Phenomenology of the Λ / Σ 0 production ratio in pp collisions

We show that the recently measured asymmetry in helicity-angle spectra of the Λ-hyperons produced in the reaction pp→K + Λp reaction and the energy dependence of the total pp→K + Λp cross section can be explained consistently by the same Λp final-state interaction. Assuming that there is no final-state interaction in the Σ 0 p channel, as suggested by the available data, we can also reproduce the energy dependence of the Λ/Σ 0 production ratio and, in particular, the rather large ratio observed near the reaction thresholds. The nominal ratio of the Λ and Σ 0 production amplitudes squared, i.e. when disregarding the final-state interaction, turns out to be about 3, which is in line with hyperon production data from proton and nuclear targets available at high energies. One of the surprising results observed at the COSY accelerator facility is the large ratio of the pp→K + Λp to pp→K + Σ 0 p cross sections near the reaction thresholds [1,2]. This ratio is as large as 28 [1] at very low energies and eventually approaches values around 3 with increasing energy. In the last few years several theoretical studies [3,4,5,6,7,8] appeared where different production scenarios were considered in order to describe simultaneously both the pp→K + Λp and pp→K + Σ 0 p reaction cross sections and, thus, the Λ/Σ 0 ratio. These included π, K as well as heavier meson exchanges, intermediate baryonic resonances coupled to the K + Λ and K + Σ 0 channels as well as the Λp and Σ 0 p final-state interaction (FSI). But, as summarized in Refs. [2,9], none of these models is able to reproduce the energy dependence of the Λ/Σ 0 ratio convincingly. A basic problem is that the relevance of the FSI on one hand side, and the interplay between the different baryonic resonances on the other side can not be resolved from considering only the total reaction cross section. Therefore, in the present paper we follow a different strategy. We look not only at the total cross sections but also at differential observables because some of them do allow to distinguish between effects from the FSI and baryonic resonances, as we pointed out recently [10]. Moreover, for the reaction pp→K + Λp data for the relevant ob-servables are already available in the literature [11]. Note that our study is completely phenomenological in the sense that we do not …