Measurement of substructure-dependent jet suppression in Pb+Pb collisions at 5.02 TeV with the ATLAS detector

The ATLAS detector at the Large Hadron Collider has been used to measure jet substructure modification and suppression in Pb+Pb collisions at a nucleon-nucleon center-of-mass energy √s_NN = 5.02 TeV in comparison with pp collisions at √s = 5.02 TeV. The Pb+Pb data, collected in 2018, have an integrated luminosity of 1.72 nb⁻¹, while the pp data, collected in 2017, have an integrated luminosity of 260 pb⁻¹. Jets used in this analysis are clustered using the anti-k_t algorithm with a radius parameter R=0.4. The jet constituents, defined by both tracking and calorimeter information, are used to determine the angular scale r_g of the first hard splitting inside the jet by reclustering them using the Cambridge-Aachen algorithm and employing the soft-drop grooming technique. The nuclear modification factor, R_AA, used to characterize jet suppression in Pb+Pb collisions, is presented differentially in r_g, jet transverse momentum, and in intervals of collision centrality. The R_AA value is observed to depend significantly on jet r_g. Jets produced with the largest measured r_g are found to be twice as suppressed as those with the smallest r_g in central Pb+Pb collisions. The R_AA values do not exhibit a strong variation with jet p_T in any of the r_g intervals. The r_g and p_T dependence of jet R_AA is qualitatively consistent with a picture of jet quenching arising from coherence and provides the most direct evidence in support of this approach.