Lech Szymanowski

We demonstrate that chiral-odd dimeson generalized distribution amplitudes (CO-GDAs)—nonperturbative objects encoding the transition of a quark-antiquark pair into two mesons—can be accessed in high-energy e − e + annihilation into two meson pairs, each with a relatively low invariant mass. While chiral-even GDAs contribute to the leading one-photon amplitude, the chiral-odd sector enters via two-photon exchange. We show that the interference between these amplitudes leads to specific effects which may be measurable at BES III or future tau-charm factories. This work opens a direct path to experimentally probing the long-missing chiral-odd sector of meson structure—specifically, the spin-orbit correlation in a spin-0 meson, in some contexts referred to as anomalous tensorial magnetic moment.

We investigate the hard exclusive photoproduction of photon-meson pairs at leading twist and leading order in perturbative QCD and focus on pseudoscalar mesons M ∈ { π ± , π 0 , η , η ′ } . Compact analytical expressions are obtained for the amplitudes involving quark generalized parton distributions, with the two-gluon components of the η and η ′ distribution amplitudes included. The numerical analysis is performed in the moderate- ξ region, where valence-quark generalized parton distributions are expected to be important. In this region, we find a strong impact of the pion-pole term in γ π ± production and a non-negligible effect for neutral mesons. We also observe a marked dependence of γ η ′ photoproduction on two-gluon contributions. This process offers enhanced sensitivity to the shape of the generalized parton distributions already at leading order, while the tested dependence on the meson distribution amplitude and the renormalization scale introduces further theoretical uncertainties, the latter emphasizing the need for next-to-leading-order corrections. Our results provide a concise analytical framework and a numerical baseline for future studies.

We introduce a new approach to modeling transition distribution amplitudes (TDAs) for the processes e p → e n π + and e p → e p π 0 . The modeling is flexible, constrained by sparsely available experimental data, and satisfies theoretical requirements, including reduction to nucleon distribution amplitudes in the appropriate limit. We study the sensitivity of observable predictions to various modeling assumptions. We discuss unpolarized cross sections, as well as the three nonvanishing polarization observables at leading twist, namely, the single transverse target spin asymmetry and the two double spin asymmetries that occur with a polarized lepton beam on either a longitudinally or transversely polarized target. The analysis is complemented by a simple Monte Carlo study to provide guidance for exploring exclusive processes in the so-called backward kinematics. Our work aims to highlight the importance of future measurements to better constrain TDAs and to support upcoming experimental proposals.

We exhibit an exclusive process, namely the photoproduction of a π 0 γ pair with large invariant mass, which violates collinear factorization. We explicitly demonstrate that this is due to the fact that there exists diagrams with gluon exchange in t channel, contributing at the leading power, for which are . This is caused by the pinching of the contour integration of the plus and minus light-cone components of the Glauber gluon momentum. We argue that this leads to the observed “end pointlike” divergence of the convolution integral at leading order and leading power when collinear factorization is naïvely assumed. Published by the American Physical Society 2025

We study the exclusive photoproduction of a π 0 γ pair with large invariant mass M γ π 2 , which is sensitive to the exchange of either two quarks or two gluons in the t channel. In this paper, we show that the process involving two-gluon exchanges does factorize in the Bjorken limit at the leading twist. This can be explicitly demonstrated by the fact that there exist diagrams, which contribute at the leading twist, for which are , due to the pinching of the contour integration of the plus and minus component of the Glauber gluon momentum. For the same reason, π 0 -nucleon scattering to two photons also suffers from the same issue. On the other hand, we stress that there are no issues with respect to collinear factorization for the quark channels. By considering an analysis of all potential reduced diagrams of leading pinch-singular surfaces, we argue that the quark channel is safe from Glauber pinches, and therefore, a collinear factorization in that case follows through without any problems. This means that processes where gluon exchanges are forbidden, such as the exclusive photoproduction of π ± γ and ρ 0 , ± γ , are unaffected by the factorization breaking effects we point out in this paper.

Exclusive diffractive meson production represents a golden channel for investigating gluonic saturation inside nucleons and nuclei. In this Letter, we settle a systematic framework to deal with beyond leading power corrections at small x , including the saturation regime, and obtain the γ * → M ( ρ , ϕ , ω ) impact factor with both incoming photon and outgoing meson carrying arbitrary polarizations. This is of particular interest since the saturation scale at modern colliders, although entering a perturbative regime, is not large enough to prevent higher-twist effects to be sizable.

We develop a framework combining the higher-twist formalism of exclusive processes in the s channel with the semiclassical effective description of small- x physics in the t channel. We apply it to transversely polarized light vector meson production, γ * p → V ( ρ , φ , ω ) p , which starts at the next-to-leading power and for which a purely collinear treatment leads to end-point singularities. The result is obtained in the most general kinematics, including both forward and nonforward cases by preserving the full impact parameter dependence in the nonperturbative correlators, in both momentum and coordinate space representations. A systematic expansion of the Wilson lines in terms of Reggeized gluon fields is performed in order to obtain the results in the weak-field BFKL approximation. These new results will allow for investigating the dilute-to-dense regime transition of quantum chromodynamics for a wide class of observables. Published by the American Physical Society 2025