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 pi pi -scatt ering data, while the third one is considered as a free parameter. The prop osed model leads to a first-order phase transition from the pion gas to a m ore dense phase at the temperature T-pt < 136 MeV. Due to relatively low te mperature this phase transition cannot be related to the deconfinement. Thi s suggests that a new phase of hadron matter - 'hot pion liquid' - may exis t. (C) 2001 Published by Elsevier Science B.V.