# Quark Matter 2019 - the XXVIIIth International Conference on Ultra-relativistic Nucleus-Nucleus Collisions

3-9 November 2019
Wanda Reign Wuhan Hotel
Asia/Shanghai timezone

## Constraining the initial stages of heavy-ion collisions with high-$p_{\perp}$ theory and data

4 Nov 2019, 17:40
20m
Wanda Han Show Theatre & Wanda Reign Wuhan Hotel

#### Wanda Han Show Theatre & Wanda Reign Wuhan Hotel

Poster Presentation Initial state and approach to equilibrium

### Speaker

Dr Bojana Ilic (Blagojevic) (Institute of Physics Belgrade)

### Description

Commonly, only low-$p_{\perp}$ sector is used to infer the features of initial stages before QGP thermalization. On the other hand, recently acquired wealth of high-$p_{\perp}$ experimental data paves the way to utilize the high-$p_{\perp}$ particles energy loss in exploring the initial stages. However, the results of such explorations are up to now either inconclusive or questionable. We here concentrate on high-$p_{\perp}$ $R_{AA}$ and $v_2$ observables, and study the effects of four different commonly considered initial stages scenarios, which have the same temperature profile after - but differ in the temperature profiles before - thermalization. For the study, we use our recently developed DREENA framework, which is a fully optimized computational procedure in which our state-of-the-art dynamical energy loss is employed. Contrary to the common expectations, we surprisingly obtain that high-$p_{\perp}$ $v_2$ is insensitive to the initial stages of medium evolution, being unable to discriminate between different scenarios. On the other hand, $R_{AA}$ is notably sensitive to these conditions, strongly preferring later thermalization times and free streaming during the initial stages of QGP formation. Moreover, we also reconsider the validity of widely-used procedure of fitting the energy loss parameters for different initial-stage cases to reproduce the experimentally observed $R_{AA}$. We here find that the reported sensitivity of $v_2$ to different initial-stage scenarios is mainly an artifact of the $R_{AA}$ fitting, with no real physical process to support it. Therefore, such a procedure may lead to erroneous conclusions, masking the underlying nature of jet-medium interactions. Consequently, the simultaneous study of high-$p_{\perp}$ $R_{AA}$ and $v_2$ is necessary for imposing reliable constraints on the initial stages.

### Primary author

Dr Bojana Ilic (Blagojevic) (Institute of Physics Belgrade)

### Co-authors

Dusan Zigic (Institute of Physics Belgrade) Magdalena Djordjevic (Institute of Physics Belgrade)