Measurement of the $t$$\bar{t}$$\gamma$ cross section and effective field theory interpretation with the CMS experiment

The standard model (SM) of particle physics, a theory of the currently known elementary particles and their fundamental interactions, provides predictions in agreement with experimental results up to the highest achievable precision. It is one of the most stringently tested theories, still known to be incomplete. If we suppose that new particles are too heavy for discovery at the Large Hadron Collider (LHC) at CERN, indirect effects can still be discernible in deviations in kinematic spectra at much lower energies. Because the top quark plays a crucial role in many beyond the SM theories, its interactions are sensitive probes of new physics phenomena. In particular, the measurement of the inclusive and differential cross sections of a top quark pair in association with a photon ($t$$\bar{t}$$\gamma$) will access the electromagnetic top quark coupling. A data set of 137 fb$^{−1}$ integrated luminosity of proton-proton collisions at a center-of- mass energy of $\sqrt{s}$ = 13 TeV, recorded by the CMS experiment during the LHC Run 2 (2016–2018) data taking, is used for the measurement of the inclusive and differential $t$$\bar{t}$$\gamma$ cross sections. Events are selected by requiring one charged lepton (electron or muon), one isolated photon, and at least three jets from the hadronization of quarks, where at least one has to originate from a bottom quark. The photon may be emitted from an initial-state quark, the top quark, or any decay product of the top quark. The inclusive cross section of the t ̄tγ process is measured for a photon transverse momentum greater than 20 GeV and absolute value of the pseudorapidity |η| < 1.4442. The measured $t$$\bar{t}$$\gamma$ cross section in the fiducial phase space of 798 ±7 (stat) ±48 (syst) fb is in good agreement with the SM prediction of 773 ±135 fb from simulations at next-to-leading order in quantum chromodynamics. Differential cross section measurements are performed in several kinematic observables and unfolded to the particle level, allowing for comparisons to theoretical predictions. The measurements are interpreted in terms of the SM effective field theory and are used to set constraints on anomalous electromagnetic dipole interactions of the top quark that are the most stringent to date.