Isospin breaking in the nucleon mass and the sensitivity of beta decays to new physics
Abstract
We discuss the consequences of the approximate conservation of the vector and axial currents for the hadronic matrix elements appearing in beta decay if non-standard interactions are present. In particular the isovector (pseudo)scalar charge gS(P) of the nucleon can be related to the difference (sum) of the nucleon masses in the absence of electromagnetic effects. Using recent determinations of these quantities from phenomenological and lattice QCD studies we obtain the accurate values gS=1.02(11) and gP=349(9) in the MS-bar scheme at mu=2 GeV. The consequences for searches of non-standard scalar interactions in nuclear beta decays are studied, finding epsilonS=0.0012(24) at 90%CL, which is significantly more stringent than current LHC bounds and previous low-energy bounds using less precise gS values. We argue that our results could be rapidly improved with updated computations and the direct calculation of certain ratios in lattice QCD. Finally we discuss the pion-pole enhancement of gP, which makes beta decays much more sensitive to non-standard pseudoscalar interactions than previously thought.
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