Beta decay of the neutron

A review is given of experimental studies of the beta decay of the free neutron carried out up to the present time. Data are given on the beta spectrum, half-life, and all four possible angular correlations in the decay of polarized neutrons. The features of the experimental apparatus are considered, and the main problems in the experimental technique of the corresponding measurements are discussed. The shape of the beta spectrum does not reveal any important deviations from a Fermi spectrum for allowed transitions. There are two values for the half-life, which do not agree with each other: $T_{1/2}=11.7\pm0.3$ min (from the work of P. E. Spivak et al. in 1959) and $T_{1/2}=10.6\pm0.16$ min (from the work of C. Christensen et al. in 1971). No angular correlation has been observed between the neutron spin and the decay plane (violating $T$ parity). The corresponding triple correlation constant is given by the latest data as $D=(-1.1\pm1.7)\times10^{-3}$ (Grenoble, 1974) and $D=(-2.7\pm3.3)\times10^{-3}$ (Moscow, 1974). Comparison of all the experimental data with each other shows that the weighted mean value of the neutrino-electron, neutron spin-electron, and neutron spin-neutrino angular-correlation constants are in good agreement with each other, and also with the result of the last measurement of the decay constant, within the framework of the $V$-$A$ theory of weak interaction, and lead to a value of the fundamental ratio $G_A/G_V=1.263\pm0.016$. However, as shown in the review, the accuracy of the measurements achieved up to the present time is completely inadequate to exclude very substatial contributions of scalar (up to 30–40%) or tensor (up to 15%) terms in the Hamiltonian of the weak interaction.