Measurement of the neutron charge radius and the role of its constituents

Abstract

The neutron is a cornerstone in our depiction of the visible universe. Despite the neutron zero-net electric charge, the asymmetric distribution of the positively- (up) and negatively-charged (down) quarks, a result of the complex quark-gluon dynamics, lead to a negative value for its squared charge radius, $⟨r_{\mathrm{n}}^2⟩$. The precise measurement of the neutron's charge radius thus emerges as an essential part of unraveling its structure. Here we report on a $⟨r_{\mathrm{n}}^2⟩$ measurement, based on the extraction of the neutron electric form factor, $G_{\mathrm{E}}^{\mathrm{n}}$, at low four-momentum transfer squared $(Q^2)$ by exploiting the long known connection between the $N\to \Delta$ quadrupole transitions and the neutron electric form factor. Our result, $⟨r_{\mathrm{n}}^2⟩=-0.110\pm 0.008({\mathrm{fm}}^2)$, addresses long standing unresolved discrepancies in the $⟨r_{\mathrm{n}}^2⟩$ determination. The dynamics of the strong nuclear force can be viewed through the precise picture of the neutron's constituent distributions that result into the non-zero $⟨r_{\mathrm{n}}^2⟩$ value.

https://doi.org/10.1038/s41467-021-22028-z

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