Neutron beta decay and the current determination of V-ud

Measurements of neutron beta decay observables required to determine /V-ud/ are reaching the 0.1% accuracy. In this Letter we review the calculation o f the decay rate of this process, discuss its relevant uncertainties, and o btain an expression that is precise at the 10(-4) level. Our analysis clear ly shows the necessity of more precise measurements of lambda, the ratio of axial/vector couplings. The current situation in neutron beta decay is tha t one cannot yet quote a single consistent value for IV,dl from it. We also discuss the region of parameter space in the /V-ud/-lambda plane where new physics effects: should lie, if they contribute to neutron beta decay. (C) 2001 Published by Elsevier Science B.V.