Evolution of baryon-free matter produced in relativistic heavy-ion collisions

A three-fluid hydrodynamic model is introduced for simulating heavy-ion collisions at incident energies between a few and about 200 A GeV. In addition to the two baryon-rich fluids of two-fluid models, a new model incorporates a third, baryon-free (i.e., with zero net baryonic charge), fluid, which is created in the midrapidity region. Its evolution is delayed due to a formation time τ, during which the baryon-free fluid neither thermalizes nor interacts with the baryon-rich fluids. After formation, it thermalizes and starts to interact with the baryon-rich fluids. It is found that, for τ=0, the interaction strongly affects the baryon-free fluid. However, at reasonable finite formation time, τ ? 1 fm/c, the effect of this interaction turns out to be substantially reduced, although still noticeable. Baryonic observables are only slightly affected by the interaction with the baryon-free fluid.