Target-Normal Single Spin Asymmetries Measured with Positrons

TL;DR

This paper discusses a proposed experiment to measure target-normal single spin asymmetries using positrons, aiming to better understand two-photon exchange effects and improve theoretical models in electron scattering.

Contribution

It introduces a new experimental approach with positrons and the Super Big-Bite Spectrometer to measure asymmetries across a critical momentum transfer range.

Findings

Potential to constrain two-photon exchange models

Distinguishes effects of two-photon exchange from time-reversal violation

Provides data to improve theoretical calculations in electron scattering

Abstract

Two-photon exchange and the larger class of hadronic box diagrams are difficult to calculate without a large degree of model-dependence. At the same time, these processes are significant radiative corrections in parity-violating electron scattering, in neutron decay, and may even be responsible for the proton's form factor ratio discrepancy. New kinds of experimental data are needed to help constrain models and guide future box-diagram calculations. The target-normal single spin asymmetry, $A_n$, formed with an unpolarized beam scattering from a target that is polarized normal to the scattering plane, is sensitive to the imaginary part of the two-photon exchange amplitude, and can provide a valuable constraint. A measurement with both electrons and positrons can reduce sources of experimental error, and distinguish between the effects of two-photon exchange and those of time-reversal symmetry violation. This article describes a proposed experiment in Hall A, using the new Super Big-Bite Spectrometer that can cover a momentum transfer range in the critical zone of uncertainty between where hadronic calculations and those based on partonic degrees of freedom are expected to be accurate.