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, langle {r}_{{rm{n}}}^{2}rangle. The precise measurement of the neutron’s charge radius thus emerges as an essential part of unraveling its structure. Here we report on a langle {r}_{{rm{n}}}^{2}rangle measurement, based on the extraction of the neutron electric form factor, {G}_{{rm{E}}}^{{rm{n}}}, 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, langle {r}_{{rm{n}}}^{2}rangle =-0.110pm 0.008,({{rm{fm}}}^{2}), addresses long standing unresolved discrepancies in the langle {r}_{{rm{n}}}^{2}rangle 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 langle {r}_{{rm{n}}}^{2}rangle value.