October 1, 2004  


Thirty Years of Quarky Nuclear Physics

By Heather Rock Woods

On the heels of discovering quarks 35 years ago, SLAC pioneered a new field called high-energy nuclear physics to delve into the quirky behavior of quarks.

Three quarks make up each proton and each neutron. Protons and neutrons form the nucleus of an atom. (Graphic by Alan Chou)

Like ecologists who want to understand the behavior and ecological niche of a newly discovered species, a SLAC user group formed by the late Benson Chertok (American University) began using high-energy electron beams to learn about quarks in their natural habitat—protons, neutrons and combinations of protons and neutrons that form the nucleus of an atom.

This small field produced rich results vital to both high-energy physics and lower-energy nuclear physics.  “The recipe for our experiments was always the same: use the unique SLAC capabilities to do brand new things to learn about how quarks and gluons work,” said long-time group leader Ray Arnold (UMass).

The group switched from American University (AU) to the University of Massachusetts, Amherst (UMass) about five years ago. The core group consisted of Arnold, Steve Rock, Zen Szalata and Peter Bosted. During its 30-year tenure, the group observed where quarks reside in the nucleus, how much momentum quarks carry and how quarks interact with each other and with gluons—the particles that carry the strong force. The strong force holds three quarks inside each proton and neutron and keeps the nucleus intact.

Nuclear physics is concerned with the nucleus; it has always been unclassified research and has nothing to do with weapons research. The small group conducted more than 20 experiments, trained legions of students and created and ran a separate injector for the linear accelerator. Their experiments, along with theoretical work done at SLAC, were among the important motivations for building a laboratory dedicated to nuclear physics—the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, Virginia.

The Strength of Quarks

The group started by challenging the conventional wisdom at the time, which said quarks were inert building blocks whose behavior did not influence a nucleus’ properties. That’s true when you look at the nucleus as a whole at low energies, where individual quarks are not visible.

Many of the participants in the E-101 experiment are shown in this photo circa 1977. Top row (left to right): Richard Zdarko, Steve Rock, Ivan Schmidt, Benson Chertok. Bottom row (left to right): Bernhard Mecking, Ray Arnold, Zen Szalata. (Photo courtesy of Ray Arnold)

However, the traditional rules don’t apply when looking at shorter distances (inside the nucleus) at high energies. For this scale, physicists were developing a set of equations called Quantum Chromodynamics (QCD) that describes strong force interactions. “We hit the quarks as hard as we can with high-energy electrons and see how they behave,” Arnold said. It turns out quarks exert a surprisingly strong effect, influencing nuclear structure—including the ability of the struck nucleus to stay intact—through electric, magnetic and spin properties.

In their first experiment, E-101, the group looked for an extremely rare event: deuterium nuclei (composed of one proton and one neutron) that remained in one piece even under heavy electron fire. It took from hours to days to catch each event. “More commonly, the electrons knocked a proton or neutron out of the nucleus,” said Rock.

However, the rare case—intact deuterium—occurred more often than expected by traditional nuclear physics. It followed the pattern predicted by dimensional counting rules, newly developed by theorists Stan Brodsky (THP) and Glennys Farrar (New York University) and extended to electron-deuterium scattering by Brodsky and Chertok.

The rules, an approximation for QCD at short-distances, say that the more quarks in the nucleus, the lower the probability that the nucleus will stay intact. For example, a hydrogen nucleus (one proton, three quarks) is much more likely to stay together than a helium-4 atom (two protons, two neutrons, 12 quarks). Like trying to keep sheep moving in the same direction after lightning has struck, the more sheep, the harder it is to keep the herd together.

What keeps the nucleus unexpectedly whole is the interaction between quarks and gluons. A proton stays intact when its three quarks keep their momentum pointed in the same direction through exchanging gluons. When an electron strikes a proton, it knocks the quarks’ momentums into new (and usually separate) directions. Only when the two gluons exchanged between the three quarks have exactly the right kinematics, the right alignment—like a flawless baton pass on a three dimensional racetrack—does the proton stay cohesive.

Picture the proton as a fuzzy ball of stuff, a smear, with particles coming and going. In the quantum smear, quarks constantly exchange gluons with each other.

—Ray Arnold

“They were the first to say you could see quark and gluon degrees of freedom controlling the physics of the nucleus,” Brodsky said. “The quarks and gluons actually make a difference.”

Fixed Target Experiments

Working with scientists from many other institutions, the AU/UMass group made prolific use of End Station A to do electron scattering experiments. Over the years, the electron beam smashed into fixed targets of deuterium, helium-3, helium-4, other nuclei, electrons, and polarized protons and neutrons. Some of the targets were very complicated and difficult to work with, like high-pressure gas targets kept near a temperature of absolute zero (Kelvin).

“It was a technical tour de force making the helium-3 target,” said Rock, referring to an early experiment. He still has an early aluminum prototype, about a foot long, with a hole blasted through it from a pressure test.

Nuclear Physics Highlights

Here are some highlights from three decades of collaborative work at the high-energy frontier of nuclear physics:

• Measured the electric and magnetic properties of quarks in neutrons and protons.

• The Nuclear Physics at SLAC (NPAS) program—an independent injector near the end of the linac that operated for six years in the 1980s. Arnold directed this program, which had its own program committee to select among novel experiment proposals from multiple user groups. The AU/UMass group conducted experiments at NPAS and also provided technical support to other users.

• Demonstrated that quarks have completely different behavior in different elements of the periodic table—a radical and surprising finding. In heavier nuclei, such as iron, quarks tend to share or overlap momentum, making it harder to find a quark at high momentum (high-energy) than in lighter nuclei. The group tested this effect on elements including carbon, aluminum, silver, gold and iron.

• In the 1990s, worked in collaboration with other groups to measure the spin structure of protons and neutrons, finding that quarks carry less than half that spin. “More spin is being carried by gluons and transitory quark-antiquark pairs than we first expected,” Arnold said of the results.

Outstanding Experimental Research

“It was an era of great accomplishments,” said Brodsky.

 Arnold received the Bonner Prize in Nuclear Physics in 2000 from the American Physical Society for outstanding experimental research. He was awarded: ‘For his leadership in pioneering measurements of the electromagnetic properties of nuclei and nucleons … that addressed the fundamental connection of nuclear physics to Quantum Chromodynamics and motivated new experimental programs.’  (See http://www.aps.org/praw/bonner/00winner.cfm for complete details.)

One program, NPAS, was explicitly funded by DOE as a feeder program to stimulate physics ideas and help train students and post docs who could move on to JLab when it began running experiments in 1994.

The high energies at SLAC provided an experimental region where one could both make theoretical calculations and take measurements to test the theory.

—Steve Rock

The SLAC-JLab connection is strong. Bosted, a long-time AU/UMass physicist now works at JLab, as well as many students who trained with the group, including Allison Lung, now JLab’s assistant director.

“Some of the first experiments done at JLab repeated and extended measurements beyond those from our earlier SLAC data,” Arnold said. JLab can achieve better precision but uses lower energies, which limits some types of experiments (like how gluons carry spin), while allowing many other types of experiments not possible at SLAC.

Budgets for high-energy nuclear physics—which falls between lower-energy nuclear physics and very high-energy particle physics—have dried up. “The experiments we did at SLAC were the first glimpses of what was in there, but there are a billion more questions,” Arnold said.

For more information on End Station A, see http://www2.slac.stanford.edu/vvc/experiments/esa.html 





The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Friday October 01, 2004 by Emily Ball