Search for new physics in the final state with a single photon and large missing transverse momentum in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search for new physics in events featuring a single photon and missing transverse momentum is presented, using proton-proton $\sqrt{s} = 13~\TeV$ collision data corresponding to an integrated luminosity of 101 \fbinv\ collected by the CMS experiment at the CERN LHC between 2017 and 2018. This analysis, combined with a previous study of 36 \fbinv of 2016 data (totaling 137 \fbinv), reveals no significant deviations from standard model expectations. The results are then used to establish 95% confidence level limits on parameters in theoretical models involving dark matter and large extra dimensions. Compared to the 2016-only analysis, this search achieves up to a 14% improvement in exclusion reach for mediator masses in simplified dark matter models, along with 11% and 10% enhancements in the limits on the effective field theory suppression scale and the fundamental Planck scale, respectively. These results are the most stringent constraints on these parameters to date in the monophoton final state.