Measurement of the $\Lambda$ hyperon polarization in $\nu_{\mu}$ charged current interactions in the NOMAD experiment

The Lambda hyperon polarization in nu /sub mu / charged current interactions has been measured in the NOMAD experiment. The event sample (8087 reconstructed Lambda 's) is more than an order of magnitude larger than that of previous bubble chamber experiments, while the quality of the event reconstruction is comparable. We observe a negative polarization along the W-boson direction which is enhanced in the target fragmentation region: P/sub x/(x/sub F/<0) =-0.21+or-0.04(stat)+or-0.02(sys). In the current fragmentation region we find P/sub x/(x/sub F/>0)=-0.09+or-0.06(stat)+or-0.03(sys). These results provide a test of different models describing the nucleon spin composition and the spin transfer mechanisms. We also observe a significant transverse polarization (in the direction orthogonal to the Lambda production plane) for the first time in a neutrino experiment: P/sub y/=-0.22+or-0.03(stat)+or-0.01(sys). The dependence of the absolute value of P/sub y/ on the Lambda transverse momentum with respect to the hadronic jet direction is in qualitative agreement with the results from unpolarized hadron-hadron experiments. The dependence of the Lambda polarization on the type of target nucleon in nu /sub mu / scattering has been also studied. (11 refs).