Medium-Induced Modification of $Z$-Tagged Charged Particle Yields in $\mathrm{Pb}+\mathrm{Pb}$ Collisions at 5.02 TeV with the ATLAS Detector

The yield of charged particles opposite to a $Z$ boson with large transverse momentum ($p_T$) is measured in $260{\mathrm{pb}}^{-1}$ of $pp$ and $1.7{\mathrm{nb}}^{-1}$ of $\mathrm{Pb}+\mathrm{Pb}$ collision data at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The $Z$ boson tag is used to select hard-scattered partons with specific kinematics, and to observe how their showers are modified as they propagate through the quark-gluon plasma created in $\mathrm{Pb}+\mathrm{Pb}$ collisions. Compared with $pp$ collisions, charged-particle yields in $\mathrm{Pb}+\mathrm{Pb}$ collisions show significant modifications as a function of charged-particle $p_T$ in a way that depends on event centrality and $Z$ boson $p_T$. The data are compared with a variety of theoretical calculations and provide new information about the medium-induced energy loss of partons in a $p_T$ regime difficult to measure through other channels.

Figure 1

Charged-particle yield per $Z$ boson as a function of $p_T^{\mathrm{ch}}$, for the selection $\mathrm{\Delta }\varphi >3\pi /4$, reported for $15<p_T^Z<30\mathrm{GeV}$, $30<p_T^Z<60\mathrm{GeV}$, and $p_T^Z>60\mathrm{GeV}$. Results are shown for $pp$ events and the three centralities of $\mathrm{Pb}+\mathrm{Pb}$ events. These are offset horizontally around the bin centers, which are located between the 0%–10% and 10%–30% points, for visibility. The vertical bars and boxes correspond to the statistical and systematic uncertainties of the data.

Figure 2

Ratio of the charged-particle yield in $\mathrm{Pb}+\mathrm{Pb}$ collisions to that in $pp$ collisions, $I_{\mathrm{AA}}$, as a function of charged-particle $p_T^{\mathrm{ch}}$, for the selection $\mathrm{\Delta }\varphi >3\pi /4$. The vertical bars and boxes correspond to the statistical and systematic uncertainties of the data. The 0%–10% and 30%–80% data are offset horizontally for visibility.

Figure 3

The $I_{\mathrm{AA}}$ ratio as a function of $p_T^{\mathrm{ch}}$ in data compared with theoretical calculations (see text), for the selection $\mathrm{\Delta }\varphi >3\pi /4$. The vertical bars and boxes correspond to the statistical and systematic uncertainties, while the shaded bands represent the theoretical uncertainty (statistical for JEWEL, Hybrid, and COLBT-hydro, parametric for ${\mathrm{SCET}}_G$). The $I_{\mathrm{AA}}$ is shown for 0%–10% $\mathrm{Pb}+\mathrm{Pb}$ events for $p_T^Z=15–30\mathrm{GeV}$ (left), 30–60 GeV (center), and $>60\mathrm{GeV}$ (right).