Abstract
Measurements of neutron beta decay observables required to determine | V ud | 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 | V ud | from it. We also discuss the region of parameter space in the | V ud |– λ plane where new physics effects should lie, if they contribute to neutron beta decay.
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