Search For Exclusive Diphoton Production with Intact Protons in pp Collisions at the LHC

Abstract

The Standard Model of particle physics has been largely successful in describing observed phenomena in the universe, but it is still believed to be incomplete.Many theories that address the shortcomings of the Standard Model also predict mechanisms of new physics that would enhance the four-photon coupling cross-section. In this work, a method to search for evidence of new physics in the diphoton final state is discussed using the CMS and PPS detectors. The quasi-real photon fluxes generated by protons at the LHC give rise to photon-photon interactions according to the Equivalent Photon Approximation. These electromagnetic interactions can create the exclusive production of two new photons while the protons remain intact in the final state and can be measured further down the LHC beam line by dedicated proton detectors. Measuring the forward protons and correlating their kinematics to that of the central two-photon system gives a strong discrimination against inclusive backgrounds. The data used in this analysis was collected with the CMS detector at the LHC from proton-proton collisions at a center-of-mass energy of 13 TeV. A study using data from 2016 only is presented before detailing an analysis of the full Run II data set -- data collected in 2016, 2017, and 2018. In both studies, no events are observed above the Standard Model background prediction. The first limits are placed on the four photon anomalous coupling process and the strongest limits are placed on axion-like particle production in the mass range of 500 - 2000 GeV