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 $pp\rightarrow pp+ \gamma \gamma, (\gamma\gamma\rightarrow VX)$ 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 $\sqrt{s} =$ 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.