Pok Man Lo

S-matrix approach to hadron gas

In this talk, we shall discuss how the S-matrix formalism can be applied to describe the system of hadrons in heavy ion collisions. The approach provides a consistent way to incorporate attractive and repulsive forces between hadrons. Using the input of empirical phase shifts from hadron scattering experiments, the important physics of resonance widths, coupling strengths and unitary constraints are naturally incorporated, beyond the capability of the standard Breit-Wigner formulation. In particular, we show how the proper treatment of the resonance widths can substantially modify the soft part of the pion momentum spectra. Lastly, we shall attempt to extend the approach to include N-body scatterings. For this a generalization of the two-body phase shift will be considered. As a numerical example we compute the generalized phase shifts of some three- and four-body processes for an interacting gas of pions.