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We study chemical equilibration in out-of-equilibrium quark-gluon plasma using the first principles method of QCD effective kinetic theory, accurate at weak coupling. In longitudinally expanding systems—relevant for relativistic nuclear collisions—we find that for realistic couplings chemical equilibration takes place after hydrodynamization, but well before local thermalization. We estimate that hadronic collisions with final state multiplicities

The experiments at the relativistic heavy ion collider and the large hadron collider (LHC) have seen signs of collective behavior in proton-proton, proton-nucleus, and nucleus-nucleus collisions, which emerge smoothly as a function of the system size measured by event multiplicities. The signals of collectivity include long range multiparticle correlations

In the context of nucleus-nucleus collisions, these observations are understood as signs of kinetically and chemically equilibrated plasma. Fluid dynamic

How approximately equilibrated plasma emerges from fundamental interactions of the medium constituents has been a topic of intense theoretical study. There have been significant developments in theoretical understanding of far-from-equilibrium dynamics

The setup we employ is the effective QCD kinetic theory of Ref.

By mapping the only free parameter of the QCD kinetic theory—the coupling constant—to the transport properties in the fluid dynamic limit, we see that the system hydrodynamizes quickly in accordance with findings in pure Yang-Mills theory

We numerically solve the Boltzmann equation for homogeneous boost invariant quark and gluon distribution functions

The particle number changing processes

We use color-glass-condensate motivated initial conditions for the gluon distribution function

In collisions with realistic center of mass energies, the QCD coupling constant for in-medium energy scale is not small

The total energy density in a multicomponent plasma is given by a sum of its parts

Fermion energy density fraction of equilibrium density

To quantify the approach to thermal equilibrium and hydrodynamization, we define two additional timescales

In Fig.

The total energy density evolution in QCD kinetic theory (red solid line) scaled by ideal asymptotics

We also compare the total energy density evolution in QCD and pure Yang-Mills kinetic theory (gray dashed line) in Fig.

Following the procedure presented in

In Fig.

Evolution of total energy density and its gluonic and fermion components in kinetic theory converted to physical units using universality of

However, the approximation of transverse translational invariance breaks when the central parts of the collision come into causal contact with the edge of the fireball and the system starts to undergo significant radial expansion. Following the logic of Ref.

Experimental measurements of strangeness enhancement in p-p, p-Pb, and Pb-Pb collisions clearly indicate a continuous increase of strangeness production, which is saturated around

Authors would like to thank Peter Arnold, Jürgen Berges, Ulrich Heinz, Jacopo Ghiglieri, Jean-François Paquet, Sören Schlichting, Derek Teaney, and Urs Wiedemann for valuable discussions. This work was supported in part by the German Research Foundation (DFG) Collaborative Research Centre (SFB) 1225 (ISOQUANT) (A. M.). Finally, A. M. would like to thank CERN Theoretical Physics Department for the hospitality during the short-term visit.

Anisotropic systems suffer from the presence of unstable plasma modes

We determine

Note that the definition of

For more accurate description, extensions of our study including transverse expansion are needed

Note that the combination

For example, our model neglects quark masses, which could delay flavor equilibration. Even then the relation of strangeness content in the QGP and hadronic phase is not trivial