Theoretical constraints on interaction amplitudes of light mesons
  We impose unitarity constraints on the S-wave isoscalar pion-pion amplitudes extracted from the analysis of the pi- p ---> pi+ pi- n data which have been measured by the CERN-Cracow-Munich collaboration on a transversely polarized target at 17.2 GeV/c pi- momentum. Two "steep" solutions contain a narrow S-wave f0(750) resonance under the rho(770) and exhibit a considerable inelasticity eta which is in disagreement with the four pion production data below the KKbar threshold. We impose eta= 1 for all data points and examine four sets of solutions for the S-wave isoscalar phase-shifts. The "down-flat" and "up-flat" solutions easily pass the eta=1 constraint but the remaining "down-steep" and "up-steep" are eliminated. We conclude that the 17.2 GeV data cannot be described by a relatively narrow f0(750). 
R. Kaminski, L. Lesniak and K. Rybicki