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 θ between the electron momentum and the neutron spin could be selected and compared with the expected cos θ 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 θ distribution a value of A0=-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 θ distribution of the asymmetry. From this result the ratio gA/gV=-1.266±0.004 can be deduced within the Standard Model. Using the recent neutron lifetime value leads to gV=1.4172 (34).10-62 J m3. This value is in good agreement with gV from ft(0+→0+) and μ decay. (orig.)