BEYOND THE NARROW RESONANCE APPROXIMATION - DECAY CONSTANT AND WIDTH OF THE FIRST PION-EXCITATION STATE

We consider the first pion excitation as a sub-continuum resonance in the pseudoscalar channel, and we obtain parameters characterizing this resonance through a global fit of the Borel-parameter dependence of t he field-theoretical pseudoscalar Laplace sum rule to its hadronic (pi on + pion-excitation + QCD-continuum) content. Our analysis incorporat es finite-width deviations from the narrow resonance approximation, in stanton effects, and higher-loop perturbative contributions to the pse udoscalar correlator. We obtain the following values (uncertainties re flect 90% confidence levels): mass M-Pi=1.15+/-0.28 GeV, width Gamma(P i)=<0.48 GeV, decay constant r=[FPiMPi2/f(pi)m(pi)(2)](2)=4.7+/-2.8. ( C) 1997 Elsevier Science B.V.