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Contributions of hyperon-hyperon scattering to subthreshold cascade production in heavy ion collisions

Using a gauged flavor SU(3)-invariant hadronic Lagrangian, we calculate the cross sections for the strangeness-exchange reactions YY to NΞ(Y=Λ, Σ) in the Born approximation. These cross sections are then used in the Relativistic Vlasov-Uehling-Uhlenbeck (RVUU) transport model to study Ξproduction 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 Ξ^{-}/(Λ+Σ^{0})=3.38E-3, which is essentially consistent with the recently measured value of $(5.6 \pm 1.2_{-1.7}^{+1.8})\times 10^{-3}$ by the HADES collaboration at GSI.