Elastic electron-neutron (e-n) scattering provides access to the neutron form factors which, when precisely measured over a range of energy scales, Q2, provide access to the spatial distribution of the charge and magnetization within the neutron. However, neutrons are only stable when bound in nuclei, making it challenging to cleanly measure scattering from a single neutron. Several techniques have been used to try and extract the form factors by isolating e-n scattering from a neutron bound in a deuteron (proton + neutron) from the dominant e-p contribution and then applying corrections for the fact that the neutron is bound in the nucleus. Modern measurements have enabled extractions with modest corrections, but yield inconsistent results between different experimental techniques [1].
A recent program at Jefferson Lab compared scattering from 3H and 3He [2]. This has two major advantages: the 3H-3He scattering difference is directly related to the difference between the proton and neutron contributions and the corrections associated with being bound in these nuclei are essentially identical due to isospin symmetry of these mirror nuclei. Data measuring quasielastic scattering (single nucleon knockout) was used to measure the neutron cross section relative to the well known proton cross section, allowing for an extraction of the neutron magnetic form factor [3]. These results overlapped with several previous measurements, including in the region where previous measurements showed systematic disagreement.
A global analysis of these measurements yields an extraction of the form factor that is consistent with the individual measurements if one allows for the possibility of small (1.5%) rescaling of the individual data sets. Such correlated uncertainties were not evaluated for most of these measurements, but these small adjustments are consistent with our estimate of such effects for the earlier measurements. Because our new data overlapped with multiple experiments, it will help determine the relative corrections for previous data sets. We are now using these new results to provide an updated evaluation of the neutron form factor and to examine the impact of these results on the spatial distribution of magnetization in the neutron.
Figure 1: The extracted neutron form factor from the 3H-3He comparison (red squares) compared to earlier measurements using a variety of different techniques.
This experiment was proposed by John Arrington and collaborators, and the data included in this paper included the PhD thesis results of RNC staff member Shujie Li, former RNC postdoc Rey Cruz-Torres, and Nathaly Santiesteban (assistant professor at UNH). The ongoing global fitting and spatial structure studies being carried out by JA, TH, NS, and her undergraduate student, Joe Jayne.
References
[1] Z. Ye and J. Arrington, Phys. Lett. B 777 (2018) 8
[2] J. Arrington, R. Cruz-Torres, T. J. Hague, L. Kurbani, S. Li, D. Meekins, and N. Santiesteban, Eur. Phys. J. A 59 (2023) 8, 188
[3] S. N. Santiesteban, Phys. Rev. Lett. (2024), arXiv:2304.13770