Linearly polarized gluon effects in unpolarized collisions

TL;DR

The paper investigates linearly polarized gluons inside unpolarized hadrons and their impact on transverse momentum distributions in Higgs and (pseudo-)scalar quarkonium production. Using TMD factorization and evolution, it derives the observable ratio ${\mathcal R}(Q_T)$ that quantifies the relative contribution of $h_1^{\perp g}$ to the unpolarized gluon distribution, exploring both perturbative tails and nonperturbative Sudakov factors. Results indicate Higgs-induced effects are at the few-percent level, while quarkonium could exhibit larger effects but with substantial theoretical uncertainties, especially from the very small-$b$ region. The work also discusses potential measurements at the EIC via heavy-quark pair production and dijet production to access $h_1^{\perp g}$ and test small-$x$/saturation dynamics, and highlights the differential sensitivity of WW and DP gluon distributions in different processes.

Abstract

Linear polarization of gluons inside unpolarized hadrons affects the transverse momentum distribution of produced spin-0 particles, such as of the Higgs or (pseudo-)scalar quarkonium states at LHC. Despite the currently unknown amount of linear gluon polarization, a range of predictions can be obtained, using TMD evolution, which indicates that their effect is on the few percent level in Higgs production, but can be much larger in quarkonium production. Together with asymmetries in open charm or bottom production in electro-production at an Electron-Ion Collider, the size and sign of the linear gluon polarization could be extracted experimentally. These processes also allow to test the behavior expected at small $x$ in and outside the saturation region.

Figures (5)

• Figure 1: nameref-RQTatHiggs fith LAB: RQTatHiggs Range of predictions for ${\@fontswitch\mathcal{R}}(Q_T)$ of Boer:2014tka (left) and of Echevarria:2015uaa (right).
• Figure 2: nameref-RQTquarkonia fith LAB: RQTquarkonia Range of predictions of ${\@fontswitch\mathcal{R}}(Q_T)$ for $\chi_{b0}$ (left) and $\chi_{c0}$ (middle) production from Boer:2014tka and for $\eta_b$ production (right) from Echevarria:2015uaa.
• Figure 3: nameref-Ints126 fith LAB: Ints126 Integrands $b \, \widetilde{W}(b)$ for the denominator and numerator of ${\@fontswitch\mathcal{R}}$ at $Q=126$ GeV. The vertical line indicates where $b=1/Q$.
• Figure 4: nameref-Ints9p9 fith LAB: Ints9p9 Same as Fig. 3 but for $Q=9.9$ GeV.
• Figure 5: nameref-Ints3p4 fith LAB: Ints3p4 Same as Fig. 3 but for $Q=3.4$ GeV.