The muonic hydrogen lamb shift experiment at PSI

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 ΔE(2⁵ P _3/2-2³ S _½) by laser spectroscopy (λ≈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.