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

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_n² ⟨ r 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_n² ⟨ r n 2 ⟩ measurement, based on the extraction of the neutron electric form factor, G_E^n G E n , at low four-momentum transfer squared ( Q 2 ) by exploiting the long known connection between the N → Δ quadrupole transitions and the neutron electric form factor. Our result, r_n² =-0.110± 0.008\,(fm²) ⟨ r n 2 ⟩ = − 0.110 ± 0.008 ( fm 2 ) , addresses long standing unresolved discrepancies in the r_n² ⟨ r 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_n² ⟨ r n 2 ⟩ value.