Towards a Lamb shift measurement in muonic hydrogen Available to Purchase

A measurement of the $2S$ Lamb shift (⁠$2S−2P$ energy difference) in muonic hydrogen $(μ−p)$ is being prepared at the Paul Scherrer Institute (PSI). The goal of the experiment is to measure the Lamb shift with 30 ppm precision and to deduce the root mean square (rms) proton charge radius with $10−3$ relative accuracy, 20 times more precise than presently known. The experiment is based on the availability of long-lived metastable muonic hydrogen atoms in the $2S$ state which has been investigated in a recent series of experiments at PSI. From the low-energy part of the initial kinetic energy distribution of $μp(2S)$ atoms we determined the fraction of long-lived $μp(2S)$ to be ∼1.5% for $H2$ gas pressures between 1 and 64 hPa. Another analysis involving $μp(1S)$ with a kinetic energy of 0.9 keV originating from quenching of thermalized $μp(2S)$ via the resonant process $μp(2S)+H2→{[(ppμ)+]*pee}*→μp(1S)+p+…+2 keV$ gives the same result. This is the first direct observation of long-lived $μp(2S)$ atoms. The realization of the μp Lamb shift experiment involves a new low-energy negative muon beam with entrance detectors for keV-muons, a xenon gas-proportional-scintillation-chamber (GPSC) read out by a microstrip-gas-chamber (MSGC) for the detection of 2 keV X-rays, and a randomly triggered 3-stage laser system providing 0.5 mJ, 7 ns laser pulses at 6.02 μm wavelength.