Jet-hadron correlations with respect to the event plane in $\sqrt{s_{NN}}=200$ GeV $\mathrm{Au}+\mathrm{Au}$ collisions in STAR

Angular distributions of charged particles relative to jet axes are studied in $\sqrt{s_{NN}}=200$ GeV Au+Au collisions as a function of the jet orientation with respect to the event plane. This differential study tests the expected path-length dependence of energy loss experienced by a hard-scattered parton as it traverses the hot and dense medium formed in heavy-ion collisions. A second-order event plane is used in the analysis as an experimental estimate of the reaction plane formed by the collision impact parameter and the beam direction. Charged-particle jets with $15<p_{\mathrm{T},\mathrm{jet}}<20$ and $20<p_{\mathrm{T},\mathrm{jet}}<40\mathrm{GeV}/c$ were reconstructed with the anti-$k_{\mathrm{T}}$ algorithm with radius parameter setting of $R=0.4$ in the 20–50% centrality bin to maximize the initial-state eccentricity of the interaction region. The reaction plane fit method is implemented to remove the flow-modulated background with better precision than prior methods. Yields and widths of jet-associated charged-hadron distributions are extracted in three angular bins between the jet axis and the event plane. The event-plane (EP) dependence is further quantified by ratios of the associated yields in different EP bins. No dependence on orientation of the jet axis with respect to the event plane is seen within the uncertainties in the kinematic regime studied. This finding is consistent with a similar experimental observation by ALICE in $\sqrt{s_{NN}}$ = 2.76 TeV Pb-Pb collision data.

Figure 1

Event-plane resolution: Second-order [fourth-order] harmonic relative to the event plane, $R_2\left({\mathrm{\Psi }}_2\right)\left[R_4\left({\mathrm{\Psi }}_2\right)\right]$, respectively. The approach follows the modified reaction plane (MRP) method [47] utilizing the charged tracks of the TPC for event-plane reconstruction and resolution calculation for tracks ranging from 0.2 to 1.0 $\mathrm{GeV}/c$.

Figure 2

Top: $\mathrm{Signal}+\mathrm{background}$ region, background-dominated region, and RPF fit to the background for the event-plane dependent jet-hadron correlations of $15<p_{\mathrm{T}}^{\mathrm{jet}}<20\mathrm{GeV}/c$ jets correlated with $1.5<p_{\mathrm{T},\mathrm{assoc}}<2.0\mathrm{GeV}/c$ charged hadrons from the 20–50% most central events. Bottom: Quality of the RPF fit to the background-dominated region expressed as (data $-$ fit)/fit.

Figure 3

Example of a background subtracted correlation function for $15<p_{\mathrm{T},\mathrm{jet}}<20\mathrm{GeV}/c$ jets correlated with $1.5<p_{\mathrm{T},\mathrm{assoc}}<2.0\mathrm{GeV}/c$ charged hadrons from the 20–50% most central collisions. Correlated scaled uncertainty from the application of mixed events is displayed by the red band, while the uncertainty associated with the RPF background fit is displayed as the gray band.

Figure 4

$p_{\mathrm{T},\mathrm{jet}}$ resolution for $15<p_{\mathrm{T},\mathrm{jet}}^{\mathrm{GEN}}<20\mathrm{GeV}/c$ (left) and $20<p_{\mathrm{T},\mathrm{jet}}^{\mathrm{GEN}}<40\mathrm{GeV}/c$ (right) $R=0.4$ full jets with the corresponding response matrix as an inset on the top right. Jets are measured from all angles relative to the event plane in the 20–50% most central events. This comparison is for matched GEN-level to RECO-level jets.

Figure 5

Near-side (left) and away-side (right) uncorrected associated yield vs $p_{\mathrm{T},\mathrm{assoc}}$ for 15—20 (top) and 20–40 $\mathrm{GeV}/c$ (bottom) full jets of 20-50% centrality in Au+Au collisions. The grey bands describe the systematic uncertainties of the background fits and are nontrivially correlated point to point. The colored bands are scale uncertainties from the mixed event acceptance shape and JES correction. There is an additional 6% global scale uncertainty. Included on the far right of the near- and away-side panels is the inclusive transverse momentum bin from 1.0 to 10.0 $\mathrm{GeV}/c$. Points are displaced for visibility.

Figure 6

Near-side (left) and away-side (right) jet widths vs $p_{\mathrm{T},\mathrm{assoc}}$ for 15–20 (top) and 20–40 $\mathrm{GeV}/c$ (bottom) full jets of 20-50% centrality in Au+Au collisions. The widths are extracted from the Gaussian fit to the jet peak. The grey bands describe the systematic uncertainties of the background fits, which are nontrivially correlated point to point. The colored bands are scale uncertainties from the mixed-event acceptance shape and JES correction. There is an additional 6% global scale uncertainty. Included on the far right of the near- and away-side panels is an inclusive transverse momentum bin from 1.0 to 10.0 $\mathrm{GeV}/c$. Points are displaced for visibility.

Figure 7

Near-side (left) and away-side (right) associated-yield ratios (of out-of-plane and mid-plane to in-plane) vs $p_{\mathrm{T},\mathrm{assoc}}$ for 15–20 (top) and 20–40 (bottom) $\mathrm{GeV}/c$ full jets in 20–50% centrality collisions. The grey bands describe the systematic uncertainties of the background fits, which are nontrivially correlated point-to-point. The colored bands are scale uncertainties from the JES correction. Points are displaced for visibility.