Observation of electroweak production of two jets in association with an isolated photon and missing transverse momentum, and search for a Higgs boson decaying into invisible particles at 13 [Formula omitted] with the ATLAS detector

This paper presents a measurement of the electroweak production of two jets in association with a [Formula omitted] pair, with the Z boson decaying into two neutrinos. It also presents a search for invisible or partially invisible decays of a Higgs boson with a mass of 125 [Formula omitted] produced through vector-boson fusion with a photon in the final state. These results use data from LHC proton-proton collisions at [Formula omitted] = 13 [Formula omitted] collected with the ATLAS detector and corresponding to an integrated luminosity of 139 [Formula omitted]. The event signature, shared by all benchmark processes considered for the measurements and searches, is characterized by a significant amount of unbalanced transverse momentum and a photon in the final state, in addition to a pair of forward jets. Electroweak [Formula omitted] production in association with two jets is observed in this final state with a significance of 5.2 (5.1 expected) standard deviations. The measured fiducial cross-section for this process is [Formula omitted] fb. An observed (expected) upper limit of 0.37 ( [Formula omitted]) at 95% confidence level is set on the branching ratio of a 125 [Formula omitted] Higgs boson to invisible particles, assuming the Standard Model production cross-section. The signature is also interpreted in the context of decays of a Higgs boson into a photon and a dark photon. An observed (expected) 95% CL upper limit on the branching ratio for this decay is set at 0.018 ( [Formula omitted]), assuming the Standard Model production cross-section for a 125 [Formula omitted] Higgs boson.