Probing the Moderate-to-Large-$x$ Gluon Helicity with $J/ψ$ Production at $\sqrt{s}=\mathrm{GeV}$
Year: 2026
Type: preprint
Abstract: We investigate the longitudinal double-spin asymmetry, $A_{LL}$, for color-singlet $J/ψ$ production in longitudinally polarized $pp$ collisions at $\sqrt{s}\simeq27~\mathrm{GeV}$, relevant to the Spin Physics Detector at NICA. At these energies, gluon fusion probes moderate-to-large gluon momentum fractions, particularly at forward rapidity. We consider the leading-order color-singlet subprocess $gg\to J/ψ+g$ within nonrelativistic QCD and calculate polarized and unpolarized cross sections using analytic partonic hard-scattering coefficients. The color-singlet long-distance matrix element cancels in $A_{LL}$ at this order. We study the transverse-momentum and rapidity dependence of the asymmetry using different polarized and unpolarized PDF combinations, including LO and NLO PDF inputs, while keeping the hard-scattering calculation at LO. Factorization- and renormalization-scale variations are examined, and PDF and scale uncertainties are combined. The asymmetry is positive and increases with transverse momentum in the kinematically stable region. Its magnitude and uncertainty become more sensitive to the polarized gluon distribution at moderate and large $x$, with substantial differences among PDF combinations at moderate and large transverse momentum. NLO PDF uncertainties are generally smaller than LO uncertainties in central regions, but this reduction is not uniform; PDF and scale uncertainties can both become sizable at high transverse momentum and forward rapidity. Differential measurements of $A_{LL}$ at SPD could therefore provide complementary constraints on gluon helicity in a relatively unexplored kinematic region. A complete NLO hard-scattering calculation is needed for precision predictions and a quantitative assessment of higher-order effects.
Source: arXiv (Cornell University)
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7163020631