Measurement of medium-induced acoplanarity in central Au-Au and pp collisions at $\sqrt{s_{\rm NN}}=200$ GeV using direct-photon+jet and $π^{0}$+jet correlations

TL;DR

This study probes how jets interact with the Quark-Gluon Plasma by measuring semi-inclusive acoplanarity of recoil jets triggered by direct photons and $\pi^0$ mesons in pp and central Au-Au collisions at $\sqrt{s_{NN}}=200$ GeV. Using jet radii $R=0.2$ and $R=0.5$, the STAR analysis employs SE/ME background subtraction, unfolding, and $\,\Delta\phi$-smearing corrections to obtain clean recoil-jet distributions. The results show a strong, $R$-dependent broadening of acoplanarity in central Au-Au, with large-angle yields enhanced for $R=0.5$ by up to a factor of $\approx20$, consistent with medium wake effects and challenging current jet-quenching models such as JEWEL, Gaussian broadening, and Hybrid approaches. The findings imply that the QGP response to jet propagation plays a significant role in jet modification at RHIC and call for improved theoretical frameworks to describe jet-medium dynamics.

Abstract

The STAR Collaboration reports measurements of acoplanarity using semi--inclusive distributions of charged--particle jets recoiling from direct photon and $π^{0}$ triggers, in central Au+Au and $pp$ collisions at $\sqrt{s_\mathrm{NN}}=200$ GeV. Significant medium--induced acoplanarity broadening is observed for large but not small recoil jet resolution parameter, corresponding to recoil jet yield enhancement up to a factor of $\approx20$ for trigger--recoil azimuthal separation far from $π$. This phenomenology is indicative of the response of the Quark--Gluon Plasma to excitation, but not the scattering of jets off of its quasiparticles. The measurements are not well--described by current theoretical models which incorporate jet quenching.

Figures (7)

• Figure 1: $Y_\mathrm{SE}$, $Y_\mathrm{MEnorm}$ and $\widetilde{Y}_{\Delta\phi}$ distributions (Eq. \ref{['eq:Ytilde']}) as a function of $p_\mathrm{T,jet}^\mathrm{reco,ch}$ from $\gamma_\mathrm{rich}$ triggers with $11<E_\mathrm{T}^\mathrm{trig}<15$ GeV in central $\mathrm{Au}\text{--}\mathrm{Au}$ collisions, for $R=0.2$ (top panels) and 0.5 (bottom panels). $\widetilde{Y}$($p_\mathrm{T,jet}^\mathrm{reco,ch}$) datapoints with a negative central value not shown. Sub-panels for each $R$: $2.2<\Delta\phi<2.5$ (left), $2.8<\Delta\phi<3.0$ (right). Upper panels show SE and MEnorm distributions; lower panels show the ratio SE/MEnorm. Insets show the ratio in the normalization region.
• Figure 2: Corrected $\widetilde{Y}_{p_\mathrm{T}}$ distributions as a function of $\Delta\phi$ for $\gamma_\mathrm{dir}$ and $\pi^0$ triggers with $11<E_\mathrm{T}^\mathrm{trig}<15$ GeV, in $pp$ and central $\mathrm{Au}\text{--}\mathrm{Au}$ collisions at $\sqrt{s_\mathrm{NN}}=200$ GeV for $R=0.2$ (left) and $R=0.5$ (right). Upper: $\gamma_\mathrm{dir}$ trigger, $10<p_\mathrm{ T,jet}^\mathrm{ch}<15$$\mathrm{GeV/}c$; middle: $\pi^0$ trigger, $10<p_\mathrm{ T,jet}^\mathrm{ch}<15$$\mathrm{GeV/}c$; lower: $\pi^0$ trigger, $15<p_\mathrm{ T,jet}^\mathrm{ch}<20$$\mathrm{GeV/}c$. Data are plotted at the spectrum-weighted bin coordinate except for low--statistics points, which are shown as 95% CL upper limits. Theoretical calculations for $pp$ collisions are described in the text.
• Figure 3: Left panels: ${I}_\mathrm{AA}(\Delta\phi)$ distributions for $11<E_\mathrm{T}^\mathrm{trig}<15$ GeV with recoil jet $R=0.2$ and 0.5. Top: $\gamma_\mathrm{dir}$ trigger, $10<p_\mathrm{ T,jet}^\mathrm{ch}<15$$\mathrm{GeV/}c$; middle: $\pi^0$ trigger, $10<p_\mathrm{ T,jet}^\mathrm{ch}<15$$\mathrm{GeV/}c$; bottom: $\pi^0$ trigger, $15<p_\mathrm{ T,jet}^\mathrm{ch}<20$$\mathrm{GeV/}c$. Arrows indicate 95% CL. Theoretical calculations are discussed in the text. Right panels: integral of ${I}_\mathrm{AA}(\Delta\phi)$ over $3\pi/4<\Delta\phi<\pi$STAR:2023ksv.
• Figure A1: Data from Fig. \ref{['fig:IAA']} for recoil jet $10<p_\mathrm{ T,jet}^\mathrm{ch}<15$$\mathrm{GeV/}c$, and for Hybrid Model calculations for both $10<p_\mathrm{ T,jet}^\mathrm{ch}<15$$\mathrm{GeV/}c$ and $5<p_\mathrm{ T,jet}^\mathrm{ch}<10$$\mathrm{GeV/}c$, both with and without wake.
• Figure B2: Distributions of $\widetilde{Y}_{\Delta\phi}$($p_\mathrm{T,jet}^\mathrm{reco,ch}$) from PRL Fig. [3] using the nominal ME normalization region (blue), and those where the upper limit of the normalization region are reduced by 1 (red) or 3 (green) $\mathrm{GeV/}c$.