UPTON, NY -- Physicists working at the U.S. Department of Energy's
Brookhaven National Laboratory have found evidence of a new and rare subatomic particle, called an "exotic meson." The finding helps validate the central theory of modern physics, called the standard model.

The team of 51 researchers, led by physicists from the University of Notre Dame and including scientists from Brookhaven and five other institutions, found the particle evidence after five years of needle-in-a-haystack searching through the reaction products of billions of particle collisions at BNL's Alternating Gradient Synchrotron (AGS) accelerator. Their paper is published in the September 1 issue of Physical Review Letters.

"To find evidence of a particle that has never been detected before, and one that's so important to our understanding of elementary particles, is hugely satisfying," said Brookhaven's Suh-Urk Chung, a co-spokesman for the collaboration. "Just as satisfying is finding it at the AGS, which has been the home to so many particle discoveries over the decades and, as our work shows, is still a great machine for high-energy physics."

Chung explained that the collaborators' experiment, known as E852, closely examined each of the particles produced when an 18-billion-electron-volt AGS particle beam hit a target of liquid hydrogen in an instrument called the Multi-Particle Spectrometer. The physicists then weeded out possible exotic mesons from the billions of particles produced in the target, and analyzed their results using sophisticated statistical techniques.

"This is a very important observation," said Ted Barnes, a theoretical physicist at Oak Ridge National Laboratory and professor of physics at the University of Tennessee who is not affiliated with the collaboration. "Theorists have predicted the existence of exotic hybrid mesons, which contain both quarks and gluons, since the late 1970s, but E852 may have found the smoking gun for their existence. It's a benchmark that will set the mathematical scales for future experimentation and theoretical study," he said.

The work depended on the teamwork of scientists, graduate students and undergraduate students from Notre Dame, BNL, Northwestern University, Rensselaer Polytechnic Institute, the University of Massachusetts Dartmouth, and Russia's Institute for High Energy Physics and Moscow State University. The collaboration is funded by DOE and the National Science Foundation.

An Odd Bird in the Particle Zoo

Many of the elementary particles studied by physicists have an internal structure. The building blocks of this structure are called quarks, anti-quarks (which are the anti-matter opposites of quarks), and gluons, named for their ability to hold quarks and anti-quarks together like glue.

Elementary particle physicists have divided these particles into two types, called baryons and mesons, depending on which building block combinations they are made of. Baryons (such as the familiar proton and neutron) contain three quarks, while mesons contain a quark and an anti-quark. The quarks in both baryons and mesons are bound together by gluons.

In one theoretical model of how a meson is structured, the gluon is pictured as a sort of string stretched between the quark and the anti-quark. The quark and anti-quark "pull" on opposited ends of the string, much like contestants in a tug-of-war.

In normal mesons, the gluon "string" is stretched tight, in what is called its ground state, and is not vibrating. But theorists have expected that mesons should also exist in states where the gluon string is oscillating, like a plucked violin string.

Some of those theoretical mesons have been predicted to have properties not allowed in normal mesons, making them "exotic." A meson could also have these exotic properties if it consisted of a pair of quarks and a pair of anti-quarks, instead of just one quark and one anti-quark.

Explained Chung, "Our new meson is exotic because it can't be a meson made of a quark and an anti-quark with a ground-state gluon string. It must be either a quark and an anti-quark with a "plucked" string, or a four-quark system bound by a ground-state string."

Trust, but Verify

Despite their confidence in their finding, and the fact that their results have been reviewed by other physicists, the E852 collaboration is now gathering and examining even more data, and plans to use an independent detection method to verify their original exotic meson evidence. They also hope to search for more exotic mesons.

They presented their findings to their peers at the recent biennial International Conference on Hadron Spectroscopy, held at Brookhaven in late August. Also at the conference, possible confirmation of the result was reported by the "Crystal Barrel" collaboration at CERN, the European particle physics laboratory.

Three Nobel prizes in physics have been given for discoveries at BNL's Alternating Gradient Synchrotron since it was built in 1960, and several new particles have been first glimpsed there. The AGS has been improved and upgraded many times in its 37-year history, and is now used by high-energy and nuclear physicists, and radiation biologists. Beginning in 1999, the AGS will also inject heavy ion and polarized proton beams into the
Relativistic Heavy Ion Collider (RHIC), BNL's newest accelerator.

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Northwestern University
Kam Seth, Principal Investigator
Chuck Loebbaka, Director of Media Relations
Pat Tremmel, Assistant Director, Media Relations
Phone: 847-491-4887
Fax: 847-491-2376
e-mail: c-loebbaka@nwu.edu, p-tremmel@nwu.edu

Rensselaer Polytechnic Institute
Gary Adams, Jim Napolitano, Co-principal Investigators
R. Bruce Adams, News and Communications
Phone: 518-276-2840
Fax: 518-276-6091
Home: 518-885-4258
E-Mail: adamsb@rpi.edu

University of Massachusetts Dartmouth
Zvi Bar-Yam, John Dowd, Wolfhard Kern, Co-principal Investigators
Maeve Hickok, Executive Director, Information/News/Publications
Phone: 508-999-8765
Fax: 508-999-9200
E-mail: mhickok@umassd.edu

University of Notre Dame
Neal Cason, Principal Investigator
Dennis Brown, Associate Director, Public Relations and Information
(219) 631-7367
fax: (219) 631-8212
E-mail: brown.18@nd.edu

Moscow State University
L.I. Sarycheva, Principle Investigator

Institute for High Energy Physics
S.P. Denisov, Principle Investigator

E 852 Collaboration home page on the World Wide Web