We present results of an exclusive measurement of the first excited state of the Sigma hyperon, Sigma(1385)^+, produced in p+p -> Sigma^+ + K^+ + n at 3.5 GeV beam energy. The extracted data allow to ...study in detail the invariant mass distribution of the Sigma(1385)^+. The mass distribution is well described by a relativistic Breit-Wigner function with a maximum at m_0 = 1383.2 +- 0.9 MeV/c^2 and a width of 40.2 +- 2.1 MeV/c^2. The exclusive production cross-section comes out to be 22.27 +- 0.89 +- 1.56 +3.07 -2.10 mu b. Angular distributions of the Sigma(1385)^+ in different reference frames are found to be compatible with the hypothesis that 33 % of Sigma(1385)^+ result from the decay of an intermediate Delta^{++} resonance.
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.
We present results on dielectron production in Ar+KCl collisions at 1.76A GeV. For the first time \(\omega\) 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 omega multiplicity has been extracted and compared to the yields of other particles, in particular of the phi 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 non-trivial 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 delta Dalitz decays.
First results of the differential cross section in dp elastic scattering at 1.25 GeV/u measured with the HADES over a large angular range are reported. The obtained data corresponds to large ...transverse momenta, where a high sensitivity to the two-nucleon and three-nucleon short-range correlations is expected.
The preliminary results on charged pion production in np collisions at an incident beam energy of 1.25 GeV measured with HADES are presented. The np reactions were isolated in dp collisions at 1.25 ...GeV/u using the Forward Wall hodoscope, which allowed to register spectator protons. The results for np -> pppi-, np -> nppi+pi- and np -> dpi+pi- channels are compared with OPE calculations. A reasonable agreement between experimental results and the predictions of the OPE+OBE model is observed.
We present transverse momentum spectra, rapidity distribution and multiplicity of Lambda-hyperons measured with the HADES spectrometer in the reaction Ar(1.76A GeV)+KCl. The yield of Xi- is ...calculated from our previously reported Xi-/(Lambda+Sigma0) ratio and compared to other strange particle multiplicities. Employing a strangeness balance equation the multiplicities of the yet unmeasured charged Sigma hyperons can be estimated. Finally a statistical hadronization model is used to fit the yields of pi-, K+, K0s, K-, phi, Lambda and Xi-. The 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 particles.
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.
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.
We present the analysis of the inclusive \(K^{0}\) production in p+p and p+Nb collisions measured with the HADES detector at a beam kinetic energy of 3.5 GeV. Data are compared to the GiBUU transport ...model. The data suggest the presence of a repulsive momentum-dependent kaon potential as predicted by the Chiral Perturbation Theory (ChPT). For the kaon at rest and at normal nuclear density, the ChPT potential amounts to \(\approx 35\) MeV. A detailed tuning of the kaon production cross sections implemented in the model has been carried out to reproduce the experimental data measured in p+p collisions. The uncertainties in the parameters of the model were examined with respect to the sensitivity of the experimental results from p+Nb collisions to the in-medium kaon potential.
Results on \(\Lambda\)p femtoscopy are reported at the lowest energy so far. 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 \(\Lambda\) p correlations at small relative momenta. The experimental correlation function is compared to corresponding model calculations allowing the determination of the space-time extent of the \(\Lambda\)p emission source. The \(\Lambda\)p source radius is found slightly smaller than the pp correlation radius for a similar collis ion system. The present \(\Lambda\)p radius is significantly smaller than that found for Au+Au/Pb+Pb collisio ns in the AGS, SPS and RHIC energy domains, but larger than that observed for electroproduction from He. Taking into account all available data, we find the \(\Lambda\)p source radius to increase almost linearly with the number of participants to the power of one-third.
