Demolition Derby of Physics Jars Loose Clues on Subatomic Glue
Particle physicists are known as the demolition crews of the very small, smashing tiny bits of matter together to find the even tinier bits that they are made of. So it may come as a surprise that the field has recently found a powerful new engine of discovery: gluing it all back together again, sometimes in weird ways that seldom occur in nature, if ever.
The glue linking these discoveries is the "strong force," which is normally relegated to holding together quarks, the building blocks of humdrum particles like protons and neutrons. But theorists have long suspected that the strong force has a wild side and that it should be able to take the subatomic equivalent of a Tudor chimney here, an Art Deco facade there, and stick them together into entirely new particle types.
Hints of those strange creations began turning up this year. Although quarks normally congregate in twos and threes, several laboratories said this summer that they were seeing what appeared to be ungainly clumps of five quarks.
In a paper to be published tomorrow, the High Energy Accelerator Research Organization in Tsukuba, Japan (called KEK for its Japanese acronym) describes what the researchers believe were two pairs of quarks dancing close enough to form a single new particle.
THe finding at KEK, which followed a related discovery at the Stanford Linear Accelerator Center this year, could also turn out to be a rare combination of just two quarks called charmonium, some theorists believe. Either way, the rush of results is expected to lend fresh insights on the strong force, widely considered among the most opaque and intractable parts of the Standard Model, the theory physicists use to explain matter's basic structure.
"We don't understand how this force really works when it gets very strong — when it makes atomic nuclei, for example, or makes these particles," said Dr. Frank Close, a professor of theoretical physics at Oxford University in England.
Dr. Close likened scattered clues on the way the strong force works to undeciphered hieroglyphs. "We need more hieroglyphs to decode them," he said. "These three discoveries are very important hieroglyphs."
The discoveries are especially welcome in a field that is in some ways adrift, lacking any big new machine to smash matter into ever-finer pieces. In 1993, Congress killed the Superconducting Super Collider, a vast particle accelerator that was to have begun collecting data by this year. A multibillion-dollar, multinational collaboration in Geneva called the Large Hadron Collider is not expected to be ready before 2007.
Dr. Robert Cahn, a particle physicist at Lawrence Berkeley National Laboratory who works on what is called the BaBar experiment at Stanford, gave the particle interregnum a name: the non-S.S.C. era.
But then, he said, "Here this thing comes in from left field."
It is not hard to see why a full understanding of the strong force has eluded theorists for decades. Although physicists know that protons and neutrons are made of groups of three quarks, not even these great demolition experts of science have been able to knock a quark free. That is because the strong force does not become weaker — in contrast to gravity or electrical forces — the farther the particles are apart. They can never escape the sticky embrace of another quark.
The swarming particles that transmit the strong force from quark to quark, called gluons, are shape-shifters. They spend part of the time in the guise of other quarks. That habit means that heavy particles like protons and neutrons are also filled with these more evanescent quarks winking in and out of existence.
Just to make things as complicated as possible, there are eight different kinds of gluons, each with a different type of "charge." And there are six varieties of quarks, each with its own cute name: up, down, top, bottom, charm and strange.
With a good-natured dig at ambitious theoretical efforts to unify the forces in the Standard Model (strong, weak and electromagnetic) with gravitation, Dr. Chris Quigg, a particle theorist at the Fermi National Accelerator Laboratory in Illinois, said: "It's only a theory of everything if you can explain all the things. The experiments are forcing us to try to understand the theory in places where the calculations are difficult."
He added, "If you call yourself a theorist and have any self-respect, you have to take the challenge."
In an episode that has become almost nonexistent among latter-day particle collaborations, usually consisting of hundreds of physicists working in antlike synchrony, that challenge began this year when a single scientist noticed something odd in the data flowing from BaBar.
Like many experiments now, when physicists have no hope of passing the high-energy frontier in collisions, BaBar uses a different strategy: creating vast numbers of particles that are closely monitored when they decay into other products, with an eye to measuring their properties precisely and finding exotica.
Scientists like to call this the luminosity frontier, said Dr. Stephen L. Olsen, a physicist at the University of Hawaii, drawing an analogy between intense particle beams and bright light beams. BaBar, for example, has created some 150 million B-meson pairs, particles made of a bottom quark and an up or down one.
In January, Dr. Antimo Palano, a collaborator on the experiment from Bari University in Italy, was checking a decay process and saw a small bump in the data sample. He added more data, and the signal kept getting bigger. The announcement of a new particle was made by the collaboration in April.
"I was looking for something new in the data, and I was lucky to see it," Dr. Palano said in a telephone interview from his office in Italy.
But what was the particle? Dr. Palano and some colleagues believe that it was a long-sought type of D-meson that contains a charm quark and a strange quark. In fact, Dr. Estia Eichten, a theorist at Fermilab, said he and others had predicted an incorrect mass for the particle — a reason no one had found it before.
If that interpretation turns out to be right, it will shed light on the workings of the strong force, Dr. Eichten said: his faulty calculations assumed that the two quarks whirled around each other like the proton and electron of a hydrogen atom. Improved calculations suggest the mesons are tethered as if by a rubber band, with one of the quarks behaving as if it were nearly massless.
But other physicists, like Dr. Close of Oxford, suggested that the particle's surprising mass could be explained more easily if it were a kind of molecule of two other mesons, whirling about each other and exchanging still other particles that help them stick together.
Dr. Close said that debate remained unresolved. But by July, the Thomas Jefferson National Accelerator Facility in Newport News, Va., had presented data that might have clinched the case that agglomerations of five quarks had been seen: two up, two down, and one strange quark swirling about in a confraternity that had never been seen before.
"At the present moment," said Dr. Lawrence Cardman, an associate director at the accelerator, "there is to the best of my knowledge no model that explains all of the data."
In one view, the up quarks bind relatively tightly to the down quarks, with the strange quark standing alone, and the whole contraption tumbles about like a three-atom molecule. Another says a vibrating, rotating clump forms, consisting of a two-quark and a three-quark nugget.
Finally, in the paper in tomorrow's issue of Physical Review Letters, a collaboration at KEK says that it has come up with another startling find, which it calls X(3872). A number of theorists believe the particle is a kind of charmonium — the term for a pair of charm quarks orbiting each other. Some scientists, including Dr. Olsen, the collaboration's spokesman, believe they have turned up another quark molecule.
This time, the molecule would contain a pair of mesons, each consisting of a charm quark and an up quark. Somehow the whole seemingly fragile collection would be sticking together long enough to be detected. Even with those questions, Dr. Olsen said, "we're really leaning toward this molecular point of view."
Add it all up, and it has been a hot time for those who study the strong force, said Dr. Close, who added that scientists had moved nearer to understanding the mysterious glue of the atomic nucleus.