Several groups of physicists may have found a new kind of particle in which gluons, evanescent "force particles," behave as ordinary matter. Gluons usually flit between quarks, the basic components of protons, neutrons, and other matter, binding them without contributing mass. But in this week's Physical Review Letters (PRL) and at a conference last week at the Brookhaven National Laboratory in Upton, New York, the physicists reported that they may have created an "exotic meson" in which the gluon has a more substantial role.
The concept of a hybrid particle of gluons and quarks was born in the early 1980s, when theorists suggested that the equations of quantum chromodynamics--the theory describing the strong force transmitted by gluons--allowed for its existence. Made of a ordinary quark, an antimatter quark, and an excited gluon, the hybrid would be short-lived, but a distinctive pattern of decay products would signal that it had been created. In 1994, an international group of physicists at Brookhaven set out to look for it with a refurbished detector attached to the Alternating Gradient Synchrotron accelerator. The experiment sends a beam of pi mesons--particles consisting of a quark and an antiquark--crashing into a liquid hydrogen target, creating a spray of secondary particles captured by the detector.
In some 200 million collisions, says Suh-Urk Chung, a spokesperson for the experiment, 40,000 showed signs of the signature decay expected by the hybrid, which existed for all of 10-23 seconds. The Brookhaven team reports in PRL that the data point to a new particle with a mass of 1400 million electron volts and a set of characteristics that can't be explained by a typical quark-antiquark meson.
Physicists say they're heartened by the nearly immediate confirmation reported last week at the Brookhaven meeting by a collaboration from CERN, the European particle physics laboratory in Geneva, and by a group from Russia.
But there is still plenty of room for doubt about the hybrid, say researchers. The physicists acknowledge that what they're seeing may not be a quark-antiquark-gluon combo but a particle made of two quarks and two antiquarks--a new kind of particle, but less exciting to theorists than one containing a gluon. "We don't know," says Chung. "All we know is that it cannot be quark and antiquark only."