Faculté des sciences

Status of the muonic hydrogen Lamb-shift experiment

Nebel, T. ; Amaro, F. D. ; Antognini, A. ; Biraben, F. ; Cardoso, J. M. R. ; Conde, C. A. N. ; Dax, A. ; Dhawan, S. ; Fernandes, L. M. P. ; Giesen, A. ; Hänsch, T. W. ; Indelicato, P. ; Julien, L. ; Knowles, Paul E. ; Kottmann, F. ; Le Bigot, E. ; Liu, Y.-W. ; Lopes, J. A. M. ; Ludhova, Livia ; Monteiro, C. M. B. ; Mulhauser, Françoise ; Nez, F. ; Pohl, R. ; Rabinowitz, P. ; Santos, J. M. F. dos ; Schaller, Lukas A. ; Schuhmann, K. ; Schwob, C. ; Taqqu, D. ; Veloso, J. F. C. A.

In: Canadian Journal of Physics, 2007, vol. 85, no. 5, p. 469-478

The Lamb-shift experiment in muonic hydrogen (μ⁻ p) aims to measure the energy difference between the 2SF=11/2-2PF=23/2 atomic levels to a precision of 30 ppm. This would allow the r.m.s. proton charge radius rp to be deduced to a precision of 10⁻³ and open a way to check bound-state quantum electrodynamics (QED) to... Plus

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    Summary
    The Lamb-shift experiment in muonic hydrogen (μ⁻ p) aims to measure the energy difference between the 2SF=11/2-2PF=23/2 atomic levels to a precision of 30 ppm. This would allow the r.m.s. proton charge radius rp 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 × 10⁻⁶, although the experimental data themselves (Lamb-shift in hydrogen) have reached a precision of × 10⁻⁶. Values for rp 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 μ⁻ atomic capture and cascade, 1% of the muonic hydrogen atoms form the metastable 2S state with a lifetime of about 1 μs. A laser pulse at λ ≈ 6 μm 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 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.