The CEBAF Large Acceptance Spectrometer (CLAS) delves into secrets of particle’s structure

Jefferson Lab researchers utilize CLAS and CEBAF’s 5.7 GeV continuous beam to gather new insights on several fundamental questions about the neutron

The CEBAF Large Acceptance Spectrometer (CLAS) is like a perfect survey instrument. Because it surrounds the interaction point in Jefferson Lab’s Hall B, it can record several particles produced in a subatomic interaction at once. More than 40,000 data channels convey information on the trajectory (measured with drift chambers), speed (time-of-flight counters) and energy (electromagnetic calorimeters) for all detected particles, up to 3,000 times a second. Often, multiple experiments run at the same time in Hall B, and data for all of them are collected simultaneously.

During the recent (February through mid-March) run dubbed “E6,” researchers used CLAS together with CEBAF’s 5.7 GeV continuous electron beam to gather new insights on several fundamental questions about the neutron. The neutron is one of the two building blocks (together with the proton) of every nucleus, and its properties are just as interesting and important as those of the proton. Unfortunately, these properties are usually obscured because neutrons are generally bound inside nuclei. E6 collaborators from several universities and Jefferson Lab, working on the experiment “Electron Scattering from a High-Momentum Nucleon in Deuterium” are seeking a clearer view of this elusive neutral partner of the proton. This experiment was proposed by co-spokespersons Keith Griffioen, College of William and Mary; and Sebastian Kuhn, Old Dominion University.

Kuhn, an ODU associate professor of physics, says that the overall results of the study, which ended March 10, appear promising.

“We’re not ready to say we’ve found new things in our data. So far, we haven’t analyzed enough data to say what ultimately we’ll discover,” he contends. “What we can say is that we’ve developed a method of extracting the true energy needed to excite a neutron resonance, even if the neutron is moving. We collected all the data we were hoping for; and I believe we’ll learn important things about the neutron’s internal structure.”

Scientists must observe neutron behavior indirectly because single neutrons are inherently unstable. That’s why researchers must use the nucleus to study neutrons, Kuhn explains. Unbound from their stable pairing with protons inside the nucleus, neutrons – which have more than 1,800 times the mass of electrons and are just slightly more massive than their partnering protons – decay by emitting radiation, in the form of a proton, an electron and a particle known as an antineutrino. Experimenters directed Hall B’s electron beam into a vial filled with deuterium liquid. Deuterium is a “heavy” isotope of hydrogen, with one proton and one neutron in its nucleus. Because both are bound together in the atomic nucleus as a pair, their movements are mirror images of each other. As either is ejected from the nucleus, the other is liberated as well, and scientists are able to infer their initial motion from the resultant trajectories.

“We basically take a snapshot of how fast and in what direction the proton, and therefore the neutron, was moving before being hit by the electron,” Kuhn explains. “Our experiment can tell us two things. First, it reveals what’s going on inside the neutron. Secondly, it tells us how being bound to a proton changes its properties.”

Kuhn suggests another method of visualization might be to compare the scattering process to using a hammer to gauge the nature of a water glass. If one would touch a hammer lightly to the glass, without breaking it, certain properties like smoothness and shape could be inferred. (This is called “elastic scattering” in physics and has been used to measure form factors of the proton and neutron in JLab’s Halls A and C.) Strike the glass lightly with the hammer, and the sound it makes reveals more about the structural composition of the glass and, therefore, its method of manufacture. (This corresponds to the excitation of neutron resonances through “inelastic scattering.”) Ultimately, the hammer could break the glass; examining the pieces could yield even more insights. “If you hit it really hard and smash the glass – in physics, when we hit a target at high energies, we call it ’deep inelastic scattering’ – you learn from the size of the pieces and how much they resist the hammer blows,” Kuhn explains. “In all of these cases, knowing how the neutron ? ’the glass’ ? was moving before striking it with the ’hammer’ allows us to get much more accurate and detailed information.” Scientists also want to compare the properties of fast and slow moving neutrons, since high initial speed means that the proton and neutron were close to each other before the neutron was struck. In this Hall B experiment, most of the observations occurred on fast-moving neutrons, which may have a modified structure because of the close proximity of protons. A slow-moving neutron, on the other hand – one that moves no faster than one-tenth the speed of light – “is as close to a free neutron as one will ever get,” Kuhn says. “Then we can really learn about neutron structure.” To do this, Kuhn and his colleagues have formed the Bound Nucleon Structure Collaboration, or BONUS, which hopes to conduct a follow-on experiment. If approved, that study would run in Hall B, during 2004.

Scientists would also like to learn more about neutrons colliding with their proton partners before both fly apart. According to a theory called “Color Transparency,” if the neutron is struck hard enough, it becomes compressed momentarily and can more easily avoid colliding with the proton on the way out. Details of this process would prove invaluable in painting a complete picture of these two building blocks of nuclear matter. Data on this process were collected for the second experiment of the E6 run, which was proposed by Kim Egiyan, Yerevan Physics Institute; Keith Griffioen, W&M; and Mark Strikman, Pennsylvania State University.