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HK: Physik der Hadronen und Kerne

HK 44: Electromagnetic and Hadronic Probes VII

HK 44.1: Gruppenbericht

Donnerstag, 11. März 2004, 16:30–17:00, B

Evidence for a narrow | S | = 1 baryon state at a mass of 1527 MeV in quasi-real photoproduction — •Klaus Rith — Universität Erlangen-Nürnberg, Physikalisches Institut II, Erwin-Rommel-Str. 1, 91058 Erlangen

The HERMES experiment at HERA has searched for a narrow baryon state in quasi-real photoproduction on a deuterium target through the decay channel p K_s⁰ → p π⁺ π⁻. A peak has been observed in the pK_s⁰ invariant mass spectrum at 1527 ± 2.3(stat) ± 1.7(syst) MeV with a width of about 21 MeV that is substantially larger than the experimental resolution of 10-15 MeV. The peak has a naive statistical significance of 5−6  σ, depending on the background model. This state may be interpreted as the predicted S=+1 exotic Θ⁺(uudds) pentaquark baryon. No signal for an hypothetical Θ⁺⁺ baryon was observed in the p K⁺ invariant mass distribution.

Supported by BMBF, contract numbers 06 ER 125I, 06 ER 928I