Observation of Exotic Baryon

Physicists Find Evidence for an Exotic Baryon

Ordinary hadrons include baryons and mesons, which are described by three-quark and quark-antiquark configurations, respectively. Unusual hadrons that do not fit into this picture would constitute new forms of hadronic matter, or exotic hadrons. Such particle states are not forbidden by the Standard Model of particle physics, but no convincing evidence has come from the myriad of experiments looking for such exotic signatures over the past 30 years.

It is well established from deep-inelastic scattering experiments that sea-quarks (quark-antiquark pairs) are part of the ground-state wavefunction of the nucleon. In addition, it is well known that excited nucleon states are surrounded by a ``cloud" of pions. In this sense, we know that five quark (qqqq qbar) configurations are mixed with the standard three quark valence configuration. It is natural to ask whether a five-quark configuration exists, where the antiquark has a different flavor than (and hence cannot annihilate with) the other four quarks. Evidence of such pentaquark states would be an important addition to our understanding of hadronic theory.

The new data show strong evidence for a five-quark baryon state at a mass of 1.54 GeV, with a narrow width of 22 MeV (FWHM). The experiment, carried out with the CLAS magnetic spectrometer in Hall B at Jefferson Lab, employed a multi-GeV beam of photons and a deuterium target. The reaction produced a K- meson and a proton in the final state, along with the five-quark object, which then decayed into a neutron and a K+ meson. One possible diagram is shown below. The valence quark configuration of the exotic state contains two up quarks, two down quarks, and an anti-strange quark.

The statistical signficance of the measurement if 5.4 standard deviations above the background. The result of the experiment was first announced in May 2003 at the Conference on the Intersections of Nuclear and Particle Physics (CIPANP) in New York by Hall B physicist Stepan Stepanyan (see Figure below). The results of the experiment are about to be submitted for publication to the Physical Review Letters journal pending completion of an internal review by the CLAS collaboration (presently in progress).

The discovery of this new exotic baryon state should have far-reaching consequences for our theory of hadronic interactions that attempt to explain the structure of matter and the possible states of matter. The interpretation of this new pentaquark state as a tightly bound five-quark object or a kaon-nucleon molecule is unclear at this time. Further investigations are continuing in a newly approved experimental program at Jefferson Lab.

Last modified: July 1, 2003
Kenneth Hicks