The associated photoproduction of positive kaons and $\pi^0 \Lambda$ or $\pi \Sigma$ pairs in the region of the $\Sigma$(1385) and $\Lambda$(1405) resonances

Abstract

The $\gamma p \to K^+ \pi^0 \Lambda$ and $\gamma p \to K^+ \pi \Sigma $ reactions are studied in the kinematic region where the $\pi^0 \Lambda$(1116) and $\pi\Sigma$(1192) pairs originate dominantly from the decay of the $\Sigma$(1385) and $\Lambda$(1405) resonances. We consider laboratory photon energies around 2 GeV, significantly above the threshold for producing the $K^+ \Sigma$(1385) and $K^+ \Lambda$(1405) final states. We compute for both reactions the process in which the ingoing photon dissociates into a real K$^+$ and a virtual K$^-$, the off-shell K$^-$ scattering subsequently off the proton target to produce the $\pi^0 \Lambda$ or $\pi \Sigma$ pair. The $K^- p \to \pi^0 \Lambda$ and $K^- p \to \pi \Sigma$ amplitudes are calculated in the framework of a chiral coupled-channel effective field theory of meson-baryon scattering. The structure of the amplitudes reflects the dominance of the $\Lambda(1405)$ in the $\pi \Sigma$ channel and of the $\Sigma(1385)$ in the $\pi \Lambda$ channel. The full pion-hyperon final state interaction is included in these amplitudes. We extract from the calculated cross section the gauge-invariant double kaon pole term. We found this term to be large and leading to sizeable cross sections for both the $\gamma p \to K^+ \pi^0 \Lambda$ and $\gamma p \to K^+ \pi \Sigma $ reactions, in qualitative agreement with the scarce data presently available. Accurate measurements of these cross sections should make it possible to extract the contribution of the double kaon pole and hence to assess the possibility of studying kaon-nucleon dynamics ust below threshold through these reactions.