Strong-Isospin Breaking in the Neutron-Proton Mass Difference
Abstract
We determine the strong-isospin violating component of the neutron-proton mass difference from fully-dynamical lattice QCD and partially-quenched QCD calculations of the nucleon mass, constrained by partially-quenched chiral perturbation theory at one-loop level. The lattice calculations were performed with domain-wall valence quarks on MILC lattices with rooted staggered sea-quarks at a lattice spacing of b ~ 0.125 fm, lattice spatial size of L~2.5 fm and pion masses ranging from mpi~290 MeV to ~350 MeV. At the physical value of the pion mass, we predict Mn - Mp |(d-u) = 2.26 +- 0.57 +- 0.42 +- 0.10 MeV where the first error is statistical, the second error is due to the uncertainty in the ratio of light-quark masses, eta=mu/md, determined by MILC, and the third error is an estimate of the systematic due to chiral extrapolation.
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