Breit interaction determination of muonic hyperfine anomalies: Large but not giant

Muon-nuclear-hyperfine anomalies are determined from Breit interaction calculations based on unrestricted Dirac-Fock solutions which explicitly include all electrons and the negative muon. We obtain anomalies of a few (3) percent, which are much larger than corresponding Bohr-Weisskopf electron-nuclear anomalies, but are much smaller than the giant, ≈(36 ± 5)%, anomaly reported by Yamazaki et al. for μ^-Pd⁺ vs. Rh. We suggest the possibility that interstitial μ^-Pd⁺ was observed and that the small anomaly for substitutional μ^-Pd⁺ may also be seen at higher temperatures.