Searching for initial state fluctuations in heavy ion collisions at FAIR energy using principal component analysis

Abstract

In high energy heavy ion collisions, the initial configurations of the colliding nuclei play an important role in determining the reaction type and the products of the reaction. The initial arrangement of nucleons within the overlap region of two colliding nuclei is generally asymmetric and such asymmetries reflect themselves in the measurement final state momentum anisotropy. Also initial distribution of the nucleons are subjected to large quantum fluctuation causing large energy deposition in a small region. The final state observables related momentum anisotropies although sensitive to such localized fluctuations but their true effect gets diluted because these observables are calculated by averaging over a set of events. Also, such fluctuations in the initial states are random and uncontrolled. Thus, identifying their effect from event-averaged final state observable is difficult. However, it would be interesting to know the origin of such fluctuations and how these fluctuation are eventually translated to the final state. In this work, we at first introduce such localized fluctuations in the initial configurations, also called hot spots, by implementing spatial rearrangements of nucleon position in the colliding nuclei in the central Pb + Pb collisions at \(\hbox {E}_{lab}\) = 20 AGeV (\(\sqrt{s}\) = 6.27 GeV) using the UrQMD event generator. Then the final state distributions of one or two dimensional variables e.g., (\(\eta \), \(\phi \), \(p_T\)) and (\(\eta -p_T\), \(\phi -p_T\), \(\eta -\phi \)) of the produced pions are analysed using the principal component analysis (PCA) technique. The eigenvalues of the principal components have been studied for various initial configurations, event fractions containing hot spots in the initial condition and for event centralities with an aim to find it’s sensitivity to the initial hot spot configurations.