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

TL;DR

This study measures how the Quark-Gluon Plasma affects jet acoplanarity in high-energy collisions, revealing significant medium-induced broadening not fully explained by existing models.

Contribution

It provides the first detailed measurement of medium-induced acoplanarity in Au+Au and pp collisions at 200 GeV using direct-photon and π0 triggers, highlighting phenomena beyond current theories.

Findings

Significant acoplanarity broadening observed in Au+Au collisions.

Recoil jet yield increases up to 20 times at large azimuthal separation.

Current models fail to fully describe the observed phenomena.

Abstract

The STAR Collaboration reports measurements of acoplanarity using semi--inclusive distributions of charged--particle jets recoiling from direct photon and $\pi^{0}$ 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 $\pi$. 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.