PSAS 2012
International Conference on Precision Physics
of Simple Atomic Systems 2012
Eltville, Germany -
Information

Title:	Dr.
Family Name:	Distler
First Name:	Michael
Gender:	male
E-Mail:	distleruni-mainz.de
Affiliation for Badge:	University of Mainz
Institute:	Institute for Nuclear Physics
Address (Street):	Becher Weg 45
Postal Code, City:	55128 Mainz
Country:	Germany
Phone:	+49 6131 39 22956
Fax:	+49 6131 39 22964
Arrival Date:	June 12
Departure Date:	June 12
Excursion with Conference Dinner:	Not selected
Payment:	Bank transfer to GSI
Title:	The Radius of the Proton from MAMI - Challenges and Implications
Abstract:	In the talk the results of a high precision measurement of the elastic electron proton scattering cross section will be presented. The experiment was performed at the Mainz Microtron (MAMI) 3-spectrometer-facility in the Q^2 range from 0.004 to 1 (GeV/c)^2. Employing a direct fit of a selection of different form factor models to the measured cross sections, the electric and magnetic form factors were extracted. The experiment adds a new, internally consistent extensive data set to the available world data. The electron proton scattering data has been used to determine the charge radius of the proton. The result is <r_e> = (0.879 +/- 0.005 stat. +/- 0.004 syst. +/- 0.002 model +/- 0.004 group) fm which is in perfect agreement with the CODATA value but constitutes a 5σ discrepancy to a recent measurement of the Lamb shift in muonic hydrogen. Although the extracted higher moments of the charge distribution are consistent with other evaluations and new polarization data from Jefferson Lab seem to confirm the Mainz results as well, one major challenge remains: The two-photon exchange (TPE). The theoretical predictions of this correction beyond the born approximation are still highly disputed, however their influence on the charge radius of the proton appears to be marginal. Nevertheless, the implications for other high precision experiments like parity violation are quite sizable.

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