Collectivity in small systems - Initial state vs. final state effects

Moritz Greif,Sören Schlichting,Björn Schenke,Zhe Xu,Carsten Greiner,A Ayala Mercado,E Cuautle Flores

doi:10.1051/epjconf/201817205007

Moritz Greif, Sören Schlichting + Show 5 more

Open Access

https://doi.org/10.1051/epjconf/201817205007

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Abstract

Observations of long rang azimuthal correlations in small collision systems (p+p/A) have triggered an enormous excitement in the heavy-ion community. However, it is presently unclear to what extent the experimentally observed correlations should be attributed to initial state momentum correlations and/or the final state response to the initial state geometry. We discuss how a consistent theoretical description of the nonequilibrium dynamics is important to address both effects within a unified framework and present first results from weakly coupled non-equilibrium simulations in [1] to quantify the relative importance of initial state and final state effects based on theoretical calculations.

Highlights

Experimental measurements of long. range azimuthal correlations in high-energy proton-proton (p+p) and proton-nucleus (p+A) collisions, have revealed many interesting features that call for a deeper theoretical understanding [2]
Event-by-event simulations of the non-equilibrium dynamics of high-energy p+p,p+A and A+A collisions, can be performed within this framework by 1) Simulating particle production and early time dynamics (τ < 0.2 f m/c) based on the IP-Glasma model [25, 26] 2) Extracting the phase space distribution of gluons dNg/dηd2xdyd2p, which includes all relevant information about initial state momentum correlations2 as well the initial state geometry 3) Simulating final state re-scattering dynamics in a parton cascade (BAMPS) [30,31,32] based on pQCD
Experimental observations of pronounced azimuthal correlations in small systems have started to challenge our current understanding of the space time evolution of high-energy collisions

Summary

Introduction

Experimental measurements of long. range (in rel. rapidity ∆η) azimuthal correlations (in rel. angle ∆φ) in high-energy proton-proton (p+p) and proton-nucleus (p+A) collisions, have revealed many interesting features that call for a deeper theoretical understanding [2]. Angle ∆φ) in high-energy proton-proton (p+p) and proton-nucleus (p+A) collisions, have revealed many interesting features that call for a deeper theoretical understanding [2]. Many features observed in these “small system” are qualitatively similar to previous observations in nucleus-nucleus (A+A) collisions, including e.g. the transverse momentum and hadron-species dependence of these correlations [3, 4] as well as the fact that the observed correlations are “collective” in the sense that many particles are correlated with each other [5,6,7]. There are some important differences between the experimental results in proton-proton/nucleus and nucleus-nucleus collisions, including e.g. the fact that (so far) no evidence of jet-quenching phenomena has been reported in small systems. Quantifying the relative importance of these two effects.

General expectations

Conclusions & Outlook