The impact of the TMD shape function on matching the transverse momentum spectrum in $J/ψ$ production at the EIC

The impact of the inclusion of TMD shape functions on the transverse momentum spectrum in $J/ψ$ production at the EIC is investigated by considering the matching of the TMD factorization description at low transverse momentum with the collinear factorization description at high transverse momentum by means of the inverse-error weighting method. Despite large uncertainties from scale variations and the $J/ψ$ long-distance matrix elements, predictions for the differential cross section and its $\cos(2ϕ_ψ)$ modulation are obtained. We find that physical constraints are satisfied in case a process-dependent term is included for color octet production, but not in all cases when it is excluded. These numerical results support the analytic calculations in \cite{Boer:2023zit}. Future experimental data can thus test the validity of TMD factorization in $J/ψ$ production and explore the presence of nontrivial process-dependent effects in the soft-gluon resummation. This will be crucial in the extraction of gluon unpolarized and linearly polarized TMD distributions of the proton. In addition, we suggest how the sign of the latter can be determined by investigating the presence of a node in the $\cos(2ϕ_ψ)$ modulation as a function of transverse momentum.