Physics Division Research Highlights

Journey to the center of the neutron

neutron charge distributionElectron scattering has long been an important tool in mapping out the spatial and momentum distributions of the quarks in nucleons. Scattering measurements at high energy provide a clean probe of the quark’s momentum distributions, extracted in the infinite momentum frame (IMF), where the nucleon travels near the speed of light. Lower energy measurements, where the electron scatters elastically from the nucleon, probe the spatial distribution of charge in the nucleon. However, model-dependent boost corrections make it difficult to fully understand the details of the spatial distribution in the nucleon’s rest frame.

The neutron is charge neutral, but the long-held picture is that fluctuations of the neutron into a system of a heavy proton and light negative pion yields a positive core with a negative ‘pion cloud’ at larger distances. Recently, Gerry Miller (Univ of Washington) provided a model-independent way to extract these distributions in the same IMF used to study momentum distributions, giving a detailed map of the transverse spatial distribution in the Lorentz-contracted nucleon. The new analysis showed the negative cloud around a region of positive charge, but also found a striking new feature; a small negative core at the very center of the neutron.

A new analysis by Miller and John Arrington of Argonne National Laboratory has explained this unusual feature. In the IMF, determining the center of the nucleon is critical to understanding the results. Just as the center of mass of an atom is determined mainly by the massive proton, the center-of-momentum, relevant in the IMF, is largely determined by the high-momentum quarks. In the limit where a quark carries the bulk of the nucleon momentum, it is by definition localized near the center of the neutron. The fact that the high momentum quarks in the neutron are dominated by negatively-charged down quarks explains the highly localized negative charge near the center of the nucleon.


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