Is the centrality dependence of the elliptic flow $v_2$ and of the average $<p_T>$ in RHIC experiments more than a Core-Corona Effect?

TL;DR

This paper demonstrates that the core-corona model can effectively explain the centrality dependence of various flow and momentum variables in RHIC experiments, challenging the exclusive use of hydrodynamics for non-central collisions.

Contribution

The study extends the core-corona model to describe kinematical variables and particle species dependence, providing an alternative explanation to hydrodynamical models for non-central heavy ion collisions.

Findings

The model reproduces the centrality dependence of $v_2/\epsilon_{part}$ and $<p_T>$ for identified particles.

Correlation between peripheral heavy ion and pp collisions supports the model's validity.

Differences in particle species ratios explain the variation in flow observables.

Abstract

Recently we have shown that the centrality dependence of the multiplicity of different hadron species observed in RHIC and SPS experiments can be well understood in a simple model, dubbed core-corona model. There it is assumed that those incoming nucleons which scatter only once produce hadrons as in pp collisions whereas those which scatter more often form an equilibrated source which decays according to phase space. In this article we show that also kinematical variables like $v_2/\epsilon_{part} (N_{part})$ as well as $v_2^i/\epsilon_{part} (N_{part})$ and $<p_T^i (N_{part})>$ of identified particles are well described in this model. The correlation of $<p_T^i>$ between peripheral heavy ion collisions and pp collisions for different hadrons, reproduced in this model, questions whether hydrodynamical calculations are the proper tool to describe non-central heavy ion collision. The model explains as well the centrality dependence of $v_2/\epsilon_{part}$ of charged particles, considered up to now as an observable which allows to determine the viscosity of the quark gluon plasma. The observed dependence of $v_2^i/\epsilon_{part}(N_{part})$ on the particle species is a simple consequence of the different ratios of core to corona particles.