CORRECTIONS TO THE PAGELS-STOKAR FORMULA FOR F-PI

Within the composite operator formalism we derive a formula for the pi on decay constant f(pi), as defined directly from the residue at the p ion pole of the meson propagator, rather than from the matrix element of the axial current. The calculation is performed under some simplify ing assumptions, and we verify the complete consistency with soft-pion results, in particular with the Adler-Dashen relation. The formula on e obtains for (the pole-defined) f(pi)(2) differs from the previous Pa gels-Stokar expression by an additive term, and it still provides f(pi )(2) in terms of the quark self-energy. We make some numerical estimat es leading to (30-40)% deviation for f(pi)(2) with respect to the Page ls-Stokar formula. (C) 1997 Elsevier Science B.V.