{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,7,3]],"date-time":"2025-07-03T04:17:58Z","timestamp":1751516278797,"version":"3.41.0"},"reference-count":18,"publisher":"Canadian Science Publishing","issue":"5","license":[{"start":{"date-parts":[[2007,5,1]],"date-time":"2007-05-01T00:00:00Z","timestamp":1177977600000},"content-version":"tdm","delay-in-days":0,"URL":"http:\/\/www.nrcresearchpress.com\/page\/about\/CorporateTextAndDataMining"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Can. J. Phys."],"published-print":{"date-parts":[[2007,5,1]]},"abstract":"<jats:p> The Lamb-shift experiment in muonic hydrogen (\u03bc<jats:sup>\u2013<\/jats:sup> p) aims to measure the energy difference between the [Formula: see text] atomic levels to a precision of 30\u2002ppm. This would allow the r.m.s. proton charge radius r<jats:sub>p<\/jats:sub> to be deduced to a precision of 10<jats:sup>\u20133<\/jats:sup> and open a way to check bound-state quantum electrodynamics (QED) to a level of 10<jats:sup>\u20137<\/jats:sup>. The poor knowledge of the proton charge radius restricts tests of bound-state QED to the precision level of about 6\u2002\u00d7\u200210<jats:sup>\u20136<\/jats:sup>, although the experimental data themselves (Lamb-shift in hydrogen) have reached a precision of \u2002\u00d7\u200210<jats:sup>\u20136<\/jats:sup>. Values for r<jats:sub>p<\/jats:sub> 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 muons are stopped in low-density hydrogen gas, where, following the \u03bc<jats:sup>\u2013<\/jats:sup> atomic capture and cascade, 1% of the muonic hydrogen atoms form the metastable 2S state with a lifetime of about 1 \u03bcs. A laser pulse at \u03bb\u2002\u2248\u20026\u2002\u03bcm is used to drive the 2S \u2192 2P transition. Following the laser excitation, we observe the 1.9\u2002keV X-ray being emitted during the subsequent de-excitation to the 1S 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. 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