Measurements of tt̄ differential cross-sections of highly boosted top quarks decaying to all-hadronic final states in pp collisions at √s̅=13 TeV using the ATLAS detector

Measurements are made of differential cross-sections of highly boosted pair-produced top quarks as a function of top-quark and tt̄ system kinematic observables using proton-proton collisions at a center-of-mass energy of √s =13  TeV. The data set corresponds to an integrated luminosity of 36.1  fb⁻¹, recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Events with two large-radius jets in the final state, one with transverse momentum p_T>500  GeV and a second with p_T>350  GeV, are used for the measurement. The top-quark candidates are separated from the multijet background using jet substructure information and association with a b-tagged jet. The measured spectra are corrected for detector effects to a particle-level fiducial phase space and a parton-level limited phase space, and are compared to several Monte Carlo simulations by means of calculated χ² values. The cross-section for tt̄  production in the fiducial phase-space region is 292±7(stat)±71(syst)  fb, to be compared to the theoretical prediction of 384±36  fb