ETA-]PI(0)GAMMA-GAMMA AND GAMMA-GAMMA-]PI(0)PI(0) IN O(P(6)) CHIRAL PERTURBATION-THEORY

Abstract

eta --> pi(0) gamma gamma and gamma gamma --> pi(0) pi(0) are considered in O(p(6)) chiral perturbation theory. In addition to the usual rho, omega contributions, there are two O(p(6)) operators (L(6,m)) arising from explicit chiral symmetry breaking. Since only one of the two operators contributes to eta --> pi(0) gamma gamma, the coefficient of this operator (= d(3)) can be determined in two ways: (i) from the measured decay rate of eta --> pi(0) gamma gamma, and (ii) by assuming the resonance saturations of the low energy coefficients in the O(p(6)) chiral lagrangian. We find that two methods lead to vastly different values of d(3), which would indicate that either the measured decay rate for eta --> pi(0) gamma gamma is too large by a factor of 2 similar to 3, or the resonance saturation assumptions do not work for d(3).