First measurement of proton-induced low-momentum dielectron radiation off cold nuclear matter

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