Generalized parton distributions (GPDs) are one of the key tools of modern particle physics. They allow us to describe how matter is structured at the deepest, subatomic level. Until now, most of our knowledge about GPDs has come from studies of phenomena such as Compton scattering and meson production. A new theoretical paper, co-authored by a scientist from the National Center for Nuclear Research, analyzes another, less well-known phenomenon—photon-meson pair photoproduction—which may provide new information about the interactions of the smallest components of matter.

GPD-based analyses allow us to study the internal structure of nucleons, including how their components carry momentum and how it is distributed among them. However, obtaining a complete picture requires combining data from different physical processes. Until now, deeply virtual Compton scattering (DVCS) and deeply virtual meson production (DVMP) have played a dominant role, and have been well described both theoretically and experimentally. This research can be complemented by photon-meson pair photoproduction, a process that is currently of interest to many research teams around the world.

The results of new theoretical research on this topic were recently published in the prestigious journal Physical Review D. One of the authors of the paper is Prof. Lech Szymanowski from the Theoretical Physics Division of the National Center for Nuclear Research (NCBJ). As he explains, the research concerns hard photoproduction of photon-pseudoscalar meson pairs, such as pion, eta, or eta', within the framework of quantum chromodynamics (QCD). A new element of the analysis was the inclusion of the contribution of gluons in the structure of eta and eta' mesons, which is particularly important in the case of photon-eta' pair production.

The team derived formulas describing in detail the individual stages of the photoproduction process, which made it possible, among other things, to estimate the cross sections for photon-pion pair production and to investigate the influence of the internal structure of mesons on the entire process. The paper also discusses the possibilities for future verification of these results in experiments conducted at research centers such as Jefferson Lab, the COMPASS experiment at CERN, the Electron-Ion Collider under construction at Brookhaven National Laboratory, and selected collisions at the Large Hadron Collider (LHC). Of particular importance here are analyses of photon-pion pairs, corresponding to the range of parameters studied at JLab and COMPASS.

As Prof. Lech Szymanowski emphasizes, the main motivation for the research was that photon-meson pair photoproduction processes are extremely sensitive to the shape of parton distributions in nucleons. The results obtained provide a solid theoretical and numerical basis for further research that may contribute to a better understanding of the structure of matter.

The results of the study are available in the publication: N. Crnkowić, G. Duplančić, S. Nabeebaccus, K. Passek-K., B. Pire, L. Szymanowski, S. Wallon, Hard exclusive photoproduction of photon-meson pairs: Pseudoscalar channels π, η and η’, Phys. Rev. D 113, 034001 (2026),  https://doi.org/10.1103/g9m4-9r9c