Interplay between core and corona components in high-energy nuclear collisions

TL;DR

This paper introduces an updated dynamical core--corona framework (DCCI2) to describe high-energy nuclear collisions across different systems and energies, revealing the interplay between equilibrated and non-equilibrated matter and its impact on observables.

Contribution

The study develops and applies DCCI2 to quantify core and corona contributions in small and large collision systems, highlighting their effects on particle yields and flow measurements.

Findings

Core contribution increases with multiplicity, surpassing corona above a certain threshold.

Corona contribution influences low-$p_T$ flow measurements, affecting $v_2$ estimates.

Small corona effects are observed even in central Pb+Pb collisions.

Abstract

We establish the updated version of dynamical core--corona initialization framework (DCCI2) as a unified description from small to large colliding systems and from low to high transverse momentum ($p_T$) regions. Using DCCI2, we investigate effects of interplay between locally equilibrated and non-equilibrated systems, in other words, core and corona components in high-energy nuclear collisions. Given experimental multiplicity distributions and yield ratios of $\Omega$ baryons to charged pions as inputs, we extract the fraction of core and corona components in p+p collisions at $\sqrt{s}=7$ TeV and Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. We find core contribution overtakes corona contribution as increasing multiplicity above $\langle dN_{\mathrm{ch}}/d\eta \rangle_{|\eta|<0.5} \sim 18$ regardless of the collision system or energy. We also see that the core contribution exceeds the corona contribution only in 0.0-0.95\% multiplicity class in p+p collisions. Notably, there is a small enhancement of corona contribution with $\sim20$\% below $p_T\sim 1$ GeV even in minimum bias Pb+Pb collisions. We find that the corona contribution at low $p_T$ gives $\sim 15$-$38$ $\%$ correction on $v_2\{2\}$ at $N_{\mathrm{ch}}\lesssim 370$. This raises a problem in conventional hydrodynamic analyses in which low $p_T$ soft hadrons originate solely from core components. We finally scrutinize the roles of string fragmentation and the longitudinal expansion in the transverse energy per unit rapidity, which is crucial in initial conditions for hydrodynamics from event generators based on string models.