Associated strangeness production at threshold

Abstract

Abstract. The associated strangeness dissociation at threshold has been studied at the COSY-11 facility measuring the hyperon -and the K + K − meson pair production. Measurements of the near threshold Λ and Σ 0 production via the pp → pK + Λ/Σ 0 reaction [1] at COSY-11 have shown that the Λ/Σ 0 cross section ratio exceeds the value at high excess energies (Q ≥ 300 MeV [2]) by an order of magnitude. For a better understanding additional data have been taken between 13 MeV and 60 MeV excess energy. The near threshold production of the charged kaon-antikaon pair is related to the discussion about the nature of the scalar states in the 1 GeV/c 2 mass range, i.e. the f0 (980) and a0 (980) [3]. The interpretation as a KK molecule is strongly dependent on the K-K interaction which can be studied via the production channel. A first total cross section value on the reaction pp → ppK + K − at an excess energy of 17 MeV [4] i.e. below the φ production threshold was measured. One of the main investigations of the COSY-11 collaboration is the associated strangeness production of the neutral Λ and Σ 0 hyperons in the reactions pp → pK + Λ/Σ 0 . Since the quark structures of these hyperons are analogous to each other one can expect similar production mechanisms. In such a case the cross section ratio PACSσ(pp→pK + Σ 0 ) should be mainly determined by the isospin relation which leads to R Λ/Σ 0 = 3 what is in line with the ratio of about 2.5 observed at high excess energies (Q ≥ 300 MeV) [2]. Very close to threshold, in the range of excess energies Q ≤ 13 MeV, the total cross sections for the Λ and Σ 0 hyperon production were measured exclusively at the COSY-11 facility [5,6] at COSY Jülich [7]. The most remarkable feature of the data [1,8] was that at the same excess energy the total cross section for the Σ 0 production appeared to be about a factor of 28 +6 −9 smaller than for the Λ particle.Enhancements in the missing mass distribution at the Λp and ΣN thresholds observed in inclusive K + production data, taken at SPES 4 [9] in proton-proton scattering at Q = 252 MeV, both having about the same magnitude, suggest a strong ΣN → Λp final state conversion. This conversion might be responsible for the decrease of the Σ 0 production yield close to threshold as seen in the COSY-11 data. Strong ΣN → Λp conversion effects are also suggested when interpreting the results of K − scattering on deuterons [10] where a sharp peak is clearly seen at an effective mass of the Λ-proton system m Λp = 2131 MeV/c 2 corresponding exactly to the Σ 0 p threshold.However, in calculations within the Jülich meson exchange model [11] the final state conversion is rather excluded as a dominant origin of the observed Σ 0 suppression. In these calculations both π and K exchange are taken into account with inclusion of the final state interaction (FSI) effects. Λ production is found to be dominated by kaon exchange, what is in line with the experimental results obtained by the DISTO collaboration [12] at higher excess energies (Q = 430 MeV), where the impo...

Resonances and final-state interactions in the reaction pp → pK+Λ

“…The results were normalized to the data at ǫ≃1 GeV by adjusting the cut-off mass of the form factor Eq. (20) to Λ≃1.7 GeV. In contrast to the pure phase space, the K-meson exchange well reproduces the energy dependence of the data also at high energies, which will be discussed later.…”

“…Over the last few years the COSY facility has provided a large amount of accurate experimental data on strangeness production in N N collisions at low energies [15][16][17][18][19][20][21][22][23]. Theoretical model studies [24][25][26][27][28][29][30] that dealt with those data suggested that the excitation of resonances in the KΛ channel could play an important if not dominant role for the reaction pp→pK + Λ in the near-treshold regime.…”