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+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 ρ-like contribution and a decrease of ω yield. These opposite trends are tentatively interpreted as a strong coupling of the ρ meson to baryonic resonances and an absorption of the ω meson, which are two aspects of in-medium modification of vector mesons.
Highlights
We present data on dielectron emission in proton induced reactions on a Nb target at 3.5 GeV kinetic beam energy measured with High Acceptance Dielectron Spectrometer (HADES) installed at GSI
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
HADES Collaboration / Physics Letters B 715 (2012) 304–309 opposite trends are tentatively interpreted as a strong coupling of the ρ meson to baryonic resonances and an absorption of the ω meson, which are two aspects of in-medium modification of vector mesons
Summary
A promising observable is the spectral shape, i.e. the e+e− invariant mass distribution, and the nuclear modification of the cross section provides valuable information since it is connected to the total widths of the hadrons inside the medium. The latter reflect the coupling of mesons to resonance hole states, which is an important element of the in-medium self energy of propagating mesons. For the ω and φ mesons, experiments [15,16,17,18] 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. 2.8 is estimated with the help of a geometrical nuclear overlap model [26]
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