arXiv subjects

Michito Okai

Publications and source records attributed to Michito Okai.

2 recordsLinked to original sources

arXiv2018

Enhancement of strange baryons in high-multiplicity proton-proton and proton-nucleus collisions

We investigate the enhancement of yields of strange and multi-strange baryons in proton-proton (p+p), proton-lead (p+Pb) and lead-lead (Pb+Pb) collisions at the Large Hadron Collider (LHC) energies from a dynamical core-corona initialization model. We first generate partons just after the collisions by using event generators. These partons dynamically generate the quark gluon plasma (QGP) fluids through the source terms in the hydrodynamic equations. According to the core-corona picture, this process tends to happen where the density of generated partons is high and their transverse momentum is low. Some partons do not fully participate in this process when they are in dilute regions or their transverse momentum is high and subsequently fragment into hadrons through string fragmentation. In this framework, the final hadrons come from either chemically equilibrated fluids as in the conventional hydrodynamic models or string fragmentation. We calculate the ratio of strange baryons to charged pions as a function of multiplicity and find that it monotonically increases up to $dN_{\mathrm{ch}}/dη\sim 100$ and then saturates above. This suggests that the QGP fluids are \textit{partly} created and that their fraction increases with multiplicity in p+p and p+Pb collisions at LHC energies.

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arXiv2017

New approach to initializing hydrodynamic fields and mini-jet propagation in quark-gluon fluids

We propose a new approach to initialize the hydrodynamic fields such as energy density distributions and four flow velocity fields in hydrodynamic modeling of high-energy nuclear collisions at the collider energies. Instead of matching the energy-momentum tensor or putting the initial conditions of quark-gluon fluids at a fixed initial time, we utilize a framework of relativistic hydrodynamic equations with source terms to describe the initial stage. Putting the energy and momentum loss rate of the initial partons into the source terms, we obtain hydrodynamic initial conditions dynamically. The resultant initial profile of the quark-gluon fluid looks highly bumpy as seen in the conventional event-by-event initial conditions. In addition, initial random flow velocity fields also are generated as a consequence of momentum deposition from the initial partons. We regard the partons that survive after the dynamical initialization process as the mini-jets and find sizable effects of both mini-jet propagation in the quark-gluon fluids and initial random transverse flow on the final momentum spectra and anisotropic flow observables. We perform event-by-event $(3+1)$-dimensional ideal hydrodynamic simulations with this new framework that enables us to describe the hydrodynamic bulk collectivity, parton energy loss, and interplay among them in a unified manner.

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