ASU Research E-Magazine: The Hunting of the Quark

Getting useful information from deep inside an atom’s nucleus is not an easy task. Particle physicists such as ASU’s Ricardo Alarcon and Joseph Comfort know that the best technique for exploring this infinitesimal world is a process of creative destruction.

It is something of a paradox: to study the smallest known constituents of the universe, one needs big science. Particle physics is an enterprise on a grand scale, requiring laboratories full of technology, large teams of scientists, and budgets to match. But in this century, the study of atoms and their bits and pieces has paid off. Nobel prizes have gone to the pioneers, and practical applications such as MRI (magnetic resonance imaging) have made it out of the laboratory and into hospitals where they are used to help people.

Ricardo Alarcon and Joseph Comfort are part of the quest to understand the smallest bits of the material world. Both are experimental nuclear physicists at Arizona State University. They lead teams of scientists and graduate students to explore the constituents of the atom, from the relatively familiar proton and neutron, to the less familiar pion which binds the protons and neutrons inside the nucleus.

Alarcon and Comfort are looking even deeper. They want information about the quarks and gluons which vibrate beneath the surface of the proton and neutron. The work involves an enormous variety of tasks. Like other researchers, the ASU physicists must write detailed proposals, compete for funds and the use of accelerator facilities, build particle detectors, write computer software to analyze data, and coordinate large teams of people to achieve a common goal.

It simply is not easy to get information from inside the atomic nucleus. Light waves are far larger than the size of the nucleus. Scientists use electron microscopy to “see” the arrangement of atoms inside a crystal. Proton and electron accelerators are the much larger “microscopes” that physicists use, paradoxically, to probe the much smaller atomic nucleus. The “pictures” physicists look at are either one form of detector, or the imprint of computer visualizations of data from the detectors.

Alarcon and Comfort explain that the only known effective technique for exploring this infinitesimal world is a process of creative destruction. Massive particle accelerators are used to fire subatomic projectiles at atomic targets. Equally large and sophisticated detectors are designed to capture the products of the resulting collisions.

The particle physicist’s main effort is devoted to finding ways to reconstruct what happened in these collisions. The process is similar to a police officer who, arriving at an accident scene, must try to reconstruct the sequence of events that led to the skid marks, broken glass, and mangled metal.

Experiments conducted by Alarcon and Comfort can last from two weeks to two months. For them to be successful, several things must occur at the same time. The detector must be operating at good efficiency, capturing as many of the events (atomic collisions) as possible. The detector can be as much as 30 feet high. It must be aligned exactly to within a millimeter in space. The computer software running the show must be bug-free. Even the weather must cooperate. Beam time, as it is called, is hard to come by, and the times when everything works are highly valued.

Kelly Craig is one of Comfort’s ASU graduate students. She tells of a time last summer when the electron beam at the Massachusetts Institute of Technology’s Bates Laboratory had to be shut down temporarily. Everyone in Boston was running air conditioning during a heat wave. The large power draw from the accelerator would have caused a brown-out.

A better understanding of nature’s building blocks has been a goal of scientists for many centuries. Alarcon and Comfort study the dynamics of the atom’s nucleus. They want to know how the nucleus changes through time.

In the language of traditional nuclear physics, there are only three main nuclear particles: protons, which carry a positive charge; neutrons, which have no charge; and pions, which carry the “strong force.” Although the positive charge of the protons causes them to repel each other, the exchange of pions between nucleons (neutrons and protons) creates an attractive force strong enough to overcome the protons’ electric repulsion.

During the past two decades, as scientists discovered more and more “elementary” particles, it took the invention of a new theory, and a new set of particles, to make sense of the new subatomic zoo. The new particles are called quarks. According to quark theory, the proton and neutron are not themselves elementary particles. Each actually are made of three quarks.

The quark theory provides a new and convincing way to understand the structure of elementary particles. However, scientists still do not know how elementary particles transform into other particles, or what effect the dynamic quark interaction has on them. Alarcon and Comfort are looking for evidence of how quarks interact with each other.

Comfort often works at the Meson Physics Facility (LAMPF) in Los Alamos, New Mexico. Los Alamos physicists have devised a method to “polarize” targets. Polarizing means controlling a particle’s “spin.” For the physicist, spin also is known as quantum angular momentum; it is one of the intrinsic properties of an atom.

Think of an atom as a spinning top. The spinning top has angular momentum—a tendency to keep spinning—which also keeps it upright. Electrons, protons, and neutrons have a similar property. They also have just two states: +1/2 and -1/2. Think of these states as analogous to clockwise and counter-clockwise.

Comfort is looking for evidence to assess “charge independence.” Mediated by pions, the strong force holds the bits of the atomic nucleus together. Researchers think that this strong force is not affected by electrical charge in any unusual way. But, at present, the experimental evidence for the hypothesis is inadequate.

“The data are a mess,” Comfort says. The ASU physicist hopes that the greater precision of the spin-controlled experiment will help.

At Los Alamos, Comfort and his colleagues send beams of pions into a target of butanol. Butanol contains a large number of hydrogen atoms. Hydrogen is a useful target because it has just one proton and one electron.

Using strong magnets, microwaves, and intense refrigeration, Comfort’s research team can orient the protons in the target either spin “up” or spin “down.” They then fire beams of negative pions at the target. Neutral pions are emitted and make signals in large detector arrays.

Alarcon and Comfort are working on a similar experiment at the National Institute for Nuclear Physics and High-Energy Physics (NIKHEF) in Amsterdam, Holland. At NIKHEF, researchers fire a high-current beam of electrons through a polarized target of deuterium gas. Deuterium also is known as “heavy hydrogen.” In a bit of scientific serendipity, they are getting two experiments for the price of one.

Alarcon wants to find the exact distance at which the nucleons in deuterium begin to repel one another. Comfort wants to know how much energy it takes to push them apart. Both answers can be found by analyzing the same data.

Alarcon looks for the rare events when a high-energy electron knocks a whole deuterium nucleus into the detectors, and is itself detected. The process is called elastic scattering, because all the kinetic energy contained in the nucleus elastically returns as kinetic energy. By analyzing the energy levels, Alarcon hopes to calculate the closest distance nucleons can get to one another inside the nucleus.

Alarcon and Comfort are part of a team installing a new detector at the Bates/MIT Laboratory in Boston. The detector is known as OOPS, short for Out-Of-Plane Spectrometer. Using OOPS, the scientists hope to learn about the dynamics of quarks within the nucleon (protons or neutrons). They will focus studies on a bit known as the “delta particle.”

“The delta particle is the first excited state of the nucleon,” Comfort explains. “In the language of quarks, one of the quarks ‘flips its spin.’ As a result, the nucleon as a whole changes from a spin of 1/2 to a spin of 3/2. Ordinarily, the spins of two quarks cancel each other out,” he adds.

The delta particle is about a third more massive than either the proton or neutron. It also is very unstable, decaying quickly into a pion then back into a nucleon. Alarcon and Comfort want to know how this transition occurs. What exactly are the quarks doing?

If it is possible for a quark’s spin to simply flip, they’ll get one type of data. If the transition is more complex, they should see evidence of that more complex mechanism.

Alarcon’s excitement for his chosen profession is evident as he speaks. “Our detectors are much better than they used to be. We can now get very precise information about the events,” he says. “The fact that we can control spin will give us much better data, better than ever before.”

In any particle physics experiment, capturing the collision events is always a matter of statistics. Many electrons pass undisturbed through a target. Many others create ordinary events that already are well understood. But a few will be different and interesting.

Much of the effort expended by experimental physicists is devoted to increasing the number of these interesting events. But they cannot be increased to the point where the detectors become so deluged with particles that nothing can be discerned. At Bates and NIKHEF, scientists often direct electrons into a circulating ring where they can pass many times through a very thin target. Using this technique, the researchers generate a fairly well-spaced flow of events. Electrons that pass through a target the first time can come around for a second try.

Both Alarcon and Comfort look forward to working at a new facility in Virginia known as the Continuous Electron Beam Accelerator Facilty (CEBAF). The new accelerator will provide a continuous, high-current polarized beam of electrons. The continuous beam will allow scientists to conduct a variety of experiments that should build a wealth of new data about the heart of the atom.—John Svetlik