Spin-parameter measurements inΛandKSproduction

The Brookhaven Alternating Gradient Synchrotron polarized proton beam incident on a beryllium target was used for inclusive Λ production at beam momenta of 13.3 and 18.5 GeV/c. The beam polarization was transverse to the beam direction with magnitude 0.63 at 13.3 GeV/c and 0.40 at 18.5 GeV/c. The Λ polarization was measured and found to be in agreement with results from earlier experiments which used unpolarized proton beams. Analyzing power $A_N$ and spin transfer $D_{\mathrm{NN}}$ of the Λ’s were both measured and compared with a hyperon-polarization model in which the polarization arises from a Thomas-precession effect. There is good agreement with its predictions: $A_N$=0 and $D_{\mathrm{NN}}$=0. In particular, our measurement of 〈$D_{\mathrm{NN}}$〉=-0.009±0.015 supports the idea that the valence quarks carry all of the hadron spin, since this assumption is implicit in the model’s use of SU(6) wave functions to form final-state hadrons from beam fragments and sea quarks. The presence of substantial $K_S$ samples at both beam momenta and Λ¯’s at 18.5 GeV/c prompted a measurement of their analyzing powers, which yielded $A_N$($K_S$)=-0.094±0.012 at 13.3 GeV/c beam momentum and -0.076±0.015 at 18.5 GeV/c, and $A_N$(Λ¯)=0.03±0.10.