Pentaquark: New evidence of an exotic five-quark particle

July 21, 2003--At the Conference on the Intersections of Particle and Nuclear Physics held May 2003 in New York City, researcher Stepan Stepanyan from the DOE Office of Science's Thomas Jefferson National Accelerator Facility revealed the most convincing evidence yet of a subatomic particle consisting of five quarks. He was representing his CLAS (CEBAF Large Acceptance Spectrometer) collaboration, a multi-national group of researchers, as he presented Jefferson Lab research supporting the existence of the "pentaquark." CEBAF--Continuous Electron Beam Accelerator Facility--is an Office of Science national user facility managed and operated by Jefferson Laboratory.

For almost 40 years, all subatomic particles have fit neatly into two categories: three-quark baryons, like protons and neutrons; or mesons, made up of one quark and one anti-quark. The new particle spotted at Jefferson Lab is a sort of baryon-meson hybrid with five quarks--or, more precisely, four quarks and one anti-quark. The pentaquark is a member of the baryon family, but it's said to be "exotic" because the anti-quark has a different "flavor" to the other quarks.

There are six known flavors of quarks, three of which are studied at CEBAF. They are called up, down, and strange quarks, with symbols u, d, and s, respectively. The other three known flavors are charm (c), top (t) and bottom (b) quarks. Each of these six quarks also has a corresponding anti-quark.

With six quarks and six anti-quarks to choose from, one could think of many possible combinations of quarks. But not all of them can exist, according to the rules of Quantum Chromodynamics (QCD), the theory that describes the strong interactions between quarks. For instance, QCD forbids four-quark configurations, while the pentaquark that left its signature on the Jefferson Lab data is an allowed state. Physicists know from conservation laws that the only possible configuration for this new particle, dubbed Theta-plus, is two up quarks, two down quarks, and an anti-strange quark (uudd s-bar).

###

Media contact: Linda Ware, JLab Public Affairs Manager, (757) 269-7689, ware@jlab.org
Technical contact: Stepan Stepanyan, JLab staff scientist, 757-269-7196, stepanya@jlab.org

Related Web Links Preprint of CLAS Results available at Los Alamos National Laboratory

Physicists Find Evidence for an Exotic Baryon, Ohio University Physics Department

Evidence for 'Pentaquark' Particle Sets Theorists Re-Joyce-ing, Science (July 11, 2003)

Quark to Pentaquark, Britannica (July 7, 2003)

Physicists discover new form of matter, Die Welt - English Translation (July 7, 2003)

Pentaquarks: New Matter Discovered, Science ORF.at (July 7, 2003)

Pentaquark: A Five Quark State Has Been Discovered, About, Inc., (July 7, 2003)

Lab finds evidence of tinier part of matter, Daily Press, (July 7, 2003)

Wild Bunch: First five-quark particle turns up, Science News (July 7, 2003)

Five Alive!, The Economist (July 3, 2003)

Pentaquark discovery confounds skeptics, NewScientist.com (July 2, 2003)

Behold the pentaquark, BBC News Online (July 2, 2003)

Physicists discover particle with five quarks, PhysicsWeb (July 2003)

Subatomic Discovery Emerges From Experiments in Japan, New York Times (July 1, 2003)

Japanese physicists' 'pentaquark' hints at answers to makeup of matter, USA Today (July 2003)

A Five-Quark State Has Been Discovered, Physics News Update (June 30, 2003)

Pentaquark, newly arrived matter, Le Figaro - English Translation (July 3, 2003)

---

Funding: This research was conducted at the Continuous Electron Beam Accelerator Facility, a DOE Office of Science national user facility, managed and operated by Thomas Jefferson National Accelerator Facility.

Thomas Jefferson National Accelerator Facility is funded by the DOE Office of Science with strong support from the City of Newport News, the Commonwealth of Virginia, and the United States Congress. As a user facility for scientists worldwide, its primary mission is to conduct basic research of the atom's nucleus at the quark level.