Measurement of top quark pair-production in association with a photon using the CMS detector and Effective Field Theory interpretation

The standard model (SM) of particle physics is one of the most stringently tested scientific theories, and provides precise predictions for a wide range of processes involving fundamental particles. Strong theoretical and experimental arguments pointing towards its incompleteness do however exist, leading experimentalists to search for new particles predicted by proposed extensions to it. As these direct searches have until now been fruitless, and large increases in the energies probed in collider experiments are not expected any time soon, attention has shifted to precision measurements of the couplings of SM particles. Deviations of these couplings from SM predictions could indirectly indicate the existence of new particles, even when these can not yet be directly produced themselves. Interpreting these deviations in a useful way can be performed using the model independent framework offered by standard model effective field theory. The cross section for the production of a top quark pair in association with a photon, with the $W$ bosons from both quarks decaying leptonically, is measured using 138 fb$^{-1}$ of proton-proton collision data. This data was collected in the 2016-2018 run period of the CMS experiment, with the LHC producing collisions at $\sqrt{s}= 13$ TeV. All leptonic final states are included ($e\mu$, $ee$, and $\mu\mu$), and the signal is defined so that it includes photons radiated by initial-state particles, top quarks, or any of their decay products. A fiducial phase space is defined with exactly one photon with a transverse momentum of above 20 GeV, and at least one jet identified as coming from the hadronization of a bottom quark. A fiducial cross section of $175.2\pm2.5$ $(stat.)$ $ \pm6.3$ $(syst.)$ is obtained using a profile likelihood fit to the measured transverse photon momentum distribution. Within this same phase space differential cross sections are measured as a function of kinematic observables of the final state particles, and compared to theoretical predictions. Both this measurement and a combination with the CMS measurement of the same process in a final state with just one lepton are interpreted in the framework of standard model effective field theory.