Constraining beyond the standard model nucleon isovector charges
R. E. Smail, M. Batelaan, R. Horsley, Yoshifumi Nakamura, H. Perlt, D. Pleiter, P. E. L. Rakow, G. Schierholz, H. Stüben, R. D. Young, J. M. Zanotti
Abstract
At the TeV scale, low-energy precision observations of neutron characteristics provide unique probes of novel physics. Precision studies of neutron decay observables are susceptible to beyond the Standard Model (BSM) tensor and scalar interactions, while the neutron electric dipole moment, ${d}_{n}$, also has high sensitivity to new BSM $CP$-violating interactions. To fully utilize the potential of future experimental neutron physics programs, matrix elements of appropriate low-energy effective operators within neutron states must be precisely calculated. We present results from the QCDSF/UKQCD/CSSM Collaboration for the isovector charges ${g}_{T}$, ${g}_{A}$ and ${g}_{S}$ of the nucleon, $\mathrm{\ensuremath{\Sigma}}$ and $\mathrm{\ensuremath{\Xi}}$ baryons using lattice QCD methods and the Feynman-Hellmann theorem. We use a flavor symmetry breaking method to systematically approach the physical quark mass using ensembles that span five lattice spacings and multiple volumes. We extend this existing flavor-breaking expansion to also account for lattice spacing and finite volume effects in order to quantify all systematic uncertainties. Our final estimates of the nucleon isovector charges are ${g}_{T}=1.010(21{)}_{\mathrm{stat}}(12{)}_{\mathrm{sys}},{g}_{A}=1.253(63{)}_{\mathrm{stat}}(41{)}_{\mathrm{sys}}$ and ${g}_{S}=1.08(21{)}_{\mathrm{stat}}(03{)}_{\mathrm{sys}}$ renormalized, where appropriate, at $\ensuremath{\mu}=2\text{ }\text{ }\mathrm{GeV}$ in the $\overline{\mathrm{MS}}$ scheme.