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...