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See paper for full list of authors,; We present data on dielectron emission in proton induced reactions on a Nb target at 3.5 GeV kinetic beam energy measured with HADES installed at GSI. The data represent the first high statistics measurement of proton-induced dielectron radiation from cold nuclear matter in a kinematic regime, where strong medium effects are expected. Combined with the good mass resolution of 2%, it is the first measurement sensitive to changes of the spectral functions of vector mesons, as predicted by models for hadrons at rest or small relative momenta. Comparing the e+e− invariant mass spectra to elementary p + p data, we observe for e+e− momenta Pee0.8 GeV/c) and are not conclusive yet. For the ρ meson, the CLAS experiment at JLab [13] reports a slight broadening and no shift of the ρ pole mass in photon induced reactions, while the E325 experiment at KEK [14] deduced a shift but no broadening in proton induced reactions. For the ω and ϕ mesons, experiments [15], [16], [17] and [18] report a sizable collisional broadening (up to a factor 16 larger than the natural line width in case of the ω) inside the medium extracted by comparing the nuclear modification of the cross sections to microscopic transport models. Besides the complications due to additional yield fed by pion driven secondary collisions inside the nucleus, the disadvantage of these indirect measurements is their model dependence. For instance, the recent analysis of [19] led to a significantly different width of the ω meson inside the medium as extracted in [15]. In this Letter we report on inclusive e+e− pair production in proton induced reactions at Ekin=3.5 GeV on the nucleus Nb, representing the first high statistics measurement with small momenta of e+e− pairs relative to the medium (Pee

Positron lines were observed in heavy ion-atom collisions at bombarding energies close to the Coulomb barrier in subcritical systems with the sum of the atomic numbers of the colliding nuclei Zu=Z1+Z2 being smaller than Zu=172. Each collision system, studied,208Pb +208Pb(Zu=164),238U+181Ta(Zu=165), and238Au(Zu=171), exhibits the emission of two positron lines with Zu-independent c.m. energies of ∼ 258 keV and ∼ 340 keV, and with widths of about 30 keV, superimposed on continuous positron spectra from nuclear pair decay and pair emission induced by the time changing Coulomb field of the collision. The production cross section of the e+-lines rises with a high power of Zu(ocZu 22), which is comparable to the Zu 20-dependence for the collision induced positrons. © 1987 Springer-Verlag. 328 2 129 145

Spectra of positrons emitted in heavy-ion-atom collisions close to the Coulomb barrier were measured. The data exhibit broad energy distributions according to expectations for induced positron creation by the strong time-changing Coulomb field. For the heavy systems U+U and U+Th superimposed narrow lines at 300 keV positron energy were observed. Possible sources for this unexpected line - such as nuclear excitations or spontaneous positron emission enhanced by a prolonged nuclear reaction time - are discussed. © 1984. 137 1-2 41 46 Cited By :218

First results on the energy and angular dependence of the (e + e − ) pairs, emitted from U + U and U + Pb collisions at a beam energy of 5.9 MeV/u, are presented. The (e + e − ) coincidence measurements were carried out utilizing a double-Orange s-spectrometer, which allows to determine the momentum vectors of the positrons and electrons and, hence, their opening angle θ e + e − . The (e + e − ) sum-energy spectrum obtained for θ e + e − =(180±20)°, which is equivalent to an invariant mass spectrum, indicates multiple-line structures in the energy range between 500 keV and 950 keV for both collision systems. The most pronounced line appears at a sum energy of ∼ 810 keV, corresponding to an invariant mass of ∼ 1.83 MeV / c 2 . No evidence for line structures was found for angles 40° ⩽ θ e + e − ⩽ 170°. The energies of the lines are consistent with our previous observations in the single p positron spectra.

High resolution positron spectroscopy in U-U collisions at bombarding energies of 5.6 MeV/u, the lowest energy studied until now as well as of 5.9 MeV/u, reveals similar multiple structures between 220 keV and 385 keV superimposed on the induced and nuclear continua. © 1987 Springer-Verlag. 326 2 235 236

The observation of a peak structure in the electron spectra resulting from e++Th,U collisions with a measured FWHM of (3.7 0.5) keV cannot be attributed to a new phenomenon, i.e., the e+e- decay of a neutral particle as recently claimed, because in this case the expected resonance width is broadened to at least 60 keV (FWHM) in the laboratory frame because of the motion of the bound electrons. © 1989 The American Physical Society. 40 4 1839 1840

Energy and angular correlations of (e + e − ) pairs emitted in HI collisions were investigated. Some evidence is found for a weak peak at 815±5keV in the (e + e − ) sum-energy spectrum obtained for θ e + c − = 180° ± 18° (FWHM). The peak energy is consistent with previous observations.

The High-Acceptance Di-Electron Spectrometer installed at GSI Darmstadt is a second generation experiment to study production of dielectron pairs from proton, pion and nucleus induced reactions at the SIS/BEVALAC energy regime. During period 2002-4 medium-resolution data have been taken with HADES on the light C+C system at 1 and 2 AGeV. The data analysis confirms former finding of the DLS collaboration. First physics run on slightly heavier system Ar+KCl was carried out will almost full HADES setup at 2005. © 2007 American Institute of Physics. 947 436 440 Sponsors: U. S. National Science Foundation Physics Latin American Center

Baryon resonance production in proton-proton collisions at a kinetic beam energy of 1.25 GeV is investigated. The multi-differential data were measured by the HADES collaboration. Exclusive channels with one pion in the final state ($np\pi^{+}$ and $pp\pi^{0}$) were put to extended studies based on various observables in the framework of a one-pion exchange model and with solutions obtained within the framework of a partial wave analysis (PWA) of the Bonn-Gatchina group. The results of the PWA confirm the dominant contribution of the $\Delta$(1232), yet with a sizable impact of the $N$(1440) and non-resonant partial waves., Comment: 12 pages, 6 figures

Following a suggestion of Bang et al. we deduce that the total energy transfer associated with the narrow positron lines observed in heavy-ion collisions is given approximately by the sum of the rest masses of the positron and the accompanied electron plus their kinetic energies. Since the same behaviour has been established previously for the production process of the dynamically induced positrons, our observation represents a strong hint that the origin of the e+ -lines may also be connected with the strong Coulomb field occuring in heavy-ion collisions. © 1987 Springer-Verlag. 328 4 499 500 Cited By :16

The excitation function for Bhabha scattering was reinvestigated in the energy range between 775 and 840 keV (c.m.) in 1.4 keV steps using a monoenergetic e+ -beam and a high-resolution kinematic coincidence detector device. Corroborating our previous results, no resonance was observed around 1.8 MeV/c2 up to an improved upper limit of 0.5 b·eV/sr (c.m.) for the energy-integrated differential cross section (97 % C.L.) The data rule out the existence of a resonance with a cross section of 5.0 b·eV/sr, recently reported elsewhere. © 1988 Springer-Verlag. 331 1 103 104 Cited By :17