Generalized distribution amplitudes (GDAs) offer valuable insights into the three-dimensional structure of hadrons, delineating the amplitudes associated with the transition from a quark–antiquark pair to a hadron pair. Currently, hadron GDAs can be probed in electron–positron collisions, with experimental feasibility demonstrated at facilities such as Belle and BESIII. In this study, we put forth the proposition that hadron GDAs can also be investigated in electron–hadron collisions at forthcoming Electron-Ion Colliders (EICs), specifically through the subprocess $$\gamma ^*\gamma \rightarrow h_1 h_2.$$ In this framework, a quasi-real photon, emitted by the ion, exhibits a photon flux proportional to the square of the ion’s electric charge. Consequently, we anticipate that the cross sections in EICs will be substantially larger than those in electron–positron collisions. We present numerical calculations pertaining to di- $$\pi ^0$$ production employing the equivalent photon approximation (EPA). Our findings suggest that, within the same kinematic region, electron-proton ($\textit{e}$-$\textit{p}$) collisions at the EIC could yield an event rate comparable to that of Belle II, while electron-gold ($\textit{e}$-Au) collisions are expected to generate an even greater number of events. This enhanced event rate facilitates a high-precision examination of di- $$\pi ^0$$ GDAs at the EIC.

China

Bing’ang Guo

Jing Han

Ya-Ping Xie

Xurong Chen

Di- $$\pi ^0$$ production and generalized distribution amplitudes at future electron-ion colliders

Exclusive photoproduction of heavy <math><mi>J</mi><mo>/</mo><mi>ψ</mi></math> meson is performed within the handbag factorization method. The general parton distributions (GPDs) for gluons, which are an essential ingredient here, are constructed using double distribution (DD) representation. The calculated cross section for few sets of gluon GPDs <math><msub><mi>H</mi><mi>g</mi></msub></math> describe fine <math><mi>J</mi><mo>/</mo><mi>ψ</mi></math> production experimental data from HERA to LHC energies. Based on this approach we estimate <math><mi>J</mi><mo>/</mo><mi>ψ</mi></math> spin asymmetries determined by <math><msub><mi>E</mi><mi>g</mi></msub></math> and <math><msub><mover><mi>H</mi><mo>˜</mo></mover><mi>g</mi></msub></math>. The GPDs factorization approach can be employed to analyze heavy vector meson production in electron-proton scattering. Investigations of this reactions at future electron-ion colliders in USA and China can give essential information on gluon GPDs.

JINR

S. V. Goloskokov

Study of gluon GPDs in exclusive <math><mrow><mi>J</mi><mo>/</mo><mi>ψ</mi></mrow></math> production in electron-proton scattering

Information about the three-dimensional description of the quark and gluon content of hadrons, described by the generalized parton distributions (GPDs), can be probed by exclusive processes in electron–proton (ep) collisions. In this letter, we investigate the timelike Compton scattering (TCS) in ep collisions at the future electron-ion collider (EIC). Such process is characterized by the exclusive dilepton production through the subprocess  $$\gamma p \rightarrow \gamma ^*p \rightarrow l^+ l^- p$$  <math> <mrow> <mi>γ</mi> <mi>p</mi> <mo>→</mo> <msup> <mi>γ</mi> <mo>∗</mo> </msup> <mi>p</mi> <mo>→</mo> <msup> <mi>l</mi> <mo>+</mo> </msup> <msup> <mi>l</mi> <mo>-</mo> </msup> <mi>p</mi> </mrow> </math>  , with the real photon in the initial state being emitted by the incoming electron. Assuming a given model for the GPDs, the TCS differential cross-section is estimated, as well the contribution associated to the interference between the TCS and Bethe–Heitler (BH) amplitudes. Predictions for the TCS, BH and interference contributions are presented considering the kinematical range expected to be covered by the EIC detectors. Moreover, the polarized photon asymmetry is also studied. Our results indicated that a future experimental analysis, considering photon circular polarizations, can be useful to probe the interference contribution and constrain the description of the GPDs for the proton.

Brazil

V. Gonçalves

Investigating the timelike Compton scattering in photon–proton interactions at the EIC

We investigate the exclusive  $$J/\psi $$  <math> <mrow> <mi>J</mi> <mo>/</mo> <mi>ψ</mi> </mrow> </math>   production at the future Electron-ion collider in China by utilizing the eSTARlight event generator. We model the cross-section and kinematics by fitting to the world data of  $$J/\psi $$  <math> <mrow> <mi>J</mi> <mo>/</mo> <mi>ψ</mi> </mrow> </math>   photoproduction. Projected statistical uncertainties on  $$J/\psi $$  <math> <mrow> <mi>J</mi> <mo>/</mo> <mi>ψ</mi> </mrow> </math>   production are based on the design of a central detector, which consists of a tracker and vertex subsystem. The precision of the pseudo-data allows us to probe the near-threshold mechanism, e.g. the re-scattering effect. The significance of the forward amplitudes is discussed as well. The design and optimization of the detector enhance the potential for exploring the near-threshold region and the realm of high four-momentum transfer squared, which is of particular interest on several physics topics.

Xue Wang

Xu Cao

Aiqiang Guo

Li Gong

Xiao-Shen Kang

Yu-Tie Liang

Jia-Jun Wu

Exclusive charmonium production at the electron-ion collider in China

The exclusive dilepton photoproduction in pp and pAu collisions at RHIC is investigated considering the contribution of the Bethe–Heitler (BH) and the timelike Compton scattering (TCS) processes. The TCS amplitude is estimated at next-to-leading-order of the strong running coupling constant  $$\alpha _s$$  <math> <msub> <mi>α</mi> <mi>s</mi> </msub> </math>   and assuming the Goloskokov–Kroll model for the quark and gluon generalized parton distributions. The TCS differential cross section is estimated as well the contribution associated to the interference between the TCS and BH amplitudes. Predictions for the TCS, BH and interference contributions are presented considering the kinematical range covered by the RHIC detectors.

Germany

Exclusive dilepton production in photon-induced interactions at RHIC

In this work, the  $$\gamma p\rightarrow \phi (1680) p$$  <math> <mrow> <mi>γ</mi> <mi>p</mi> <mo>→</mo> <mi>ϕ</mi> <mo>(</mo> <mn>1680</mn> <mo>)</mo> <mi>p</mi> </mrow> </math>   reaction is systematically studied based on the two gluon exchange model and the pomeron model. The calculation results show that the total cross section of  $$\phi (1680)$$  <math> <mrow> <mi>ϕ</mi> <mo>(</mo> <mn>1680</mn> <mo>)</mo> </mrow> </math>   photoproduction can reach more than 80 nb near the center-of-mass energy  $$W=4$$  <math> <mrow> <mi>W</mi> <mo>=</mo> <mn>4</mn> </mrow> </math>   GeV, so it is suggested that JLab and other experiments carry out relevant measurements. Moreover, under the framework of the vector meson dominance assumption, the scattering length of  $$\phi (1680)$$  <math> <mrow> <mi>ϕ</mi> <mo>(</mo> <mn>1680</mn> <mo>)</mo> </mrow> </math>   and proton interaction is estimated as  $$0.07\pm 0.01$$  <math> <mrow> <mn>0.07</mn> <mo>±</mo> <mn>0.01</mn> </mrow> </math>   fm from the cross sections predicted by the theoretical models, which is on the same order of magnitude as the  $$\phi (1020)$$  <math> <mrow> <mi>ϕ</mi> <mo>(</mo> <mn>1020</mn> <mo>)</mo> </mrow> </math>  -p scattering length. In addition, the total cross section, number of events and rapidity distribution of  $$\phi (1680)$$  <math> <mrow> <mi>ϕ</mi> <mo>(</mo> <mn>1680</mn> <mo>)</mo> </mrow> </math>   production are predicted at Electron-Ion colliders (EICs) and ultra-peripheral collisions (UPCs) by using eSTARlight and STARlight code. The study of  $$\phi (1680)$$  <math> <mrow> <mi>ϕ</mi> <mo>(</mo> <mn>1680</mn> <mo>)</mo> </mrow> </math>   and relevant properties is of enlightening significance in better understanding the strangeonium family spectrum, and provides necessary data support for the future experiment of EICs and UPCs.

Chen Dong

Yuhang Hou

Xiao-Yun Wang

Li Zhang

Production of strangeonium  $$\phi (1680)$$  <math> <mrow> <mi>ϕ</mi> <mo>(</mo> <mn>1680</mn> <mo>)</mo> </mrow> </math>   state in electromagnetic probe-driven scatterings and hadron–hadron ultraperipheral collisions

The study of exclusive processes in photon-induced interactions allows us to probe the generalized parton distributions (GPDs), which provide information about the 3-dimensional description of the quark and gluon content of hadrons. In this paper we investigate the timelike Compton scattering (TCS) in ultraperipheral pPb collisions at the LHC. Such process is characterized by the exclusive dilepton production through the subprocess <math><mi>γ</mi><mi>p</mi><mo>→</mo><msup><mrow><mi>γ</mi></mrow><mrow><mo>⁎</mo></mrow></msup><mi>p</mi><mo>→</mo><msup><mrow><mi>l</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>l</mi></mrow><mrow><mo>−</mo></mrow></msup><mi>p</mi></math>, with the real photon in the initial state being emitted by the nucleus. Assuming a given model for the GPDs, the TCS differential cross section is estimated as well the contribution associated to the interference between the TCS and Bethe–Heitler (BH) amplitudes. Predictions for the TCS, BH and interference contributions are presented considering the kinematical range covered by the LHC detectors.

V.P. Gonçalves

Timelike Compton scattering in ultraperipheral Collisions

Exclusive   $ \pi^0 $   electroproduction is analyzed within the handbag approach based on Generalized Parton Distribution (GPDs) factorization. We consider the leading-twist contribution together with the transversity effects. It is shown that the transversity GPDs   $ \pi^0 $   and   $ \pi^0 $   are essential in the description of the   $ \pi^0 $   cross section. Predictions for the future Electron-Ion Collider of China (EicC) energy range are provided. It is found that transversity dominance   $ \pi^0 $  , observed at low energies, is valid up to the EicC energy range.

Russia

S.V. Goloskokov

Exclusive  production at EIC of China within handbag approach  

Exclusive vector meson production is an excellent probe for describing the structure of protons. In this study, based on the dipole model, the differential cross sections, total cross sections, and ratios of the longitudinal to transverse cross section of the   $ J/\psi $   and   $ J/\psi $   productions are calculated with the analytical solution of the Balitsky-Kovchegov (BK) equation. In addition, we consider the influences of two meson wave function models on the results. Our predictions, which are slightly sensitive to meson wave functions, agree with the experimental data. The analytical solution of the BK equation is reliable for description of exclusive vector meson productions in a certain range of   $ J/\psi $  .

Xiao-Peng Wang

Wei Kou

Gang Xie

Xu-Rong Chen

Exclusive vector meson production with the analytical solution of Balitsky-Kovchegov equation  

The exclusive production of vector mesons and photons in <math><mi>e</mi><mi>p</mi></math> collisions is investigated considering three phenomenological saturation models based on distinct assumptions for the treatment of the dipole-hadron scattering amplitude. The latest high precision HERA data for the reduced and vector meson cross sections are used to update the saturation model proposed by Marquet Peschanski, and Soyez, which predicts that the saturation scale is dependent of the squared momentum transfer <math><mi>t</mi></math>. The Marquet-Peschanski-Soyez predictions for the photon virtuality, energy and <math><mi>t</mi></math> dependencies of the exclusive <math><mi>ρ</mi></math>, <math><mi>J</mi><mo>/</mo><mi>Ψ</mi></math>, and deeply virtual Compton scattering cross sections are presented and a detailed comparison with the results derived using the impact parameter saturation models is performed. Our results indicate that a future experimental analysis of the <math><mi>t</mi></math> distribution <math><mi>d</mi><mi>σ</mi><mo>/</mo><mi>d</mi><mi>t</mi></math> for exclusive processes in the kinematical range that will covered by the EIC and LHeC, considering the distinct photon polarizations and large values of <math><mi>t</mi></math>, will be able to discriminate between the distinct approaches for the QCD dynamics at high energies.