Why is the Lamb shift in muonic hydrogen five sigma (which is an absolute error of 4%) different from the predicted value? (Muonic hydrogen is smaller than predicted by QED.)
One answer, according to a pre-print by a Spanish physicist (Antonio Pineda), may be that there are subtle issues in the calculation of polarization effects that show up on muonic hydrogen calculations which must be about 200 times as precise as those for proton size calculations in ordinary hydrogen.
Still, the issue deserves attention as it is one of the only five sigma or greater deviations from a Standard Model prediction that is out there.
A reanalysis of the overestimated accuracy in the electron case and other calcuation issues is found here but does not fully resolve the gap.
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