Search

We extend our studies of a new class of $2 \to 3$ exclusive processes using the collinear factorisation framework by considering the exclusive photoproduction of a $\gamma\,\rho$ pair, in the kinematics where the pair has a large invariant mass, {and the outgoing $ \rho $-meson has a sufficiently large transverse momentum to not resonate with the nucleon}. We cover the whole kinematical range from medium energies in fixed target experiments to very large energies of colliders, by considering the experimental conditions of JLab~12-GeV, COMPASS, future EIC and LHC (in ultra-peripheral collisions) cases. One of the main interest in studying the present process is that it provides access to both chiral-even and chiral-odd GPDs, depending on the polarisation of the outgoing $\rho$-meson, both at leading twist. Our analysis covers both neutral and charged $ \rho $-mesons. We find that the order of magnitude of the obtained cross sections are sufficiently large for a dedicated experimental analysis to be performed, especially at JLab, for both longitudinally and transversely polarised $\rho$. Furthermore, we compute the linear photon beam polarisation asymmetry {which is} sizeable for a longitudinally polarised meson. These predictions are obtained for both asymptotic distribution amplitude (DA) and the \textit{holographic} DA., Comment: 84 pages, 56 figures. arXiv admin note: substantial text overlap with arXiv:2212.00655

We study the exclusive photoproduction of a photon-meson pair with a large invariant mass, working in the QCD factorisation framework. Explicitly, we consider a $ \rho $-meson or a charged $ \pi $ in the final state. This process gives access to chiral-even GPDs as well as chiral-odd GPDs. We focus here on the chiral-even sector. The computation is performed at leading order and leading twist. We discuss the prospects of measuring them in various experiments such as JLab 12-GeV, COMPASS, future EIC and LHC (in ultraperipheral collisions). In particular, the high centre of mass energies available at collider experiments can be used to probe GPDs at small skewness $ \xi $. We also compute the polarisation asymmetries with respect to the incoming photon. The results for an alternative distribution amplitude (`holographic' form) are also compared with the predictions obtained with an asymptotic distribution amplitude., Comment: 6 pages, 2 figures, Presented by S. Nabeebaccus at `Diffraction and Low-x 2022', Corigliano Calabro (Italy), September 24-30, 2022. arXiv admin note: substantial text overlap with arXiv:2209.05380

We compute the exclusive photoproduction of a $\gamma\,\pi^\pm$ pair using the collinear factorisation framework, in the kinematic regime where the pair has a large invariant mass. This exclusive channel presents a new avenue for the investigation of GPDs. It is particularly interesting as the high centre of mass energies available at future experiments will allow the study of GPDs at small skewness $ \xi $. We compute the scattering amplitude of the process, at leading twist and leading order in $\alpha_s$, which is used to estimate its cross-section and linear polarisation asymmetries with respect to the incoming photon, for JLab~12-GeV, COMPASS, future EIC and LHC (in ultra-peripheral collisions) kinematics. We find that the order of magnitude of estimates are sufficiently large for a dedicated experimental analysis to be performed, especially at JLab. We also compare the results from an asymptotic distribution amplitude (DA) to those using a recently proposed \textit{holographic} DA., Comment: 50 pages, 32 figures

We investigate exclusive electroproduction of $\pi^-\Delta^{++}$ within the handbag approach in which the helicity amplitudes factorize into generalized parton distributions (GPDs) and hard partonic subprocesses. We define the $p-\Delta$ transversity GPDs while the helicity non-flip GPDs are taken from the literature. For the numerical estimates of observables we utilize large-$N_C$ results in order to relate a few of the $p-\Delta$ GPDs to the proton-proton ones and neglect all other GPDs. In the calculation of the twist-2 and twist-3 subprocess amplitudes we take into account quark transverse momenta in combination with Sudakov suppressions. The partial cross sections for $\gamma^*p\to \pi^-\Delta^{++}$ are predicted in the large-$N_C$ limit., Comment: 31 pages, 6 figures

We consider the exclusive photo-production of a photon-meson pair with a large invariant mass, working in the QCD factorisation framework. Explicitly, we consider a $ \rho $-meson or a charged $ \pi $ in the final state. This process gives access both to chiral-even GPDs and chiral-odd GPDs, which are not well-known experimentally, especially the latter ones. The computation is performed at leading order and leading twist. We discuss the prospects of measuring them in experiments, focusing on the kinematics at the JLab 12-GeV experiment, and pPb ultra-peripheral collisions at LHC. In particular, the latter gives access to the small $ \xi $ regime of GPDs., Comment: 6 pages, 3 figures. Presented by S. Nabeebaccus at DIS2022: XXIX International Workshop on Deep-Inelastic Scattering and Related Subjects, Santiago de Compostela, Spain, May 2-6 2022

We investigate wide-angle photoproduction of the $\eta'$ meson within the handbag approach to twist-3 accuracy. It turns out that, due to the gluon content of the $\eta'$, this process is dominated by twist-2 in contrast with pion and $\eta$ photoproduction. Using the presently available information on the twist-2 and twist-3 distribution amplitudes and on the $\eta - \eta'$ mixing, we provide prediction for the $\eta'$ cross section and helicity correlations. It is argued that $\eta'$ photoproduction is well suited to improve our knowledge of the two-gluon distribution amplitude., Comment: 18 pages, 5 figures

We investigate wide-angle photo- and electroproduction of pions within the handbag mechanism in which the $\gamma^{(*)} N\to \pi N'$ amplitudes factorize into hard partonic subprocess amplitudes, $\gamma^{(*)} q\to \pi q'$, and form factors representing $1/x$-moments of generalized parton distributions (GPDs). The subprocess is calculated to twist-3 accuracy taking into account the 2- and 3-body Fock components of the pion. Both components are required for achieving gauge invariance in QED and QCD. The twist-2 and twist-3 distribution amplitudes (DAs) of the pion as well as the form factors are taken from our study of $\pi^0$ photoproduction. Extensive results on photoproduction of charged pions are presented and compared to experiment. Predictions on electroproduction of pions as well as on spin effects are also given. As a byproduct of our analysis we also obtain the complete twist-3 subprocess amplitudes contributing to deeply-virtual electroproduction of pions., Comment: 50 pages, 11 figures; version as published in PRD up to corrected typo in Eq. (52)

At leading-twist accuracy the form factors for the transitions from a virtual photon to the $\eta$ or $\eta'$ can be expanded into a power series of the variable $\omega$, being related to the difference of two photon virtualities. The series possess the remarkable feature that only the Gegenbauer coefficients of the meson distribution amplitudes of order $l\leq m$ contribute to the term $\sim \omega^m$. Thus, for $\omega\to 0$ only the asymptotic meson distribution amplitude contributes, allowing for a test of the mixing of the $\eta$ and $\eta'$ decay constants. Employing the Gegenbauer coefficients determined in analysis of the form factors in the real photon limit, we present predictions for the $\gamma^*\eta$ and $\gamma^*\eta'$ form factors and compare them to the BaBar data., Comment: 14 pages, 2 figures

Based on the collinear factorization approach, we present a comprehensive perturbative next-to-leading (NLO) analysis of deeply virtual meson production (DVMP). Our representation in conformal Mellin space can serve as basis for a global fitting procedure to access generalized parton distributions from experimental measurements of DVMP and deeply virtual Compton scattering (DVCS). We introduce a rather general formalism for the evaluation of conformal moments that can be developed further beyond the considered order. We also confirm previous diagrammatical findings in the pure singlet quark channel. Finally, we use the analytic properties of the hard scattering amplitudes to estimate qualitatively the size of radiative corrections and illustrate these considerations with some numerical examples. The results suggest that global NLO GPD fits, including both DVMP and DVCS data, could be more stable than often feared., Comment: 125 pages, 7 figures, 8 tables

The eta-gamma and eta'-gamma transition form factors are analyzed to leading-twist accuracy and next-to-leading order (NLO) of perturbative QCD. Using an eta-eta' mixing scheme and all currently available experimental data the lowest Gegenbauer coefficients of the distribution amplitudes for the valence octet and singlet quark-antiquark and the gluon-gluon Fock components are extracted. Predictions for the g*g*eta' vertex function are presented. We also comment on the new BELLE results for the pi-gamma transition form factor., Comment: 28 pages, 13 figures, the paper is shortened, the references are updated and the title changed

We discuss modeling of generalized parton distributions (GPDs), their access from present experiments, and the phenomenological potential of an electron-ion collider. In particular, we present a comparison of phenomenological models of GPD H, extracted from hard exclusive meson and photon production. Specific emphasis is given to the utilization of evolution effects at moderate x_Bj in a future high-luminosity experiment within a larger Q^2 lever arm., Comment: LaTeX, 23 pages, 11 figures, extended version of contribution to the proceedings of the INT 10-3 program "Gluons and the quark sea at high energy: distribution, polarization, tomography", INT, Seattle, Sep.13-Nov.19, 2010

We describe small-x_Bj deeply virtual Compton scattering measurements at HERA in terms of generalized parton distributions at leading order of perturbation series., Comment: LaTeX, 8 pages, 2 figures, talks by K.K. at DIS 2008, London and by D.M. at Diffraction Workshop 2008 in La Londe-les-Maures, more results included and text essentially modified

To leading order approximation, the physical content of generalized parton distributions (GPDs) that is accessible in deep virtual electroproduction of photons or mesons is contained in their value on the cross-over trajectory. This trajectory separates the t-channel and s-channel dominated GPD regions. The underlying Lorentz covariance implies correspondence between these two regions through their relation to GPDs on the cross-over trajectory. This point of view leads to a family of GPD sum rules which are a quark analogue of finite energy sum rules and it guides us to a new phenomenological GPD concept. As an example, we discuss the constraints from the JLab/Hall A data on the dominant u-quark GPD H. The question arises whether GPDs are governed by some kind of holographic principle., Comment: 45 pages, 4 figures, Sect. 2 reorganized for clarity. Typos in Eq. (20) corrected. 4 new refs. Matches published version

The short review of the higher order corrections to the hard exclusive processes is given. Different approaches are discussed and the importance of higher-order calculations is stressed., Comment: 17 pages; talk given at the 9th Adriatic Meeting, Dubrovnik 2003

We critically reexamine the formalism for treating the leading-twist contributions from the two-gluon Fock components occurring in hard processes that involve eta and eta' mesons and establish a consistent set of conventions for the definition of the gluon distribution amplitudes, the anomalous dimensions as well as the projector of a two-gluon state onto an eta or eta' state. We calculate the eta, eta'--photon transition form factor to order alpha_s and show the cancellation of the collinear and UV singularities explicitly. An estimate of the lowest Gegenbauer coefficients of the gluon and quark distribution amplitudes is obtained from a fit to the eta, eta'--photon transition form factor data. In order to elucidate the role of the two-gluon Fock component further, we analyze electroproduction of eta, eta' mesons and the g*g* eta (eta') vertex., Comment: 20 pages(revtex4 two-column format), 14 figures; a typo corrected; version published in Phys. Rev. D