Study of the Σ ( 1385 ) and Λ ( 1405 ) resonances in K + photoproduction processes

The γ p → K + π 0 Λ and γ p → K + π Σ reactions are studied in the kinematic region where the π 0 Λ ( 1116 ) and π Σ ( 1192 ) pairs originate dominantly from the decay of the Σ ( 1385 ) and Λ ( 1405 ) resonances. We consider laboratory photon energies around 2 GeV, i.e. total center of mass energies above the known resonance region. We compute the t-channel kaon-exchange contribution to these reactions using K − p → π 0 Λ and K − p → π Σ amplitudes calculated in the framework of a chiral coupled-channel effective field theory of meson–baryon scattering. We extract from the calculated cross section the gauge-invariant double kaon pole term. We find this term to be large and likely to drive significantly the γ p → K + π 0 Λ and γ p → K + π Σ reactions in the kinematics under investigation. Accurate measurements of t-distributions for these processes, in progress or planned at ELSA and at SPring-8, are needed to confirm this expectation and assess the possibility of studying antikaon–nucleon dynamics just below threshold through these reactions.