The Virginia laboratory's Experiment Hall A, which houses two identical spectrometers, is where scientists will use an electron beam to probe the nature of atomic subunits called quarks.

At the Frontier / The Quirks Of Quarks

Jefferson Lab Aims To Understand The Microcosmic World Within A Single Proton

One of the lesser known outposts of particle physics is named for Thomas Jefferson, a politician, revolutionary and polymath who knew nothing of quarks but was a keen proponent of the sort of scientific inquiry that has helped revolutionize our view of the atom and the cosmos.

At the Thomas Jefferson National Accelerator Facility here, just down the road from the colonial capital of Williamsburg and the hallowed battleground of Yorktown, physicists are trying to make a little history of their own.

Using a continuous beam of electrons as a hair-thin scalpel, they have begun to probe the interior of the common protons and neutrons that make up the core of atoms.

Over the past three decades, it has become clear that such nuclear particles harbor a microworld of their own that, in many ways, is analogous to the atom itself. Each proton and neutron has three smaller subunits called quarks rattling around within them.

Some theorists now refer to the proton as a "quark atom," and they are eager to explore its interior structure, a landscape where quarks are bound together by the action of force carriers called gluons and where all sorts of strange things can happen.

The possibilities are described by a theory called quantum chromodynamics, a mathematical model of quark and gluon behavior that many physicists are convinced is true but only now is becoming amenable to detailed experimental investigation, thanks to new tools such as the electron beam machine at the Jefferson lab.

By the late 1970s, physicists had concluded that a continuous, high-energy beam of electrons would offer distinct advantages in probing the nucleus compared to the intermittent beams then available. A group of researchers at the University of Virginia, led by physicist James McCarthy, worked on a design for such an accelerator.

Their brainchild, developed and backed by a consortium of southeastern universities, was selected in 1983 by the U.S. Department of Energy over proposals for continuous electron beam machines by the Massachusetts Institute of Technology, Argonne National Laboratory near Chicago and others. The Virginia-based team, considered the dark horse in the competition, lured Hermann Grunder - an experienced accelerator builder and manager at the Lawrence Berkeley Laboratory in California - to head the $600-million project.

Early on, Grunder and his colleagues concluded that superconducting technology developed at Cornell University and elsewhere could make the proposed Virginia accelerator even more powerful and versatile. Construction began in 1987, and the machine delivered full-energy electron beams to all three experiment halls for the first time in 1997.

Although the design, construction and testing of the accelerator took nearly 15 years, physicists say it was well worth the wait. They now have a powerful new tool to study the quirks of quarks in their natural habitat.

While such research is pursued for its own reward and without regard to practical applications, Grunder says it is not inconceivable that studying the inner world of the proton might someday lead to new technologies. "I, for one, am convinced," Grunder said. "The best guide we have is history." As science moved to smaller scales in the past - to the level of molecules, atoms and atomic nuclei - the increased understanding led to such practical advances as transistors, computers, lasers and nuclear medicines.

Still, the world of the quark poses difficult and unforgiving obstacles for the experimentalist. Most vexing: Pairs and trios of quarks are bound to each other by gluon forces that do not diminish as the quarks are pulled apart. The farther the quarks are tugged apart, the greater the energy that builds between them and the more futile is the effort to sever their bonds.

That is just the opposite of what happens with the more familiar electromagnetic force that binds electrons to the nucleus by the action of force carriers called photons. The attraction between a negatively charged electron and the positively charged nucleus falls off quickly as they are separated. Freely roaming electrons are no big deal. They are the reason that electricity can flow.

A freely roaming quark is another matter altogether. The interaction that binds quarks in a tight embrace is truly remarkable, according to Nathan Isgur, a theorist at the Jefferson lab. The pull, acting at distances of less than a trillionth of a meter, is calculated to be 20 tons of force, Isgur said.

"If you could actually take a quark and put a hook on it and pull, you could measure that force directly," he said. "You could hang a truck off that quark." Given such forces, free quarks never appear in ordinary matter. The practical result is that the reductionist effort to take nature apart in ever smaller pieces to discover what makes it tick - an approach that has served science well since the days of the ancient Greeks - appears to have reached its limits.

"In every previous step down the staircase into the microcosm," Isgur said, "we've been able to actually get those smaller pieces in our hands, if you like, and study their properties individually and the forces between them." Now, for the first time, physicists are confronted with the a situation where the pieces cannot be separated.

It is hard to imagine, Isgur admits, that something can be made of pieces "but you can't take the pieces apart like the pieces of a car." But that is the hand the researchers have been dealt, and they've been learning to cope.

They have found convincing evidence for quarks by sifting the subatomic debris that results when individual particles are smashed together at tremendous energies in accelerators such as the Tevatron at the Fermi National Accelerator Laboratory near Chicago. Such evidence, while indirect, is convincing, physicists say.

Researchers at Long Island's Brookhaven National Laboratory will take a similar approach when they smash together heftier bits of matter - such as gold atoms that have been stripped of their electrons - in the new relativistic heavy ion collider, or RHIC. The teams expect to create a superhot, superdense patch of plasma - similar to what existed a few microseconds after the birth of the universe - where quarks and gluons are able to roam free for an instant.

The Jefferson lab, on the other hand, studies quarks under the less extreme conditions of ordinary matter. "Because the nucleus is so complex and the particles that make it up are even more complex, we are trying to get at it from a lot of different directions," said Barbara Jacak, a physicist at the State University at Stony Brook who is on a research team at RHIC and also serves on the outside advisory panel for the Jefferson lab.

"At the Jefferson lab, they are turning up the microscope and looking at a single proton at a time," Jacak said. "What are the quarks doing in there?" And when that proton is put inside a collection of other nuclear particles, how do the neighbors - with their quarks - affect it? How tightly can two nuclear particles, or nucleons, be squeezed together before they start to lose their individual identity and act more like a collection of six quarks? To attack such questions, researchers at the Virginia lab use a beam of electrons that are accelerated in an underground tunnel by successive "kicks" of microwave radiation. After a few laps around the 7/8-mile track, the beam is split and directed to three experiment halls where elaborate detectors can take a "snapshot" when the beam interacts with a target.

In some of the experiments, the beam is used to probe and tickle individual protons, looking for hints of what happens to the quarks and gluons within. In other experiments, the researchers want to study more closely what happens when quark-containing protons and neutrons come together to form atomic nuclei.

An accelerator operator monitors the electron beams delivered to the three experiment halls in the $600 million lab.

"Our focus is not on large nuclei or the complexity of nuclear physics," Isgur said. "Our focus is on understanding the fundamentals: How does a proton work? And when you put two of these simplest 'atoms' together, what are the mechanisms by which they stick together? If you understand that, all the rest follows." As a start, the Jefferson lab researchers want to study the lay of the land within the proton in its so-called ground state (when no additional energy is being pumped in.) Each of the three quarks within the proton has an electrical charge and acts like a tiny bar magnet as it orbits within the particle. By measuring the distribution of charge and magnetization within the proton, researchers can say something about where quarks might be lurking at a given moment - at the proton's center or farther out.

Such studies set the stage for a deeper understanding of what may be happening inside the proton. Theorists are convinced that there is a lot more "stuff," if you will, inside the proton than just its three constituent quarks and their associated gluons. The proton is composed of two "up" quarks and a "down" quark. But computer simulations, based on the quantum chromodynamics theory, suggest that a strange quark and its antimatter counterpart can spontaneously bubble into existence within the proton for a brief time.

A "snapshot" of the proton's interior at the right moment should reveal some of these strange quarks. Many theorists thought these ephemeral particles would account for at least 10 or 20 percent of the charge and magnetic field of the proton. Experiments so far suggest the number is substantially less - only a few percent.

Just where do such particles come from anyway? Quarks are connected to each other by elongated bundles of gluon energy called flux tubes. The tubes - essentially the raceways by which quarks are constantly exchanging gluons with each other - harbor substantial amounts of energy. Isgur said that under the right conditions, as allowed by Heisenberg's uncertainty principle, a quark-antiquark pair can pop into existence by borrowing energy from the flux tube. The energy is converted to mass according to Einstein's famous equation E=mc2. The proton re-absorbs the newly created pair almost instantaneously, Isgur said.

Studies suggest that roughly half of the mass of the proton may be stored in gluon flux tubes, Isgur said. "That underlines why you can't understand protons unless you understand gluons," he said, adding, "It's absolutely essential that we find some direct experimental confirmation of our understanding of glue and its properties." Theoretical calculations say the gluons that hold protons together can, under certain circumstances, clump together into rings of pure glue. Physicists at Brookhaven and elsewhere have mounted experiments to search for evidence of such "glueballs." But the lightest glueball allowed by the theory - and the most amenable to production in an atom smasher - has quantum properties that it shares with some quark pairs. That means interpreting the evidence can be tricky.

Isgur said there are other ways to study the properties of the glue, including using the Jefferson lab's electron beam to help "pluck" a gluon flux tube and see whether the vibrating bundle of energy makes exotic states of unadulterated glue that can't otherwise be seen. Such an experiment is planned for next year.

With experiments at the Jefferson lab now well under way and the first experiments at RHIC expected to begin early next year, these are heady times for nuclear physicists as they seek to explain the world at its most fundamental level.

"It's always dangerous and difficult to say what is going to happen with new knowledge," Isgur said. "The only truthful answer is that we've never known what value will come out of basic research." But he and others at the lab are confident this latest investment, in Jefferson's name, will be worthwhile.