Study of ambiguities in $π^-p\to ΛK^0$ scattering amplitudes

Abstract

Amplitudes for the reaction $π^-p\to ΛK^0$ are reconstructed from data on the differential cross section $dσ/dΩ$, the recoil polarization $P$, and on the spin rotation parameter $β$. At low energies, no data on $β$ exist, resulting in ambiguities. An approximation using $S$ and $P$ waves leads only to a fair description of the data on $dσ/dΩ$ and $P$; in this case, there are two sets of amplitudes. Including $D$ waves, the data on $dσ/dΩ$ and $P$ are well reproduced by the fit but now, there are several distinct solutions which describe the data with identical precision. In the range where the spin rotation parameter $β$ was measured, a full and unambiguous reconstruction of the partial wave amplitudes is possible. The energy-independent amplitudes are compared to the energy dependent amplitudes which resulted from a coupled channel fit (BnGa2011-02) to a large data set including both pion and photo-induced reactions. Significant deviations are observed. Consistency between energy dependent and energy independent solutions by choosing the energy independent solution which is closest to the energy dependent solution. In a second step, the {\it known} energy dependent solution for low (or high) partial waves is imposed and only the high (or low) partial waves are fitted leading to smaller uncertainties.