Andrei Afanasev

This feature issue covers recent developments in the field of optics and photonics that involve interactions of twisted light with matter at a fundamental level, as defined by the laws of quantum physics. In this introduction, we provide a brief description of the published articles. Twisted light (TL) fields and their quanta—the twisted photons—possess unique properties due to their controllable spin and angular momentum along the direction of propagation. This feature issue includes original research articles that report on the studies of TL and its interaction with matter.

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.

Electron fields (and more generally spinor fields) with a vortex structure in free space that allows them to have arbitrary integer orbital angular momentum along the direction of motion have been studied for some time. We point out that there are several ways to calculate the local velocity of the electron field, defined as the ratio of momentum density to energy density, and that all but one show a singular vorticity at the vortex line. That one, using the Dirac bilinear current with no derivatives, is the only one so far (to our knowledge) studied in the literature in this context and we further show how to understand an apparent conflict in the existing results. The momentum densities corresponding to the three possible velocity fields give different physical results, in particular regarding the electron induced quantum superkicks given to small electron-absorbing test objects.

We report universal skyrmionic spin textures in the cores of optical and acoustic vortex beams, described within the framework of Laguerre-Gaussian modes. We analytically demonstrate nondiffractive propagating spin merons, independent of whether the field is transverse or longitudinal, with their sign controlled by the wavefront helicity. Experimental confirmation is provided in acoustics through full three-dimensional measurements of the velocity vector field. Although these phenomena are intrinsic to vortex cores, we also show that the claimed universality breaks down for higher-order topological charges, depending on the carrier mode, here exemplified using the Bessel framework.

At the energies of present and future electron accelerators designed to study the structure of hadrons, there is a regime where hard pion electroproduction proceeds by a perturbatively calculable process in QCD. The process is not the leading twist fragmentation one but rather a higher-twist process that produces kinematically isolated pions. Semi-inclusive data may teach us more about parton distribution functions of the target and the pion distribution amplitude. In addition, there is a connection to generalized parton distribution calculations of exclusive electroproduction of mesons in that the perturbative kernel is the same.

It is demonstrated that two-photon exchange (TPE) corrections to the cross-section of unpolarized semi-inclusive deep-inelastic scattering generate azimuthal-dependent terms and the corresponding ⟨ cos ( n ϕ ) ⟩ moments. A quark-diquark model of a nucleon was used in the calculations along with a soft-photon approximation. The infrared divergences in the intermediate steps of calculation are regularized with the fictitious photon mass that cancels out in the final result when the soft-photon bremsstrahlung process and interference terms are added. The calculations employ the oopools package to evaluate the loop integrals. TPE corrections are analyzed in the kinematics of planned experiments at Jefferson Lab.

Elastic scattering of protons on , , , and nuclei is analyzed in the framework of the Born approximation and an optical model for energies of 150 to 320 MeV. Analytical expressions for the scattering amplitudes, differential cross sections and polarization observables are obtained both in the Born approximation and within an optical model approach. The differences between outcomes of these approaches are discussed.

In this work, we examine the extraordinary behavior of polarization and spin angular momentum density in the vicinity of longitudinal field zeros in three-dimensional monochromatic acoustic fields. We demonstrate that, as governed by the continuity equation, the velocity fields of arbitrary acoustic sources maintain nondiffractive elliptical polarization structures that enclose longitudinal field zeros, despite having divergent transverse spatial profiles of intensity. Furthermore, embedded in these nonparaxial field contours, for infinite distance, are threads of circular polarization singularities. We illuminate these inherent properties in acoustic vortex fields, dipole arrays, and the famous Young's double-slit experiment. Our results reveal spin characteristics of sound waves that provide a platform for future studies and applications of structured acoustic waves and chiral acoustic phenomena.