Measurements of differential cross-sections in top-quark pair events with a high transverse momentum top quark and limits on beyond the Standard Model contributions to top quark pair production with the ATLAS detector

Cross-section measurements of top-quark pair production where the hadronically decaying top quark has transverse momentum greater than $355$ GeV are presented using 139 fb$^{-1}$ of data collected by the ATLAS experiment during proton-proton collisions at the LHC. The fiducial cross-section at $\sqrt{s}=13$ TeV is measured to be $\sigma = 1.267 \pm 0.005 \pm 0.053$ pb, where the uncertainties reflect the limited number of data events and the systematic uncertainties, giving a total uncertainty of $4.2\%$. The cross-section is measured differentially as a function of kinematic variables characterising the $t\bar{t}$ system and also as a function of variables that characterise the additional radiation in the events. The results are compared with various Monte Carlo generators, including comparisons where the generators are reweighted to match a parton-level calculation at next-to-next-to-leading order. The reweighting improves the agreement between data and theory. The measured distribution of the top-quark transverse momentum is used to set limits on the Wilson coefficients of the dimension-six operators $O_{tG}$ and $O_{tq}^{8}$ in the effective field theory framework. The obtained $95\%$ credibility intervals are $C_{tG} \in [-0.68, 0.21]$ and $C_{tq}^{8} \in [-0.30, 0.36]$.