The experimental evidence for pentaquarks

Fading, but not extinguished. The proton, the neutron, and all the other well-established baryons can be described as bound states of three quarks. But pentaquarks—if they exist—have combinations of quantum numbers that require five quarks. The oldest candidate is the θ⁺(1540), a putative positive-strangeness baryon with a mass of 1.54 GeV (see September 2003, page 19 ). Since 2003, 10 low-statistics experiments have claimed to see it. At the April 2005 meeting of the American Physical Society in Tampa, Florida, however, Raffaella De Vita of the CLAS collaboration at the Thomas Jefferson National Accelerator Facility reported that the group found no sign of the θ⁺(1540) in a new photoproduction experiment with at least 100 times as many gamma–proton scattering events as any of the previous experiments. A heavier pentaquark candidate, the anticharmed θ⁰ _c(3100), although embattled, is still in the running. On 27 April, at the International Workshop on Deep Inelastic Scattering in Madison, Wisconsin, Karin Daum of the H1-detector collaboration at the HERA electron–proton collider ring in Hamburg, Germany, reported that the group’s evidence for the anticharmed pentaquark, first reported in 2004, is getting stronger. But the rival ZEUS collaboration, also at HERA, has found no evidence for it. The two groups have put their heads together, but have not as yet reconciled their difference.