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The Search for QCD Exotics

Particles predicted by the theory of quantum chromodynamics help explain why the fundamental building blocks of matter are impossible to isolate

Alex Dzierba, Curtis Meyer, Eric Swanson

For the past quarter-century, physicists have suspected that a subatomic particle unlike any other must exist?one made of the very glue that holds matter together at the most fundamental level. This glue acts within the nucleus of every atom, binding together charged particles that would otherwise repel one another. But might this force itself exist as a form of matter? Recently, the search for this elusive particle, nicknamed the "glueball," has intensified as tantalizing hints of its presence have appeared. And investigators have found evidence of related particles that are just as exotic. Now, more than a century after the first subatomic building block, the electron, was discovered, physicists may be on the verge of uncovering a whole new class of matter.

To appreciate why physicists like ourselves are keen to study glueballs and other similarly exotic particles requires a little patience and best begins with a short detour into the history of our discipline.

After the electron became known in 1897, 14 years elapsed before scientists discovered the proton, and another 21 passed before they recognized the neutron. By 1932, the electron, proton and neutron were sufficient to explain all of particle physics, and there was a comforting feeling that the subatomic world had been fully mapped out. Alas, that smugness was about to give way to exasperation and confusion.

The trouble really had begun in 1910, when Theodor Wulf, a Jesuit priest and physicist, climbed the Eiffel Tower with an electrometer strapped to his back. This device, which Father Wulf designed and built himself, detected energetic charged particles. Knowing that radioactive minerals give off such charges, he expected that his electrometer would be less affected when he raised it high off the ground. But he was surprised to find an increased level of activity after he scaled the tower. The explanation: Subatomic particles rain down from space.

Although the source of these particles remains something of a mystery to this day, their reality was apparent to all physicists by the 1920s and 1930s. And after the dislocations of World War II, young men and women began climbing mountains in the Pyrenees and Alps to study these "cosmic rays" more carefully. The detectors then used consisted of large stacks of photographic plates, which literally photographed the miniature trails of destruction the speeding charges left behind. These efforts, along with the analysis of particles created soon afterward in giant atom-smashing accelerators, revealed an ever-growing list of fundamental particles that arise under extreme conditions?kaons, pions and lambdas, to name just a few.

Figure 1. Curving lines . . .Click to Enlarge Image

Roughly 200 such particles are now known. Physicists initially separated them into two classes according to their mass: Mesons (from the Greek μεσο, meaning "medium") weigh more than an electron but less than a proton; baryons (βαριο, "heavy") weigh as much or more than a proton. The modern division depends not on mass but on the spin of the particle. Mesons carry integer spin, and baryons carry half-integer spin, measured in units of Planck's constant, h, divided by 2π. (The closest analogue to this purely quantum mechanical quantity would be the speed of a spinning top.)

Together the mesons and baryons are called "hadrons" (which comes from αδρος, "strong"), because these particles all feel the strong force, which along with the electromagnetic force, the weak force and gravity constitute the four basic forces of nature. The first indication for the strong force came in the 1930s when it became obvious that the nuclei of atoms contain tight groups of protons and neutrons. This fact was difficult to understand, because the mutual electrostatic repulsion of the positively charged protons should cause nuclei to fly apart. Physicists soon decided that another fundamental force was needed, one that must act over just a short range, because it had never been detected outside the nucleus.

Figure 2. Among the many subatomic particles . . .Click to Enlarge Image

So it was clear enough early on that the strong force exists and that hadrons all feel the strong force. But why were scores of hadrons cluttering the formerly tidy subatomic world? That puzzle remained unsolved until 1961 when Murray Gell-Mann (then a professor at the California Institute of Technology) and Yuval Ne'eman (then an Israeli military attach? in London who was also studying physics at Imperial College) independently proposed the solution. Each realized that the known subatomic particles could be grouped according to a certain mathematical symmetry, which Gell-Mann called the eightfold way, in reference to the Buddhist "eightfold path" to enlightenment.

Support mounted surprisingly swiftly. Just a few months after conceiving the new theory, Gell-Mann attended a conference at CERN, the European particle physics laboratory in Geneva, and was in the audience when a group from the University of California, Los Angeles announced the discovery of two new baryons, Ξ*? (xi-star-minus) and Ξ*0 (xi-star-naught). Gell-Mann realized that this pair nearly completed a group of ten related particles. Not only did he know immediately that another particle in that family had to exist, but he could estimate its properties. Gell-Mann made this bold prediction in front of the assembled physicists, and the race to discover the new particle?called Ω? (omega-minus)?was on. By February 1964 a team at Brookhaven National Laboratory in New York had seen evidence for it. Confirmation arrived from CERN within a few weeks.

Figure 3. Seven particles . . .Click to Enlarge Image

That same year Gell-Mann and George Zweig (who was working at CERN at the time) separately suggested that the symmetry of the known hadrons existed because all are constructed from three fundamental subparticles. Gell-Mann dubbed them quarks, a playful word that he liked, in part, because of the way that James Joyce had used it in Finnegan's Wake ("Three quarks for Muster Mark!").

The proposition was a risky one, because the hypothetical quarks were like no other particles. Quarks were said to come in three varieties, whimsically called "flavors" and named up, down and strange. Like other particles, quarks had a mass and a spin, but unlike all others they carried fractional electric charge. The up had charge +2/3, the down and the strange quarks, ?1/3. The crux of Zweig and Gell-Mann's idea was that hadrons are bound states of quarks, just as atoms are bound states of electrons, protons and neutrons. For example, a proton was said to consist of two up quarks and one down, making for a total electric charge of 2/3 + 2/3 ? 1/3 = 1. A neutron has two down quarks and one up, making a charge of ?1/3 ?1/3 + 2/3 = 0. Fractional charge was a wild notion, but perhaps the most disturbing aspect of the theory was that no quark had ever been seen?something for which Gell-Mann and Zweig offered no explanation.

Figure 4. Quark models . . .Click to Enlarge Image

Nonetheless, quark theory gained support. It got a big boost just four years later, when investigators at the Stanford Linear Accelerator Center showed definitively that protons have substructure. Those results came from what was essentially a modern rendition of experiments Ernest Rutherford and Ernest Marsden performed in 1909. At the time, Rutherford and Marsden were shooting alpha particles through thin sheets of mica. What they found was that the particles were only slightly affected the vast majority of the times they penetrated the material. On rare occasions, however, an alpha particle would carom off at a large angle. Rutherford was thunderstruck and declared "It was as though you had fired a 15-inch shell at a piece of tissue paper and it had bounced back and hit you." He soon realized that the bizarre behavior implied that atoms have small but massive cores?that is, he had discovered the nucleus.

In the Stanford experiments of 1968, physicists were scattering electrons off protons when they observed that a small but significant fraction of the electrons made large deflections?revealing the substructure of protons and, by extension, other hadrons. Further probing provided evidence that the subparticles carried fractional charge.

These discoveries mark a watershed. Before that time, many physicists (including Gell-Mann) believed that quarks were merely a mathematical contrivance that helped to systematize the hadronic world. Now it was starting to look as though quarks might exist after all. Finally, in 1974, even the most recalcitrant were won over by the announcement that a new particle had been discovered. This particle, a meson, was made of a fourth flavor of quark, called charm. (Since then two more flavors have been added to the menu: bottom, in 1976, and top, in 1995. Each of these six quarks has a corresponding "antiquark," bringing the total to 12.)

Figure 5. Set of building blocks for matter . . .Click to Enlarge Image

Although the quark hypothesis was enjoying a brilliant success, there were several lingering worries. For one thing, no one had ever seen an isolated quark. A more nagging problem was that the measured properties of a baryon called the Δ++ (delta-plus-plus) seemed to disagree with a general theorem of quantum field theory. That theorem states that the quantum-mechanical wavefunctions that describe hadrons must be antisymmetric if the constituent subparticles are identical. The Δ++ was known to consist of three identical up quarks, yet its observed properties pointed to a wavefunction that was symmetric.

Theorists went to great lengths to resolve the conundrum. Oscar Greenberg of the University of Maryland offered one of the more creative solutions. He proposed that quarks carry a new type of charge that forms itself into an antisymmetric wavefunction. Greenberg's mathematical legerdemain skirted the problem, in essence by declaring that the total wavefunction of the Δ++ was antisymmetric after all; physicists had just missed counting some of it. Gell-Mann dubbed the new attribute color, although it had nothing to do with the usual definition of the word. Such "color charge" presumably came in three varieties, often labeled red, green and blue.

Figure 6. Quarks were endowed with Click to Enlarge Image

But experimentalists had never observed color. So it seemed that one problem (the symmetry of the Δ++) had just been replaced with another (the absence of color). Yet Greenberg's invention had achieved a certain economy: The inability to observe isolated quarks and the inability to observe color combined to become the "color confinement hypothesis," which states that color is cloistered inside hadrons and can never emerge to interact with any sort of detector. Rather, colors must always aggregate so as to produce a colorless object. Protons, neutrons and other baryons are a combination of three quarks of different colors (red, green and blue), which, like the red, green and blue phosphors of a television screen, combine to produce a colorless mixture. Pions, kaons and other mesons are formed from a quark of a given color and its antiquark made of its "anticolor."

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