Physicists Find New Way to Reveal Neutron's Secrets

A team of researchers at Thomas Jefferson National Accelerator Facility, including Sebastian Kuhn, Old Dominion University Eminent Scholar and professor of physics, has come up with an ingenious way to study neutrons, which, along with protons, are the building blocks of the atomic nucleus.

Results gleaned from an experiment, which was conducted in 2005, were published by the team in the April 8 issue of the journal Physical Review Letters. On April 23, a news article about the findings appeared in DOE Pulse, published by the U.S. Department of Energy.

Kuhn, a spokesperson for the experiment team as well as chairman of the Jefferson Lab Users Group, was interviewed (together with Wally Melnitchouk of the Jefferson Lab's Center for Theoretical and Computational Physics) for the DOE Pulse article.

Physicists are interested in the differences between the proton and neutron, but have had difficulty studying the latter in isolation. When the Jefferson Lab's electron beam, which travels near the speed of light, collides with a nucleus, the neutrons that usually are bound to protons can be probed. But they decay in a few minutes when they are isolated and therefore are difficult to study.

Kuhn said the experiment - dubbed BoNuS (for Barely Off-Shell Neutron Structure) - took a traditional tack by focusing on an isotope of hydrogen, called deuterium. "The simplest nucleus that we can possibly use is the deuteron; it's just one proton and one neutron. And that's what people have tried to study as an approximation to a free neutron. Subtract the proton and hope to get the free neutron," Kuhn explained in the DOE Pulse article.

But the BoNuS team chose to look at accelerator-collision results in which the proton is not taking part in that collision, but instead is "a spectator," only loosely bound with the neutron. "What we set out to do with this experiment is to eliminate as much as possible any effects on the neutron, due to the fact that it's bound in a nucleus," Kuhn said.

"There's a proton and a neutron: you hit the neutron, and the proton becomes a spectator. It just watches what happens to the neutron. We only chose events where the proton was moving backwards relative to the electron beam, and at low momentum. Those are the two signatures we use to distinguish spectator protons from anything that was involved in the reaction: backward and slow."

The scientists developed a new piece of equipment, the Radial Time Projection Chamber, to capture and measure the slowly moving protons. ODU physics faculty member Stephen Bültmann and former graduate students Jixie Zhang and Svyatoslav Tkachenko helped build this detector and run the experiment. The experiment was carried out seven years ago in Jefferson Lab's Experimental Hall B, where the CLAS spectrometer detects the scattered electrons. Results were carefully analyzed in the intervening years.

Among the new insights gained from the BoNuS experiment, the team studied the likelihood that scattering electrons can alter the neutron to form so-called resonances.

"A resonance is an excited state, which basically means that the internal configuration of the quarks changes in a well-defined manner. For instance, one quark changes its spin, or two quarks start moving around each other, or some other changes of the internal motion occur that lead to a new, more energetic state of the neutron," Kuhn said.

While many of the proton resonances are well-documented, there has been less exploration of the neutron resonances, since, until now, the neutron has been difficult to probe. This is the first experiment to reveal a sequence of neutron resonances in "inclusive" electron scattering. Encouragingly, the sequence appears, in some ways, to mirror the sequence of proton resonances.

"This will allow us to study the differences between the spectrum of resonance excitations of the neutron and the proton," said Melnitchouk. "We now have a way of directly studying the structure of the free neutron; this is just the first glimpse. But this experiment demonstrates that this method works and opens up a whole new range of experiments to study the structure of the neutron."

Beyond new information of resonances of the neutron, the BoNuS experiment offers a direct view of the motion of the quarks inside the neutron, in particular which type of quarks tends to move the fastest (carrying most of the neutron's momentum and energy).

The scientists said now that the experimental method has been confirmed, they plan to use it in future experiments with the energy-upgraded electron beam at Jefferson Lab to continue probing the neutron, in hopes of revealing more of its structure and of the behavior of the quarks inside.

"For the first time, we can directly compare the structure of the neutron to that of the proton, and we can then start explaining the differences in terms of fundamental properties of bound quark states," Kuhn said.

The DOE Pulse article is at http://www.ornl.gov/info/news/pulse/no361/feature.shtml.