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Employing the Bonn-Gatchina partial wave analysis framework (PWA), we have analyzed HADES data of the reaction p(3.5 GeV) + p → pK + Λ. This reaction might contain information about the kaonic cluster "pp K − " (with quantum numbers J P = 0 − and total isospin I = 1/2) via its decay into pΛ. Due to interference effects in our coherent description of the data, a hypothetical K N N (or, specifically "pp K − ") cluster signal need not necessarily show up as a pronounced feature (e.g. a peak) in an invariant mass spectrum like pΛ. Our PWA analysis includes a variety of resonant and non-resonant intermediate states and delivers a good description of our data (various angular distributions and two-hadron invariant mass spectra) without a contribution of a K N N cluster. At a confidence level of CL s = 95% such a cluster cannot contribute more than 2-12% to the total cross section with a pK + Λ final state, which translates into a production cross-section between 0.7 μb and 4.2 μb, respectively. The range of the upper limit depends on the assumed cluster mass, width and production process.

Abstract. We present transverse momentum spectra, rapidity distribution and multiplicity of Λ-hyperons measured with the HADES spectrometer in the reaction Ar(1.76A GeV) + KCl. The yield of Ξ − is calculated from our previously reported Ξ − /(Λ+Σ 0 ) ratio and compared to other strange particle multiplicities. Employing a strangeness balance equation the multiplicities of the yet unmeasured Σ ± -hyperons can be estimated. Finally a statistical hadronization model is used to fit the yields of πThe resulting chemical freeze-out temperature of T = (76 ± 2) MeV is compared to the measured slope parameters obtained from fits to the transverse mass distributions of the different particles.

We present results on dielectron production in 40 Ar + KCl collisions at 1.76A GeV. For the first time ω mesons could be reconstructed in a heavy-ion reaction at a bombarding energy which is well below the production threshold in free nucleon-nucleon collisions. The ω multiplicity has been extracted and compared to the yields of other particles, in particular of the φ meson. At intermediate e + e − invariant masses, we find a strong enhancement of the pair yield over a reference spectrum from elementary nucleon-nucleon reactions, suggesting the onset of nontrivial effects of the nuclear medium. Transverse-mass spectra and angular distributions have been reconstructed in three invariant mass bins. In the former unexpectedly large slopes are found for high-mass pairs. The latter, in particular the helicity-angle distributions, are largely consistent with expectations for a pair cocktail dominated at intermediate masses by Dalitz decays.