Neutron beta decay and the current determination of Vud

Measurements of neutron beta decay observables required to determine |Vud| are reaching the 0.1% accuracy. In this Letter we review the calculation of the decay rate of this process, discuss its relevant uncertainties, and obtain an expression that is precise at the 10−4 level. Our analysis clearly shows the necessity of more precise measurements of λ, the ratio of axial/vector couplings. The current situation in neutron beta decay is that one cannot yet quote a single consistent value for |Vud| from it. We also discuss the region of parameter space in the |Vud|–λ plane where new physics effects should lie, if they contribute to neutron beta decay.