The pentaquark rush

In 2003, results published by three experimental collaborations initiated a flood of papers about a class of particles known as pentaquarks. Physicists working on the LEPS, DIANA, and CLAS experiments had observed subatomic processes that seemed to indicate the existence of composite particles consisting of five quarks. Ordinary matter particles, such as protons and neutrons, only contain three quarks. The existence of pentaquarks would provide an important test for the theory of the strong nuclear force.

Since then more than 445 papers with the word "pentaquark" in the title have been recorded in the spires database (see graphic), including over 50 papers with experimental results. The majority of the papers has come from theorists performing analyses and computations to provide further insight into the observations and to make predictions for additional five-quark signals.

Standard particle theory allows for many types of quark-composite particles, a fact that theorists have known for more than 30 years. While some quark combinations are forbidden, the quark model permits the existence of pentaquarks. Yet physicists published few papers dedicated to pentaquarks before the 2003 announcements. For the period from 1974 to 2002, the spires database contains only 32 papers that refer to "pentaquark" in their titles. The first of these papers was published in 1989, twenty years after Murray Gell-Mann received the Nobel prize for developing the quark model.

The pentaquark, however, might return to oblivion. Experimental results of the last three years have raised doubts whether the signals published in 2003 represent pentaquarks (see story). The CLAS experiment at Jefferson Laboratory as well as other experiments with large data samples that should have confirmed the initial results have found no significant signals.

Heath O'Connell

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