Quarkonium production in p+p collisions at RHIC and LHC

Abstract

Quarkonium production mechanism is still an unresolved question. This paper provides an overview of recent quarkonium production studies performed at RHIC and LHC in p + p collisions at a wide range of collision energies. After a brief explanation of representative theoretical models, experimental results of the quarkonium spectra and polarizations are presented with comparison to the model calculations. The spectra are overall rather well described by production model calculations, with a few exceptions, while no significant polarization is observed beyond large uncertainties for J/ψ and ϒ, and thus no constraint on the models is obtained at present. Recently, new observables have been measured such as charged particle multiplicity dependence of quarkonium production, which provides a way to understand the interplay between hard and soft processes, and associated production of quarkonium (J/ψ in jets and double production), which provides more information about production mechanism. A trend of non-linear increase of quarkonium production with increasing the particle multiplicity is observed both at RHIC and LHC, but it is reproduced by most models and further studies and more data are needed in order to differentiate them. Meanwhile, the first measurements of J/ψ in jets show that it is produced with larger activity within a jet than simulated by PYTHIA. A comment on the study of J/ψ-hadron femtoscopic correlations to measure their mutual interaction is also given. In summary, the production mechanism remains a puzzle, but more precise studies will be possible with upcoming RHIC and LHC runs.