Muonic Lamb shift

Willis Lamb’s 1947 measurement of the tiny splitting between the 2s and 2p states of atomic hydrogen gave a crucial impetus to the development of quantum electrodynamics (QED). That “Lamb shift” from the Dirac hydrogen spectrum is a 4-µeV increase in the 2s energy level due primarily to vacuum fluctuations of the electromagnetic field. Now Randolf Pohl (Max Plank Institute for Quantum Optics, Garching, Germany) and coworkers at the Paul Scherrer Institute (PSI) in Switzerland have finally measured the analogue of the Lamb shift in the muonic H atom—a proton orbited by a µ⁻ instead of an e⁻. Muons live only microseconds, but they are 200 times heavier than electrons, and their atomic orbits are correspondingly tighter. The muonic Lamb shift is about 200 meV, and its precise value is particularly sensitive to the proton’s finite size. The PSI experiment was accomplished with precision laser excitation of µ⁻ p atoms created by an intense µ⁻ beam stopping in a small volume of H₂ gas at very low pressure. The team measured the muonic Lamb shift to a part in 10⁵ and compared it with elaborate QED calculations that parameterize the proton’s finite size with an effective charge radius R _p. They find an R _p about 4% smaller than that measured, with less precision, by conventional H spectroscopy and e⁻-p scattering experiments. The discrepancy is 5 standard deviations. Either the proton really is smaller than previously thought, argue Pohl and company, or there’s something wrong with the QED calculations or their input constants. But the proton is a quark composite whose size and shape are quantum-chromodynamic manifestations beyond the purview of QED. Several QCD theorists suggest that at the extraordinary precision achieved by the PSI experiment, it may not be possible to describe proton-size effects adequately with a single length parameter. (R. Pohl et al., Nature 466 , 213, 2010 http://dx.doi.org/10.1038/nature09250 .)