A search is conducted in proton-proton collisions at the Large Hadron Collider for photon-induced production pp pp+ γγ, (γγ VX) 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, 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.
Aad et al. (Thu,) studied this question.