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

Abstract

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 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 θ 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 μ decay.