Experiment to measure the Lamb shift in muonic hydrogen

The contribution of the root mean square (RMS) proton charge radius to the Lamb shift (2S-2P energy difference) in muonic hydrogen (mu p) amounts to 2 %. Apart from the uncertainty on this charge radius, theory predicts the La mb shift with a precision on the ppm level. We are going to measure Delta E (2 S-1/2(F=1)-2 P-3/2(F=2)) in a laser resonance experiment to a precision of 30 ppm (i.e., 10% of the natural linewidth) and to deduce the RMS proto n charge radius with 10(-3) relative accuracy, 20 times more precise than p resently known. The most important requirement for the feasibility of such an experiment, n amely the availability of a sufficient amount of long-lived metastable mu p atoms in the 2S state, has been investigated in a recent experiment at PSI . Our analysis shows that in the order of one percent of all muons stopped in low-pressure hydrogen gas form a long-lived mu p(2S) with a lifetime of the order of 1 mu s. The technical realization of our experiment involves a new high-intensity l ow-energy muon beam, an efficient low-energy muon entrance detector, a rand omly triggered 3-stage laser system providing the 0.5 mJ, 7 ns laser pulses at 6.02 mu m wavelength, and a combination of a xenon gas-proportional-sci ntillation-chamber (GPSC) and a microstrip-gas-chamber (MSGC) with a CsI-co ated surface to detect the 2 keV X-rays from the mu p(2P-->1S) transition.