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The vector analyzing power for the n + d-* n + n+p breakup reaction was measured at 12 MeV. Data for n-p final-state interaction and n-p quasifree scattering, along with elastic-scattering data, are compared to rigorous three-nucleon calculations using the Paris and Bonn TV-TV potentials. Calculations agree with the quasifree data and with the elastic and n-p final-state-interaction data except around 0cm. = 120°. PACS numbers: 25.10,+s, 24.70. +s, 25.40.Dn, 25.40.Fq Recently, considerable progress has been achieved both in the calculation of the three-nucleon (37V) scattering observables 1 and in the accuracy of neutrondeuteron (n-d) elastic-scattering data. 2 Such 37V scattering calculations are now done with use of nucleon-nucleon (TV-TV) potentials based on realistic mesonexchange theory, e.g., Paris 3 and Bonn 4 potentials, and the Faddeev equations 5 are solved rigorously. In previous works by other groups various approximations were made. For elastic scattering, separable finite-rank approximations to the Paris and Bonn potentials 2,6 and phenomenological TV-TV potentials 7 were used, while for the breakup process, calculations were performed with phenomenological separable 8 and local (treating higher partial waves perturbatively) 9 potentials. We are now able to solve the three-nucleon continuum problem rigorously (i.e., with high numerical accuracy 10 ) using TV-TV forces based on meson-exchange theory. Only for this case can one be sure that discrepancies with experimental data indicate a defect of the input nuclear interaction. Our calculations are restricted to n-d scattering, thus avoiding the still pending Coulomb problem. The comparison between 3TV calculations and n-d data is essential, since some three-body observables, e.g., analyzing powers, display more sensitivity to certain N-N partial-wave interactions than the two-body observables. Furthermore, 37V force effects can be determined only from studies of the 37V system. In the present Letter we report n-d predictions for the vector analyzing power A y (6) using the Paris potential 3 and the Bonn relativistic one-boson-exchange potential in q space (OBEPQ) 4 and compare them with our elastic-scattering and breakup data obtained at an incident neutron energy of 12 MeV.These data are the highest-accuracy data for these observables.While n-d breakup cross sections have been measured extensively, 11 the A y (0) have been measured only for the neutron-proton (n-p) final-state interaction (FSI) at 14.3 and 29.6 MeV (Ref. 12) with monoenergetic neutron beams and at nine energies from 20 to 50 MeV with use of a continuous-energy neutron beam. 13 Kinematically complete measurements of cross section and A y (0) for n+d breakup reactions are in progress by Geissdorfer et al. at 13.9 MeV. 14 The experimental study of the 2 H(n,np)n reaction was conducted at the Triangle Universities Nuclear Laboratory with polarized neutrons bombarding a deuterated scatterer (C6D12) in the form of a liquid scintillator, which allowed a measurement of the energy of th...