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We discuss the consequences of the approximate conservation of the vector andaxial currents for the hadronic matrix elements appearing in beta decay ifnon-standard interactions are present. In particular the isovector(pseudo)scalar charge g_S(P) of the nucleon can be related to the difference(sum) of the nucleon masses in the absence of electromagnetic effects. Usingrecent determinations of these quantities from phenomenological and lattice QCDstudies we obtain the accurate values g_S=1.02(11) and g_P=349(9) in the MS-barscheme at mu=2 GeV. The consequences for searches of non-standard scalarinteractions in nuclear beta decays are studied, finding epsilon_S=0.0012(24)at 90%CL, which is significantly more stringent than current LHC bounds andprevious low-energy bounds using less precise g_S values. We argue that ourresults could be rapidly improved with updated computations and the directcalculation of certain ratios in lattice QCD. Finally we discuss the pion-poleenhancement of g_P, which makes beta decays much more sensitive to non-standardpseudoscalar interactions than previously thought.

Isospin breaking in the nucleon mass and the sensitivity of beta decays to new physics