Possible Pentaquark Evidence

A Japanese group announced that they see evidence for either a meson-baryon molecular resonance or a five-quark (u u d d sbar) state. Confirming evidence is also being claimed by a groups at Jefferson Lab and ITEP in Moscow.

Using GeV energy gamma rays onto a plastic target (Carbon and Hydrogen), the Japanase experiment looks for events of the type: 

gamma n -> K- K+ n

where they only detect the K+ and K-. They see a peak in the missing mass plot recoiling from the K-, i.e. that the (K+ n) is coming from some exotic resonance (Z+) at 1.54 GeV/c^2.

The signal goes away if they use a proton target, and they have cross checks from the conventional reactions: 

  gamma n -> K+ Sigma- -> K+ pi- n

  gamma p -> K+ Lambda -> K+ pi- p

Note that they create the gamma-ray source (2.4 GeV max energy) by backscattering a laser from an eight GeV electron beam.

If you have questions or more information, please contact Doug.Wright@llnl.gov.