Results on spin sum rules and polarizabilities at low $Q^2$

TL;DR

This paper presents experimental measurements of nucleon spin sum rules and polarizabilities at very low momentum transfer, testing the predictions of Chiral Effective Field Theory and revealing areas of agreement and tension.

Contribution

It provides new low-$Q^2$ experimental data on spin polarizabilities for proton and neutron, challenging the current $ ext{Chiral EFT}$ predictions.

Findings

Some observables agree with $ ext{Chiral EFT}$ predictions

Others, including $ ext{delta}_{LT}^n$, show significant discrepancies

Results indicate $ ext{Chiral EFT}$ does not fully describe nucleon spin observables at low $Q^2$

Abstract

We report on recently published experimental results on spin sum rules, and particularly on the generalized spin polarizabilities $\gamma_0(Q^2)$ (for both the proton and neutron) and $\delta_\mathrm{LT}(Q^2)$ (for the neutron). The data were taken at Jefferson Lab in Hall A by experiment E97110 (neutron) and in Hall B by experiments E03006 and E05111 (proton and deuteron, respectively). The experiments covered the very low $Q^2$ domain, down to $Q^2 \simeq 0.02$ GeV$^2$. This is well into the domain where Chiral Effective Field Theory ($\chi$EFT) predictions should be valid. Some measured observables agree with the state-of-the-art $\chi$EFT predictions but others are in tension, including $\delta_\mathrm{LT}^n(Q^2)$ which $\chi$EFT prediction was expected to be robust. This suggests that $\chi$EFT does not yet consistently describe nucleon spin observables, even at the very low $Q^2$ covered by the experiments.