Proton puzzle puts physicists in a whirl.

Proton puzzle puts physicists in a whirl

The more deeply particle physicists probe the proton's structure, the more complicated it seems to get. The lastest experimental result to spur theorists into a flurry of speculation concerns the proton's spin, which is the source of its magnetism. This experiment suggests that very little -- perhaps none -- of the proton's spin comes from the spins of the quarks that are thought to make up a proton. The result raises serious questions about how the proton is put together.

"This is not an experiment telling us about esoteric things that happened in the first microsecond of the Big Bang or in some remote part of the universe," says Francis E. Close of the University of Tennessee in Knoxville. "This is the stuff we're made of, and it's showing that maybe we don't understand it as well as we thought."

Although scientists have raised questions about the validity of the experimental results, reported last year in PHYSICS LETTERS B, Dozens of theoritical papers have addressed the puzzle in recent months. "There is no denying . . . that something new and unexpected is there to be investigated," Close says. "A great deal of work is now being done to examine possible explanations."

Over the years, theorists have developed a number of models for picturing a proton's internal structure. One of the more sophisticated models suggests that each proton contains three principal, or "valence," quarks (two "up" quarks and one "down" quark), tightly bound together by the strong nuclear force, which is carried by electrically neutral particles called gluons. The proton also contains an unruly "sea" of pairs of quarks and antiquarks. All of a proton's constituents are in continual motion.

"The proton has a very well-defined value for spin, but it's got a lot of internal structure," Close says. "The question is how the individual bits and pieces of that structure add up to the proton's spin."

In their experiment at CERN, near Geneva, Switzerland, researchers belonging to the European Muon Collaboration fired muons (particles with the same properties as electrons but with masses about 200 times larger) at protons in the nuclei of ammonia molecules held in a magnetic field. Their surprising results indicate that the spins from all the quarks in the proton--both the valence and "sea" quarks -- cancel out. Moreover, in addition to the "up" and "down" valence quarks, "strange" quark and antiquark pairs seem to play an important role within the proton.

So far, no clear consensus has emerged to explain the findings. One possibility is that a proton's quark constituents are not only spinning but also orbiting one another. Another is that the gluons themselves are spining uniformly in a particular direction. Other experimental data already indicate that gluons carry most of the proton's mass and about 50 percent of a proton's momentum. But the same data also show that "strange" quarks make only a small contribution to a proton's momentum.

"The interesting question is how do you make a picture of the proton in which strange quarks enter in an intelligent way, and how does that dynamics manifest itself in spin and certain other quantities but not in momentum and energy," says Robert L. Jaffe of the Massachusetts Institute of Technology. "Simple rules are not going to work." Jaffe and MIT colleague Aneesh V. Manohar have prepared a lengthy, soon-to-be-published paper on the whole question of proton structure and spin.

"What's being discovered here is the intrinsic complexity of the proton," Jaffe says. "We had hoped to get by without having to deal with this complexity, but we're finding we can't get by without it." In other words, simple models don't work very well for explaining all of a proton's properties.

"In some ways, we're in a very peculiar situation," says Ta-Pei Cheng of the University of Missouri in St. Louis. "We have a theory [quantum chromodynamics and the quark-gluon picture] that most of us believe very strongly to be the correct theory, but we don't know how to derive many basic properties [of particles such as protons]."