Ratio of bulk to shear viscosity in a quasigluon plasma: From weak to strong coupling

Abstract

The ratio of bulk to shear viscosity is expected to exhibit a different behaviour in weakly and in strongly coupled systems. This can be expressed by its dependence on the squared sound velocity. In the high temperature QCD plasma at small running coupling, the viscosity ratio is uniquely determined by a quadratic dependence on the conformality measure, whereas in certain strongly coupled and nearly conformal theories this dependence is linear. Employing an effective kinetic theory of quasiparticle excitations with medium-modified dispersion relation, we analyze the ratio of bulk to shear viscosity of the gluon plasma. We show that in this approach, depending on the temperature, the viscosity ratio exhibits either of these dependencies found by means of weak coupling perturbative or strong coupling holographic techniques. The turning point between the two different dependencies is located around the maximum in the scaled interaction measure.

Temperature effect on shear and bulk viscosities of QCD matter

“…It seems that our result near the phase transition region favors the estimation from the strong-coupled nonconformal theory (similar to that given in Ref. [53] for gluon plasma) than of the leading-order pQCD result [35].…”

“…Controversial results also exist for its ð1/3 − c 2 s Þ (c 2 s , the sound speed squired) dependence (see, e.g., Refs. [35,46,[50][51][52][53]). One may then doubt the validity for the η/s and ζ/s to label a phase transition, even the power of statistical physics principle to describe properties of strong interaction matters.…”

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“…This, however, does not necessarily imply that any picture based on quasiparticle excitations is inadequate for describing the properties of the produced matter. Within the QPM [13,14], based on an effective kinetic theory approach of BoltzmannVlasov type, the shear and bulk viscosities have been determined in [21] for the pure gluon plasma in relaxation time approximation and their ratio was studied in [22]. In this approach, the medium is viewed as being composed of quasigluons with the same quantum numbers and obeying the same quantum statistics as partonic gluons.…”

Equation of state and viscosities from a gravity dual of the gluon plasma

“…transport coefficients. (This is in contrast to quasiparticle approaches which require additional input to access transport coefficients [4].) In considering an ansatz of gravity + scalar as framework of effective dual models to pure non-abelian gauge thermo-field theories within a bottom-up approach one has to adjust either the potential of the dilaton field, or a metric function, or the dilaton profile.…”

“…of transport coefficients. Our goal is accordingly the quantification of the bulk viscosity in the LHC relevant region, in particular near to T + c , and a comparison with results of the quasiparticle model [4].…”