In-medium ππ correlation induced by partial restoration of chiral symmetry -: art. no. 011901

We show that both the linear and the nonlinear chiral models give an enhancement of the pi pi cross section near the 2 pi threshold in the scalar-isoscalar (I=J=0) channel in nuclear matter. The reduction of the chiral condensate, i.e., the partial chiral restoration in nuclear matter, is responsible for the enhancement in both cases. We extract an effective 4 pi -nucleon vertex, which is responsible for the enhancement but has not been considered in the nonlinear models for in-medium pi pi interaction. The relation of this vertex and a next-to-leading order term in the heavy-baryon chiral Lagrangian L-piN((2)) is also discussed.