# Importance of initial and final state effects for azimuthal correlations in p+Pb collisions

## TL;DR

This paper investigates how initial and final state effects influence azimuthal correlations in p+Pb collisions, introducing a new combined model to analyze their interplay across different multiplicities and momentum ranges.

## Contribution

It presents a novel model combining classical Yang-Mills dynamics with a perturbative QCD cascade to study initial and final state effects in small system collisions.

## Key findings

- Initial state effects dominate at high transverse momentum in low-multiplicity events.
- Final state interactions are more influential at low momentum in high-multiplicity events.
- Approximately half of the azimuthal correlation signal in low-multiplicity events originates from initial state effects.

## Abstract

We study the influence and interplay of initial state and final state effects in the dynamics of small systems, focusing on azimuthal correlations at different multiplicities. To this end we introduce a new model, matching the classical Yang-Mills dynamics of pre-equilibrium gluon fields (IP-GLASMA) to a perturbative QCD based parton cascade for the final state evolution (BAMPS) on an event-by-event basis. Depending on multiplicity of the event, we see transverse momentum dependent signatures of the initial, but also the final state in azimuthal correlation observables, such as $v_2\left\lbrace 2PC\right\rbrace(p_T)$. In low-multiplicity events, initial state correlations dominate for transverse momenta $p_T>2~\mathrm{GeV}$, whereas in high-multiplicity events and at low momenta final state interactions dominate and initial state correlations strongly affect $v_2\left\lbrace 2PC\right\rbrace(p_T)$ for $p_T>2~\mathrm{GeV}$ as well as the $p_T$ integrated $v_2\left\lbrace 2PC\right\rbrace$. Nearly half of the final pT integrated $v_2\left\lbrace 2PC\right\rbrace$ is contributed by the initial state in low-multiplicity events, whereas in high-multiplicity the share is much less. Based on Ref. [1], we are now able to carry out a systematic multiplicity scan, probing the dynamics on the border of initial state dominated to final state dominated - but not yet hydrodynamic regime.