Soft Contributions Stabilize NNLO QCD Corrections to Quarkonium Production and Decay

Next-to-next-to-leading order (NNLO) QCD corrections to quarkonium production and decay are known to exhibit perturbative instabilities within non-relativistic QCD. We identify the origin of this problem and propose a simple remedy. Applying our approach to $S$-wave color-singlet quarkonium processes, we achieve substantially improved perturbative convergence and agreement with experimental data.