First measurement of low momentum dielectrons radiated off cold nuclear matter
Keywords
p nucleus: scattering, niobium, electron: pair production, momentum: low, vector meson: spectral representation, nuclear matter: effect, baryon resonance, electron positron: mass spectrum, HADES, rho(770): coupling, omega(783): absorption, multiple scattering, experimental methods, Darmstadt SIS, 3.5 GeV-kin
Abstract
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_s16e invariant mass spectra to elementary p_s16p data, we observe for e_s16e 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.
