Single-spin asymmetries in $p^\uparrow p \to J/\psi +X$ within a TMD approach: role of the color octet mechanism

Understand

We calculate the transverse single-spin asymmetry (SSA) for $J/\psi$ production in proton-proton collisions, within non-relativistic QCD, employing the transverse momentum dependent, generalized parton model, which includes both spin and intrinsic motion effects.

• In particular, we study the role of the color octet mechanism in accessing the gluon Sivers function.
• In doing that, we also show, within this approach, how the singularities coming from color octet terms in the low-$P_T$ region can be handled, leading to finite cross sections.
• Predictions for both unpolarized cross sections and SSAs are given and compared against PHENIX data.