The Ridge and Di-Hadron Correlations from the Beam Energy Scan Program at Rhic
Song, Liao 1988-
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Relativistic heavy-ion collisions aim to create an extremely hot and dense medium in which the hadronic matter undergoes a phase transition into a unique form of matter known as the Quark Gluon Plasma (QGP). Di-hadron correlations are a key observable in heavy-ion collisions, and play a critical role in establishing the equation of state of the QGP. Long range di-hadron correlations, also known as the ridge, have been observed at the top energies in the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC) in Au+Au, p+Au, and d+Au collisions. They are crucial in probing the collectivity of the various systems. In this thesis, the first set of results of 2D di-hadron correlations for Au+Au collisions with center-of-mass energies per nucleon of 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV from the STAR experiment is presented. Measurements of these correlations incorporate extensive coverage in azimuth and pseudo-rapidity, where the ranges extend to $0 < \phi < 2\pi$ and $0 < |\eta| < 1$ respectively, and are integrated over transverse momenta $0.2<p_T<2.0$ GeV/c. The anisotropy coefficients of various orders from the ridge are obtained, and they show correlations with the collision energy and the centrality. In particular, when scaled by the multiplicity density, the triangular coefficient shows a non-monotonic behaviour as a function of energy. Hints of similar behaviour are observed when the triangular and quadrature coefficients are scaled by the elliptic coefficients. Evidence of a pressure minimum around a center-of-mass energy per nucleon of 10-20 GeV is found.