Quarks, diquarks and pentaquarks

Pentaquarks do not fit into the traditional quark model of hadrons, but understanding these exotic new particles is offering fresh insights into the subtleties of the strong interaction. In the early 1960s particle physicists found that they had wandered into a jungle. They were confronted by a bewildering array of particles called hadrons that did not appear to follow any discernible pattern. Murray Gell-Mann eventually tamed this hadronic jungle by proposing that all hadrons are made up of combinations of more fundamental particles called quarks: mesons contain a quark and an antiquark, while baryons contain three quarks. Some physicists found this bold hypothesis difficult to swallow at first, but for four decades these two basic 'body plans' have been more or less adequate to accommodate the many new hadrons that have turned up in experiments. (U.K.)