In the (critical) chiral phase transition of two-flaver QCD at finite temperature, the scalar quark condensate is identified as the order parameter, and the excitation of its fluctuation (sigma meson) becomes gapless. In the temperature-quark chemical potential plane, we expect a tri-critical point on the phase boundary line, where the 2nd order singularity changes to the first order. When the small, but finite quark mass is introduced, the 2nd order singularity is smeared out whereas the 1st order line survives with the critical endpoint. In this talk we discuss the nature of the critical eigen mode associated with this endpoint.

University of Tokyo, Komaba Institute of Physics