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

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 $P_{ee}$ < 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.