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Search for Exotic Matter

Pentaquark
Figure 1. A schematic picture of the Theta+ pentaquark discovery reaction in 2003.

All known hadronic matter is composed of two kinds of quark configurations: baryons, such as protons and neutrons, which are combinations of 3 quarks (qqq), and mesons, such as pions, which are combinations of a quark and an anti-quark (q ). However states of matter consisting of four quarks and an anti-quark (qqqq ), called pentaquarks, are allowed by the theory of strong interactions, Quantum Chromodynamics, and have been the subject of experimental searches since the 1980’s. Spurred by an initial report from Japan in 2003, a total of 11 experimental collaborations from many countries reported possible evidence for a pentaquark state composed of 2 up quarks, 2 down quarks and 1 anti-strange quark, i.e. udud , based on the re-analyses of existing data sets. The exotic nature (strangeness +1, indicating the presence of an anti-strange quark) and the predicted narrow width of this state, called Theta+, makes its identification as a pentaquark state relatively straightforward. However the observed signals had very few counts and were challenged by 13 other experiments which did not see any signal (but the groups were looking at various different reactions, with widely varying experimental setups). The controversy regarding the existence of the pentaquark can best be resolved by new dedicated measurements conducted under the same or similar conditions as the previously positive findings, but with at least 10 times as many counts. This report presents the first result of an experiment dedicated to the pentaquark search undertaken at the Thomas Jefferson National Accelerator Facility. Evidence of the exotic baryon, which is expected to decay into a neutron (n) and positive K meson, (Theta+ -> n K+) was sought in a reaction in which an energetic photon interacts with a liquid hydrogen target yielding an uncharged antikaon meson, K0, a charged kaon, K+, and a neutron, n. In contrast to the positive results from Germany for this specific reaction, no evidence for a Theta+ pentaquark was observed even though the CEBAF experiment was more than 50 times more sensitive. This result is the first to refute a published positive finding of the Theta+ with an experiment designed specifically to search for pentaquark states using similar experimental conditions.

CLAS
Figure 2. A schematic diagram of the CLAS detector at the Thomas Jefferson National Accelerator Facility at Newport News, Virginia.

Citation is: Physical Review Letters 96, 042001, 2006