QCD corrections to J/ψ polarisation in pp collisions at RHIC

We update the study of the polarisation of J/ψ produced in proton-proton collisions at RHIC at s=200 GeV using the QCD-based Colour-Singlet Model (CSM), including next-to-leading order partonic matrix elements from gluon and light quark fusion and leading-order contributions from charm-quark initiated processes. To do so, we also evaluate the corresponding cross section differential in PT which agrees qualitatively with the measurements of PHENIX in the central and forward regions at low PT - for instance below 2 GeV, while emphasising the need for Initial State Radiation (ISR) resummation. At mid PT, we also compare the measurements from PHENIX and STAR with the same evaluation complemented with the dominant αS5 contributions (NNLO⋆). We find a reasonable agreement with the data. Regarding the polarisation, as shown for previous studies at larger s and PT, the polarisation pattern from gluon and light quark fusion in the helicity frame is drastically modified at NLO and is shown to be increasingly longitudinal. The yield from charm-gluon fusion is found to be slightly transversally polarised. Combining both these contributions with a data-driven range for the polarisation of J/ψ from χc, we eventually provide an evaluation of the polarisation of the prompt J/ψ yield which is in a good agreement with the experimental data from PHENIX both in the central and forward regions.