The size of the proton

Randolf Pohl (), Aldo Antognini, François Nez, Fernando D. Amaro, François Biraben, João M. R. Cardoso, Daniel S. Covita, Andreas Dax, Satish Dhawan, Luis M. P. Fernandes, Adolf Giesen, Thomas Graf, Theodor W. Hänsch, Paul Indelicato, Lucile Julien, Cheng-Yang Kao, Paul Knowles, Eric-Olivier Le Bigot, Yi-Wei Liu, José A. M. Lopes, Livia Ludhova, Cristina M. B. Monteiro, Françoise Mulhauser, Tobias Nebel, Paul Rabinowitz, Joaquim M. F. dos Santos, Lukas A. Schaller, Karsten Schuhmann, Catherine Schwob, David Taqqu, João F. C. A. Veloso and Franz Kottmann
Additional contact information
Randolf Pohl: Max-Planck-Institut für Quantenoptik
Aldo Antognini: Max-Planck-Institut für Quantenoptik
François Nez: Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France
Fernando D. Amaro: Universidade de Coimbra
François Biraben: Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France
João M. R. Cardoso: Universidade de Coimbra
Daniel S. Covita: Universidade de Coimbra
Andreas Dax: Yale University
Satish Dhawan: Yale University
Luis M. P. Fernandes: Universidade de Coimbra
Adolf Giesen: Institut für Strahlwerkzeuge, Universität Stuttgart
Thomas Graf: Institut für Strahlwerkzeuge, Universität Stuttgart
Theodor W. Hänsch: Max-Planck-Institut für Quantenoptik
Paul Indelicato: Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France
Lucile Julien: Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France
Cheng-Yang Kao: National Tsing Hua University
Paul Knowles: Université de Fribourg
Eric-Olivier Le Bigot: Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France
Yi-Wei Liu: National Tsing Hua University
José A. M. Lopes: Universidade de Coimbra
Livia Ludhova: Université de Fribourg
Cristina M. B. Monteiro: Universidade de Coimbra
Françoise Mulhauser: Université de Fribourg
Tobias Nebel: Max-Planck-Institut für Quantenoptik
Paul Rabinowitz: Princeton University
Joaquim M. F. dos Santos: Universidade de Coimbra
Lukas A. Schaller: Université de Fribourg
Karsten Schuhmann: Dausinger & Giesen GmbH, Rotebühlstr. 87, 70178 Stuttgart, Germany
Catherine Schwob: Laboratoire Kastler Brossel, École Normale Supérieure, CNRS, and Université P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France
David Taqqu: Paul Scherrer Institute
João F. C. A. Veloso: I3N, Universidade de Aveiro
Franz Kottmann: Institut für Teilchenphysik, ETH Zürich, 8093 Zürich, Switzerland

Nature, 2010, vol. 466, issue 7303, 213-216

Abstract: Shrinking the proton Considering that the proton is a basic subatomic component of all ordinary matter — as well as being ubiquitous in its solo role as the hydrogen ion H+ — there are some surprising gaps in our knowledge of its structure and behaviour. A collaborative project to determine the root-mean-square charge radius of the proton to better than the 1% accuracy of the current 'best' value suggests that those knowledge gaps may be greater than was thought. The new determination comes from a technically challenging spectroscopic experiment — the measurement of the Lamb shift (the energy difference between a specific pair of energy states) in 'muonic hydrogen', an exotic atom in which the electron is replaced by its heavier twin, the muon. The result is unexpected: a charge radius about 4% smaller than the previous value. The discrepancy remains unexplained. Possible implications are that the value of the most accurately determined fundamental constant, the Rydberg constant, will need to be revised — or that the validity of quantum electrodynamics theory is called into question.

Date: 2010
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DOI: 10.1038/nature09250

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