Two-particle correlation studies in heavy ion collisions at the Large Hadron Collider
Results on two-particle angular correlations in pPb at [square root sNN] =5.02 TeV and PbPb collisions at [square root sNN] =2.76 TeV are reported. The long-range correlations in pseudorapidity (“ridge'') has been seen in various systems, including PbPb, pPb and pp systems. In this thesis, two-particle correlations in pPb collisions at nucleon-nucleon center-of-mass energy of 5.02 TeV are studied as a function of the pseudorapidity separation (Δ η) of the particle pair at small azimuthal angle separation (:Δ&phis;: < π/3). The correlations are decomposed into a jet component that dominates the short-range correlations (:Δη: < 1), and a component that persists at large Δη, which is the ridge. The ridge may be associated with collective behavior of the produced system, well described by hydrodynamics. The azimuthal correlations, after subtraction of the jet component, are characterized by the V 2 and V3 coefficient. The single-particle anisotropy parameters v2 and v 3 are extracted and normalized by their mid-rapidity value. The normalized v2(ηcm)/v 2(ηcm=-0.465) distribution as a function of the center-of-mass pseudorapidity ηcm is found to be asymmetric about ηcm = 0, with smaller values observed at forward proton direction pseudorapidity, and smaller value at backward Pb direction, but the decreasing trend towards the two sides is different. The normalized v3(η cm)/v3(η cm=-0.465) distribution has no significant pseudorapidity dependence within the statistical uncertainties. The underlying physics for the η dependence of the v2 parameter is under extensive research at the time of this thesis writing. The two-particle correlation method is widely used also in jet-like correlation studies. The jet axis direction can be identified effectively by a high pT particle. The away-side partner jet is quenched in heavy ion collisions due to medium interactions. The biggest challenge in jet correlation studies is the subtraction of the large underlying anisotropy flow backgroud. In previous studies, the flow background is calculated from the measured Fourier coefficients, which results in large uncertainties. In this thesis, the away-side jet shape of PbPb collisions at [square root sNN] =2.76 TeV is studied utilizing a novel method of subtracting flow background using the data itself. The away-side is enhanced by a relatively large recoil transverse momentum in a given η range. The two-particle correlation function is constructed from different η regions, one is close and the other far away from the away-side jet. These two η regions are symmetric about η=0, so the flow background is the same. The correlation function difference between these two regions, therefore, measures the away-side jet shape. The jet width is studied as a function of multiplicity and pT. It is found that the jet width increases with multiplicity, indicating jet broadening in medium.
Wang, Purdue University.
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