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The size of the proton

机译：质子的大小

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摘要

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, r_p, has been determined with an accuracy of 2 per cent (at best) by electron-proton scattering experiments. The present most accurate value of r_p (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 r_p 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 r_p 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 2S_(1/2) and 2P_(1/2) 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 r_p = 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 - 110kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.
机译：质子是可见宇宙的主要组成部分，但是它的许多特性（例如其电荷半径和反常的磁矩）尚未得到很好的理解。均方根电荷半径r_p已通过电子-质子散射实验确定为2％（最佳）的精度。 r_p的当前最准确值（不确定性为1％）由物理常数的CODATA编译给出。该值主要基于原子氢的精密光谱学和键合态量子电动力学的计算（QED；参考文献8、9）。由电子-质子散射推论得出的r_p精度限制了原子氢中键态QED的测试以及Rydberg常数（当前最精确测量的基本物理常数）的确定。离子氢（质子由负μ子环绕）提供了一种有吸引力的提高r_p测量精度的方法。与普通原子氢相比，它的玻尔半径小得多，从而增强了与质子有限尺寸有关的效应。尤其是兰姆位移（2S_（1/2）和2P_（1/2）状态之间的能量差）受到的影响高达2％。在这里，我们使用脉冲激光光谱法来测量49,881.88（76）GHz的muonic Lamb位移。根据当前精细和超精细分裂以及QED项的计算，我们发现r_p = 0.84184（67）fm，与CODATA值0.8768（69）fm相差5.0个标准差。我们的结果表明，要么必须将Rydberg常数偏移-110kHz / c（4.9个标准偏差），要么要计算原子氢或muonic氢原子中的QED效应是不够的。

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  • 来源
    《Nature》 |2010年第7303期|P.213-216|共4页
  • 作者

    Randolf Pohl; rnAldo Antognini; rnFrancois Nez; rnFernando D. Amaro; rnFrancois Biraben; rnJoao M. R. Cardoso; Daniel S. Covita; rnAndreas Dax; rnSatish Dhawan; rnLuis M. P. Fernandes; rnAdolf Giesen; rnThomas Graf; Theodor W. Haensch; rnPaul Indelicato; rnLucile Julien; rnCheng-Yang Kao; rnPaul Knowles; rnEric-Olivier Le Bigot; Yi-Wei Liu; rnJose A. M. Lopes; rnLivia Ludhova; rnCristina M. B. Monteiro; rnFrancoise Mulhauser; rnTobias Nebel;

  • 作者单位

    Max-Planck-Institut fuer Quantenoptik, 85748 Garching, Germany;

    rnMax-Planck-Institut fuer Quantenoptik, 85748 Garching, Germany;

    rnLaboratoire Kastler Brossel, Ecole Normale Superieure, CNRS, and Universite P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France;

    rnDepartamento de Fisica, Universidade de Coimbra, 3004-516 Coimbra, Portugal;

    rnLaboratoire Kastler Brossel, Ecole Normale Superieure, CNRS, and Universite P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France;

    rnDepartamento de Fisica, Universidade de Coimbra, 3004-516 Coimbra, Portugal;

    rnDepartamento de Fisica, Universidade de Coimbra, 3004-516 Coimbra, Portugal I3N, Departamento de Fisica, Universidade de Aweiro, 3810-193 Aveiro, Portugal;

    rnPhysics Department, Yale University, New Haven, Connecticut 06520-8121, USA;

    rnPhysics Department, Yale University, New Haven, Connecticut 06520-8121, USA;

    rnDepartamento de Fisica, Universidade de Coimbra, 3004-516 Coimbra, Portugal;

    rnInstitut fuer Strahlwerkzeuge, Universitaet Stuttgart, 70569 Stuttgart, Germany Deutsches Zentrum fuer Luft- und Raumfahrt e.V. in der Helmholtz-Gemeinschaft, 70569 Stuttgart, Germany;

    rnInstitut fuer Strahlwerkzeuge, Universitaet Stuttgart, 70569 Stuttgart, Germany;

    rnMax-Planck-lnstitut fuer Quantenoptik, 85748 Garching, Germany;

    rnLaboratoire Kastler Brossel, Ecole Normale Superieure, CNRS, and Universite P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France;

    rnLaboratoire Kastler Brossel, Ecole Normale Superieure, CNRS, and Universite P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France;

    rnPhysics Department, National Tsing Hua University, Hsinchu 300, Taiwan;

    rnDepartement de Physique, Universite de Fribourg, 1700 Fribourg, Switzerland;

    rnLaboratoire Kastler Brossel, Ecole Normale Superieure, CNRS, and Universite P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France;

    rnPhysics Department, National Tsing Hua University, Hsinchu 300, Taiwan;

    rnDepartamento de Fisica, Universidade de Coimbra, 3004-516 Coimbra, Portugal;

    rnDepartement de Physique, Universite de Fribourg, 1700 Fribourg, Switzerland;

    rnDepartamento de Fisica, Universidade de Coimbra, 3004-516 Coimbra, Portugal;

    rnDepartement de Physique, Universite de Fribourg, 1700 Fribourg, Switzerland International Atomic Energy Agency, A-1400 Vienna, Austria;

    rnMax-Planck-lnstitut fuer Quantenoptik, 85748 Garching, Germany;

    rnDepartment of Chemistry, Princeton University, Princeton, New Jersey 08544-1009, USA;

    rnDepartamento de Fisica, Universidade de Coimbra, 3004-516 Coimbra, Portugal;

    rnDepartement de Physique, Universite de Fribourg, 1700 Fribourg, Switzerland;

    rnDausinger & Giesen GmbH, Rotebuehlstr. 87, 70178 Stuttgart, Germany;

    rnLaboratoire Kastler Brossel, Ecole Normale Superieure, CNRS, and Universite P. et M. Curie-Paris 6, 75252 Paris, Cedex 05, France;

    rnPaul Scherrer Institute, 5232 Villigen-PSI, Switzerland;

    rnI3N, Departamento de Fisica, Universidade de Aweiro, 3810-193 Aveiro, Portugal;

    rnInstitut fuer Teilchenphysik, ETH Zuerich, 8093 Zuerich, Switzerland;

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