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

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

Wanda Han Show Theatre & Wanda Reign Wuhan Hotel

Poster Presentation Initial state and approach to equilibrium Poster Session


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


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)


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

Presentation Materials