Jet-triggered photons from back-scattering kinematics for quark gluon plasma tomography

High energy photons created from back-scattering of jets in quark gluon plasma are a valuable probe of the temperature of the plasma, and of the energy loss mechanism of quarks in the plasma. An unambiguous identification of these photons through single-inclusive photon measurements and photon azimuthal anisotropies has so far been elusive. We estimate the spectra of back-scattering photons in coincidence with trigger jets for typical kinematic situations at the Large Hadron Collider and the BNL Relativistic Heavy Ion Collider. We find that the separation of back-scattering photons from other photon sources using trigger jets depends crucially on our ability to reliably estimate the initial trigger-jet energy. We estimate that jet-reconstruction techniques in heavy ion experiments need to be able to get to jet $R_{AA}\gtrsim 0.7$ in central collisions for viable back-scattering signals.

Figure 1

Upper panel shows $R_{AA}$ of single-inclusive jets as a function of jet $p_T$ in central $\text{Au}+\text{Au}$ collisions at RHIC for two values of $\widehat{r}$ corresponding to “raa” values of roughly 1.0 $\left(\widehat{r}=0\right)$ and 0.7 at 30 GeV, respectively. Lower panel shows the same for central collisions of lead nuclei at LHC energy, for four values of $\widehat{r}$, corresponding to raa values of roughly 1.0, 0.9, 0.7, and 0.5 at 100 GeV, respectively. Data from the STAR [29], ALICE [30], and CMS [31] collaborations for jet-cone radii of 0.4, 0.2, and 0.4, respectively, are also shown for comparison.

Figure 2

Upper panel shows the yield $dN/d^2{p}_{T}dy$ of photons opposite of a jet with energy between 30 and 35 GeV in central $\text{Au}+\text{Au}$ collisions at $\sqrt{s_{NN}}=200$ GeV. We show the sum of prompt hard photons and fragmentation photons (solid and dashed lines) and back-scattering photons (solid lines with marks) at LO accuracy for the hard process for different parton and trigger energy-loss scenarios. Triangles show signal for no parton energy loss and no trigger-jet energy loss (raa 1.0). Squares show parton energy loss on and no trigger-jet energy loss (raa 1.0). Circles show both parton energy loss and trigger-jet energy loss at realistic strength (raa 0.7). Lower panel shows the same as the upper panel except that background (prompt $\text{hard}+\text{fragmentation})$ calculated at NLO accuracy for the case of raa 1.0 (solid line). Back-scattering photons for raa 1.0 (dashed line) at LO accuracy is multiplied by a $K$ factor.

Figure 3

Upper panel shows nuclear modification factor $R_{AA}$ calculated from the results in Fig. 2 for background and signal at LO accuracy for (i) jet raa 1.0 and (ii) jet raa 0.7 (scaled by 0.5 for better visibility). In both cases $\left(\text{signal}+\text{background}\right)$/background and the reference background/background are shown. Lower panel shows the same as the upper panel but for raa 1.0 at NLO accuracy.

Figure 4

Upper panel shows the same as Fig. 2 for trigger jets between 60 and 65 GeV energy in central $\text{Pb}+\text{Pb}$ collisions at $\sqrt{s_{NN}}=2760$ GeV for four different trigger-jet energy-loss scenarios: raa 1.0 (solid line, circles), raa 0.9 (dashed line, diamonds), raa 0.7 (dash-dotted line, triangles), and 0.5 (dotted line, squares). Both background and back-scattering signal are calculated at LO accuracy for the hard process. All scenarios have parton energy loss taken into account. Lower panel shows background (prompt $\text{hard}+\text{fragmentation})$ calculated at NLO accuracy for the case raa 1.0 (solid line), whereas back-scattering photons are estimated at LO accuracy for raa 1.0 (dashed line), multiplied by a $K$ factor.

Figure 5

Upper panel shows nuclear modification factor $R_{AA}$ for central collisions of lead nuclei at LHC at LO accuracy for raa 1.0, raa 0.7, and raa 0.5. We again show $R_{AA}$ with and without the inclusion of back-scattering photons. Lower panel shows the same thing, but calculated for raa 1.0 at NLO accuracy.