Open Access

The study of meson production in proton-proton collisions in the energy range up to one GeV above the production threshold provides valuable information about the nature of the nucleon-nucleon interaction. Theoretical models describe the interaction between nucleons via the exchange of mesons. In such models, different mechanisms contribute to the production of the mesons in nucleon-nucleon collisions. The measurement of total and differential production cross sections provide information which can help in determining the magnitude of the various mechanisms. Moreover, such cross section information serves as an input to the transport calculations which describe e.g. the production of e+e− pairs in proton- and pion-induced reactions as well as in heavy ion collisions. In this thesis, the production of ω and η mesons in proton-proton collisions at 3.5 GeV beam energy was studied using the High Acceptance DiElectron Spectrometer (HADES) installed at the Schwerionensynchrotron (SIS 18) at the Helmholtzzenturm f¨ur Schwerionenforschung in Darmstadt. About 80 000 ω mesons and 35 000 η mesons were reconstructed. Total production cross sections of both mesons were determined. Furthermore, the collected statistics allowed for extracting angular distributions of both mesons as well as performing Dalitz plot studies. The ω and η mesons were reconstructed via their decay into three pions (π+π−π0) in the exclusive reaction pp −→ ppπ+π−π0. The charged particles were identified via their characteristic energy loss, via the measurement of their time of flight and momentum, or using kinematics. The neutral pion was reconstructed using the missing mass method. A kinematic fit was applied to improve the resolution and to select events in which a π0 was produced. The correction of measured yields for the effects of spectrometer acceptance was done as a function of four variables (two invariant masses and two angles). Systematic studies of the acceptance for different input distributions were performed. The measured yields were normalized to the number of measured events of elastic scattering. Systematic errors due to the methods of the data analysis and the background subtraction were investigated. Production angular distributions of ω and η mesons were measured. Both mesons exhibit a slightly anisotropic angular distribution. The Dalitz plot of ω meson production shows indications of resonant production. However, the deviation of the distribution from the one expected by phase space simulations is not large. The Dalitz plot of η meson production shows a signal of the production via the N(1535) resonance, The contribution of N(1535) to the production was quantified to be about 47%. The angular distribution of η mesons does not show significant differences between resonant and non resonant production. The total production cross section of ω mesons in the reaction pp −→ ppω was determined to be 106.5 ± 0.9 (stat) ± 7.9 (sys) [μb] where stat indicates statistical error and sys indicates systematic error, while that of η mesons was determined to be 136.9 ± 0.9 (stat) ± 10.1 (sys) [μb] in the reaction pp −→ ppη

We present measurements of exclusive ensuremathπ+,0 and η production in pp reactions at 1.25GeV and 2.2GeV beam kinetic energy in hadron and dielectron channels. In the case of π+ and π0 , high-statistics invariant-mass and angular distributions are obtained within the HADES acceptance as well as acceptance-corrected distributions, which are compared to a resonance model. The sensitivity of the data to the yield and production angular distribution of Δ (1232) and higher-lying baryon resonances is shown, and an improved parameterization is proposed. The extracted cross-sections are of special interest in the case of pp → pp η , since controversial data exist at 2.0GeV; we find \ensuremathσ=0.142±0.022 mb. Using the dielectron channels, the π0 and η Dalitz decay signals are reconstructed with yields fully consistent with the hadronic channels. The electron invariant masses and acceptance-corrected helicity angle distributions are found in good agreement with model predictions.

The knowledge of baryonic resonance properties and production cross sections plays an important role for the extraction and understanding of medium modifications of mesons in hot and/or dense nuclear matter. We present and discuss systematics on dielectron and strangeness production obtained with HADES on p+p, p+A and A+A collisions in the few GeV energy regime with respect to these resonances.

We present the results of two-pion production in tagged quasi-free np collisions at a deutron incident beam energy of 1.25 GeV/c measured with the High-Acceptance Di-Electron Spectrometer (HADES) installed at GSI. The specific acceptance of HADES allowed for the first time to obtain high-precision data on π+π− and π−π0 production in np collisions in a region corresponding to large transverse momenta of the secondary particles. The obtained differential cross section data provide strong constraints on the production mechanisms and on the various baryon resonance contributions (∆∆, N(1440), N(1520), ∆(1600)). The invariant mass and angular distributions from the np → npπ+π −and np → ppπ−π0 reactions are compared with different theoretical model predictions.

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 “ppK −” (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 KNN (or, specifically “ppK −”) 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 KNN cluster. At a confidence level of CLs = 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.