Quasifree Ξ- production in the 9Be(K-, K+) reaction

We studied the 9Be(K-, K+) reaction at PK-=1.8 GeV/c with and without an explicit identification of a Ξ- hyperon. The Ξ- hyperon, whose typical momentum was around 550 MeV/c, was identified by using the cylindrical detector system (CDS), which was newly constructed to detect the charged particles from the Ξ- decay in a wide angular range. It had a large solid angle of 65% of 4π, and a good momentum resolution of dP/P=3.3% in sigma for 100 MeV/c particles. Under the classical cascade assumption in which a Ξ- hyperon is produced in the quasifree process and then interacts with a nucleon in the target nucleus, we identified the events in which the produced Ξ- hyperon was further scattered by a nucleon in the same nucleus. From these data, we obtained the mean free path of a Ξ- hyperon in the nucleus, and the isospin-averaged Ξ-N elastic scattering cross section, found to be 1.7-0.4+0.6(stat)-0.4+0.6(syst) fm and 30.7±6.7(stat)-3.6+3.7(syst) mb, respectively. From the measured probability in detecting a proton in addition to a Ξ- hyperon, the ratio of Ξp and Ξn scattering cross sections was found to be 1.1-0.7+1.4(stat)-0.4+0.7(syst). (author)