Would You Believe Lemon Leptons And Magic Muons?

I deplore articles

On nuclear particles.

I see no reason

To extol the meson.

Colored or albino,

I dislike the neutrino.

I never carry on

With the bare baryon,

Nor can I have it on

With the glum graviton.

I remain in the dark

On the charms of the quark,

Nor am I adept on

The loves of the lepton.

Strangeness and charm

Cause me alarm.

I deplore articles

Pro- or anti-particles. --Allen Herzog

Don't pick on small particles

You find in my articles.

My next theory, if true,

Has some bigger than you!   --Sheldon L. Glashow

Sheldon Lee Glashow is a charming man. Five days a week the tall, pot-bellied, cigar-puffing physics professor shuffles into his third-floor office, plunks himself down behind his desk, strikes a match to a gigantic role of cured tobacco leaves and plies his trade--pondering the fundamental constituents of matter. And the profundity of Glashow's thoughts have made him a founding father of the theory of "charmed particles."

What is modern particle physics--or, for that matter, Sheldon Glashow--all about? Glashow gives a pithy, if somewhat superficial account in the introduction to his Oct. 1, 1975 article in Scientific American, "Quarks with Color and Flavor." Glashow writes:

Atomos, the Greek root of 'atom,' means indivisible, and it was once thought that atoms were the ultimate, indivisible constituents of matter, that is, they were regarded as elementary particles. One of the principal achievements of physics in the 20th century has been the revelation that the atom is not indivisible or elementary at all but has a complex structure. In 1911 Ernest Rutherford showed that the atom consists of a small, dense nucleus surrounded by a cloud of electrons. It was subsequently revealed that the nucleus itself can be broken down into discrete particles, the protons and neutrons, and since then a great many related particles have been identified. During the past decade it has become apparent that those particles too are complex rather than elementary. They are now thought to be made up of the simpler things called quarks. A solitary quark has never been observed, in spite of many attempts to isolate one. Nevertheless, there are excellent grounds for believing they do exist. More important, quarks may be the last in the long series of progressively finer structures. They seem to be truly elementary.

Glashow has a large, well-lit office in Lyman laboratory, near the Law School. A huge floor-to-ceiling bookcase filled with copies of the Physics Review and unbound notes lines one wall. Some colorful charts of fish from a local food-packing company and a map of Boston decorate another. The third has a blackboard on it covered with scientific-type scribbling and a picture of Glashow and Howard M. Georgi III, associate professor of Physics and frequent collaborator with Glashow. Georgi and Glashow face each other in the picture, bemused. A cartoon-type bubble pasted on the picture depicts them berating each other with the caustic phrase, "You dummy!" A sketch of Einstein and an enlarged photograph of a delicate white web of spirals on a black background decorate the fourth wall. Cross-country skis stand in the corner.

Glashow sauntered in, put his feet up on his desk, and began to explain atomic physics:

The quantum mechanics of the '30s, formulated by Schroedinger, Heisenberg and others, made astonishingly successfuly predictions about such atoms. Physicists turned their attention from the atom to the nucleus.

The force holding the nucleus together was still an enigma. Theoretically, a nucleus should fly apart since it is composed of identically charge particles. The gravitational attraction of two protons is negligible when compared to their electric, or electromagnetic, repulsion. In fact, atomic nuclei are very tightly bound. Confronted with an otherwise successful theory and this apparent empirical contradiction, physicists simply invented another force, which protons and neutrons feel, but to which electrons are immune. Because it is so strong, they named it the "strong" force.

Here a short digression is necessary. Modern quantum mechanics describes the forces between particles in terms of other, "mediating" particles. The mediating particle of gravity is called the graviton. The mediating particle of the electromagnetic force is called the photon. And the mediating particle of the "strong" force, proposed in 1934 by the Japanese physicist Hideki Yukawa, is called the "pion."

In 1947, the pion was actually observed. Much had happened in the interim, not the least of which was the discovery of the muon in 1937, and of the "strange" particles in 1944, so named for their inexplicably long lifetimes. In the '50s, many more new particles were discovered. Suddenly, "the elementary particles" did not seem quite so elementary.

One gets the impression of a jumble of particles and physicists. In fact, things were somewhat more orderly than they might seem. Glashow explains the scheme in a diagram accompanying an article he wrote last July. He first divides elementary particles into "carriers of force" and "carriers of mass." Carriers of force are the mediating particles described above (although the pion is not a member of this class). Carriers of mass comprise everything else.

The carriers of mass can be further divided into leptons, which do not feel the "strong" force, and hadrons, which do. There are four leptons: the electron, the muon, the electron neutrino and the muon neutrino. There are hundreds of hadrons. The neutron and proton are both hadrons and so are subject to the paull of the "strong" force.

Leptons and carriers of force are orderly families. Hadrons are particles of definite mass.

In an attempt to bring some order to the unruly world of hadrons, Murray Gell-Mann, a physicist at the California Institute of Technology, proposed the quark theory. All hadrons, said Gell-Mann in 1963, can be composed of two or three quarks or their anti-particles. Quarks come in three "flavors"--up, down and sideways. Down quarks ('d') have an electric charge of -1/3, up quarks ('u') have charge + 2/3. And then there are sideways quarks ('s'). Sideways quarks are used to make "strange" particles, the ones with the long lifetimes, and are said to have "strangeness" 1. All other quarks have strangeness zero. The anti-particle of each quark (designated here by an *) has properties exactly opposite the appropriate quark. Thus the 's*' quark has charge +1/3 and strangeness+1.

Gell-Mann's theory, modified so that each quark also comes in three different "colors" (no kidding), proved tremendouly successful. One could then describe every hadron by some quark combination, and every quark combination by an observed hadron. (A proton, for instance, is the combination 'uud.' The neutral K-meson, which has strangeness *1, is the combination 's*d.')

Finally the story moves to Glashow's contribution, which came in 1964. Motivated by considerations of grace and elegance in theories which combined the electromagnetic force and a force called the "weak force" (to distinguish it from the "strong force"), Glashow and another physicist, James D. Bjorken, postulated an additional flavor of quark, and named it the "charmed" quark. Glashow writes in a New York Times article:

"The case for charm--or the fourth quark--became much firmer when it was realized that there was a serious flaw in the familiar three-quark [flavor] theory, which predicted that "strange" particles would sometimes decay in ways that they did not. In an almost magical way, the existence of the charmed quark prohibits these unwanted and unseen decays, and brings the theory into agreement with experiment. Thus did my recent [1970] collaborators, John Iliopoulos, Luciano Maiani and I justify another definition of charm as a magical device to avert evil."

More support for Glashow's work came in 1974 with the discovery of the J or psi particle at Brookhaven National Laboratory and the Stanford Linear Accelerator Center. The particle was eventually interpreted as a combination of a charmed quark (c) and a charmed anti-quark (c*). But final confirmation came in May 1976, when the Stanford team, led by Gerson Goldhaber, found incontrovertible evidence that charmed particles exist.

"I saw Gerson at a conference in April," Glashow said, "and told him he'd better get on the stick. I said, 'Look, Gerson, this is getting embarassing for you. You'd better find some charmed particles.' Three weeks later he called me up and told me he'd found them."

Glashow is currently examining the ways in which quarks combine to form elementary particles, a subject he calls "chromodynamics," an allusion to the "color" attributed to quarks. Actually, neither "color", "flavor," "charm," nor "strangeness" has any correlation to the common-sense meaning of the words. They are just ways of labeling the various attributes of quarks and could just as easily be called "beauty," "faith" or "hope."

***

Glashow attended the Bronx High School of Science, a public school for the scientifically gifted. He once claimed that the school made the single most important contribution to his education. He no longer holds this view, although he still calls his high school experiences "particularly significant."

Glashow's closest friends in high school were Steven Weinberg, who is now Higgins Professor of Physics, and Gary Feinberg, now a professor of physics at Cornell. "We learned a great deal by talking and competing with one another. We'd ride the subway to school together. On the way, one of us would say something like 'Quantum mechanics isn't so hard--I learned it last night,' and then explain whatever he'd read," Glashow said. Not to be outdone, someone else would read about another topic that evening and explain it the following morning. "In this way, we certainly learned the language of physics, although nothing in detail," Glashow said.

Glashow's memories of school are somewhat less sublime. "For 'physics' we had a choice between automotive and electrical engineering...My teacher was 'Mad-Dog' Tyson, who had a mania for keeping students quiet in the halls. He was also preoccupied with neat notebooks."

Glashow recalls reading a good deal of science on his own, but does not describe himself as a prodigy. "I frankly don't remember knowing very much, although my teachers were impressed with how much I knew."

His first serious scientific interest was chemistry. "There was my period of explosives," he says, somewhat wistfully. "in those days you could get any chemicals you wanted. I would mix them together in various random ways. At one time I had a basement full of liquid bromine."

Glashow synthesized nitrogen tetra-iodide, an extremely unstable compound which explodes when anything is dropped on it, and took to soaking dollar bills in nitric acid. This turns the dollar bill into flashpaper, which burns very rapidly and leaves no ash. Glashow gave this up when it became too expensive.

Eventually, Glashow dropped chemistry altogether. "Chemistry is good for fun--it's like baseball," Glashow said. "It has its role for small children, but I can't see an adult being concerned with it."

Glashow ultimately went to Cornell, where he received a B.A. in physics. "I was mostly interested in avoiding difficult problem sets," Glashow said of Cornell. In 1954 he came to Harvard, where he studied physics under Nobel laureate Julian Schwinger.

"From 1958 to 1966 I was in exile," Glashow said. "I just wandered around teaching, waiting for an offer from Harvard." Since 1966, Glashow has worked on particle physics at Harvard.

Glashow fears that the startling successes of contemporary physicists may eventually render physics a "complete" science in the same way that he considers chemistry a complete science, lacking any "interesting" questions. He laments the lack of great contradictions such as those evident in early 20th century physics. From such contradictions the theory of relativity and quantum mechanics grew. In the absence of such contradictions, physics proceeds too smoothly, Glashow reasons, and becomes less interesting.

Glashow draws an analogy with biology, saying that once people agree biology is chemistry it is completely known. "The fact that it's known doesn't make it trivial, but we're interested in the rules of the game, not the play," he says.

What important physical questions remain unanswered? Glashow points to "quark confinement"--the fact that quarks, although an amazingly successful theoretical construct, have never been seen. A good explanation of their reclusive nature is desireable.

Glashow does not think about the social implications of his work, of which he believes there are none. In a 1974 lecture entitled "Something exciting Is Happening in Particle Physics" Glashow asked, "What are the implications of the charmed quark for--anything?"

"The problem we're faced with is we're given a world, which we're not terribly responsible for ourselves, and we're trying to understand it. We hit these particles against each other, or, in the beginning [of particle physics], we just looked at the cosmic rays as they came at us. The particles hit particles and strange particles come out. These particles are totally unnecessary--you can't make toothpaste out of them.

"But they're there! And you want to understand, whatever 'why' means, why they're there... Charmed particles don't help anything at the practical level, except our understanding of how this whole thing is put together.

"I could make very attractive theories with just u quarks, d quarks, electrons and neutrinos. We don't need strangeness, we don't need charm and we don't need muons. They seem to come together as a package of mysterious unexplained things, none of which have anything to do with toothpaste. But they're all there.

"I wish they weren't, too, because I could make theories without them. But they're there, some of them, at least. And we have to try to explain them."