We compute the bulk viscosity of a gas of pions at temperatures below the QCD crossover temperature, for the physical value of m_π, to lowest order in chiral perturbation theory. Bulk viscosity is controlled by number-changing processes which become exponentially slow at low temperatures when the pions become exponentially dilute, leading to an exponentially large bulk viscosity ζ~(F₀⁸/m_π⁵)exp(2m_π/T), where F₀93MeV is the pion decay constant.
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Lu et al. (2011) studied this question.
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