| A strongly interacting Quark Gluon Plasma (QGP) is created in relativistic heavy ion collision at the Relativistic Heavy Ion Collider (RHIC). Large transverse momentum (pT) partons in the high-density system result from the initial hard scattering of nucleon constituents. After a hard scattering, the parton fragments to create a high-energy clus-ter (jet) of particles. The energetic parton is predicted to experience substantial energy loss when traversing the dense colored medium and therefore can be used a probe to detect the medium property. Since the medium interaction probability increases with path length through the medium, high-pT particle production is strongly biased towards surface of the collision zone. The study of single particle production at high pT gives limited information about the interior of the QCD medium and about jet-medium inter-actions. However, the away-side jet partner opposite to a high-pT particle traverses the entire volume suffering maximal interactions with the medium. Therefore, di-hadron and multi-hadron correlations with high pT trigger particles can be used as valuable tools to study the energy loss mechanisms due to interactions of energetic partons with the QGP medium.Two novel phenomena have been observed in di-hadron correlations:(1) the long-range pseudo-rapidity correlation on the near side (azimuthal angle difference within π/2) of high-pT trigger particles, and (2) the double-peak correlation structure in az-imuthal angle on the away side (di-hadron azimuthal angle difference larger than π/2). The former is often referred to as the "ridge" because the pseudo-rapidity distribution of the near-side correlated particles is approximately uniform. The latter is often re-ferred to as the "cone" correlation because the leading explanation of the double-peak structure is Mach cone shock wave excitations emitting particles in a preferred direction dependent of the speed of sound of the medium. However, only the elliptic flow v2 back-ground is subtracted in these correlation measurements. The ridge and the away-side double-peaked correlations maybe mainly caused by higher order hydrodynamic flow harmonics, particularly v3 that have been overlooked until recently. The subtraction of the elliptic flow v2 background involves determination of the elliptic flow parameters v2 and the amplitude of the flow background (normalization). The v2 parameters are from separate flow measurements. The elliptic flow background has been normalized to the data around|△φ|= 1 assuming that the background-subtracted signal is zero at minimum. This is referred to as the ZYAM (zero yield at minimum) method. Those measurements are usually coming with large uncertainties.We devise a novel method to subtract the flow background using data. No assump- tions on the flow shape and amplitude are made and we are going to subtract all orders of flow background. We select events with a large recoil transverse momentum (Px) within a pseudorapidity (77) window of 0.5<|η|< 1 from a high-pT trigger particle to en-hance the away-side jet population. Di-hadron azimuthal correlations are analyzed with associated particles in two 77 ranges (-0.5<η< 0 and 0< η< 0.5) symmetric about midrapidity, one ("close-region") close to and the other ("far-region") far away from the Px selection η window. The away-side jet contributes to the close-region but not as much to the far-region due to the large η gap, while the flow contributions are equal. Assuming the △φ shape of jet-like correlations does not depend on △η, the correlation difference measures the away-side jet shape where the anisotropic flow background is cleanly subtracted.The away-side jet correlation width is studied in Au+Au collisions at (?)= 200 GeV as a function of centrality and associated particle pT. The width is found to increase with centrality at modest to high associated particle pT.The increase can arise from jet-medium modifications, event averaging of away-side jets deflected by medium flow, and/or simply nuclear kT broadening. To further discriminate various physics mechanisms, a three-particle correlation analysis is conducted with robust flow background subtraction also using data. Based on the three-particle correlation analysis we discuss possible physics mechanisms of away-side broadening of jet-like correla-tions. |
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