Measurement of the Lambda polarization in nu/mu charged current interactions in the NOMAD experiment

Abstract

The Λ polarization in ν μ charged current interactions has been measured in the NOMAD experiment. The event sample (8087 reconstructed Λ '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 (in the direction orthogonal to the Λ 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 Λ transverse momentum with respect to the hadronic jet direction is in qualitative agreement with the results from unpolarized hadron–hadron experiments.