Medium effects in the production and decay of ω- and ϱ-resonances in pion-nucleus interactions

Abstract

The ω-and ρ-resonance production and their dileptonic decay in π − A reactions at GSI energies are calculated within the intranuclear cascade (INC) approach. The invariant mass distribution of the dilepton pair for each resonance is found to have two components which correspond to the decay of the resonances outside and inside the target nucleus. The latter components are strongly distorted by the nuclear medium due to resonance-nucleon scattering and a possible mass shift at finite baryon density. These medium modifications are compared to background sources in the dilepton spectrum from πN bremsstrahlung and the Dalitz decays of ∆'s, ω and η mesons produced in the reaction. * Supported by DFG 1The question about the properties of hadronic resonances in the nuclear medium has received a vivid attention during the last years (cf. Refs. [1,2,3,4]). Here, QCD inspired effective Lagrangian models [1,2] or approaches based on QCD sum rules [3,4] predict that the masses of the vector mesons ρ, ω and φ should decrease with the nuclear density. Furthermore, along with a dropping mass the phase space for the resonance decay also decreases which results in a modification of the resonance width in matter. On the other hand, due to collisional broadening -which depends on the nuclear density and the resonance-nucleon interaction cross section ( cf. Refs. [5,6]) -the resonance width should increase again.The in-medium properties of vector mesons have been addressed experimentally so far by dilepton measurements at the SPS, both for proton-nucleus and nucleusnucleus collisions [7,8,9]. As proposed by Li et al. [10], the enhancement in S + Au reactions compared to p + Au collisions in the invariant mass range 0.3 ≤ M ≤ 0.7GeV might be due to a shift of the ρ meson mass. The microscopic transport studies in Refs. [11,12] for these systems point in the same direction, however, also more conventional selfenergy effects cannot be ruled out at the present stage [11,13]. It is therefore necessary to have independent information on the vector meson proporties from reactions, where the dynamical picture is more transparent, i.e. in pion-nucleus collisions. Here, especially the ω meson can be produced with low momenta in the laboratory system, such that a substantial fraction of them will still decay inside a heavy nucleus [14].The mass distributions of the vector mesons in the latter case are expected to have a two component structure [6] in the dilepton invariant mass spectrum: the first component corresponds to resonances decaying in the vacuum, thus showing the free spectral function which is very narrow in case of the ω meson; the second (broad) component then corresponds to the resonance decay inside the nucleus. We will use that (in first order) the in-medium resonance can also be described by a Breit-Wigner formula with a mass and width distorted by the nuclear environment.In this letter we present first microscopic calculations for the production and dilep- The vector mesons ρ, ω are produced in the first hard pion-nu...

ρ-meson properties at finite nuclear density

“…In the low density approximation one now can express the correction of the ρ-meson mass and width at finite nuclear density ρ N through f ρN using Eqs. (7) and (8). We show the corresponding results for the mass and width of the ρ-meson in Fig.…”

Current correlation functions, QCD sum rules and vector mesons in baryonic matter

“…Such processes can be explored with the upcoming HADES lepton pair spectrometer at GSI. A reaction of particular interest is π − p → ωn in heavy nuclei with subsequent decay ω → e + e − and a kinematic cut focusing on low ω meson momenta [39,40].…”

Studying the ωN elastic and inelastic cross section with nucleons

“…An earlier version of this approach, denoted as Intra-Nuclear Cascade (INC) model, has been applied to the analysis of η and ω production in pA and pA interactions in [22,23], respectively. Recently this version of the INC model has been extended to incorporate in-medium modifications of the mesons produced [4,5] in hadron-nuclear collisions. For an explicit presentation of the production cross sections employed we refer the reader to [5] or the review [1].…”

“…at density ρ 0 ≈0. 16 fm −3 [3][4][5][6]. However, the inmedium properties of the vector mesons -as reflected in their spectral functions -are presently a matter of strong debate.…”