Demolition Derby of Physics Jars Loose Clues on Subatomic Glue
By JAMES GLANZ
Published: December 30, 2003
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.