The muonic hydrogen Lamb shift experiment at PSI

A measurement of the 2S Lamb shift in muonic hydrogen (mu(-)p) is being prepared at the Paul Scherrer institute (PSI). The goal of the experiment is to measure the energy difference DeltaE(2(5) P-3/2 - 2(3)S(1/2)) by laser spectroscopy (lambda approximate to 6mum) to a precision of 30 ppm and to deduce the root mean square (rms) proton charge radius with 10(-3) relative accuracy, 20 times more precise than presently known. An important prerequisite to this experiment is the availability of long-lived mup(2S)-atoms. A 2S-lifetime of similar to1 mus - sufficiently long to perform the laser experiment - at H-2 gas pressures of 1-2 hPa was deduced from recent measurements of the collisional 2S-quenching rate. A new low-energy negative muon beam yields an order of magnitude more muon stops in a small low-density gas volume than a conventional cloud muon beam. A stack of ultra-thin carbon foils is the key element of a fast detector for keV-muons. The development of a 2 keV X-ray detector and a 3-stage laser system providing 0.5 mJ laser pulses at 6 mum is on the way.