Four international collaborations have recently announced strong experimental evidence for a five-quark exotic baryon named the theta-plus (Θ+), composed of two up quarks, two down quarks and a strange antiquark (uuddsbar). Originally called the Z+ by the Particle Data Group, but renamed the Θ+ in April this year, as suggested by Russian theorist Dmitri Diakonov, the exact nature of the so-called "pentaquark" is not yet clear. Further experiments should reveal whether it is a tightly bound five-quark object or a molecular meson-baryon state, and will provide measurements of its spin, and the angular distribution and energy dependence of its production.

Physicists have searched for a five-quark state for more than 35 years. Recent experimental efforts were largely motivated by Diakonov and colleagues Victor Petrov and Maxim Polyakov. In 1997 they predicted an exotic isoscalar baryon having spin-parity (1/2)+ and strangeness S = +1 (D Diakonov et al. 1997). In the antidecuplet of five-quark resonances that they predicted, the Θ+ is the lowest mass member at about 1530 MeV, having a width of less than 15 MeV. The theorists suggested that the particle would be seen as a sharp peak in the nK+ or pK0 mass spectrum.

The first publicly announced experimental evidence emerged from the Laser Electron Photon Facility at SPring-8 (LEPS) collaboration in Osaka, Japan (T Nakano et al. 2003). The LEPS involvement began at a conference in 2000, when Diakonov convinced collaboration members to search for the exotic Θ+ state. Using data from an unrelated experiment on φ-meson production, the SPring-8 team studied the inclusive reaction γn→K+K-n on 12C by measuring both K+ and K- at forward angles. They realized that they had a signal in August 2002, but kept their result quiet until the Particles and Nuclei International Conference (PANIC) in October 2002. After months of independent analyses to confirm the result, and after correcting for Fermi momentum, they reported a 4.6 σ nK+ peak at 1540 MeV, less than 25 MeV wide and consistent with the exotic baryon predicted by Diakonov et al.

Meanwhile, the DIANA collaboration from the Institute of Theoretical and Experimental Physics (ITEP) in Moscow, Russia, was examining a 1986 data set from low-energy K+Xe collisions in a xenon bubble chamber. They analysed the effective mass of the pK0 system in the charge-exchange reaction K+Xe→K0pXe', finding a baryon resonance with a mass of 1539 MeV and a width less than 9 MeV at an estimated statistical significance of 4.4 σ. Their findings will appear in Physics of Atomic Nuclei (V V Barmin et al. 2003).

The CLAS collaboration at the US Department of Energy's Thomas Jefferson National Accelerator Facility (Jefferson Lab) revealed the most statistically significant result to date at the Conference on the Intersections of Particle and Nuclear Physics (CIPANP) in May. Their results have been submitted for publication in Physical Review Letters (S Stepanyan et al. 2003). Using data from August 1999, the CLAS team studied an exclusive measurement of the reaction γd→K+K-pn. Energy-tagged photons struck a liquid-deuterium target and the particles generated were detected in the CEBAF large-acceptance spectrometer. In the final state, the reaction produced a K- meson and a proton, along with the five-quark object, which then decayed into a neutron (identified by missing mass) and a K+ meson. The CLAS collaboration reports a 5.3 σ Θ+ peak in the nK+ invariant mass spectrum at around 1542 MeV, with a measured width of 21 MeV. They have received approval for 30 days of beam time from the Program Advisory Committee, so as to characterize fully the exotic Θ+ baryon, and the experiment could be conducted as soon as early 2004.

The most recent experimental evidence for the pentaquark comes from the SAPHIR collaboration at the Electron Stretcher Accelerator (ELSA) in Bonn, Germany. Again using older data, taken in 1997 and 1998, they measured the reaction γp→nK0sK+ with the decay K0s→π+π- in the SAPHIR detector at ELSA. In an upcoming issue of Physics Letters B they report evidence for the Θ+ in the invariant mass spectrum of the nK+ system (J Barth et al. 2003). They observe a 4.8 σ peak with a mass of 1540 MeV and a width of less than 25 MeV. After searching for a signal in the pK+ invariant mass distribution in γp→pK+K-, they conclude that the Θ+ must be isoscalar.

The details of the theory proposed by Diakonov et al. are hotly debated, as a brief scan of the pre-print servers will confirm. However, it is undeniable that the four collaborations that have announced convincing evidence of the Θ+ baryon report consistent experimental results. This would seem to confirm the existence of the particle. If the groups' analyses are correct, this new exotic baryon could have profound implications for baryon spectroscopy and hadronic physics in general.

Further reading

V V Barmin et al. 2003
J Barth et al. 2003 hep-ex/0307083.
D Diakonov et al. 1997 Z. Phys. A 359 305.
T Nakano et al. 2003 Phys. Rev. Lett. 91 012002.
S Stepanyan et al. 2003 hep-ex/0307018.