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.
Highlights
The neutron is a cornerstone in our depiction of the visible universe
The study of the nucleon charge radius has been historically instrumental towards the understanding of the nucleon structure. It is the highly complicated dynamics of the strong force between quarks and gluons, the fermionic nature of quarks and spin-orbit correlations that leads to an asymmetric distribution of u- and d-quarks in it, resulting in a negative value for hr2ni
The extraction of hr2ni has been uniquely based on the measurement of the neutron-electron scattering length bne, where low-energy neutrons are scattered by electrons bound in ddaiatma aggronuetpic(PatDomG)s3.–T6heexhhrib2niit measurements adopted discrepancies, with the by the values particle ranging from hr2ni 1⁄4 À0:114 ± 0:0034 to hr2ni 1⁄4 À0:134 ± 0:009 ðfm2Þ5
Summary
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 negativelycharged (down) quarks, a result of the complex quark-gluon dynamics, lead to a negative value for its squared charge radius, hr2ni. Employing new, different techniques in extracting this fundamental quantity has proven most valuable, as recently exhibited in the proton’s case: the disagreement of the proton charge radius, rp, as determined using the Lamb shift measurement in the muonic hydrogen atom[1], with the earlier results based on the hydrogen atom and the electron scattering measurement, gave rise to the proton radius puzzle[2] This led to a significant reassessment of the methods and analyses utilized in the proton radius extraction, and to the consideration of physics beyond the standard model as potential solutions to this discrepancy. These discrepancies have not been fully resolved, a direct indication of the limitations of this method
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