Zhun Lu

The sin ( 2 ϕ ) azimuthal angular correlation between the transverse momenta of the scattered electron and the recoil proton in the e p → e ′ p ′ π 0 process provides a probe for quark orbital angular momentum. We numerically calculate this asymmetry for the future Electron-Ion Collider (EIC) in the U.S. and China (EicC) kinematics using a light-front quark-scalar-diquark model, in which the light-front wave functions are derived from the soft-wall AdS/QCD framework. We also investigate the properties of the valence quark angular momentum expressed in terms of helicity-independent and helicity-dependent parton distributions. This study aims to establish theoretical constraints on the asymmetry sensitive to the quark orbital angular momentum prior to its first experimental measurement.

We study the near-threshold exclusive photoproduction of heavy vector mesons (quarkonia J / ψ and ϒ ) off the proton within the framework of generalized parton distribution (GPD) factorization. The gluon GPDs are computed using a spectator model in which the proton emits a gluon and the remaining constituents are treated as a single spectator particle. Model parameters are determined by fitting the gluon unpolarized and helicity collinear parton distribution functions from global analyses. We compare our results with the latest near-threshold J / ψ production data from the GlueX and J / ψ − 007 experiments at Jefferson Laboratory, finding good agreement for both differential and total cross sections. Predictions are also provided for ϒ photoproduction, which can be tested at future electron-ion colliders.

We present the perturbative calculation of the leading-twist gluon transverse momentum dependent distribution functions (TMDPDFs) at one-loop level within large-momentum effective theory. By employing the basis of circular polarizations, we generalize the gluon-gluon correlator to a matrix Φ Λ Λ ′ i j ( x , k T 2 ; S ) in the gluon ⊗ hadron spin space. Incorporating the transverse momentum of external states, we derive the leading-twist gluon TMDPDFs and quasi-TMDPDFs at one-loop level. Our calculations reveal nonzero distributions for the gluon TMDPDFs f 1 T ⊥ g ( x , k T 2 ) , h 1 ⊥ g ( x , k T 2 ) , h 1 T g ( x , k T 2 ) , and h 1 T ⊥ g ( x , k T 2 ) . These findings enable future lattice quantum chromodynamics simulations to extract these gluon TMDPDFs, which will facilitate a complete determination of nucleon tomography.

We present a model calculation of the T-odd transverse momentum dependent (TMD) gluon fragmentation functions for a spin-1/2 hadron. Our model is based on the postulation that a time-like off-shell gluon can fragment into a hadron and a single spectator particle, which is considered to be on-shell. We consider the effect of the gluon exchange to calculate all necessary one-loop diagrams for the gluon-gluon correlation functions. Two out of four one-loop diagrams give sizeable contributions to the fragmentation functions. We obtain analytical expressions for the four T-odd TMD fragmentation functions of the gluon. We also provide numerical results on the $z$-dependence and $k_T$-dependence of the fragmentation functions.

We study the longitudinal beam single spin asymmetry A L U sin ϕ R of dihadron production in the semi-inclusive deep inelastic scattering process with a polarized electron beam scattering off an unpolarized proton target. The asymmetry arises from the convolutions of the twist-3 parton distribution function (PDF) e ( x ) and the twist-2 dihadron fragmentation function (DiFF) H 1 ∢ as well as the twist-2 PDF f 1 ( x ) and the twist-3 DiFF G ˜ ∢ . Using the spectator model calculations for the twist-3 PDF e ( x ) and the DiFFs D 1 , H 1 ∢ , and G ˜ ∢ , we estimate the beam single spin asymmetry (BSA) A L U sin ϕ R at the kinematical configuration of CLAS and CLAS12. We find good agreement with z - and M h -dependencies data, though x - and Q 2 -dependencies show discrepancies. We include the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution of e ( x ) and provide predictions for COMPASS, EIC, and EicC. The negligible contribution from f 1 ( x ) ⊗ G ˜ ∢ supports the Wandzura-Wilczek approximation, while our results highlight the importance of QCD evolution in twist-3 PDF analyses.