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 + γγ, (γγ → V X) of a visible particle V decaying into a pair of same-flavour charged leptons (e+e− or µ+µ−) 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 √
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, modelindependent 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.