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

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 $\approx 20$ 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.

Comments: Submitted to PRL; 3 figures+1 figure in End Matter, accepted by PRC