The Muonic Hydrogen Lamb Shift Experiment at PSI

Abstract

Abstract A measurement of the 2S Lamb shift in muonic hydrogen (µ-p) is being prepared at the Paul Scherrer Institute (PSI). The goal of the experiment is to measure the energy difference ?E(25 P 3/2-23 S 1/2) by laser spectroscopy (?˜6µm) 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 µp2S -atoms. A 2S-lifetime of ~1 µs – sufficiently long to perform the laser experiment – at H2 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 µm is on the way.