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The positive-strangeness baryon resonance + is observed in photoproduction of the nK + K 0 s final state with the SAPHIR detector at the Bonn ELectron Stretcher Accelerator ELSA. It is seen as a peak in the nK + invariant mass distribution with a 4.8σ confidence level. We find a mass M + = 1540 ± 4 ± 2 MeV and an upper limit of the width Γ + < 25 MeV at 90% c.l. The photoproduction cross section for K 0 + is in the order of 300 nb. From the absence of a signal in the pK + invariant mass distribution in γ p → pK + K − at the expected strength we conclude that the + must be isoscalar.

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Abstract. Quasi-free photoproduction of η -mesons off nucleons bound in the deuteron has been measured with the combined Crystal Barrel -TAPS detector. The experiment was done at a tagged photon beam of the ELSA electron accelerator in Bonn for incident photon energies from the production threshold up to 2.5 GeV. The η -mesons have been detected in coincidence with recoil protons and recoil neutrons. The quasi-free proton data are in good agreement with the results for free protons, indicating that nuclear effects have no significant impact. The coincidence with recoil neutrons provides the first data for the γn → nη reaction. In addition, also first estimates for coherent η -production off the deuteron have been obtained. In agreement with model predictions, the total cross-section for this channel is found to be very small, at most at the level of a few nb. The data are compared to model calculations taking into account contributions from nucleon resonances and t-channel exchanges.

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