{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,25]],"date-time":"2026-03-25T12:54:37Z","timestamp":1774443277775,"version":"3.50.1"},"reference-count":26,"publisher":"Canadian Science Publishing","issue":"4","license":[{"start":{"date-parts":[[2005,4,1]],"date-time":"2005-04-01T00:00:00Z","timestamp":1112313600000},"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":[[2005,4,1]]},"abstract":"<jats:p> The charge radius of the proton, the simplest nucleus, is known from electron-scattering experiments only with a surprisingly low precision of about 2%. The poor knowledge of the proton charge radius restricts tests of bound-state quantum electrodynamics (QED) to the precision level of about 6 \u00d7 10<jats:sup>\u00966<\/jats:sup>, although the experimental data themselves (1S Lamb shift in hydrogen) have reached a precision of 2 \u00d7 10<jats:sup>\u00966<\/jats:sup>. The determination of the proton charge radius with an accuracy of 10<jats:sup>\u00963<\/jats:sup> is the main goal of our experiment, opening a way to check bound-state QED predictions to a level of 10<jats:sup>\u00967<\/jats:sup>. The principle is to measure the 2S\u00962P energy difference in muonic hydrogen (\u00b5<jats:sup>\u0096<\/jats:sup>p) by infrared laser spectroscopy. The first data were taken in the second half of 2003. Muons from our unique very-low-energy muon beam are stopped at a rate of ~100\u2002s<jats:sup>\u00961<\/jats:sup> in 0.6 mbar H<jats:sub>2<\/jats:sub> gas where the lifetime of the formed \u00b5p(2S) atoms is about 1.3\u2002\u00b5s. An incoming muon triggers a pulsed multistage laser system that delivers ~0.2\u2002mJ at \u03bb\u2002\u2248\u20026\u2002\u00b5m. Following the laser excitation \u00b5p(2S) \u2192 \u00b5p(2P) we observe the 1.9\u2002keV X-rays from 2P\u00961S transitions using large area avalanche photodiodes. The resonance frequency, and, hence, the Lamb shift and the proton radius, is determined by measuring the intensity of these X-rays as a function of the laser wavelength. A broad range of laser frequencies was scanned in 2003 and the analysis is currently under way. 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