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 $U(1{)}_Y\mathrm{\text{−}}U(1{)}_d$ 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 $\sim 20–100\mathrm{MeV}$. 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 $Z_d\to e^{+}{e}^{-}$ 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.

• Received 23 August 2012

DOI:https://doi.org/10.1103/PhysRevD.86.095009