Vector-boson-tagged jet production in heavy ion collisions at energies available at the CERN Large Hadron Collider

Vector-boson-tagged jet production in collisions of heavy nuclei opens new opportunities to study parton shower formation and propagation in strongly interacting matter. It has been argued to provide a golden channel that can constrain the energy loss of jets in the quark-gluon plasma created in heavy ion reactions. We present theoretical results for isolated-photon-tagged and $Z^0$-boson-tagged jet production in Pb + Pb collisions with $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ at the LHC. Specifically, we evaluate the transverse momentum imbalance ${\mathrm{x}}_{\mathrm{JV}}$ distribution and nuclear modification factor ${\mathrm{I}}_{\mathrm{AA}}$ of tagged jets and compare our theoretical calculations to recent experimental measurements by ATLAS and CMS collaborations. Our analysis, which includes both collisional and radiative energy losses, sheds light on their relative importance versus the strength of jet-medium interactions and helps quantify the amount of out-of-cone radiation of predominantly prompt quark-initiated jets.

Figure 1

Comparison between pythia8 simulations and CMS measurements of $V$ + jet production in $p+p$ collisions at the LHC. Left: the $Z^0+\text{jet}$ differential cross section at $\sqrt{s}=7\mathrm{TeV}$ as a function of $p_T^J$. Right: the isolated photon+jet differential cross section at $\sqrt{s}=8\mathrm{TeV}$ as a function of $p_T^{\gamma }$. The blue curves are from pythia8 simulations, the red data points are from the CMS collaboration [6, 7].

Figure 2

The fractional contributions of different subprocesses to the $Z^0+\text{jet}$ (left) and isolated-$\gamma +\text{jet}$ (right) production cross sections in $p+p$ collisions at $\sqrt{s}=5.02\mathrm{TeV}$. Kinematical cuts are implemented in our simulations as in CMS measurements, see Ref. [24] for $Z^0+\text{jet}$ and Ref. [25] for isolated-$\gamma +\text{jet}$.

Figure 3

The $Z^0$-tagged jet asymmetry distribution at $\sqrt{s}=5.02\mathrm{TeV}$ in $p+p$ (black) and Pb + Pb (red) collisions at the LHC. The jet radius parameter is $R=0.3$, and the transverse momenta of the $Z^0$ boson and the jet are $p_T^Z>60$ GeV and $p_T^J>30$ GeV, respectively. The $p+p$ baseline is simulated by pythia8 and shown by the black dashed line. The theoretical results for Pb + Pb collisions with two different jet-medium coupling strength are shown by the green ($g=2.0$) and magenta ($g=2.2$) histograms. The data is from the CMS collaboration [24].

Figure 4

The isolated-photon-tagged jet asymmetry distributions are shown and compared to CMS data in central (left) and semicentral (right) collisions [25]. The transverse momenta for the isolated photon and the jet are $p_T^{\gamma }>60$ GeV and $p_T^J>30$ GeV, respectively. The jet radius parameter is $R=0.3$. The $p+p$ baseline, simulated by pythia8, is shown in the black dashed line. The theoretical results for Pb + Pb collisions with two different jet-medium coupling strengths are shown by green ($g=2.0$) and magenta ($g=2.2$) lines.

Figure 5

Same as in Fig. 4, but for comparison to ATLAS data with jet radius $R=0.4$ [26].

Figure 6

The transverse momentum cuts dependence of ${\mathrm{I}}_{\mathrm{AA}}$ are shown comparing to CMS data. We have chosen four different setups of energy loss effects: with and without collisional energy loss for both $g=2.0$ and $g=2.2$.

Figure 7

The predicted transverse momentum cuts dependence of ${\mathrm{I}}_{\mathrm{AA}}$ for $Z^0+\text{jet}$ in central ($0–30\%$) Pb + Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$. The jet radius parameter is $R=0.3$.