Thomas Cohen, University of Maryland

How the Hedgehog was Harried and
Other Fables From the Land of Pentaquarks

A hedgehog is a small nocturnal insect-eating mammal with a spiny coat, able to roll itself into a ball for defense; it is also a whimsically named mathematical configuration in which fields point outward like the spines of a frightened hedgehog. Pentaquarks are supposed exotic particles composed of four quarks and an anti-quark (in contrast to usual particles such as protons or neutrons which are made of three quarks). Such exotic particles had been searched for and not seen for more than three decades. In 1996 a pentaquark with very specific properties was predicted with a hedgehog based model. There was great excitement and significant publicity in 2003 and 2004 when ten different experiments looked for pentaquarks where they were predicted and reported seeing them with properties virtually identical to those predicted. This seems to be a textbook case of how science is supposed to work: a theory leads to an experimental discovery and then multiple confirmations. However, as will be discussed in this talk, the theory was inconsistent at a fundamental level. The key mathematical issue is the behavior of the hedgehog when perturbed (hence the title). The discovery of pentaquarks based on a wrong theory could be an example of serendipity in science---a surprisingly common occurrence in the history of science. Alternatively, all ten experiments could be mistaken, essentially by reading into inadequate data what one expects to see. Some recent high statistics experiments which do not see the pentaquark suggest that this second possibility may well be correct. This talk explores the science---and the sociology---of pentaquarks.