The GMO sumrule and the pi NN coupling constant

The isovector GMO sumrule for forward pi N scattering is critically evaluat ed using the precise pi(-)p and pi(-)d scattering lengths obtained recently from pionic atom measurements. The charged pi NN coupling constant is then deduced with careful analysis of systematic and statistical sources of unc ertainties. This determination gives directly from data g(c)(2)(GMO)/4 pi = 14.17+/-0.09 (statistic) +/-0.17 (systematic) or f(c)(2)/4 pi = 0.078(11). This value is half-way between that of indirect methods (phase-shift analy ses) and the direct evaluation from from backward np differential scatterin g cross sections (extrapolation to pion pole). From the pi(-)p and pi(-)d s cattering lengths our analysis leads also to accurate values for (1/2)(a(pi -p)+a(pi-n)) and (1/2) (a(pi-p)-a(pi-n)).