High-Precision Muonic X-Ray-Measurement of the Rms Charge Radius of C-12 with a Crystal Spectrometer

A precision measurement with the SIN bent-crystal spectrometer of the wave length of the 2P/sub 3/2/-1S/sub 1/2/ transition in muonic /sup 12/C yields lambda = 16.473 766(89) pm; the accuracy being an order of magnitude higher than that of earlier investigations. The rms charge radius of /sup 12/C is deduced as /sup 1/2/ = 2.482 6(18) fm, differing by 2.3 standard deviations from the most accurate electron-scattering results. Consequences of attributing this discrepancy to a ..mu..-N interaction beyond QED are discussed.