Measurement of the Λ polarization in νμ charged current interactions in the NOMAD experiment

The Lambda polarization in upsilon (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 event reconstruction is comparable. We observe negative polarization along the W-boson direction which is enhanced in the target fragmentation region: P-x(x(F) < 0) = -0.21 +/- 0.04(stat) +/- 0.02(sys). In the current fragmentation region we find P-x(x(F) > 0) = -0.09 +/- 0.06(stat) +/- 0.03(sys). These results provide a test of different models describing the nucleon spin composition and the spin transfer mechanisms. A significant transverse polarization tin the direction orthogonal to the Lambda production plane! has been observed for the first time in a neutrino experiment: P-y = -0.22 +/- 0.03(stat) +/- 0.01(sys). The dependence of the absolute value of P-y on the Lambda transverse momentum with respect to the hadronic jet direction is in qualitative agreement with the resuits from unpolarized hadron-hadron experiments.