Lambda collective flow in heavy-ion reactions

Collective flow of lambda hyperons in heavy-ion reactions at SIS energies is investigated. It is found that a Λ mean field constructed on the basis of the quark model leads to a good description of the experimental data of the in-plane transverse flow of Λ's. The attractive mean field can also give rise to an additional 'virtual' Λ radial flow directed inwards, which is reflected by a 'concave' structure of the transverse mass spectrum of the Λ hyperons emitted at midrapidity. The Λ radial flow is found to exhibit a strong mass dependence: The flow is visible in the Ni + Ni system, but is strongly reduced in the system of Au on Au