Missing-mass search in forward-proton-tagged dilepton events with the ATLAS detector

A search is conducted in proton–proton collisions at the Large Hadron Collider for photon-induced production pp -> pp + gamma gamma, (gamma gamma -> VX) of a visible particle V decaying into a pair of same-flavour charged leptons (e+e− or mu+ mu−) and an undetected invisible component X. Measurements of the outgoing proton energies by the ATLAS forward proton spectrometer allow the full photon-photon four-momentum to be reconstructed. By subtracting the visible four-momentum of the central system measured with the ATLAS detector, the “missing mass” of any event components not detected in the central region can be reconstructed, enabling the reconstruction of X without knowing its properties, thus allowing the search to be model-independent. A search for a narrow resonance is performed in the missing-mass spectrum between 100 GeV and 900 GeV. The analysis uses data collected in 2017 from proton–proton collisions at a centre-of-mass energy of root s = 13 TeV, corresponding to an integrated luminosity of 14.7 fb-1. No significant excess over the Standard Model expectation is observed and upper limits at 95% confidence level are set on the fiducial cross sections for three different signal models in the range between 128 and 2.5 fb. Additionally, model-independent limits are set on the visible cross section of BSM processes, for two sets of selection criteria. Both individual lepton flavour decay channels of the visible boson and a combination of the two channels are considered.