Semi-inclusive direct photon + jet and π⁰+jet correlations measured in p + p and central Au + Au collisions at ^√s_NN = 200 GeV

The Solenoidal Tracker at RHIC (STAR) experiment at the Relativistic Heavy Ion Collider reports new measurements of jet quenching based on the semi-inclusive distribution of charged-particle jets recoiling from direct photon (γ_dir) and neutral pion (π⁰) triggers in pp and central Au + Au collisions at ^√s_NN = 200 GeV for triggers in the range 9 < E_T ^trig < 20 GeV. The datasets have integrated luminosities of 3.9 nb⁻¹ for Au + Au and 23 pb⁻¹ for pp collisions. Jets are reconstructed using the anti-k_T algorithm with resolution parameters R = 0.2 and 0.5. The large uncorrelated jet background in central Au + Au collisions is corrected using a mixed-event approach, which enables precise charged-particle jet measurements at low transverse momentum p^ch _T,jet and large R. Recoil-jet distributions are reported in the range p^ch _T,jet < 25 GeV/c. Comparison of the distributions measured in pp and Au + Au collisions reveals strong medium-induced jet yield suppression for R = 0.2 with markedly less suppression for R = 0.5. Comparison is also made to theoretical models incorporating jet quenching. These data provide new insight into the mechanisms underlying jet quenching and the angular dependence of medium-induced jet-energy transport and provide new constraints on modeling such effects.