Speaker
Description
Incorporating two-photon exchange (TPE) effects into the study of elastic lepton-nucleon/lepton-nucleus scattering is crucial for improving our description of the internal structure of nucleons and atomic nuclei, particularly in light of the inconsistencies in the proton electromagnetic form factor measurements. Current models accurately account for TPE effects in the calculation of beam-normal single-spin asymmetries (SSAs) in light nuclei; however, recent experiments at Jefferson Lab reveal substantial discrepancies between theoretical predictions and measured SSA values for the lead target.
The current research aims to address this issue by refining TPE models and advancing a more accurate framework for SSA calculations in elastic scattering regimes. Specifically, the project focuses on providing quantitative estimations of the contribution of dispersive effects to the beam-normal SSA at forward angles. For that, direct calculations of the TPE amplitude's imaginary part arising from the excitation of specific, low-lying nuclear intermediate resonance states will be performed. Predominantly, the states with natural parity will be considered, since the magnetic excitations of the states with odd parity are generally much weaker. Through these efforts, the role of dipole and quadrupole intermediate resonance states in the suppression of beam-normal SSA values in lead will be determined. The results might provide potential explanations for existing discrepancies between the theory and experiment and offer a more comprehensive understanding of the transverse beam-normal single spin asymmetry generation mechanisms.
