The Lamb-shift experiment in muonic hydrogen (μ– p) aims to measure the energy difference between the [Formula: see text] atomic levels to a precision of 30 ppm. This would allow the r.m.s. proton charge radius rp to be deduced to a precision of 10–3 and open a way to check bound-state quantum electrodynamics (QED) to a level of 10–7. The poor knowledge of the proton charge radius restricts tests of bound-state QED to the precision level of about 6 × 10–6, although the experimental data themselves (Lamb-shift in hydrogen) have reached a precision of × 10–6. Values for rp not depending on bound-state QED results from electron scattering experiments have a surprisingly large uncertainty of 2%. In our Lamb-shift experiment, low-energy negative muons are stopped in low-density hydrogen gas, where, following the μ– atomic capture and cascade, 1% of the muonic hydrogen atoms form the metastable 2S state with a lifetime of about 1 μs. A laser pulse at λ ≈ 6 μm is used to drive the 2S → 2P transition. Following the laser excitation, we observe the 1.9 keV X-ray being emitted during the subsequent de-excitation to the 1S state using large-area avalanche photodiodes. The resonance frequency and, hence, the Lamb shift and the proton charge radius are determined by measuring the intensity of the X-ray fluorescence as a function of the laser wavelength. The results of the run in December 2003 were negative but, nevertheless, promising. One by-product of the 2003 run was the first observation of the short-lived 2S component in muonic hydrogen. Currently, improvements in the laser-system, the experimental apparatus, and the data acquisition are being implemented. PACS Nos.: 36.10.Dr, 14.20.Dh, 42.62.Fi
New Parity-Violating Muonic Forces and the Proton Charge Radius