Bulletin of the American Physical Society
APS April Meeting 2013
Volume 58, Number 4
Saturday–Tuesday, April 13–16, 2013; Denver, Colorado
Session Q5: Invited Session: Few-Body Physics and the Proton Charge Radius |
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Sponsoring Units: GFB Chair: Charlotte Elster, Ohio University Room: Governor's Square 14 |
Monday, April 15, 2013 10:45AM - 11:21AM |
Q5.00001: Proton Radius, Darwin-Foldy Term and Radiative Corrections Invited Speaker: Ulrich Jentschura It is not an easy task to define the proton charge radius. Namely, by definition, the proton radius is the slope of the G\_E Sachs form factor of the proton at zero momentum transfer, provided one has subtracted from the scattering cross sections, all effects due to QED. That means that radiative corrections must be subtracted; these otherwise ``mask'' the proton structure from the surroundings. On the other hand, the self-energy of the proton (not of the electron or of the muon) also influence the spectrum of atomic hydrogen, or muonic hydrogen, respectively. In the talk, we shall review the difficulties faced by a consistent definition, offer a way to resolve them, and review the current status of Lamb shift predictions in muonic hydrogen, with a special reference to the current experimental-theoretical discrepancy, as reported by the CREMA collaboration. [Preview Abstract] |
Monday, April 15, 2013 11:21AM - 11:57AM |
Q5.00002: Proton polarisability contribution to the Lamb shift in muonic hydrogen at fourth order in chiral perturbation theory Invited Speaker: Judith McGovern The recent determination of the proton charge radius from the Lamb shift in muonic hydrogen [1] gives a value that differs by many standard deviations from the CODATA value [2] and from the results of recent electron scattering experiments [3]. In the theoretical calculations [4], the least-well-determined contribution is the ``proton polarisability'' contribution. This is the part of the two-photon exchange which involves proton excitations. The dominant effect can be determined via dispersion relations from the proton structure functions, but a subtraction term remains [5,6]. This subtraction term is the amplitude $T_1(0,Q^2)$ for forward, zero-energy, doubly-virtual Compton scattering, which we calculate in heavy-baryon chiral perturbation theory, to fourth order in the chiral expansion and with the leading contribution of the $\gamma N\Delta$ form factor. This provides a model-independent expression for the amplitude in the low-momentum region, which is the dominant one for its contribution to the Lamb shift, and allows us to significantly reduce the theoretical uncertainty in the latter [7].\\[4pt] [1] R. Pohl \textit{et al.}, Nature \textbf{466}, 213 (2010).\\[0pt] [2] P. J. Mohr, B. N. Taylor and D. B. Newell, Rev. Mod. Phys. \textbf{80}, 633 (2008) [arXiv:0801.0028].\\[0pt] [3] J. C. Bernauer \textit{et al.} (A1 Collaboration), Phys. Rev. Lett. \textbf{105}, 242001 (2010) [arXiv:1007.5076].\\[0pt] [4] U. D. Jentschura, Ann. Phys. \textbf{326}, 500 (2011) [arXiv:1011.5275]; E. Borie, Ann. Phys. \textbf{327}, 733 (2012) [arXiv:1103.1772].\\[0pt] [5] K. Pachucki, Phys. Rev. A \textbf{60}, 3593 (1999) [arXiv:physics/9906002].\\[0pt] [6] C. E. Carlson and M. Vanderhaeghen, Phys. Rev. A \textbf{84}, 020102 (2011) [arXiv:1101.5965]; also [arXiv:1109.3779].\\[0pt] [7] M. C. Birse and J. A. McGovern, Eur.\ Phys.\ J. A48, 120 (2012) [arXiv:1206.3030]. [Preview Abstract] |
Monday, April 15, 2013 11:57AM - 12:33PM |
Q5.00003: What electron (and muon) scattering tell us about the proton radius Invited Speaker: John Arrington The nature of QCD is such that quarks cannot be observed in isolation, but only in tightly bound states. Because of this, the proton plays an important dual role as a basic building block of matter and the most accessible bound state of QCD. Elastic electron-proton scattering is one of the oldest and most widely used tools for studying the spatial structure of the proton, with the rms charge radius taken to be the slope of the form factor at $Q^2=0$. However, recent extractions of the radius from the Lamb shift in muonic hydrogen are significantly smaller than extractions from both earlier and more recent electron scattering data. The extraction of the proton charge and magnetization radii requires precise data taken at very low energies, a detailed understanding of two-photon exchange corrections, and a careful analysis of the uncertainty in extracting the slope at $Q^2=0$ from data at finite $Q^2$. I will summarize the state of existing extractions of the radius and present plans for future measurements which will address some of the issues mentioned above and improve the scattering-based extractions of the radius. [Preview Abstract] |
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