Measurements of the azimuthal anisotropy and substructure of jets in Pb+Pb collisions with the ATLAS detector

t has been shown that high-energy partons lose energy when traversing the hot, dense medium produced in heavy-ion collisions. However, the mechanism of the energy loss, including its dependence on the path-length of the shower in the medium and sensitivity to the jet substructure, is not fully understood. This talk presents a new measurement of single jet yields as a function of the azimuthal angle with respect to the event plane in Pb+Pb collisions at 𝑠𝑁𝑁‾‾‾‾√=5.02 TeV. Because partons produced at different angles with respect to the event plane traverse, on average, different path lengths of the medium, this measurement gives insight into the path-length dependence of parton energy loss. The azimuthal angle dependence of the yields is characterized by the parameter 𝑣𝑗𝑒𝑡𝑛, which quantifies the magnitude of the modulation of the azimuthal angle distribution with respect to the 𝑛𝑡ℎ order event plane. While ATLAS has previously reported the 𝑣𝑗𝑒𝑡2 in Pb+Pb at 2.76 TeV, this is the first ATLAS measurement of higher-order 𝑣𝑗𝑒𝑡𝑛. The 𝑣2, 𝑣3, and 𝑣4 are measured for jets with 𝑝𝑇=71−398 GeV as a function of 𝑝𝑇 and collision centrality. A nonzero value of 𝑣2 is observed in all but the most central collisions. A smaller nonzero value of 𝑣3 is measured, suggesting that fluctuations in the initial state play a small but distinct role in jet energy loss. This talk also presents measurements of jet substructure performed using various jet (de)clustering and grooming techniques. Measurements of inclusive jet suppression (𝑅𝐴𝐴) in heavy-ion collisions are presented for the first time as a function of the jet substructure using both nominal (𝑅=0.4) and large-radius (𝑅=1.0) jets in Pb+Pb and 𝑝𝑝 collisions at 5.02 TeV. The jet substructure is characterized using the Soft-Drop grooming procedure in order to identify subjects corresponding to the hardest parton splitting in the jet. The dynamics of jet quenching is measured and presented as a function of the transverse momentum scale and the angle of the hardest splitting in the jet. These measurements provide new information about the path-length dependence of jet quenching and the sensitivity of jet suppression to its substructure.