Search for dark photons in VBF Higgs events in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search is presented for a Higgs boson that is produced via vector boson fusion and that decays to an undetected particle and an isolated photon. The search is performed by the CMS Collaboration at the Large Hadron Collider, using a data set corresponding to an integrated luminosity of $130~\mathrm{fb}^{-1}$ recorded at a center-of-mass energy of 13 TeV. No significant excess of events above the expectation from the standard model background is found. The results are interpreted in the context of a theoretical model in which the undetected particle is a massless dark photon. An upper limit is set on the product of the cross section for vector boson fusion production and the branching fraction for such a Higgs boson decay, as a function of the Higgs boson mass. For a Higgs boson mass of 125 GeV, assuming the standard model production rates, a combination with previous searches further reduces the observed (expected) $95\%$ confidence level upper limit on the branching fraction to $2.9\ (2.1)\%$. This is the first search for such decays in the vector boson fusion channel. The results improve significantly the constraints on this decay mode.