Dilepton production by bremsstrahlung of meson fields in nuclear collisions

We study the bremsstrahlung of virtual ω mesons due to the collective deceleration of nuclei at the initial stage of an ultrarelativistic heavy-ion collision. It is shown that electromagnetic decays of these mesons may give an important contribution to the observed yields of dileptons. Mass spectra of $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ pairs produced in central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions are calculated under some simplifying assumptions on the space-time variation of the baryonic current in a nuclear collision process. Comparison with the CERES data for $160A\mathrm{GeV}$ $\mathrm{P}\mathrm{b}+\mathrm{A}\mathrm{u}$ collisions shows that the proposed mechanism gives a noticeable fraction of the observed $e^{+}{e}^{-}$ pairs in the intermediate region of invariant masses. Sensitivity of the dilepton yield to the in-medium modification of masses and widths of vector mesons is demonstrated.