Exotic needle found in particle haystack.

In an experimental triumph that helps validate the standard theory of the particles and forces of matter, physicists have found evidence of a rare particle known as an exotic meson.

"We think we are the first group to have finally identified such a particle," says Suh-Urk Chung of the Brookhaven National Laboratory in Upton, N.Y. Physical Chung and his collaborators report their results in the Sept. 1 Physical Review Letters.

"This is a very important observation," says theoretical physicist Ted Barnes of the Oak Ridge (Tenn.) National Laboratory. "It's the first evidence of a new combination of quarks and gluons."

A meson is a particle made up of a quark paired with an antiquark. The pion, for example, consists of a down quark and an up antiquark. The quarks are tied together by gluons, which embody the so-called strong force.

The mathematical relationships of the theory of quantum chromodynamics describe the behavior of quarks and gluons. However, quark-gluon interactions are so complicated that physicists have invented simple models that capture the main features of the phenomena in different energy ranges without requiring them to tangle with the full theory's mathematical details (SN: 8/27/94, p. 140).

When pions crash into protons at an energy of about 18 gigaelectron-volts (GeV), the gluons holding the quarks together behave as if they were elastic strings stretched between the quarks. In contrast, electrons colliding with protons at much higher energies reveal that, in this range, gluons act much more like particles than strings (SN: 9/6/97, p. 158).

In the 1970s, theorists used simplified models to predict the existence of what they called exotic mesons, in which the quark-antiquark pair is linked by gluon strings that vibrate at particular frequencies as if plucked. Another possibility is that the exotic meson contains extra quark-antiquark pairs. Finding evidence of such particle states, however, proved extraordinarily difficult.

Chung and his collaborators analyzed data obtained at Brookhaven's Alternating Gradient Synchrotron in collisions of fast-moving pions; with protons in a liquid hydrogen target. Out of the billions of particles created in the crashes, they found 47,200 instances in which the interaction produced a particle known as an eta meson.

The unexpectedly high yield of the eta meson suggests that it formed via an intermediate stage in which the pion is excited info a very short-lived state--the exotic meson. From the data, the researchers determined the exotic meson's mass, expressed in energy units, to be about 1.4 GeV. "It required a lot of sophisticated analysis, so it took a long time," Chung notes.

At the 7th International Conference on Hadron Spectroscopy, held last week at Brookhaven, members of the Crystal Barrel collaboration at the European Laboratory for Particle Physics (CERN) in Geneva presented preliminary findings that appear to confirm the Brookhaven discovery. Taking a different approach, the physicists used the Crystal Barrel detector to study the products created when antiprotons and protons collide and annihilate each other, and they also found evidence of an exotic meson with a mass of 1.4 GeV.

Theorists still have some puzzles to ponder. For example, the newly discovered exotic meson is less massive than they had predicted. "This is all a little worrying," Barnes says.