Last year, considerable skepticism greeted the first experimental reports of a new kind of hadron: a baryon whose combination of charge and flavor could not be made by a triplet of quarks. No fewer than five quarks (one of them an antiquark) would do (see Physics Today, Physics Today 0031-9228 56 9 2003 19 https://doi.org/10.1063/1.1620820 September 2003, page 19 ). Why, the skeptics asked, had no one previously seen this alleged “pentaquark” or, for that matter, any other pentaquark?
With charge and strangeness +1 and a modest mass of 1.54 GeV, the Ξ+(1540), as it was named, should have appeared decades ago as a prominent resonance in kaon–nucleon scattering experiments. For three decades, every fledgling particle physicist knew that no positive-strangeness baryon had ever been seen because you couldn’t make such a thing with three quarks.
By year’s end, however, the number of experiments claiming to have...
