The muonic hydrogen Lamb-shift experiment

• Published in: Canadian journal of physics Vol. 83; no. 4; pp. 339 - 349
• Main Authors: Pohl, R, Antognini, A, Amaro, F D, Biraben, F, Cardoso, J MR, Conde, C AN, Dax, A, Dhawan, S, Fernandes, L MP, Hänsch, T W, Hartmann, F J, Hughes, V W, Huot, O, Indelicato, P, Julien, L, Knowles, P E, Kottmann, F, Liu, Y -W, Ludhova, L, Monteiro, C MB, Mulhauser, F, Nez, F, Rabinowitz, P, dos Santos, J MF, Schaller, L A, Schwob, C, Taqqu, D, Veloso, JF CA
• Format: Journal Article
• Language: English
• Published: Ottawa, Canada NRC Research Press 01.04.2005; Canadian Science Publishing NRC Research Press
• Subjects: Energy; Experimental data; Hydrogen; Measurement; Physics; Protons; Resonance; Spectroscopy; Spectrum analysis
• ISSN: 0008-4204; 1208-6045
• DOI: 10.1139/p05-016

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 × 10 –6 , although the experimental data themselves (1S Lamb shift in hydrogen) have reached a precision of 2 × 10 –6 . The determination of the proton charge radius with an accuracy of 10 –3 is the main goal of our experiment, opening a way to check bound-state QED predictions to a level of 10 –7 . The principle is to measure the 2S–2P energy difference in muonic hydrogen (µ – 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 s –1 in 0.6 mbar H 2 gas where the lifetime of the formed µp(2S) atoms is about 1.3 µs. An incoming muon triggers a pulsed multistage laser system that delivers ~0.2 mJ at λ ≈ 6 µm. Following the laser excitation µp(2S) → µp(2P) we observe the 1.9 keV X-rays from 2P–1S 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. PACS Nos.: 36.10.Dr, 14.20.Dh, 42.62.Fi