Momentum transport away from a jet in an expanding nuclear medium

We study the transport dynamics of momenta deposited from jets in ultrarelativistic heavy-ion collisions. Assuming that the high-energy partons traverse expanding quark-gluon fluids and are subject to lose their energy and momentum, we simulate dijet asymmetric events by solving relativistic hydrodynamic equations numerically without linearization in the fully $\left(3+1\right)$-dimensional coordinate. Mach cones are formed and strongly broadened by radial flow of the background medium. As a result, the yield of low-$p_T$ particles increases at large angles from the jet axis and compensates the dijet momentum imbalance inside the jet cone. This provides an intimate link between the medium excitation by jets and results in dijet asymmetric events observed by the CMS Collaboration.

Figure 1

Energy-density distribution of the expanding QGP fluid at $\tau =9.6\mathrm{fm}/c$ (a) in transverse plane at ${\eta }_s=0$ and (b) in reaction plane at $y=0$. A pair of energetic partons is created at $(\tau =0,x=1.5\mathrm{fm},y=0,{\eta }_s=0)$ and travels in the opposite direction along the $x$-axis at the speed of light.

Figure 2

$\langle {p/}_T^{\left|\right|}\rangle$ as a function of the dijet asymmetry ratio $A_J$ for (a) the whole region, (b) inside the leading and subleading jet cones $\left(\Delta R<0.8\right)$, and (c) outside both of the leading and subleading jet cones $\left(\Delta R\ge 0.8\right)$. Each band represents the contribution in six transverse-momentum ranges: 0–$0.5$, $0.5$–1, 1–2, 2–4, 4–8 GeV/$c$, and $p_T>8\mathrm{GeV}/c$. The solid circles show the total $\langle {p/}_T^{\left|\right|}\rangle$ including the contributions mentioned above.