Speaker
Description
It 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 $\sqrt{s_{NN}} = 5.02$ 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 $v_n^{jet}$, which quantifies the magnitude of the modulation of the azimuthal angle distribution with respect to the $n^{th}$ order event plane. While ATLAS has previously reported the $v_2^{jet}$ in Pb+Pb at $\sqrt{s_{NN}} = 2.76$ TeV, this is the first ATLAS measurement of higher-order $v_n^{jet}$. The $v_2$, $v_3$, and $v_4$ are measured for jets with $p_T = 71-398$ GeV as a function of $p_T$ and collision centrality. A nonzero value of $v_2$ is observed in all but the most central collisions. A smaller nonzero value of $v_3$ 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 ($R_{AA}$) 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 $\sqrt{s_{NN}} = 5.02$ 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 ($\sqrt{d_{12}}$) 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.
