TMD evolution study of the cos 2? azimuthal asymmetry in unpolarized J/ψ production at EIC

Semi-inclusive J/iy production in electron-proton collisions is a promising process to study gluon transverse momentum distributions (TMDs) at the future electron-ion collider (EIC). In this article, we improve on previous studies of the cos 20 azimuthal asymmetry that arises from the linear polarization of gluons inside unpolarized protons by including TMD evolution. We find that in the TMD regime the asymmetry grows monotonically with increasing transverse momentum of the outgoing J/iy, in contrast to tree level calculations with Gaussian TMDs. Our predictions for the asymmetry at the EIC can become very large at larger x, Q, and transverse momenta, even larger than the positivity bound. This problem stems from the very small b region and implies a range of validity of TMD factorization that is more restricted than usually expected. We also include an estimate of the nonperturbative uncertainty from the large b region and we conclude that it is smaller than the largest source of uncertainty, which stems from the choice of color octet long-distance matrix elements.

Automated summary

The article studies gluon transverse momentum distributions in electron-proton collisions. By including TMD evolution, the authors improve on previous studies of the cos 20 azimuthal asymmetry. They find that in the TMD regime, the asymmetry grows monotonically with increasing transverse momentum of the outgoing J/iy, contrary to tree level calculations. The predictions for the asymmetry at the EIC can be very large, exceeding the positivity bound.