Viscosity, nonconformal equation of state, and sound velocity in Landau hydrodynamics

We find an analytical solution to relativistic viscous hydrodynamics for a (1 + 1)-dimensional Landau flow profile. We consider relativistic Navier-Stokes form of the dissipative hydrodynamic equation, for a nonconformal system with a constant speed of sound, and employ the obtained solution to fit rapidity spectrum of observed pions in root s(NN) = 200, 17.3, 12.3, 8.76, 7.62, 6.27, 4.29, 3.83, 3.28, and 2.63 GeV collision energies. We find that at the freeze-out hypersurface with improved Landau's freeze-out prescription, the viscous corrections do not affect the rapidity spectra. We demonstrate that the solution of the nonconformal Landau flow leads to a better agreement with the experimental data compared to the conformal ideal solution. We also extract speed of sound from fit to the rapidity spectra for various collision energies and find a monotonous decrease with decreasing collision energies. Appealing to the fact that viscosity has negligible effect on rapidity spectra for Landau's freeze-out scenario, we argue that our calculations provides a framework for extracting the average value of speed of sound in relativistic heavy-ion collisions.