Roy A. Lacey, Yi Gu, X. Gong, D. Reynolds, N. N. Ajitanand, J. M. Alexander, A. Mwai, A. Taranenko
The flow harmonics for charged hadrons ($v_{n}$) and their ratios $(v_n/v_2)_{n\geq 3}$, are studied for a broad range of transverse momenta ($p_T$) and centrality ($\text{cent}$) in Pb+Pb collisions at $\sqrt{s_{NN}}= 2.76$ TeV. They indicate a characteristic pattern of viscous damping consistent with the dispersion relation for sound propagation in the plasma produced in these collisions. This pattern is not only a unique signature for anisotropic expansion modulated by viscosity, it provides essential constraints for the relaxation time and a distinction between two of the leading models for initial eccentricity. These constraints could be important for a more precise determination of the specific shear viscosity $\eta/s$.
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http://arxiv.org/abs/1301.0165
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