Improved determination of color-singlet nonrelativistic QCD matrix elements for S-wave charmonium

We present a new computation of S-wave color-singlet nonrelativistic QCD matrix elements for the J/psi and the eta(c). We compute the matrix elements of leading order in the heavy-quark velocity nu and the matrix elements of relative order nu(2). Our computation is based on the electromagnetic decay rates of the J/psi and the eta(c) and on a potential model that employs the Cornell potential. We include relativistic corrections to the electromagnetic decay rates, resumming a class of corrections to all orders in nu, and find that they significantly increase the values of the matrix elements of leading order in nu. This increase could have important implications for theoretical predictions for a number of quarkonium decay and production processes. The values that we find for the matrix elements of relative order nu(2) are somewhat smaller than the values that one obtains from estimates that are based on the velocity-scaling rules of nonrelativistic QCD.