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