R_{\gamma\gamma} = 1.5$ (bottom) $-\ 2.0$ (top).