History of Neutron Mean-Square Charge Radius Measurements

The neutron, being composed of charged quarks, exhibits a known spherically symmetric charge distribution.  This charge distribution is quantified by the mean-square charge radius (〈r_n² 〉).  For the neutron 〈r_n²〉 < 0 reflecting the fact that it has a slightly negative outer shell.  The mean-square charge radius has been measured experimentally by its proportionality, through a series of known constants, to the neutron-electron scattering length (b_ne).  The difficulty in measuring 〈r_n² 〉 is that b_ne is ~1000x smaller than that of scattering off the nucleus.   Prior measurements of 〈r_n²〉 utilized neutron scattering from targets of noble gases or dense liquids. The Particle Data Group (PDG) considers 5 such experiments and reports an average 〈r_n²〉_PDG =   -0.1161(22) fm² where the error has been inflated by 30% to account for minor discrepancies.   Note that the most recent measurement considered by the PDG was published in 1997. There has been renewed interest in this quantity, however, as there have been several new determinations of 〈r_n² 〉. These include a χEFT approach using isotope shifts by Filin et al. (PRL 2020), an interpretation of electron scattering data by Atac et al. (Nat Com 2021), and a direct neutron measurement 〈r_n² 〉 by Heacock et al. (arXiv 2021).  For context on how these new determinations improve our understanding of 〈r_n²〉, the speaker will discuss the current measurements making up the Particle Data Group’s average.