The measurement of the beta asymmetry in the decay of polarized neutrons

The beta-decay asymmetry parameter A was measured for the free neutron with high precision. A time projection chamber (TPC) and plastic scintillators were used as a track and energy detector for the decay electrons. Thus the decay volume was well defined. Electron tracks with different angle theta between the electron momentum and the neutron spin could be selected and compared with the expected cos theta distribution. A deviation from this distribution was observed and remains unexplained. Using the electron track detector only to delimit the fiducial volume and adopting the theoretically predicted cos theta distribution a value of A(0) = -0.1160 +/- 0.0015 was determined including corrections for recoil and weak magnetism. To our best knowledge this result is not influenced by the observed deviation from the cos theta distribution of the asymmetry. From this result the ratio g(A)/g(V) = - 1.266 +/- 0.004 can be deduced within the Standard Model. Using the recent neutron lifetime value leads to g(V) = 1.4172 (34). 10(-62) J m(3). This value is in good agreement with g(V) from ft(0(+) --> 0(+)) and mu decay.