  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 Pee < 0.8 GeV/c a strong modification of the shape of
the spectrum, which we attribute to an additional rho-like contribution and a
decrease of omega yield. These opposite trends are tentatively interpreted as a
strong coupling of the rho meson to baryonic resonances and an absorption of
the omega meson, which are two aspects of in-medium modification of vector
mesons.
