Phase transition in hot pion matter

The equation of state for the pion gas is analyzed within the third virial approximation. The second virial coefficient is found from the ππ-scattering data, while the third one is considered as a free parameter. The proposed model leads to a first-order phase transition from the pion gas to a more dense phase at the temperature Tpt<136 MeV. Due to relatively low temperature this phase transition cannot be related to the deconfinement. This suggests that a new phase of hadron matter — ‘hot pion liquid’ — may exist.