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Register a new user$Sigma^0$ production in proton nucleus collisions near threshold
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The production of $Sigma^{0}$ baryons in the nuclear reaction p (3.5 GeV) + Nb (corresponding to $sqrt{s_{NN}}=3.18$ GeV) is studied with the detector set-up HADES at GSI, Darmstadt. $Sigma^{0}$s were identified via the decay $Sigma^{0} rightarrow Lambda gamma$ with subsequent decays $Lambda rightarrow p pi^{-}$ in coincidence with a $e^{+}e^{-}$ pair from either external ($gamma rightarrow e^{+} e^{-}$) or internal (Dalitz decay $gamma^{*}rightarrow e^{+} e^{-}$) gamma
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Threshold measurements of the associated strangeness production reactions pp --> p K(+) Lambda and pp --> p K(+) Sigma(0) are presented. Although slight differences in the shapes of the excitation functions are observed, the most remarkable feature of the data is that at the same excess energy the total cross section for the Sigma(0) production appears to be about a factor of 28 smaller than the one for the Lambda particle. It is concluded that strong Sigma(0)-p final state interactions, and in particular the Sigma-N --> Lambda-p conversion reaction, are the likely cause of the depletion for the yield in the Sigma signal. This hypothesis is in line with other experimental evidence in the literature.
This paper reports results on simultaneous measurements of the reaction channels pp -> pK+Lambda and pp -> pK+Sigma^0 at excess energies of 204, 239, and 284 MeV (Lambda) and 127, 162, and 207 MeV (Sigma^0). Total and differential cross sections are given for both reactions. It is concluded from the measured total cross sections that the high energy limit of the cross section ratio is almost reached at an excess energy of only about 200 MeV. From the differential distributions observed in the overall CMS as well as in the Jackson and helicity frames, a significant contribution of interfering nucleon resonances to the Lambda production mechanism is concluded while resonant Sigma^0-production seems to be of lesser importance and takes place only through specific partial waves of the entrance channel. The data also indicate that kaon exchange plays a minor role in the case of Lambda- but an important role for Sigma^0-production. Thus the peculiar energy dependence of the Lambda-to-Sigma^0 cross section ratio appears in a new light as its explanation requires more than mere differences between the pLambda and the pSigma^0 final state interaction. The data provide a benchmark for theoretical models already available or yet to come.
The near threshold total cross section and angular distributions of K+K- pair production via the reaction pp --> ppK+K- have been studied at an excess energy of Q = 17 MeV using the COSY-11 facility at the cooler synchrotron COSY. The obtained cross section as well as an upper limit at an excess energy of Q = 3 MeV represent the first measurements on the K+K- production in the region of small excess energies where production via the channel pp --> pp Phi --> ppK+K- is energetically forbidden. The possible influence of a resonant production via intermediate scalar states f0(980) and a0(980) is discussed.
Exclusive data on the reactions pp -> ppK+K- and pp -> pK+ Lambda/Sigma0 have been taken at the cooler synchrotron COSY close to threshold. At equal excess energies, an enhancement of the Lambda/Sigma0 ratio by one order of magnitude has been observed compared to data at higher excess energies. New results obtained at the COSY-11 facility explore the transition region between this low-energy Sigma0 suppression and excess energies of 60 MeV. A first total cross section for elementary antikaon production below the phi threshold has been determined, two orders of magnitude smaller compared to kaon production at the same excess energy.
Motivated by the ongoing discussion concerning the nature of the scalar resonances f0(980) and a0(980), the COSY-11 collaboration has taken exclusive data on the pp->ppK+K- reaction near the production threshold. A first total cross section sigma=(1.80+-0.27+0.28-0.35)nb for the excess energy Q=17 MeV has been determined. In contrary to the eta, omega and eta single meson production studies which clearly show the strong pp final state interaction (FSI), the cross section values obtained at COSY-11 and DISTO can be both described by a fit with a four-body phase space including the proton-proton final state interaction as well as with one-meson exchange calculations neglecting FSI effects. Therefore, one might think about a compensation of the strong pp interaction through a pK- FSI effect or an additional degree of freedom caused by the four-body final state. In the latter case, strong FSI effects can be expected at Q-values very close to the K+K- production threshold. Such a motivation triggered -- in combination with the investigation of the kaon-Antikaon interaction being relevant to the structure of the f0(980) -- further measurements at the excess energies Q=10 and Q=28 MeV at COSY-11.
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