Proton tensor charges from a Poincaré-covariant Faddeev equation

Abstract

The proton's tensor charges are calculated at leading order in a symmetry-preserving truncation of all matter-sector equations relevant to the associated bound-state and scattering problems. In particular, the nucleon three-body bound-state equation is solved without using a diquark approximation of the two-body scattering kernel. The computed charges are similar to those obtained in contemporary simulations of lattice-regularised quantum chromodynamics, an outcome which increases the tension between theory and phenomenology. Curiously, the theoretical calculations produce a value of the scale-invariant ratio $(-\delta_T d/\delta_T u)$ which matches that obtained in simple quark models, even though the individual charges are themselves different. The proton's tensor charges can be used to constrain extensions of the Standard Model using empirical limits on nucleon electric dipole moments.