Heavy ion anisotropies: a closer look at the angular power spectrum

Abstract

Anisotropies in the final state of heavy-ion collisions carry information on the creation, expansion and evolution of the quark-gluon plasma. Currently, there is an abundance of studies on azimuthal anisotropies in comparison to longitudinal ones. The purpose of this work is to quantify angular $(\theta, \phi)$ correlations to further the understanding of the full spatial 3-D picture of emitted hadrons. Therefore, public ALICE data from Run 1 (2010) of Pb-Pb collisions at $\sqrt{s_{NN}} = 2.76\mathrm{~TeV}$ is analyzed through the estimation of an angular power spectrum. Issues with $|\eta| < 0.9$ limitation are tackled, as well as event multiplicity and detector efficiency. Firstly, spectra are calculated for toy Monte Carlo samples. Secondly, heavy-ion data spectra are presented for the full momentum phase space $0.15 < p_T < 100\mathrm{~GeV}$ and also separate intervals $p_T < 0.54\mathrm{~GeV}$ and $p_T > 0.54\mathrm{~GeV}$. The latter reveal how different geometries dominate at distinct scales and transverse momentum. Finally, the study submits particles generated through the AMPT model to the same power spectrum analysis. This comparison shows that in scales dominated by flow geometry, AMPT qualitatively describes the data spectra, while the opposite is true for smaller scales.