The 2S Lamb shift in muonic hydrogen and the proton rms charge radius

The determination of the proton rms charge radius with an accuracy of 10⁻³ is the main goal of our experiment, opening the way to check bound‐state QED predictions in hydrogen to a level of 10⁻⁷. The principle is to measure the 2S_1/2(F = 1) − 2P_3/2(F = 2) energy difference in muonic hydrogen (μ⁻p) by infrared laser spectroscopy to a precision of 30 ppm. Very low‐energy negative muons are stopped in 0.6 mbar of hydrogen gas, where, following the μ⁻ atomic capture and cascade, 1% of the muonic hydrogen atoms form the metastable 2S state with a lifetime of 1.3 μs. A 6 μm laser pulse is used to drive the 2S → 2P transition. When on resonance, the laser induces the transition, and the subsequent muonic deexcitation to the 1S state emits a 1.9 keV x ray which is detected by avalanche photodiodes. The resonance frequency, and hence the Lamb shift and the proton charge radius, is determined by measuring the rate of laser‐induced x rays as a function of the laser wavelength. Some details of the experiment, recent measurements and improvements will be presented.