Towards a Measurement of the Lamb Shift in Muonic Hydrogen

The availability of long-lived metastable muonic hydrogen atoms (μp) in the 2S state has been investigated in a recent series of experiments at PSI. From the low-energy part of the initial kinetic energy distribution of μp(1S) we determined the fraction of long-lived μp(2S) to be ~ 1.5% for pressures between 1 and 64 hPa. Another analysis involving μp(1S) with a kinetic energy of ~ 1 keV originating from quenching of thermalized μp(2S) via the resonant process mp( 2S ) + \textH\text2 ® { [ ( ppm )+ ]* pee }* ® mp( 1S ) + p + ... + 2 \mu p\left( {2S} \right) + {\text{H}}_{\text{2}} \to \left\{ {\left[ {\left( {pp\mu } \right)^ + } \right]^* pee} \right\}^* \to \mu p\left( {1S} \right) + p + ... + 2 keV gives the same result. This is the first direct observation of long-lived μp(2S) atoms. We are preparing a measurement of the 2S Lamb shift in muonic hydrogen, which will improve the uncertainty on the RMS proton charge radius by more than one order of magnitude. Technical aspects of our experiment are presented, including a new low-energy negative muon beam, an efficient low-energy muon entrance detector, a randomly triggered 3-stage laser system providing 0.5 mJ, 7 ns laser pulses at 6.02 μm wavelength, and a large solid angle xenon gas-proportional-scintillation-chamber (GPSC) read out by a microstrip-gas-chamber (MSGC) with a CsI-coated surface for the detection of 2 keV X-rays.