Status of the muonic hydrogen Lamb shift experiment
Résumé
The Lamb-shift experiment in muonic hydrogen (mu(-)p) aims to measure the energy difference between the 2S(1/2)(F=1)-2P(3/2)(F=2) atomic levels to a precision of 30 ppm. This would allow the r.m.s. proton charge radius r(p) to be deduced to a precision of 10(-3) and open a way to check bound-state quantum electrodynamics (QED) to a level of 10(-7). The poor knowledge of the proton charge radius restricts tests of bound-state QED to the precision level of about 6 x 10(-6), although the experimental data themselves (Lamb-shift in hydrogen) have reached a precision of 2 x 10(-6). Values for r(p) not depending on bound-state QED results from electron scattering experiments have a surprisingly large uncertainty of 2%. In our Lamb-shift experiment, low-energy negative mucus are stopped in low-density hydrogen gas, where, following the mu(-) atomic capture and cascade, 1% of the muonic hydrogen atoms form the metastable 2S state with a lifetime of about 1 mu s. A laser pulse at lambda approximate to 6 mu m is used to drive the 2S -> 2P transition. Following the laser excitation, we observe the 1.9 keV X-ray being emitted during the subsequent de-excitation to the I S state using large-area avalanche photodiodes. The resonance frequency and, hence, the Lamb shift and the proton charge radius are determined by measuring the intensity of the X-ray fluorescence as a function of the laser wavelength. The results of the run in December 2003 were negative but, nevertheless, promising. One by-product of the 2003 run was the first observation of the short-lived 2S component in muonic hydrogen. Currently, improvements in the laser-system, the experimental apparatus, and the data acquisition are being implemented.