by James Schultz

It took effort, but a team led by Cynthia Keppel, a Jefferson Lab staff scientist and assistant professor of physics at Hampton University, begged and borrowed time over two years from individual spectrometers in Hall C. "We were scavengers," she says. "We collected data whenever we could."

The payoff for their persistence is the experimental validation of a phenomenon known as quark-hadron duality. The confirmation is an important step on the road to an intersection between certain nuclear physics theories and quantum chromodynamics (QCD), the theory which describes the ways quarks make up and interact with ordinary matter.

"The theorists are excited by this," Keppel says. "These data are telling us that some surprisingly simple physics governs complex particle states. Hopefully, one day, we'll be able to describe nuclear interactions completely in terms of the fundamental interplay between quarks and gluons."

Duality -- the idea that properties of reactions calculated at the quark level are retained at the level of the observed particles, the so-called hadrons -- appears to be an intrinsic property of sub-nuclear matter. Physicists know that when electrons scatter from intact hadrons, such as the single proton that makes up the hydrogen nucleus, the proton can just recoil as a whole.

Ioana Niculescu (left) and Thia Keppel review inclusive resonance spectra from their quark-hadron duality test.

However, the energy transferred from the electrons to the proton can also cause the proton to be excited into a higher energy state, a process known as resonance formation or to emit new particles (for example, pions). The JLab findings indicate that these various processes apparently yield the same result on average as expected from single quark scattering. This appears to confirm duality: the notion that the basic electron-quark interaction is the dominant process.

But the experimental results come with a "twist" " literally. QCD theory, as it explains the interactions of the quarks and the gluons that bind quarks together, allows for interactions between the struck quark in the electron-quark scattering process and the other quarks in the nucleon. Such interactions are termed higher twist effects, and are predicted to be large in the JLab energy regime. Yet, the new results indicate that such effects are surprisingly small, or perhaps cancel each other out. (See the diagram.)

Single Quark Scattering Dominates Interactions

"We observed minimal higher twist effects on the average. That's quite surprising," she says. "It implies that single quark scattering is the dominant effect, even in a multi-quark system such as a bound nucleon or nucleon resonance."

An additional surprising result was that the duality-averaged data at low momentum transfers (the momentum passed from the electron to the struck quark) are sensitive only to the quarks which contribute to the charge of the nucleon. Because there is a substantial amount of energy available, there also exists a sea of additional quarks within the nucleon. This sea is apparently invisible in the new results.

To further understand these new findings, the research team of which Keppel is a member will conduct a Hall C experiment slated to begin in mid-July and conclude by mid-August. The experiment will extend previous studies of duality by investigating whether it persists if one considers the electric and magnetic pieces of the electron-quark and electron-nucleon interaction separately.

Armed with their research results, Keppel, Rolf Ent and Ioana Niculescu re-evaluated older research from other labs. It seems to confirm their results and, further, indicates that duality may hold in the longitudinal channel as well as the transverse.

"If we understand duality, we expect to observe in the longitudinal channel what we've already observed in the transverse," she says. "Otherwise, it may be back to the theoretical drawing board."

Keppel presented the quark-hadron duality results at the American Physical Society's Centennial Meeting in Atlanta, Georgia, this past March. She has also spoken on the experiment at Deep Inelastic Scattering 1999, a conference in Zeuthen, Germany, and has been invited to discuss the research team's findings at additional scientific meetings this summer.

Three separate publications have just been submitted on this work, which comprises the Ph.D. thesis of Ioana Niculescu who is now a Hall B postdoctoral research associate.

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