σ meson in hot and dense matter

An important role of the scalar isoscalar σ meson in low-energy physics is discussed. The behavior of the σ meson in a hot and dense medium is studied. It is shown that, in the vicinity of the critical values of the temperature (T) and the chemical potential (μ), the σ meson can become a sharp resonance. This effect can lead to a strong enhancement of the processes ππ → γγ and ππ → ππ near the two-pion threshold. Experimental observation of this phenomenon can be interpreted as a signal of approaching the domain where the chiral symmetry restoration and the phase transition of hadron matter into quark-gluon plasma take place