Baryon-antibaryon pair production in time-dependent meson fields.

Abstract

Strong mean meson fields, which are known to exist in normal nuclei, experience a violent deformation in the course of a heavy-ion collision at relativistic energies. This may give rise to a new collective mechanism of the particle production, not reducible to the superposition of elementary nucleon-nucleon collisions. We study the baryon-antibaryon (NN\ifmmode\bar\else\textasciimacron\fi{},\ensuremath{\Lambda}\ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{},...) pair production under some simplifying assumptions about the space and time variation of meson fields in the nuclear collision process. The mutual deceleration of nuclei is described schematically by introducing the time-dependent relative velocity. For comparison the yield of NN\ifmmode\bar\else\textasciimacron\fi{} pairs is also calculated within a convolution model assuming the same deceleration scenario. Due to the specific Lorentz structure, the vector meson field turns out to be more efficient for producing pairs than the scalar field. Within the perturbative approach we study two processes: the bremsstrahlung of a virtual meson and the fusion of two virtual mesons, both leading to the baryon-antibaryon final states. The calculated multiplicities of NN\ifmmode\bar\else\textasciimacron\fi{} and \ensuremath{\Lambda}\ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{} pairs grow fast with the bombarding energy, reaching a saturation above the RHIC energy (\ensuremath{\surd}s=200A GeV). At lower energies the coherent production mechanism gives higher pair yields than predicted by microscopic cascade-like models. The rapidity spectra of antibaryons exhibit a characteristic two-hump structure which may serve as an observable signature of the bremsstrahlung mechanism. A strong sensitivity of the predicted yield to the baryon effective mass and the parameters characterizing the stopping power of nuclear matter is demonstrated.