Collective mechanism of dilepton production in high–energy nuclear collisions

Collective bremsstrahlung of vector meson fields in relativistic nuclear collisions is studied within the time–dependent Walecka model. Mutual deceleration of the colliding nuclei is described by introducing the effective stopping time and average rapidity loss of baryons. It is shown that electromagnetic decays of virtual ω–mesons produced by bremsstrahlung mechanism can provide a substantial contribution to the soft dilepton yield at the SPS bombarding energies. In particular, it may be responsible for the dilepton enhancement observed in 160 AGev central Pb+Au collisions. Suggestions for future experiments to estimate the relative contribution of the collective mechanism are given. A high degree of baryon stopping observed in relativistic nuclear collisions implies a strong space–time variation of the collective baryon current in these reactions. According to the time– dependent Walecka model [1], in this situation it is natural to expect the excitation of propagating waves (bremsstrahlung) of classical meson fields. Conversion of virtual mesons into real particles may provide an additional important source of secondary particles in nuclear collisions. Unlike the conventional mechanism of incoherent hadron–hadron collisions the contribution of this source increases strongly (quadratically) with the number of participating nucleons. Following this idea in Refs. [2-4] we considered the production of pions, baryon–antibaryon pairs and dileptons by the collective bremsstrahlung and decay of virtual ω–mesons. It was shown that the role of the Collective Bremsstrahlung Mechanism (CBM) grows with bombarding energy and that it becomes important in central collisions of heavy nuclei already at the SPS energies. The study of dilepton production is especially interesting because of the strong enhancement of soft dileptons observed in nuclear collisions by the CERES [5] and HELIOS–3 [6] collaborations. It was shown [7–11] that the contribution of ππ → ρ → ll processes may explain the enhancement only if one assumes a strong modification of ρ–mesons at intermediate stages of