Pb208 Nuclear Charge Radius Revisited: Closing the Fine-Structure-Anomaly Gap

A comprehensive reevaluation of the root-mean-square nuclear charge radius is presented for the doubly magic <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mmultiscripts><a:mrow><a:mi>Pb</a:mi></a:mrow><a:mprescripts/><a:none/><a:mrow><a:mn>208</a:mn></a:mrow></a:mmultiscripts></a:mrow></a:math> extracted from muonic spectroscopy measurements. By integrating rigorous theoretical quantum electrodynamics calculations, state-of-the-art numerical methods, and a systematic reanalysis of the uncertainties, we reduced the long-standing muonic fine-structure anomaly and improved the goodness of fit by a factor of 20. The resulting value of 5.5062(5) fm for a Fermi distribution is fairly consistent with the previously reported muonic spectroscopy value, and 3 standard deviations larger than the commonly used compilation data, which indicates that the current value and its uncertainty could be significantly underestimated. Attributing the remaining discrepancy to theory errors which cannot be rigorously calculated, we suggest the rms charge radius with reduced model dependence to be 5.5062(17) fm. This Letter sets an improved benchmark for charge radius extraction in heavy nuclei and paves a path for systematic reevaluations across the nuclear chart.