Gluon TMDs in Quarkonium Production

Abstract

Quarkonium production offers good possibilities to study gluon TMDs. In this proceedings contribution this topic is explored for the linearly polarized gluons inside unpolarized hadrons and unpolarized gluons inside transversely polarized hadrons. It is argued that χ b0/2 and η b production at LHC are best to study the effects of linearly polarized gluons in hadronic collisions, by means of angular independent ratios of ratios of cross sections. This can be directly compared to cos 2φ asymmetries in heavy quark pair and dijet production in DIS at a future high-energy Electron-Ion Collider (EIC), which probe the same TMDs. In the small-x limit this corresponds to the Weizsäcker-Williams (WW) gluon distributions, which should show a change in behavior for transverse momenta around the saturation scale. Together with investigations of the dipole (DP) gluon distributions, this can provide valuable information about the polarization of the Color Glass Condensate if sufficiently small x-values are reached. Quarkonia can also be useful in the study of single transverse spin asymmetries. For transversely polarized hadrons the gluon distribution can be asymmetric, which is referred to as the Sivers effect. It leads to single spin asymmetries in for instance J/ψ (pair) production at AFTER@LHC, which probe the WW or f -type gluon Sivers TMD. It allows for a test of a sign-change relation w.r.t. the gluon Sivers TMD probed at an EIC in open heavy quark pair production. Single spin asymmetries in backward inclusive C-odd quarkonium production, such as J/ψ production, may offer probes of the DP or d-type gluon Sivers TMD at small x-values in the polarized proton, which in that limit corresponds to a correlator of a single Wilson loop, describing the spin-dependent odderon.

Proper TMD factorization for quarkonia production: pp → ηc, b as a study case

“…All and all, quarkonium production seems the most promising way to probe gluon TMDs. Indeed, there has been a growing interest lately, with numerous proposals based on treelevel ansatzs for TMD factorization for quarkonium production [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] and even several next-to-leading-order (NLO) calculations [41][42][43][44].…”

Feasibility Studies for Single Transverse-Spin Asymmetry Measurements at a Fixed-Target Experiment Using the LHC Proton and Lead Beams (AFTER@LHC)

“…These processes are sensitive to the gluon content of the colliding hadrons which can then be measured with an outstanding precision. Associated-production channels [13,19,23,40,103,109,110] are fundamental tools to access the Gluon Sivers effect, and also probing the gluon TMD sector and their evolution [84,111]. A few different processes are potentially interesting in this context, for instance J/ψ − J/ψ , J/ψ − γ, γ − γ, Υ − γ.…”

Subfemtometer scale color charge correlations in the proton

“…[9]. The WW gluon TMDs appear in a variety of processes such as production of a dijet or heavy quark pair in hadronic collisions [10] or DIS at moderate [11] or high energies [12][13][14]; photoproduction of three jets [15]; photon pair [16], quarkonium [17], quarkonium pair [18], or quarkonium plus dilepton [19] production in hadronic collisions. These gluon distributions also determine the fluctuations of the divergence of the Chern-Simons current at the initial time of a relativistic heavy-ion collision [20].…”