Pion induced coherent strangeness production

S. Bender† 1, H. Lenske1,2, and R. Shyam3 1Institut für Theoretische Physik, Universität Gießen, Germany; 2GSI, Darmstadt, Germany; 3Saha Institute of Nuclear Physics, Kolkata, India We investigated pion induced coherent strangeness production on light (C) and medium-heavy (Ca, V, Y) nuclei with a covariant model. This model was based on a Lagrangian approach, where resonance excitation and decay is the driving mechanism behind the elementary process. The aim of this work was to apply this model, which was already successfully applied to strangeness production on nuclei in proton [1, 2] and photon [3] induced reactions, to pion-nucleus reactions. Since the Λ is not subject to Pauli blocking with respect to the other nucleons, it provides an excellent probe for the interplay of single-particle and many-body degrees of freedom in nuclei. They are, therefore, a suitable choice when it comes to verify our models of these bound states and for doing spectroscopy. The theoretical description, however, gets rather involved since one is dealing with interacting many-body systems and approximations are necessary when dealing with them. Most of the theoretical models used so far to describe the (π,K) reaction employ a non-relativistic distorted wave impulse approximation (DWIA) framework [4, 5] without resolving the production vertex. In these calculations, the Λ bound states are generated by solving the Schrödinger equation with Woods–Saxon or harmonic oscillator potentials. However, for processes involving momentum transfers of typically 300 MeV/c or more, a nonrelativistic treatment of the corresponding wave functions may not be adequate as in this region the lower component of the Dirac spinor is no longer negligible in comparison to its upper component. In our work [6, 7], we studied the A(π,K)ΛA reaction within a fully covariant model by retaining the field theoretical structure of the interaction vertices and by treating the baryons as Dirac particles. In this model, the kaon production proceeds via the collision of the projectile pion with one of the target nucleons. This excites intermediate baryon resonance states (N∗) which decay into a kaon and a Λ hyperon. The hyperon is captured in the respective nuclear orbit while the kaon rescatters onto its mass shell. The intermediate resonance states included are N∗(1650)[ 1 2 − ], N∗(1710)[ 1 2 + ], and N∗(1720)[ 3 2 + ] which have dominant branching ratios for the decay to the KΛ channel. Terms corresponding to the interference among various resonance excitations are included in the total reaction amplitude. We find that excitations of N∗(1650), andN∗(1710) resonant states dominate the cross sections for the (π,K) reaction for beam energies below 2 GeV. Our model describes well the shapes of the experimental angular dis-