The size of the proton

Pohl, Randolf; Antognini, Aldo; Nez, F.; Amaro, F. D.; Biraben, F.; Cardoso, J. M. R.; Covita, D. S.; Dax, A.; Dhawan, S.; Fernandes, L. M. P.; Giesen, Adolf; Graf, Thomas; Hänsch, Theodor W.; Indelicato, P.; Julién, L.; Kao, Cheng-Yang; Knowles, P.; Bigot, Éric-Olivier Le; Liu, Yi-Wei; Lopes, J. A. M.; Ludhová, L.; Monteiro, C. M. B.; Mulhauser, F.; Nebel, Tobias; Rabinowitz, P.; Santos, J.M.F. dos; Schaller, L. A.; Schuhmann, Karsten; Schwob, Catherine; Taqqu, D.; Veloso, J.F.C.A.; Kottmann, F.

doi:10.1038/nature09250

Cited by 1,270 publications

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“…Experiments with atoms that do not exist naturally can be powerful tools for the study of fundamental physics [ 1,2,3,4,5 ]. A major experimental challenge for such studies is the short intrinsic lifetimes of the exotic atoms.…”

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Confinement of antihydrogen for 1,000 seconds

et al. 2011

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Reported trapping times of magnetically confined (matter) atoms range from <1 s in the first, room temperature, traps [ 18 ] to 10 to 30 minutes in cryogenic devices [ 19,20,21,22 ]. However, antimatter atoms can annihilate on background gases. Also, the loading of our trap (i.e., anti-atom production via merging of cold plasmas) is different from that of ordinary atom traps, and the loading dynamics could adversely affect the trapping and orbit dynamics. Mechanisms exist for temporary magnetic trapping of particles (e.g., in quasi-stable trapping orbits [ 23 ], or in excited internal states [ 24 ]); such particles could be short-lived with a trapping time of a few 100 ms. Thus, it is not a priori obvious what trapping time should be expected for antihydrogen. 5In this article, we report the first systematic investigations of the characteristics of trapped antihydrogen. These studies were made possible by significant advances in our trapping techniques subsequent to Ref. [ 17 ]. These developments, including incorporation of evaporative antiproton cooling[ 25 ] into our trapping operation, and optimisation of autoresonant mixing [ 26 ], resulted in up to a factor of five increase in the number of trapped atoms per attempt. A total sample of 309 trapped antihydrogen annihilation events was studied, a large increase from the previously published 38 events.Here we report trapping of antihydrogen for 1000 s, extending earlier results [ 17 ] by nearly four orders of magnitude. Further, we have exploited the temporal and spatial resolution of our detector system to perform a detailed analysis of the antihydrogen release process, from which we infer information on the trapped antihydrogen kinetic energy distribution.The ALPHA antihydrogen trap [ 27,28 ] is comprised of the superposition of a Penning trap for antihydrogen production and a magnetic field configuration that has a three-dimensional minimum in magnitude (Fig. 1). For ground-state antihydrogen, our trap well-depth is 0.54 K (in temperature units).The large discrepancy in the energy scales between the magnetic trap depth (~50 eV), and the characteristic energy scale of the trapped plasmas (a few eV) presents a formidable challenge to trapping neutral anti-atoms. antiprotons at ~100K, with radius 0.4 mm and density 7x10 7 cm -3 is prepared for mixing with positrons.Independently, the positron plasma, accumulated in a Surko-type buffer gas accumulator [ 33 ,34 ], is transferred to the mixing region, and is also radially compressed. The magnetic trap is then energized, 6 and the positron plasma is cooled further via evaporation, resulting in a plasma with a radius of 0.8 mm and containing 1x10 6 positrons at a density of 5x10 7 cm -3 and a temperature of ~40 K. The silicon vertex detector, surrounding the mixing trap in three layers (Fig. 1 a) ]. Knowledge of annihilation positions also provides sensitivity to the antihydrogen energy distribution, as we will show.In Table 1 and Fig. 2, we present the results for a series of measurements, wherein the confinemen...

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Confinement of antihydrogen for 1,000 seconds

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“…The recent measurement of the muonic hydrogen Lamb shift [1] yielded a proton charge radius 5σ smaller than the 2006 CODATA value available at the time of its publication [2], and 7σ smaller than the 2010 CODATA update [3], which incorporates the latest proton radius determinations from electron scattering [4]. The data used in the CODATA determinations is all electronic.…”

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New physics and the proton radius problem

2012

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Background: The recent disagreement between the proton charge radius extracted from Lamb shift measurements of muonic and electronic hydrogen invites speculation that new physics may be to blame. Several proposals have been made for new particles that account for both the Lamb shift and the muon anomalous moment discrepancies. Purpose: We explore the possibility that new particles' couplings to the muon can be fine-tuned to account for all experimental constraints. Method: We consider two fine-tuned models, the first involving new particles with scalar and pseudoscalar couplings, and the second involving new particles with vector and axial couplings. The couplings are constrained by the Lamb shift and muon magnetic moments measurements while mass constraints are obtained by kaon decay rate data. Results: For the scalar-pseudoscalar model, masses between 100 to 200 MeV are not allowed. For the vector model, masses below about 200 MeV are not allowed. The strength of the couplings for both models approach that of electrodynamics for particle masses of about 2 GeV. Conclusions: New physics with fine tuned couplings may be entertained as a possible explanation for the Lamb shift discrepancy.

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“…The absolute frequency of the line is determined from the absolute calibration of the light at 6 mm using the well-known H 2 O spectroscopy and the free spectral range of the Fabry-Perot interferometer (figure 3). A careful and detailed analysis of the uncertainty budget of the centroid position of this line has been done in Nebel [37] and Pohl et al [39].…”

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“…The cw-laser is permanently locked on a Fabry-Perot fringe. The step of scanning the laser frequency is a multiple of the free spectral range of this Fabry-Perot interferometer [39]. Whereas the design of the TiSa ensemble has remained the same between the first and the last runs [40], the design of the green pump laser has had to be changed drastically.…”

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Is the proton radius a player in the redefinition of the International System of Units?

Nez

Antognini

Amaro

et al. 2011

Phil. Trans. R. Soc. A.

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International audienceIt is now recognized that the International System of Units (SI units) will be redefined in terms of fundamental constants, even if the date when this will occur is still under debate. Actually, the best estimate of fundamental constant values is given by a least-squares adjustment, carried out under the auspices of the Committee on Data for Science and Technology (CODATA) Task Group on Fundamental Constants. This adjustment provides a significant measure of the correctness and overall consistency of the basic theories and experimental methods of physics using the values of the constants obtained from widely differing experiments. The physical theories that underlie this adjustment are assumed to be valid, such as quantum electrodynamics (QED). Testing QED, one of the most precise theories is the aim of many accurate experiments. The calculations and the corresponding experiments can be carried out either on a boundless system, such as the electron magnetic moment anomaly, or on a bound system, such as atomic hydrogen. The value of fundamental constants can be deduced from the comparison of theory and experiment. For example, using QED calculations, the value of the fine structure constant given by the CODATA is mainly inferred from the measurement of the electron magnetic moment anomaly carried out by Gabrielse's group. (Hanneke et al. 2008 Phys. Rev. Lett. 100, 120801) The value of the Rydberg constant is known from two-photon spectroscopy of hydrogen combined with accurate theoretical quantities. The Rydberg constant, determined by the comparison of theory and experiment using atomic hydrogen, is known with a relative uncertainty of 6.6 x 10(-12). It is one of the most accurate fundamental constants to date. A careful analysis shows that knowledge of the electrical size of the proton is nowadays a limitation in this comparison. The aim of muonic hydrogen spectroscopy was to obtain an accurate value of the proton charge radius. However, the value deduced from this experiment contradicts other less accurate determinations. This problem is known as the proton radius puzzle. This new determination of the proton radius may affect the value of the Rydberg constant R-infinity. This constant is related to many fundamental constants; in particular, R-infinity links the two possible ways proposed for the redefinition of the kilogram, the Avogadro constant N-A and the Planck constant h. However, the current relative uncertainty on the experimental determinations of N-A or h is three orders of magnitude larger than the 'possible' shift of the Rydberg constant, which may be shown by the new value of the size of the proton radius determined from muonic hydrogen. The proton radius puzzle will not interfere in the redefinition of the kilogram. After a short introduction to the properties of the proton, we will describe the muonic hydrogen experiment. There is intense theoretical activity as a result of our observation. A brief summary of possible theoretical explanations at the date of writing of the pap...

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The size of the proton

Cited by 1,270 publications

References 33 publications

Confinement of antihydrogen for 1,000 seconds

Confinement of antihydrogen for 1,000 seconds

New physics and the proton radius problem

Is the proton radius a player in the redefinition of the International System of Units?

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