Charm diffusion in a pion gas implementing unitarity, chiral and heavy quark symmetries.

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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 D and D* 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 p(2)-independent at a fixed temperature which simplifies phenomenological analysis. At the higher end of reliability of our calculation, T similar or equal to 150 MeV, we report a charm relaxation length lambda(c) similar or equal to 40 fm, 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.
© 2011 Elsevier Inc. All rights reserved. This work was supported by grants FPA 2008-00592, FIS2008-01323, FPA2007-29115-E, FIS2006-03438, PR34-1856-BSCH, UCM-BSCH, GR58/08 910309, PR34/07-15875 (Spain) and by the EU Integrated Infrastructure Initiative Hadron Physics Project under Grant Agreement n. 227431. The authors are grateful to Li Sheng Geng for updating them on the current D pi meson effective Lagrangians. Luciano Abreu is grateful for the hospitality at Univ. Complutense of Madrid where this work has been completed and acknowledges financial support from CAPES/Fundacion Carolina. Daniel Cabrera acknowledges financial support from Centro Nacional de Fisica de Particulas, Astroparticulas y Nuclear (CPAN, Consolider-Ingenio 2010). Juan M. Torres-Rincon is recipient of an FPU scholarship from the Spanish Ministry of Education.
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