Measurement of the 2P3/2−S1/2 energy difference in the (μ−4He)+ muonic ion by means of a tunable infrared dye-laser

Abstract Results of an experiment to measure the 2P 3/2 −2S 1/2 energy difference δ 1 in the muonic (μ −4 H e) + ion are presented; we found δ 1 exp = 1.5274 ± 0.0009 eV. Comparison with QED predictions shows that there is agreement between theory and experiment within 0.0087 eV; this limitation is mainly due to the uncertainty with which the r.m.s. charge radius of the 4 He nucleus is known.