Speaker
Description
Motivated by experimental results implying the possible QGP (quark-gluon plasma) formation in small colliding systems, we extend the hydro-based framework incorporating non-equilibrated components which play an essential role in small colliding systems. It has been widely accepted that relativistic hydrodynamics well describes the dynamics of the QGP at low $p_{\mathrm{T}}$ regimes in large colliding systems. Hence hydro-based frameworks are used to tease out properties of the QGP in high-energy heavy-ion collisions. In contrast, particle productions in small colliding systems have been studied through QCD-motivated phenomenological models such as perturbative QCD (semi-)hard processes followed by string fragmentation. As keeping these pictures in each regime, the ``marriage" of relativistic hydrodynamics and QCD-motivated phenomenological framework is indispensable to explore the dynamics over wide ranges of colliding systems.
We realize this as the dynamical core--corona initialization framework (DCCI) [1-3]. In DCCI, QGP fluids are generated from initial partons obtained from PYTHIA/PYTHIA Angantyr [4-5] which reflects the total energy-momentum of incoming nuclei. We phenomenologically describe the fluidization of the initial partons with the dynamical aspects of the core--corona picture. Partons with sufficient secondary scatterings tend to generate QGP fluids (core) as equilibrated matter.
While partons with insufficient secondary scatterings tend to survive as non-equilibrated matter (corona). This framework is, so to speak, the hydrodynamic afterburner for PYTHIA. By treating both locally equilibrated QGP fluids and non-equilibrated matter, the DCCI, as a hydro-based Monte Carlo event generator, is capable of describing from low to high $p_{\mathrm{T}}$, from backward to forward rapidity, and from small to large colliding systems.
In this talk, we investigate the interplay between core and corona components in high-energy nuclear collisions using DCCI. We reveal that the particle production from the core becomes dominant above $\langle dN_{\mathrm{ch}}/d\eta \rangle \sim 18$ regardless of the system size and the collision energy. Remarkably, the corona components turn out to dilute $\langle p_T \rangle$ and $v_2\{2\}$ obtained from the core components even in Pb--Pb collisions in which the entire system is often assumed to be locally equilibrated. These results suggest the importance of both equilibrated and non-equilibrated contributions in both small and large colliding systems towards an accurate understanding of the QGP properties.
[1] Y. Kanakubo, M. Okai, Y. Tachibana, and T.~Hirano, PTEP 2018, 121D01 (2018).
[2] Y. Kanakubo, Y. Tachibana, and T. Hirano, Phys.~Rev.~C101, 024912 (2020).
[3] Y. Kanakubo, Y. Tachibana, and T. Hirano, arXiv:2108.07943 [nucl-th].
[4] T. Sj\"{o}strand, S. Mrenna, and P. Z. Skands, Comput. Phys. Commun. 178, 852 (2008).
[5] C. Bierlich, G. Gustafson, L. L\"{o}nnblad, H. Shah, JHEP 10 134 (2018).
