Remarks about spin measurements in p̄p → ΛΛ

We comment on the proposed measurement of the spin-depolarisation parameter Dnn in the strangeness-exchange reaction p̄p → ΛΛ. It is shown that the existing data on the correlation coefficients Cij limit the range allowed for Dnn. There is a renewed interest in the strangeness-exchange reaction p̄p → ΛΛ, due to the results of PS185 collaboration at CERN, which has measured the cross-section σ, the angular distribution I0(θcm), polarisation P (θcm), and correlation coefficients Cij(θcm), at various energies [1]. Here θcm is the angle of Λ with respect to p̄ in the centre-of-mass frame. Some of these data [2] are reminded in Fig. 1, at two selected momenta. So far, the spin measurements are given for free, thanks to the intrinsic asymmetry in the decay of Λ or Λ. The experimental investigation will be resumed using a transversely-polarised proton target [3], giving access to new observables, in particular the depolarisation Dnn and spin-transfer Knn coefficients, not to mention quantities involving more than two spins. The Dnn and Knn parameters measure by how much the transverse polarisation of the proton target modifies the transverse polarisation of the outcoming Λ and Λ, respectively. As for the theory of p̄p → ΛΛ, there are many interesting contributions, of which we shall only give a brief survey, before focusing on those where depolarisation and spin-transfer are discussed. A comprehensive bibliography can be found in Refs. [1, 4]. There are two main approaches. The first one relies on the exchange of strange e-mail: [email protected]