Search for Jet interactions with quark-gluon plasma

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Chen, Chin-Hao
The Graduate School, Stony Brook University: Stony Brook, NY.
A hot, dense QCD medium is created in heavy ion collisions at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. This new type of matter is opaque to energetic partons, which suffer a strong energy loss in the medium. Two particle correlations are a powerful tool to study the jet properties in the medium and provide information about the energy loss mechanism and jet-medium interactions. When triggering on high $p_T$ particles, the away-side shape depends strongly on the $p_T$ of the associated particles. In this analysis, we present the inclusive photon-hadron two particle azimuthal correlations measured in Au+Au collisions at $sqrt{s_{NN}}~=~200$ GeV by PHENIX experiment. In order to study jet-medium interactions, we focus on intermediate $p_T$, and subtract particle pairs from the underlying event. Jet-like correlations appear modified in central Au+Au compared to p+p, in both the trigger and opposing jet. The trigger jet is elongated in pseudo-rapidity (the ``ridge''), while the opposing jet shows a double peak structure (''head'' and ``shoulder''). We decompose the structures in $Deltaeta$ and $Deltaphi$ to disentangle contributions from the medium and the punch-through and trigger jets. Upon correcting the underlying event for elliptic flow, the ridge is observed for associated particle $p_T$ below 3 GeV/c; it is broad in rapidity and narrow in $Deltaphi$. The away side correlated particle yield is enhanced in central collisions. The yield of particles in the shoulder grows with centrality while the away side punch-through jet is suppressed. Remarkably, the ridge closely resembles the shoulder in the centrality dependence of particle yield and spectra. There has been great debate about the origin of the ridge and shoulder. A favored explanation is that the structure is due to features of the collective flow of particles in the underlying event, particularly the fluctuation-driven triangular flow, quantified by the third Fourier component, $v_3$. We measure higher order Fourier harmonics in two ways, and use the results to give the shape of particle correlations in the underlying event. We decompose the power spectrum for the medium and for jets measured in p+p collisions. When including the higher harmonics of the collective flow ($v_3$, $v_4$) in the shape of the underlying events in two particle correlations, the ridge and shoulder no longer exist after subtraction. The jet function in Au+Au looks like p+p in which the away side jet is suppressed and broadened. There is also a pedestal-like structure in the jet function. Since the higher harmonics only change the shape of the underlying background, the pedestal is simply the redistribution of the ridge and shoulder particle yields. In conclusion, when jets pass through the medium, the away side jet is suppressed and the shape is broadened. This also brings out extra particles with spectra slightly harder than the medium, but softer than jet fragments. These are probably from the jet-medium interaction.