Do pentaquarks really exist?

Results from a growing number of experiments at laboratories around the world are casting doubt on the recent discovery of particles containing five quarks. In 2003 evidence for a novel family of particles called pentaquarks was reported by researchers working on a number of different experiments. The prospect of such a particle - which contains four quarks and one antiquark - has generated a huge amount of interest among theorists because, if confirmed, it would prove that quarks experience powerful correlations that had not been anticipated. Last year, however, the experimental pendulum swung the other way: several experiments with large data samples saw no evidence for the claimed pentaquark. As 2005 begins, we anxiously await news from what could prove to be a definitive experiment with the CLAS detector at the Jefferson Laboratory in Virginia. (U.K.)