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The size of the proton

Pohl, Randolf and Antognini, Aldo and Nez, Francois and Amaro, Fernando and Biraben, Francois and Cardoso, Joao and Covita, Daniel and Dax, Andreas and Dhawan, Satish and Fernandes, Luis and Giesen, Adolf and Graf, Thomas and Hänsch, Theodor and Indelicato, Paul and Julien, Lucile and Kao, Cheng-Yang and Knowles, Paul and Le Bigot, Eric and Liu, Yi-Wei and Lopes, Jose and Ludhova, Livia and Monteiro, Christina and Mulhauser, Francoise and Nebel, Tobias and Rabinowitz, Paul and dos Santos, Joaquim and Schaller, Lukas and Schuhmann, Karsten and Schwob, Catherine and Taqqu, David and Veloso, Joao and Kottmann, Franz (2010) The size of the proton. Nature, 466, pp. 213-217. Macmillan Publishers Limited. DOI: 10.1038/nature09250.

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Abstract

The proton is the primary building block of the visible Universe, but many of its properties—such as its charge radius and its anomalous magneticmoment—are not well understood. The root-meansquare 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. 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 themeasurement of rp is provided bymuonic 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, theLamb 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 50.84184(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 2110 kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.

Document Type:Article
Title:The size of the proton
Authors:
AuthorsInstitution or Email of Authors
Pohl, RandolfMax Plank Institut für Quantenoptik
Antognini, AldoMax Plank Institut für Quantenoptik
Nez, FrancoisLaboratoire Kastler Brossel
Amaro, FernandoUniversidade de Coimbra
Biraben, FrancoisLaboratoire Kastler Brossel
Cardoso, JoaoUniversidade de Coimbra
Covita, DanielUniversidade de Coimbra
Dax, AndreasYale University
Dhawan, SatishYale University
Fernandes, LuisUniversidade de Coimbra
Giesen, AdolfUNSPECIFIED
Graf, ThomasInstitut für Strahlwerkzeuge
Hänsch, TheodorMax Plank Institut für Quantenoptik
Indelicato, PaulLaboratoire Kastler Brossel
Julien, LucileLaboratoire Kastler Brossel
Kao, Cheng-YangTsing Hua University, Taiwan
Knowles, PaulUniversity de Fribourg
Le Bigot, EricLaboratoire Kastler Brossel
Liu, Yi-WeiNational Tsing Hua University
Lopes, JoseUniversidade de Coimbra
Ludhova, LiviaUniversity de Fribourg
Monteiro, ChristinaUniversidade de Coimbra
Mulhauser, FrancoiseUniversity de Fribourg
Nebel, TobiasMax Plank Institut für Quantenoptik
Rabinowitz, PaulPrinceton University
dos Santos, JoaquimUniversidade de Coimbra
Schaller, LukasUniversity de Fribourg
Schuhmann, KarstenTechnologiegesellschaft für Strahlwerkzeuge
Schwob, CatherineLaboratoire Kastler Brossel
Taqqu, DavidPaul Scherrer Institute
Veloso, JoaoUniveristy of Aveiro, Portugal
Kottmann, FranzPaul Scherrer Institute
Date:8 July 2010
Journal or Publication Title:Nature
Refereed publication:Yes
In Open Access:No
In SCOPUS:Yes
In ISI Web of Science:Yes
Volume:466
DOI:10.1038/nature09250
Page Range:pp. 213-217
Publisher:Macmillan Publishers Limited
Series Name:Letters
Status:Published
Keywords:proton size
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Aeronautics
HGF - Program Themes:Aircraft Research
DLR - Research area:Aeronautics
DLR - Program:L AR - Aircraft Research
DLR - Research theme (Project):L - Laser Research and Technology
Location: Stuttgart
Institutes and Institutions:Institute of Technical Physics
Deposited By: Dr.rer.nat. Hans-Albert Eckel
Deposited On:08 Jul 2010 11:33
Last Modified:26 Mar 2013 13:18

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