title: Isospin breaking in the nucleon mass and the sensitivity of β 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 β decay if nonstandard 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 modified minimal subtraction scheme at μ=2  GeV. The consequences for searches of nonstandard scalar interactions in nuclear β decays are studied, finding for the corresponding Wilson coefficient εS=0.0012(24) at 90% C.L., 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 β decays much more sensitive to nonstandard pseudoscalar interactions than previously thought.
authors.affiliation: Department of Physics, University of Wisconsin-Madison, 1150 University Avenue, Madison, Wisconsin 53706, USA and INFN, Laboratori Nazionali di Frascati, I-00044 Frascati, Italy.
authors.forename: M
authors.initials: M
authors.lastname: González-Alonso
authors.affiliation: Department of Physics and Astronomy, University of Sussex, BN1 9QH Brighton, United Kingdom and Department of Physics, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0319, USA and PRISMA Cluster of Excellence Institut für Kernphysik, Johannes Gutenberg-Universität Mainz, 55128 Mainz, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Martin Camalich