Igor I. Strakovsky

A formalism for radiative correction calculations in exclusive η electroproduction on the proton is presented, extending the treatment developed for the pion channel. The code is used in the radiative correction procedure with EtaMAID-2023 multipole amplitudes. The cross-section correction factor δ varies by up to ∼ 30 % across the resonance region W = 1.49 – 2.0 GeV at E beam = 6.535 GeV , with a local maximum near W ≃ 1.66 GeV driven by the S 11 ( 1535 ) and S 11 ( 1650 ) resonances. The beam-spin asymmetry is suppressed by 15%–25% at the same kinematics. Numerical results covering Q 2 = 0.3 – 4.0 GeV 2 and the full angular range are provided for kinematics relevant to CLAS12 experiments at Jefferson Lab.

New threshold measurements for γ p → p J / ψ → p ( μ + μ − ) by 007 and γ * p → p J / ψ → p ( e + e − ) by CLAS12 allow us to extend the previous phenomenological determination of the J / ψ -proton scattering length, α J / ψ p , using GlueX threshold data for γ p → p J / ψ → p ( e + e − ) . The agreement between all three J / ψ datasets shows no indication of systematic differences between methodologies. Furthermore, perturbative QCD predictions support the phenomenological determination of heavy vector meson-nucleon scattering lengths. The role of the nucleon form factor, F N , is discussed, and a possible correction to the phenomenological scattering length for heavy vector meson photoproduction is calculated using the pole form of F N . Upcoming J-PARC threshold measurements of the reaction π − p → n J / ψ → n ( l + l − ) will help to evaluate the possible role of heavy pentaquark, P c , states in low-energy J / ψ production and the effects caused by the nucleon form factors.

The quality of recent Short-Range Correlations/Color-Transparency (SRC/CT) Collaboration J / ψ photoproduction data off a He 4 target from Hall D at Jefferson Laboratory, combined with the feasibility of measuring the reaction close to the free-nucleon energy threshold, opens the door to using incoherent J / ψ photoproduction to explore a variety of interesting physics aspects. An example is the estimation of the J / ψ p scattering length | α J / ψ p | on the bound proton, obtained using the vector meson dominance model. The new experiment E12–25–002, conducted by the SRC/CT Collaboration, was recently approved in Hall D with the GlueX spectrometer, which will significantly enhance the number of reconstructed J / ψ mesons. Using a plane-wave theoretical model to generate quasidata, we estimated that the experiment could achieve a result of | α J / ψ p | = 3.08 ± 0.45 mfm . This value can be compared to an extraction based on the free proton data from the GlueX Collaboration, | α J / ψ p | = 3.08 ± 0.55 mfm . A comparison between the two would provide an opportunity to evaluate the effects of medium modification in light nuclei.

The experiment at Jefferson Lab has observed p p ¯ and, for the first time, Λ Λ ¯ and p Λ ¯ photoproduction from a proton target at photon energies up to 11.6 GeV. The angular distributions are forward peaked for all produced pairs, consistent with Regge-like t -channel exchange. Asymmetric wide-angle antibaryon distributions show the presence of additional processes. In a phenomenological model, we find consistency with a double- t -channel exchange process where antibaryons are created only at the middle vertex. The model matches all observed distributions with a small number of free parameters. In the hyperon channels, we observe a clear distinction between photoproduction of the Λ Λ ¯ and p Λ ¯ systems but general similarity to the p p ¯ system. We report both total cross sections and cross sections differential with respect to momentum transfer and the invariant masses of the created particle pairs. No narrow resonant structures were found in these reaction channels. The suppression of s s ¯ quark pairs relative to d d ¯ quark pairs is similar to what has been seen in other reactions.

Sixty years ago, the first excited state of a proton/neutron was “born”. During this time, we learned a lot about it, specifically, how unique this case is: a single resonance with two-pole positions on different Riemann sheets. Let me provide a brief history to remind the readers how development progressed. Sure, history is sometimes what never happened, described by those who were never there … Published by the Jagiellonian University 2026 authors

Sixty years ago, the first excited state of a proton/neutron was ``born.'' During this time, we learned a lot about it, specifically - how unique this case is: a single resonance with two pole positions on different Riemann sheets. Let me present a brief history to remind readers how development progressed. Sure, history is sometimes something that never happened, described by those who were never there...