The role of the soft scale for $J/ψ$ production in the transverse momentum dependent framework

Abstract: We use vNRQCD to study power corrections in the ${\rm v}$ expansion due to soft gluon radiation during $J/\psi$ production at small transverse momentum. We categorize four new $J/\psi$ production operators that mediate the transition of perturbatively produced color-octet charm quark/anti-quark pairs to charm quarks in a $^3S_1^{[1]}$ state via soft gluon emission. We then use Soft Collinear Effective Theory and vNRQCD to derive a factorization theorem for $J/\psi$ production in SIDIS in terms of the gluon transverse momentum dependent (TMD) PDFs in the proton and new objects which we call TMD soft transition functions. We show that the TMD soft transition function leads in the ${\rm v}$ power-counting with respect to the color-octet TMD shape functions that have been used in previous studies of $J/\psi$ production at small transverse momentum.