Gluon Sivers function from forward exclusive ${\chi }_{c1}$ photoproduction on unpolarized protons

Journal Article · Tue Mar 11 00:00:00 EDT 2025 · Physical Review. D.

DOI:https://doi.org/10.1103/PhysRevD.111.054008· OSTI ID:2530437

Benić, Sanjin; Dumitru, Adrian; Motyka, Leszek; Stebel, Tomasz

Exclusive production of a ${\chi }_{c1}$ axial vector quarkonia in photon-proton scattering at high energies requires a $C$ -odd $t$ -channel exchange. In the limit of vanishing momentum transfer this occurs either via the exchange of a photon, the Primakoff process, where the spin of the proton does not change. For axial-vector meson production, as a consequence of the Landau-Yang theorem, the Primakoff cross section is finite as $t\to 0$ . Alternatively, a $C$ -odd spin dependent Odderon can be exchanged, which involves a spin flip of the proton. The resulting cross section is related to the square of the collinear trigluon correlator or the $k_{\perp }$ -moment of the gluon Sivers function. Using two models for the gluon Sivers function from the literature we compute the ratio of Sivers to Primakoff cross sections and the angular coefficient ${\lambda }_{\theta }$ governing the angular distribution of the ${\chi }_{c1}\to J/\psi +\gamma$ decay as functions of $x$ . We point out that these observables constrain the magnitude of the gluon Sivers function at small $x$ which could be accessed in electron-proton scattering and ultraperipheral proton-proton and nucleus-proton collisions.

Published by the American Physical Society 2025

• Sponsoring Organization: USDOE Office of Science (SC), Nuclear Physics (NP)
• Grant/Contract Number: SC0002307
• OSTI ID: 2530437
• Alternate ID(s): OSTI ID: 2530277
• Journal Information: Physical Review. D., Journal Name: Physical Review. D. Journal Issue: 5 Vol. 111; ISSN PRVDAQ; ISSN 2470-0010
• Publisher: American Physical Society
• Country of Publication: United States
• Language: English