Moments of the Spin Structure Functions g_1^p and g_1^d for 0.05 < Q^2 < 3.0 GeV^2

TL;DR

This paper reports measurements of the spin structure functions g_1 for the proton and deuteron over a broad Q^2 range, providing insights into their moments, sum rules, and spin polarizabilities, with comparisons to theoretical predictions.

Contribution

It presents the first measurements of the first moments of g_1 and the generalized forward spin polarizability gamma_0^p over a wide Q^2 range, testing theoretical models.

Findings

First moments of g_1 have a negative slope at low Q^2.

Gamma_0^p shows strong Q^2 dependence and flattens at high Q^2.

Discrepancy between gamma_0^p data and ChPT at low Q^2.

Abstract

The spin structure functions g_1 for the proton and the deuteron have been measured over a wide kinematic range in x and Q2 using 1.6 and 5.7 GeV longitudinally polarized electrons incident upon polarized NH_3 and ND_3 targets at Jefferson Lab. Scattered electrons were detected in the CEBAF Large Acceptance Spectrometer, for 0.05 < Q^2 < 5 GeV^2 and W < 3 GeV. The first moments of g_1 for the proton and deuteron are presented -- both have a negative slope at low Q^2, as predicted by the extended Gerasimov-Drell-Hearn sum rule. The first result for the generalized forward spin polarizability of the proton gamma_0^p is also reported. This quantity shows strong Q^2 dependence at low Q^2, while Q^6\gamma_0^p seems to flatten out at the highest Q^2 accessed by our experiment. Although the first moments of g_1 are consistent with Chiral Perturbation Theory (ChPT) calculations up to approximately Q^2 = 0.06 GeV^2, a significant discrepancy is observed between the \gamma_0^p data and ChPT for gamma_0^p, even at the lowest Q2.