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

Abstract

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 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. The QCD vacuum is characterized by nonzero expectation values of various quark and gluon operators. Most notable is the finite chiral quark condensate, signaling the spontaneous breaking of chiral symmetry. In a cold strongly interacting medium, the chiral condensate is expected to be modified (in linear density approximation, by about 30% at nuclear saturation density). As predicted by various models [1], [2], [3], [4], [5], [6], [7], [8] and [9], vector meson properties should be affected by changes of the condensates. Such phenomena can be studied in leptonic decays of the vector mesons (e.g. V→e+e−, with V=ρ,ω,ϕ). According to [10] the ρ meson is especially sensitive to changes of those condensates which break chiral symmetry (also called chirally odd condensates). In hadronic many-body approaches many hints have been found for a broadening of the ρ meson spectral function in an ambient nuclear medium [5], [6], [11] and [12]. Experimentally, in-medium properties can be studied in heavy-ion collisions (probing hot and dense hadronic matter) or in proton-, pion- or photon-induced reactions on nuclei (probing cold nuclear matter). For recent reviews on medium effects in cold nuclear matter see in particular [6] and [7]. Medium modifications are expected to be stronger in heavy-ion collisions due to the higher densities and temperatures. Measured observables represent an average over the complete space-time evolution of the temperature and the density of the system. On the other hand, in proton-, pion- or photon-induced reactions the system does not undergo a noticeable density and temperature evolution in time hence conditions of the system are better defined. Electron pair (e+e−) decay of vector mesons is an ideal probe for such studies since electrons and positrons are not affected by strong final state interactions. A promising observable is the spectral shape, i.e. the e+e− invariant mass distribution, but also the nuclear modification of the cross section provides valuable information since i...

Dilepton production and resonance properties within a new hadronic transport approach in the context of the GSI-HADES experimental data

“…The dilepton production in proton-nucleus (pA) collisions, as a cold nuclear matter scenario, is discussed for a proton projectile that scatters on a niobium target with E Kin = 3.5 GeV (pNb). To obtain the cross-section the value reported in [21] (σ pNb = 848 ± 127 mb) is used.…”

“…A comparison to HADES data [21] of the invariant electron-pair mass spectrum is displayed in Fig. 12. In the low-mass region, the π and η contributions are prominent.…”

Experiments on Hadrons in Finite Density Medium

“…2. The spectrum for p+p collisions (open symbols) is shown together with a dilepton cocktail obtained from a tuned PYTHIA simulation in combination with the PLUTO decay generator [12]. The latter takes care of the (semi) leptonic decay processes using proper mass dependent electromagnetic transition form factors (cf.…”

Medium effects in proton-inducedK0production at 3.5 GeV

“…For the p + Nb case the total reaction cross section σ pNb = 848 ± 127 mb is provided by the analysis of the charged pion production [28,29].…”

“…The ratio in terms of R Nb/p (p) has already been used in the comparative analysis of the electron-positron pair production in p + p and p + Nb collisions [29]. There, it 10. (Color online) Momentum dependence of the nuclear modification factor R Nb/p (p) ∝ σ K 0 pNb /σ K 0 pp .…”