The key to the "not-so-neutral neutron" is in the quarks. In a simple quark picture, neutrons are made up of three quarks - one "up" quark with an electric charge of +2/3 and two "down" quarks, each with a charge of -1/3. Quick mental arithmetic will tell you that the total electric charge of the neutron is zero, as expected. But taking a closer look shows a more complex story. When the neutron is viewed with a probe that cannot see objects smaller than the size of a neutron, the net charge of the neutron is zero; but a closer look with a probe that can examine objects smaller than the neutron reveals the distribution of charge within the neutron, which depends on the spatial distribution of the quarks.
Jefferson Lab experiment 93-038 found that distributions of oppositely charged quarks don't quite cancel each other out, leaving a positively charged interior and negatively charged surface. These findings agree qualitatively with the theory of quark-quark interactions, but rigorous theoretical calculations of neutron (and proton) structure will be required.
A research team, led by Dick Madey, research professor and Professor Emeritus of Physics at Kent State University, used Jefferson Lab's unique high-intensity, highly polarized, continuous electron beam to probe the neutron's structure. Since neutrons are not found in isolation, the team used the next-best target for their scattering experiments - a deuterium nucleus.
Deuterium, an isotope of hydrogen, has a neutron and a proton bound loosely together in its nucleus. The target was kept cryogenically cold to maintain the deuterium in a liquid state. "We needed the density of a liquid to obtain a sufficient number of scattering events to measure the relatively small effect of the neutron charge," explains James J. Kelly of the University of Maryland in College Park and a member of the E93-038 team. After firing electrons at the "neutron" target, Madey's team selected scattering events where the neutron had been nearly at rest, and therefore received the entire momentum transfer from the electron.
Madey and his colleagues measured the polarization of the scattered neutron using a neutron polarimeter, a special detector designed by Madey. It is a "stand-alone" device that can function simultaneously as a neutron and a proton polarimeter. From this they determined the neutron's so-called electric form factor, which is a measure of how spread out the neutron is in space. Then, from the electric form factor the density of the charge within the neutron was deduced.
"E93-038 has been able to measure the neutron electric form factor more precisely than ever before and to infer the charge density with much better resolution," Kelly says.
The new data extend scientists' knowledge of the neutron electric form factor to higher momentum transfer and improves their understanding of the charge distribution within the neutron. "This is a unique experiment because the technique used provides extremely small systematic uncertainties," says Andrei Semenov, of Kent State University in Ohio, and another member of the 93-038 collaboration. "The results are extremely reliable."
Hall C Scientist Roger Carlini agrees, calling this a "flagship experiment" for Jefferson Lab. He says the Lab's measurement of the neutron form factor is "already a textbook measurement" and that it will likely remain so for the foreseeable future. Measurements of the neutron electric form factor at even higher momentum transfer are planned using a polarized target in Hall A at Jefferson Lab.
Team member Andrei Semenov, presented the group's recent findings, in an invited talk, at a joint meeting of the American Physical Society and the High Energy Astrophysics Division of the American Astrophysical Society in Albuquerque, New Mexico in April. Four of the graduate students participating in the experiment also contributed to talks during the meeting, based on the analyses of their respective parts of the experiment. The collaborators have presented their results at a number of international workshops already this year, and have been invited to present their results at several more symposia and workshops.
By Melanie Cooper
contributing writer