Measurement of the generalized spin polarizabilities of the neutron in the low $Q^2$ region

TL;DR

This paper reports measurements of the neutron's generalized spin polarizabilities at very low energy scales, revealing significant discrepancies with theoretical predictions from chiral effective field theory, thus challenging current models of neutron spin structure.

Contribution

First experimental measurement of neutron's generalized spin polarizabilities at low $Q^2$, testing the applicability of chiral effective field theory in this regime.

Findings

Data shows strong discrepancy with chiral EFT predictions

Highlights need for improved theoretical models of neutron spin structure

Provides new constraints for non-perturbative QCD approaches

Abstract

Understanding the nucleon spin structure in the regime where the strong interaction becomes truly strong poses a challenge to both experiment and theory. At energy scales below the nucleon mass of about 1 GeV, the intense interaction among the quarks and gluons inside the nucleon makes them highly correlated. Their coherent behaviour causes the emergence of effective degrees of freedom, requiring the application of non-perturbative techniques, such as chiral effective field theory. Here, we present measurements of the neutron's generalized spin-polarizabilities that quantify the neutron's spin precession under electromagnetic fields at very low energy-momentum transfer squared down to 0.035 GeV$^2$. In this regime, chiral effective field theory calculations are expected to be applicable. Our data, however, show a strong discrepancy with these predictions, presenting a challenge to the current description of the neutron's spin properties.