Search

The STAR Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum hadron trigger, in central and peripheral $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathrm{NN}}}=200$ GeV. Charged jets are reconstructed with the anti-${k}_{\mathrm{T}}$ algorithm for jet radii $R$ between 0.2 and 0.5 and with low infrared cutoff of track constituents (${p}_{\mathrm{T}}g0.2 \mathrm{GeV}/\mathrm{c}$). A novel mixed-event technique is used to correct the large uncorrelated background present in heavy ion collisions. Corrected recoil jet distributions are reported at midrapidity, for charged-jet transverse momentum ${p}_{\mathrm{T},\mathrm{jet}}^{\mathrm{ch}}l30 \mathrm{GeV}/\mathrm{c}$. Comparison is made to similar measurements for $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s}=2.76$ TeV, to calculations for $p+p$ collisions at $\sqrt{s}=200$ GeV based on the pythia Monte Carlo generator and on a next-to-leading order perturbative QCD approach, and to theoretical calculations incorporating jet quenching. The recoil jet yield is suppressed in central relative to peripheral collisions, with the magnitude of the suppression corresponding to medium-induced charged energy transport out of the jet cone of $2.8\ifmmode\pm\else\textpm\fi{}0.2(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}1.5(\mathrm{sys})$ $\mathrm{GeV}/c$, for $10l{p}_{\mathrm{T},\mathrm{jet}}^{\mathrm{ch}}l20$ $\mathrm{GeV}/c$ and $R=0.5$. No medium-induced change in jet shape is observed for $Rl0.5$. The azimuthal distribution of low-${p}_{\mathrm{T},\mathrm{jet}}^{\mathrm{ch}}$ recoil jets may be enhanced at large azimuthal angles to the trigger axis, due to scattering off quasiparticles in the hot QCD medium. Measurement of this distribution gives a 90% statistical confidence upper limit to the yield enhancement at large deflection angles in central $\mathrm{Au}+\mathrm{Au}$ collisions of $50\ifmmode\pm\else\textpm\fi{}30(\mathrm{sys})%$ of the large-angle yield in $p+p$ collisions predicted by pythia.

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured φ meson production and its nuclear modification in asymmetric Cu + Au heavy