Interpretation of top-antitop-photon production rate in effective field theories

Abstract

The top quark, the heaviest elementary particle, was discovered about 25 years ago. However, many of its fundamental properties, including the nature of its gauge couplings, are not yet accurately determined. The Standard Model (SM) predicts the top-photon vertex to be vectorially coupled. Given that the SM does not explain all observed phenomena, this analysis looks for additional axial-vector or tensor couplings using the Standard Model Effective Field Theory (SMEFT) approach. Three EFT operators that can affect the top-photon vertex are added to the SM Lagrangian with a coupling coefficient. By varying contributions from these three operators, 27 Monte Carlo samples are generated for the tt¯γ process. Changes were observed in the shape of the distribution of various kinematic variables. The differential cross-sections with respect to these observables are modelled as a second order polynomial function of the coupling coefficients to interpolate between the generated SMEFT tt¯γ samples. With these polynomial functions, limits are extracted on the coupling coefficients from the differential crosssection measurements using the full run 2 data (L = 139 fb−1), from the ATLAS detector at the Large Hadron Collider.