Speaker
Description
We report the first direct evidence of thermalization of the Quark-Gluon Plasma (QGP) formed in ultra-relativistic heavy-ion collision, by studying the fluctuation of mean transverse momentum per particle ($\langle p_t \rangle$) in ultra-central Pb+Pb collision. The recent experimental data from the ATLAS collaboration at the Large Hadron Collider (LHC), provides measurement of variance of $\langle p_t \rangle$ at fixed multiplicity ($N_{ch}$) and a steep fall of the variance is observed over a narrow range of $N_{ch}$, for most of the central collision events (Fig. 14 and 15, \href{https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2021-01/}{ATLAS : HION-2021-01}). Such a behaviour cannot be reproduced by previously existing models, such as HIJING which treats Pb+Pb collisions as a superposition of independent nucleon-nucleon collisions. However, our model results can accurately reproduce the peculiar pattern in the variance of $\langle p_t \rangle$ that is observed by ATLAS. To explain such a novel phenomenon, we argue that at a given multiplicity, the impact parameter ($b$) fluctuation plays an important role; the transverse momentum per particle increases with the increasing impact parameter. A larger $b$ corresponds to a smaller collision volume resulting in a higher density. In relativistic thermodyanimcs, a higher density corresponds to a higher initial temperature which implies a larger energy per particle leading to a larger $\langle p_t \rangle$. Thus at a fixed $N_{ch}$, a higher $\langle p_t \rangle$ at a larger b is the effect of the thermalization of the system at a higher temperature. To illustrate this point further, we provide results for Pb+Pb collision from hydrodynamic simulation with TRENTO + MUSIC at b=0 and compare with corresponding HIJING results. From the hydro results, we find a very strong correlation between $\langle p_t \rangle$ and $N_{ch}$ at fixed impact parameter, which is not observed in HIJING where there is no thermalization. The strong correlation is the consequence of the thermalization that is assumed in the hydro simulation.
( R. Samanta, S. Bhatta, J. Jia, M. Luzum and J-Y Ollitrault, \href{https://arxiv.org/pdf/2303.15323.pdf}{arXiv: 2303.15323} )
| Category | Theory |
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