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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

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.