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, ⟨rn2⟩. The precise measurement of the neutron’s charge radius thus emerges as an essential part of unraveling its structure. Here we report on a ⟨rn2⟩ measurement, based on the extraction of the neutron electric form factor, GEn, at low four-momentum transfer squared (Q2) by exploiting the long known connection between the N → Δ quadrupole transitions and the neutron electric form factor. Our result, ⟨rn2⟩=−0.110±0.008(fm2), addresses long standing unresolved discrepancies in the ⟨rn2⟩ 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 ⟨rn2⟩ value.