Single-nucleon experiments

TL;DR

This paper reviews low momentum transfer data on nucleon spin structure functions and pion electroproduction, highlighting discrepancies with Chiral Perturbation theory predictions and emphasizing the need for further theoretical calculations.

Contribution

It provides a comprehensive overview of experimental data and compares it with theoretical calculations, identifying areas where theory and experiment do not align.

Findings

Discrepancies between experimental data and Chiral Perturbation theory for higher moments.

The Bjorken sum agrees with theory, but other observables do not.

Preliminary pion electroproduction data are compared with phenomenological models.

Abstract

We discuss the Jefferson Lab low momentum transfer data on moments of the nucleon spin structure functions $g_1$ and $g_2$ and on single charged pion electroproduction off polarized proton and polarized neutron. A wealth of data is now available, while more is being analyzed or expected to be taken in the upcoming years. Given the low momentum transfer selected by the experiments, these data can be compared to calculations from Chiral Perturbation theory, the effective theory of strong force that should describe it at low momentum transfer. The data on various moments and the respective calculations do not consistently agree. In particular, experimental data for higher moments disagree with the calculations.The absence of contribution from the $\Delta$ resonance in the various observables was expected to facilitate the calculations and hence make the theory predictions either more robust or valid over a larger $Q^2$ range. Such expectation is verified only for the Bjorken sum, but not for other observables in which the $\Delta$ is suppressed. Preliminary results on pion electroproduction off polarized nucleons are also presented and compared to phenomenological models for which contributions from different resonances are varied. Chiral Perturbation calculations of these observables, while not yet available, would be valuable and, together with these data, would provide an extensive test of the effective theory.