Interplay between Mach cone and radial expansion and its signal in $\gamma$-jet events

We study the hydrodynamic response to jet quenching in expanding quark-gluon plasma (QGP) and its signal in the resulting particle distribution. The ideal hydrodynamic simulations of the $\gamma$-jet events in heavy-ion collisions are performed in a full (3 + 1)-dimensional setup. The jet-induced Mach cone and the radial expansion of the background mutually push and distort each other. As a result, the particle emission is suppressed in the direction in which the radial flow is pushed back by the Mach cone when the jet path is an off-central one. This is the direct signal of hydrodynamic response to the jet and, moreover, includes information about the jet path in the expanding QGP fluid.

Figure 1

Energy density distribution of the medium fluid in the transverse plane at midrapidity ${\eta }_{\mathrm{s}}=0$ for different jet production points. The snapshots are taken at $\tau =12.0$ fm/$c$. The solid lines show the higher energy density region compared to the case without jet propagation.

Figure 2

Event-averaged azimuthal angle distributions of charged pions with $1<p_T<2$ GeV/$c$ subtracted by the background. The trigger threshold for the jet transverse momentum at final state is $p_{T\mathrm{jet}}>80$ GeV/$c$. The solid line is the result averaged over all the triggered events. The dashed line and the dotted line are the results averaged over events with the jet production points in the region $y\le 0$ and in the region $y\ge 0$, respectively.

Figure 3

Azimuthal angle distributions of charged pions with $1<p_T<2$ GeV/$c$ subtracted by the background. The solid line, the dashed line, and the dotted line are the results for the events with the jet production point at $\left(x_0,y_0\right)=\left(3.0\mathrm{fm},0\mathrm{fm}\right),\left(3.0\mathrm{fm},-3.0\mathrm{fm}\right)$, and $\left(3.0\mathrm{fm},-6.0\mathrm{fm}\right)$, respectively.

Figure 4

Azimuthal angle distributions of charged pions with $1<p_T<2$ GeV/$c$ subtracted by the background. The solid line, the dashed line, and the dotted line are the results for the events with the jet production point at $\left(x_0,y_0\right)=\left(0\mathrm{fm},0\mathrm{fm}\right),\left(0\mathrm{fm},-3.0\mathrm{fm}\right)$, and $\left(0\mathrm{fm},-6.0\mathrm{fm}\right)$, respectively.

Figure 5

Azimuthal angle distributions of charged pions with $1<p_T<2$ GeV/$c$ subtracted by the background. The solid line, the dashed line, and the dotted line are the results for the events with the jet production point at $\left(x_0,y_0\right)=\left(-3.0\mathrm{fm},0\mathrm{fm}\right),\left(-3.0\mathrm{fm},-3.0\mathrm{fm}\right)$, and $\left(-3.0\mathrm{fm},-6.0\mathrm{fm}\right)$, respectively.

Figure 6

Schematic picture of the transverse plane ${\eta }_{\mathrm{s}}=0$ to illustrate how the Mach cone propagating in the expanding QGP suppresses the particles in a certain direction. The radial flow is induced by the expansion of the background QGP medium (arrows pointing in the radial direction). The jet (small circle with a small arrow at the top of the V-shaped region) travels through the lower half region of the QGP and induces the Mach cone (V-shaped region). During propagation, the wave front of the Mach cone pushes back the radial flow (large arrow from the side of the V-shaped region). As a result, the particles emitted in the direction opposite to the large arrow are suppressed.

Figure 7

Distribution of the jet production point for the $\gamma$-jet events when the trigger threshold for the jet transverse momentum in the final state is set to 80 GeV/$c$.

Figure 8

Distribution of the jet production point for different transverse momentum ranges of the jet trigger. The trigger range of the photon transverse momentum is 110–120 GeV/$c$. The trigger ranges of the jet transverse momentum are (a) 110–120, (b) 100–110, (c) 90–100, and (d) 80–90 GeV/$c$.

Figure 9

Azimuthal angle distributions of charged pions with $1<p_T<2$ GeV/$c$ subtracted by the background. The trigger range of the photon transverse momentum is 110–120 GeV/$c$. The trigger ranges for jet transverse momentum at final state are 100–110 GeV/$c$ (solid line) and 110–120 GeV/$c$ (dashed line).

Figure 10

Azimuthal angle distributions of charged pions with $1<p_T<2$ GeV/$c$ subtracted by the background. The trigger range of the photon transverse momentum is 110–120 GeV/$c$. The trigger ranges for jet transverse momentum at final state are 80–90 GeV/$c$ (solid line) and 90–100 GeV/$c$ (dashed line).