Charm diffusion in a pion gas implementing unitarity, chiral and heavy quark symmetries Original Research Article

We compute the charm drag and diffusion coefficients in a hot pion gas, such as is formed in a heavy ion collision after the system cools sufficiently to transit into the hadron phase. We fully exploit heavy quark effective theory (with both image and image mesons as elementary degrees of freedom during the collision) and chiral perturbation theory, and employ standard unitarization to reach higher temperatures. We find that a certain friction and shear diffusion coefficients are almost image-independent at a fixed temperature which simplifies phenomenological analysis. At the higher end of reliability of our calculation, image, we report a charm relaxation length image, in agreement with the model estimate of He, Fries and Rapp. The momentum of a 1 GeV charm quark decreases about 50 MeV per fermi when crossing the hadron phase.