Differential cross-section measurement of 𝑡𝑡𝛾̄ production and interpretation in the context of effective field theory using ATLAS detector data at √𝑠 = 13 TeV

The study of the production of top quark pairs in association with a photon (𝑡𝑡𝛾̄ ) probes the top-photon coupling (𝑡𝛾). This coupling is sensitive to new particle interactions Beyond the Standard Model (BSM) physics. The measurement of the absolute differential cross-section in the single-lepton channel of 𝑡𝑡𝛾̄ process is performed in a fiducial region at the particle level. The measurement is carried out using the Run-2 data collected at the ATLAS detector of the Large Hadron Collider (LHC) that corresponds to the centre-of-mass-energy of √𝑠 = 13 TeV at the instantaneous luminosity of 139 fb⁻¹. This measurement is employed to study the sensitivity of the 𝑡𝑡𝛾̄ process BSM by using the effective field theory (EFT) framework. The fiducial region is defined by requiring one lepton (𝑒 or 𝜇), one photon, and at least four jets where one of them is identified as a b-jet. The measurement is performed as a function of the transverse momentum of the photon (𝑝T(𝛾)), and the absolute pseudorapidity of the photon (|𝜂(𝛾)|). The EFT framework used for the interpretation provides higher-order operators that parameterize the potential deviations from the Standard Model (SM). Three of these are considered here for the 𝑡𝑡𝛾̄ process: $O_{tB}$, $O_{tG}$, and $O_{tW}$ to model the sensitivity of the process. The photon 𝑝T in the 𝑡𝑡𝛾̄ process is sensitive to these operators, therefore the cross-section measurement study as a function of $p_{\mathrm{T}}(\gamma)$ is exploited in the EFT interpretation. The estimated 1𝜎 limits for the EFT operators $O_{tB}$, $O_{tG}$, and $O_{tW}$ are [-0.82, 1.02], [-0.24, 0.14], and [-0.46, 0.97], respectively.