• DOI:10.1038/nature09250
• Corpus ID: 4424731

The size of the proton

@article{Pohl2010TheSO,
  title={The size of the proton},
  author={Randolf Pohl and Aldo Antognini and François Nez and Fernando Domingues Amaro and Francois Biraben and Jo{\~a}o M. R. Cardoso and Daniel S. Covita and Andreas J Dax and S. K. Dhawan and Luis M P Fernandes and Adolf Giesen and Thomas Graf and Theodor W. H{\"a}nsch and Paul Indelicato and Lucile Julien and Cheng-Yang Kao and Paul E. Knowles and E.-O. Le Bigot and Yi-Wei Liu and Jos{\'e} A. M. Lopes and Livia Ludhova and Cristina Maria Bernardes Monteiro and Françoise Mulhauser and Tobias Nebel and Paul Rabinowitz and Joaquim M.F. dos Santos and Lukas A. Schaller and Karsten Schuhmann and Catherine Schwob and David Taqqu and Jo{\~a}o Veloso and Franz Kottmann},
  journal={Nature},
  year={2010},
  volume={466},
  pages={213-216}
}

• R. Pohl, A. Antognini, +29 authors F. Kottmann
• Published 8 July 2010
• Physics
• Nature

The proton is the primary building block of the visible Universe, but many of its properties—such as its charge radius and its anomalous magnetic moment—are not well understood. The root-mean-square charge radius, rp, has been determined with an accuracy of 2 per cent (at best) by electron–proton scattering experiments. The present most accurate value of rp (with an uncertainty of 1 per cent) is given by the CODATA compilation of physical constants. This value is based mainly on precision spectroscopy of atomic hydrogen and calculations of bound-state quantum electrodynamics (QED; refs 8, 9). The accuracy of rp as deduced from electron–proton scattering limits the testing of bound-state QED in atomic hydrogen as well as the determination of the Rydberg constant (currently the most accurately measured fundamental physical constant). An attractive means to improve the accuracy in the measurement of rp is provided by muonic hydrogen (a proton orbited by a negative muon); its much smaller Bohr radius compared to ordinary atomic hydrogen causes enhancement of effects related to the finite size of the proton. In particular, the Lamb shift (the energy difference between the 2S1/2 and 2P1/2 states) is affected by as much as 2 per cent. Here we use pulsed laser spectroscopy to measure a muonic Lamb shift of 49,881.88(76) GHz. On the basis of present calculations of fine and hyperfine splittings and QED terms, we find rp = 0.84184(67) fm, which differs by 5.0 standard deviations from the CODATA value of 0.8768(69) fm. Our result implies that either the Rydberg constant has to be shifted by −110 kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.