The Muonic Hydrogen Lamb-Shift Experiment

The charge radius of the proton, the simplest nucleus, is known from electron-scattering experiments only with a surprisingly low precision of about 2%. The poor knowledge of the proton charge radius restricts tests of bound-state quantum electrodynamics (QED) to the precision level of about 6 x 10(-6), although the experimental data themselves (1S Lamb shift in hydrogen) have reached a precision of 2 x 10(-6). The determination of the proton charge radius with an accuracy of 10(-3) is the main goal of our experiment, opening a way to check bound-state QED predictions to a level of 10(-7). The principle is to measure the 2S-2P energy difference in muonic hydrogen (mu(-)p) by infrared laser spectroscopy. The first data were taken in the second half of 2003. Muons from our unique very-low-energy muon beam are stopped at a rate of similar to 100 s(-1) in 0.6 mbar H-2 gas where the lifetime of the formed mu p(2S) atoms is about 1.3 mu s. An incoming muon triggers a pulsed multistage laser system that delivers similar to 0.2 mJ at lambda 6 mu m. Following the laser excitation mu p(2S) -> mu p(2P) we observe the 1.9 keV X-rays from 2P-1S transitions using large area avalanche photodiodes. The resonance frequency, and, hence, the Lamb shift and the proton radius, is determined by measuring the intensity of these X-rays as a function of the laser wavelength. A broad range of laser frequencies was scanned in 2003 and the analysis is currently under way.