Magnetic systems at criticality: different signatures of scaling

Different aspects of critical behaviour of magnetic materials are presented and discussed. The scaling ideas are shown to arise in the context of purely magnetic properties as well as in that of thermal properties as demonstrated by magnetocaloric effect or combined scaling of excess entropy and order parameter. Two non-standard approaches to scaling phenomena are described. The presented concepts are exemplified by experimental data gathered on four representatives of molecular magnets.Comment: 33 pages, 16 figure

Deep sub-threshold Ξ−\Xi^- production in Ar+KCl reactions at 1.76A GeV

We report first results on a deep sub-threshold production of the doubly strange hyperon $\Xi^-$ in a heavy-ion reaction. At a beam energy of 1.76A GeV the reaction Ar+KCl was studied with the High Acceptance Di-Electron Spectrometer (HADES) at SIS18/GSI. A high-statistics and high-purity $\Lambda$ sample was collected, allowing for the investigation of the decay channel $\Xi^- \to \Lambda \pi^-$. The deduced $\Xi^-/(\Lambda+\Sigma^0)$ production ratio of $(5.6 \pm 1.2 ^{+1.8}_{-1.7})\cdot 10^{-3}$ is significantly larger than available model predictions.Comment: 4 pages, including 4 figure

In-Medium Effects on K0 Mesons in Relativistic Heavy-Ion Collisions

We present the transverse momentum spectra and rapidity distributions of $\pi^{-}$ and K$^0_S$ in Ar+KCl reactions at a beam kinetic energy of 1.756 A GeV measured with the spectrometer HADES. The reconstructed K$^0_S$ sample is characterized by good event statistics for a wide range in momentum and rapidity. We compare the experimental $\pi^{-}$ and K$^0_S$ distributions to predictions by the IQMD model. The model calculations show that K$^0_S$ at low tranverse momenta constitute a particularly well suited tool to investigate the kaon in-medium potential. Our K$^0_S$ data suggest a strong repulsive in-medium K$^0$ potential of about 40 MeV strength.Comment: 10 pages, 10 figures, accepted by Phys. Rev.

Study of the quasi-free np→npπ+π−np \to np \pi^+\pi^- reaction with a deuterium beam at 1.25 GeV/nucleon

The tagged quasi-free $np \to np\pi^+\pi^-$ reaction has been studied experimentally with the High Acceptance Di-Electron Spectrometer (HADES) at GSI at a deuteron incident beam energy of 1.25 GeV/nucleon ($\sqrt s \sim$ 2.42 GeV/c for the quasi-free collision). For the first time, differential distributions for $\pi^{+}\pi^{-}$ production in $np$ collisions have been collected in the region corresponding to the large transverse momenta of the secondary particles. The invariant mass and angular distributions for the $np\rightarrow np\pi^{+}\pi^{-}$ reaction are compared with different models. This comparison confirms the dominance of the $t$-channel with $\Delta\Delta$ contribution. It also validates the changes previously introduced in the Valencia model to describe two-pion production data in other isospin channels, although some deviations are observed, especially for the $\pi^{+}\pi^{-}$ invariant mass spectrum. The extracted total cross section is also in much better agreement with this model. Our new measurement puts useful constraints for the existence of the conjectured dibaryon resonance at mass M$\sim$ 2.38 GeV and with width $\Gamma\sim$ 70 MeV

Inclusive dielectron production in proton-proton collisions at 2.2 GeV beam energy

Data on inclusive dielectron production are presented for the reaction p+p at 2.2 GeV measured with the High Acceptance DiElectron Spectrometer (HADES). Our results supplement data obtained earlier in this bombarding energy regime by DLS and HADES. The comparison with the 2.09 GeV DLS data is discussed. The reconstructed e+e- distributions are confronted with simulated pair cocktails, revealing an excess yield at invariant masses around 0.5 GeV/c2. Inclusive cross sections of neutral pion and eta production are obtained

Inclusive pion and eta production in p+Nb collisions at 3.5 GeV beam energy

Data on inclusive pion and eta production measured with the dielectron spectrometer HADES in the reaction p+93Nb at a kinetic beam energy of 3.5 GeV are presented. Our results, obtained with the photon conversion method, supplement the rather sparse information on neutral meson production in proton-nucleus reactions existing for this bombarding energy regime. The reconstructed e+e-e+e- transverse-momentum and rapidity distributions are confronted with transport model calculations, which account fairly well for both pi0 and eta production.Comment: 12 pages, 9 figures, submitted to Physical Review

Dilepton production in pp and CC collisions with HADES

Dilepton production has been measured with HADES, the "High Acceptance DiElectron Spectrometer". In pp collisions at 2.2GeV kinetic beam energy, exclusive eta production and the Dalitz decay eta -> gamma e+e- has been reconstructed. The electromagnetic form factor is well in agreement with existing data. In addition, an inclusive e+e- spectrum from the C+C reaction at 2AGeV is presented and compared with a thermal model.Comment: 11 pages, 3 figures, proceedings of the IVth International Conference on Quarks and Nuclear Physics, Madrid, June 5th-10th, submitted to Eur.Phys.J.

Partial Wave Analysis of the Reaction p(3.5GeV)+p→pK+Λp(3.5 GeV)+p \to pK^+\Lambda to Search for the "ppK−ppK^-" Bound State

Employing the Bonn-Gatchina partial wave analysis framework (PWA), we have analyzed HADES data of the reaction $p(3.5GeV)+p\to pK^{+}\Lambda$. This reaction might contain information about the kaonic cluster "$ppK^-$" via its decay into $p\Lambda$. Due to interference effects in our coherent description of the data, a hypothetical $\overline{K}NN$ (or, specifically "$ppK^-$") cluster signal must not necessarily show up as a pronounced feature (e.g. a peak) in an invariant mass spectra like $p\Lambda$. 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 $\overline{K}NN$ cluster. At a confidence level of CL$_{s}$=95\% such a cluster can not contribute more than 2-12\% to the total cross section with a $pK^{+}\Lambda$ final state, which translates into a production cross-section between 0.7 $\mu b$ and 4.2 $\mu b$, respectively. The range of the upper limit depends on the assumed cluster mass, width and production process.Comment: 7 Pages, 5 Figure