The reactions pp → pΛK + and pp → pΣ 0 K + are studied near their thresholds. The strangeness production process is described by the π-and K exchange mechanisms. Effects from the final state interaction in the hyperon-nucleon system are taken into account rigorously. The Λ production turns out to be dominated by K exchange whereas K-as well as π exchange play an important role for the Σ 0 case. It is shown that the experimentally observed strong suppression of Σ 0 production compared to Λ production at the same excess energy can be explained by a destructive interference between π and K exchange in the reaction pp → pΣ 0 K + . Implications of such an interference on the reaction pp → nΣ + K + are pointed out.Recently the total cross sections for the reactions pp → pΛK + and pp → pΣ 0 K + were measured for the first time in the threshold region [1][2][3]. Certainly the most interesting aspect of these new data is the observed strong suppression of the Σ 0 production in comparison to the Λ channel: at the same excess energy the cross section for the Σ 0 production is about a factor of 25 smaller than the one for the Λ production [3]. This is indeed rather surprising, specifically because data at higher energies [4,5] indicate that the cross section for Λ production exceeds the one for Σ 0 production only by a factor of around 2.5.In principle, this strong suppression of the Σ 0 production compared to the Λ Preprint submitted to Elsevier Preprint 16 April 2018case can be understood if one assumes that the hyperon production is solely due to the K-exchange diagram, depicted in Fig. 1(a). In this case the same elementary (K + p) re-scattering amplitude enters in the two reactions. Therefore the ratio σ Λ /σ Σ 0 := σ pp→pΛK + /σ pp→pΣ 0 K + will be given essentially by the ratio of the coupling constants at the vertices from which the K meson emerges, i. e. by g 2 ΛN K /g 2 ΣN K . These coupling constants are not very well known experimentally. However, they can be inferred from SU(3) flavour symmetrya symmetry which, so far, has been rather successfully employed in investigations of reactions involving hyperons. Specifically according to SU(6) this ratio is 27 [6], a value which coincides almost exactly with the experimental cross section ratio.None-the-less, already a simple estimation of the elementary scattering pro-, respectively) based on experimental amplitudes reveals that the contribution of the π-exchange diagram ( Fig. 1(b)) to hyperon production should be not negligible, cf. Ref. [3]. Strong evidence for the relevance of π-exchange comes also from more detailed model calculations [7][8][9][10][11][12]. (It should be said, however, that most of these studies focus on data at rather large excess energies or look at the Λ channel only.) Indeed the Λ/Σ 0 production ratio estimated in Ref.[3], considering K-as well as π exchange, is roughly 3.6, i.e. about a factor of 8 below the measurement.Because of this situation another explanation for the observed suppression of the Σ 0 production was suggested in...

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We calculate the momentum dependence of the ρ-meson selfenergy based on the dispersion relation for the ρN scattering amplitude f (ω) at low nuclear density. The imaginary part of f (ω) is determined from the optical theorem, while the total ρN cross section is obtained within the VDM at high energy and within the resonance model at low energy. Our numerical results indicate a sizeable broadening of the ρ-meson width in the medium especially for low relative momenta p while the real part of the ρ selfenergy is found to change its sign and to become repulsive already at momenta above 100 MeV/c. Extrapolating to nuclear saturation density ρ 0 we find a dropping of the ρ-mass for p ≈ 0 roughly in line with the QCD sumrule analysis of Hatsuda while at high energy an increase of the ρ-mass close to the prediction by Eletsky and Joffe is obtained. However, when including a broadening of the baryonic resonances in the medium, the ρmeson mass shift at p ≈ 0 becomes slightly repulsive whereas the width increases substantially.