Last year Takashi Nakano and colleagues in the Laser Electron Photon experiment at SPring-8 (LEPS) collaboration reported evidence for a so-called pentaquark with a mass of 1.54 GeV at a conference in Japan. The particle was observed in experiments in which high-energy gamma rays were scattered off neutrons in a carbon nucleus. Both the mass of the particle and the width of the particle peak - less than 25 MeV - were in agreement with theoretical predictions made by Dmitri Diakonov of the Petersburg Nuclear Physics Institute and co-workers in 1997.

Now the CLAS collaboration at the Thomas Jefferson National Accelerator Facility in the US has reported evidence for a pentaquark with a similar mass and width. The US team scattered gamma-rays from a deuterium nucleus. The DIANA collaboration at the ITEP laboratory in Moscow has also found evidence for pentaquarks, and other groups searching for particles containing five quarks include the HERMES experiment at DESY in Germany.

The statistical significance of the Japanese and US experiments are 4.6 and 5.4 standard deviations respectively, which means that the chances of the observations being statistical flukes are extremely low. It is not yet clear if the pentaquark observed in the experiments is a tightly bound five-quark state or a sort of molecule made of a kaon and a neutron.

In the Japanese experiment low-energy photons from a laser were scattered from 8 GeV electrons in the storage ring of the SPring-8 synchrotron radiation facility to produce gamma-rays. These gamma-rays, which had energies of up to 2.4 GeV, were then directed at a plastic target. The LEPS team searched for evidence of collisions in which a gamma-ray photon interacted with a neutron to produce a negative kaon and a pentaquark, which subsequently decayed into a positive kaon and a neutron.

The gamma-rays in the CLAS experiment were produced by sending an electron beam into a solid cryogenic target, and the gamma-ray-neutron interactions took place in a separate target made of deuterium. Like the LEPS experiment the CLAS team also searched for evidence of collisions that produced two kaons in the final state.

In April physicists at the BaBar experiment at Stanford reported evidence for an new D-meson that might contain four quarks, although this interpretation has not been confirmed. And earlier this month three experiments at the Relativistic Heavy Ion Collider at Brookhaven reported that they might have produced a quark-gluon plasma - a state of matter in which quarks are no longer confined inside other particles. However, the properties of the strong force mean that single quarks are unlikely to be seen in experiments any time soon.