Subthreshold cascade production in heavy ion collisions

Using a gauged avor SU(3)-invariant hadronic Lagrangian, we have calculated the cross sections for the strangeness-exchange reactions Y Y → NΞ (Y = λ,∑) in the Born approximation. These cross sections are then used in the Relativistic Vlasov-Uehling-Uhlenbeck (RVUU) transport model to study Ξ production in Ar+KCl collisions at incident energy of 1.76A GeV and impact parameter b = 3:5 fm. We find that including the contributions of hyperon-hyperon scattering channels strongly enhances the yield of Ξ, leading to the abundance ratio Ξ̄/(λ + ∑⁰) = 3:38 × 10^-3, which is essentially consistent with the recently measured value of (5.6 ± 1.2^+1.8 _-1.7) × 10 ^-3 by the HADES collaboration at GSI.