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We report measurements of electron pair production in elementary p + p and d + p reactions at 1.25 GeV/mu with the HADES spectrometer. For the first time, the electron pairs were reconstructed for n + p reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass Me+e- > 0.15 GeV/c(2) is about an order of magnitude larger in n + p reactions as compared to p + p. A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in C + C reactions are compatible with a superposition of elementary n + p and p + p collisions, leaving little room for additional electron pair sources in such light collision systems., The collaboration gratefully acknowledges the support by BMBF grants 06TM970I, 06GI146I, 06F-140, and 06DR135 (Germany), by GSI (TM-FR1,GI/ME3,OF/STR), by Helmholtz Alliance HA216/EMMI, by grants GA CR 202/00/1668 and GA AS CR IAA1048304 (Czech Republic), by grant KBN 1P03B 056 29 (Poland), by INFN (Italy), by CNRS/IN2P3 (France), by grants MCYT FPA2000-2041-C02-02 and XUGA PGID T02PXIC20605PN (Spain), by grant UCY-10.3.11.12 (Cyprus), by INTAS grant 03-51-3208 and by EU contract RII3-CT-2004-506078.

Two high-accuracy measurements of cross sections and analyzing powers for the proton-proton bremsstrahlung reaction have been performed at 190 MeV beam energy. These measurements provide not only the most accurate exclusive cross-section and analyzing-power data to date, but also a wide phase-space coverage. Thus, an accurate test of nucleon-nucleon interaction models is made feasible. The experimental setup and the analysis procedure are discussed in detail. Microscopic calculations and calculations based on the soft-photon theorem show deficiencies in explaining the bulk of the data.

The first high-statistics measurement of double-photon and dilepton yields in proton-proton scattering below the pion threshold has been performed. The data obtained will allow a detailed study of the proton-proton interaction., Izveli smo prvo mjerenje s dobrom statistikom emisije dvaju fotona i dileptona pri proton-proton raspršenju na energiji ispod praga za produkciju piona. Dobiveni podaci omogućuju dodatno proučavanje svojstava međudjelovanja protona.

real- and virtual-photon emission during interactions between few-nucleon systems have been investigated at KVI with a 190 MeV proton beam. Here I will concentrate the discussion on the results of the virtual-photon emission for the proton-proton system and proton-deuteron capture. Predictions of a fully-relativistic microscopic-model of the proton-proton interaction are discussed. For the proton-deuteron capture process the data is compared with predictions of a relativistic gauge-invariant impulse approximation and a Faddeev calculation. For the virtual photon processes, the nucleonic electromagnetic response functions were obtained for the first time and are compared to model predictions.

real- and virtual-photon emission during interactions between few-nucleon systems have been investigated at KVI with a 190 MeV proton beam. Here I will concentrate the discussion on the results of the virtual-photon emission for the proton-proton system and proton-deuteron capture. Predictions of a fully-relativistic microscopic-model of the proton-proton interaction are discussed. For the proton-deuteron capture process the data is compared with predictions of a relativistic gauge-invariant impulse approximation and a Faddeev calculation. For the virtual photon processes, the nucleonic electromagnetic response functions were obtained for the first time and are compared to model predictions.

real- and virtual-photon emission during interactions between few-nucleon systems have been investigated at KVI with a 190 MeV proton beam. Here I will concentrate the discussion on the results of the virtual-photon emission for the proton-proton system and proton-deuteron capture. Predictions of a fully-relativistic microscopic-model of the proton-proton interaction are discussed. For the proton-deuteron capture process the data is compared with predictions of a relativistic gauge-invariant impulse approximation and a Faddeev calculation. For the virtual photon processes, the nucleonic electromagnetic response functions were obtained for the first time and are compared to model predictions.

The real- and virtual-photon emission during interactions between few-nucleon systems have been investigated at KVI with a 190 MeV proton beam. Here I will concentrate on the discussion of the results of the virtual-photon emission for the proton-proton system and proton-deuteron capture. Predictions of a fully-relativistic microscopic-model of the proton-proton interaction are discussed. For the proton-deuteron capture process the data is compared with predictions of a relativistic gauge-invariant impulse approximation and a Faddeev calculation. For the virtual photon processes, the nucleonic electromagnetic response functions were obtained for the first time and are compared to model predictions.

The real- and virtual-photon emission during interactions between few-nucleon systems have been investigated at KVI with a 190 MeV proton beam. Here I will concentrate on the discussion of the results of the virtual-photon emission for the proton-proton system and proton-deuteron capture. Predictions of a fully-relativistic microscopic-model of the proton-proton interaction are discussed. For the proton-deuteron capture process the data is compared with predictions of a relativistic gauge-invariant impulse approximation and a Faddeev calculation. For the virtual photon processes, the nucleonic electromagnetic response functions were obtained for the first time and are compared to model predictions.

The first high-statistics measurement of dilepton and double-photon yields in proton-proton scattering below the pion threshold has been performed. The data obtained allow a detailed study of off-shell effects in the proton-proton interaction.

The first high-statistics measurement of dilepton and double-photon yields in proton-proton scattering below the pion threshold has been performed. The data obtained allow a detailed study of off-shell effects in the proton-proton interaction.