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

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 $\sqrt{s_{\mathrm{N}\mathrm{N}}}=200$ 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.