Measurement of the Generalized Forward Spin Polarizabilities of the Neutron

The generalized forward spin polarizabilities ${\gamma }_0$ and ${\delta }_{LT}$ of the neutron have been extracted for the first time in a $Q^2$ range from 0.1 to $0.9{\mathrm{G}\mathrm{e}\mathrm{V}}^2$. Since ${\gamma }_0$ is sensitive to nucleon resonances and ${\delta }_{LT}$ is insensitive to the $\Delta$ resonance, it is expected that the pair of forward spin polarizabilities should provide benchmark tests of the current understanding of the chiral dynamics of QCD. The new results on ${\delta }_{LT}$ show significant disagreement with chiral perturbation theory calculations, while the data for ${\gamma }_0$ at low $Q^2$ are in good agreement with a next-to-leading-order relativistic baryon chiral perturbation theory calculation. The data show good agreement with the phenomenological MAID model.

Figure 1

Forward spin polarizabilities ${\gamma }_0$ (top panel) and ${\delta }_{LT}$ (bottom panel). Solid squares are the results with statistical uncertainties. The light bands are the systematic uncertainties. The dashed curves are the heavy baryon $\chi \mathrm{P}\mathrm{T}$ calculation from Ref. [12]. The dot-dashed curves and the dark bands are the relativistic baryon $\chi \mathrm{P}\mathrm{T}$ calculation from Ref. [13], without and with the $\Delta$ and vector meson contributions, respectively. Solid curves are from the MAID model [14].

Figure 2

Forward spin polarizability ${\gamma }_0$ (top panel) and ${\delta }_{LT}$ (bottom panel) with $Q^6$ weighting. The solid squares are the results with statistical uncertainties. The light bands are the systematic uncertainties. The open squares are the Stanford Linear Accerelator Center data [15], and the open diamonds are the lattice QCD calculations [16]. The curves are the same as in Fig. 1.