Dark photons $\overline{\gamma }$ mediating long-range forces in a dark sector are predicted by various new physics scenarios, and are being intensively searched for in experiments. We extend a previous study of a new discovery process for dark photons proceeding via Higgs-boson production at the LHC. Thanks to the nondecoupling properties of the Higgs boson, $\mathrm{BR}(H\to \gamma \overline{\gamma })$ values up to a few percent are possible for a massless dark photon, even for heavy dark-sector scenarios. The corresponding signature consists (for a Higgs boson at rest) of a striking monochromatic photon with energy $E_{\gamma }=m_H/2$, and a similar amount of missing energy. We perform a model-independent analysis at the LHC of both the gluon-fusion and vector-boson fusion (VBF) Higgs production mechanisms at 14 TeV, including parton-shower effects, and updating our previous parton-level analysis at 8 TeV in the gluon-fusion channel by a more realistic background modeling. We find that a $5\sigma$ sensitivity can be reached in the gluon-fusion channel for $\mathrm{BR}(H\to \gamma \overline{\gamma })\simeq 0.1\%$ with an integrated luminosity of $L\simeq 300{\mathrm{fb}}^{-1}$. The corresponding VBF reach is instead restricted to 1%. Such decay rates can be naturally obtained in dark-photon scenarios arising from unbroken $U(1{)}_F$ models explaining the origin and hierarchy of the Yukawa couplings, strongly motivating the search for this exotic Higgs decay at the LHC.

• Received 9 March 2016

DOI:https://doi.org/10.1103/PhysRevD.93.093011