Transport coefficients of QCD matter

Measuring shear viscosity using correlations

“…HBT and resonance effects omitted here may contribute only at the 10% and 15% levels, respectively [10]. Minijets, color glass, and other particle production effects modify σ o in (16). We assume that any modification cancels in studying the centrality dependence at fixed beam energy.…”

“…Observe that (16) and (17) are exact for our diffusion model. To see that this is the case, we apply the theoretical framework developed by Van Kampen and others [25] to viscous diffusion.…”

“…We have used this framework elsewhere to study the effect of diffusion on net charge fluctuations [26]. Since the key results of this section, (16) and (17), are "obvious," this section may be skipped on a first reading. However, the details are amusing.…”

“…The broadening in rapidity of ∆r g depends on the shear viscosity via (16). Equation (29) implies that the rapidity dependence of ∆r g can be measured by studying the dependence of (27) on the rapidity window in which particles are measured.…”

“…Theorists had long anticipated a large collisional viscosity based on weak coupling QCD [16] and hadronic computations [17], with values of η/s roughly of order unity for both phases near the crossover temperature ∼ 170 MeV. Supersymmetic Yang Mills calculations give the significantly smaller ratio η/s = 1/4π in the strong coupling limit [18].…”