A non-perturbative chiral approach for meson-meson interactions

A non-perturbative method [1] which combines constraints from chiral symmet ry breaking and coupled channel unitarity is used to describe meson-meson i nteractions up to root s less than or similar to 1.2 GeV, extending in this way the range of applicability of the information contained in Chiral Pert urbation Theory (chi PT) [2], since this perturbative series is typically r estricted to root s less than or similar to 500 MeV. The approach uses the O(p(2)) and O(p(4)) chi PT Lagrangians. The seven free parameters resulting from the O(p4) Lagrangian are fitted to the experimental data. The approac h makes use of the expansion of T-l instead of the amplitude itself as done in chi PT. The former expansion is suggested by analogy with the effective range approximation in Quantum Mechanics and it appears to be very useful. The results, in fact, are in good agreement with a vast amount of experime ntal analyses [3,4]. The amplitudes develop poles corresponding to the f(0) (980), a(0)(980), rho(770), K-*(890), the octet contribution to the phi, f( 0)(400 - 1200) - sigma and kappa [4]. The total and partial decay widths of the resonances are also well reproduced.