Speaker
Kun Jiang
(University of Science and Technology of China)
Description
Jets are modified in relativistic heavy-ion collisions due to jet-medium interactions. Measurements of jet medium modifications have so far been obscure because of the large underlying anisotropic flow background. In this analysis we devise a novel method to subtract the flow background using data themselves. We select events with a large recoil momentum ($P_x$) within a pseudorapidity ($\eta$) window of $0.5<|\eta|<1$ from a high-$p_T$ trigger particle to enhance the away-side jet population. Di-hadron azimuthal correlations are analyzed with associated particles in two $\eta$ ranges ($-0.5<\eta<0$ and $0<\eta<0.5$) symmetric about midrapidity, one ("close-region") close to and the other ("far-region") far away from the $P_{x}$ selection $\eta$ window. The away-side jet contributes to the close-region but not as much to the far-region due to the large $\eta$ gap, while the flow contributions are equal. Assuming the $\Delta\phi$ shape of jet-like correlations does not depend on $\Delta\eta$, the correlation difference measures the away-side jet shape where the anisotropic flow background is cleanly subtracted.
The away-side jet correlation width is studied as a function of centrality and associated particle $p_T$. The width is found to increase with centrality at modest to high associated particle $p_T$. The increase can arise from jet-medium modifications, event averaging of away-side jets deflected by medium flow, and/or simply nuclear $k_T$ broadening. To further discriminate various physics mechanisms, a three-particle correlation analysis is conducted with robust flow background subtraction also using data themselves. Based on this analysis we discuss possible physics mechanisms of away-side broadening of jet-like correlations.
Primary authors
Kun Jiang
(University of Science and Technology of China)
for the STAR Collaboration
