Heidelberg 2015 – scientific programme

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HK: Fachverband Physik der Hadronen und Kerne

HK 39: Invited Talks 3

HK 39.2: Invited Talk

Wednesday, March 25, 2015, 11:40–12:20, T/HS1

Recent results on the proton: Two photon exchange and the radius puzzle — •Jan Bernauer — Massachusetts Institute of Technology, Cambridge, USA

The proton form factors and radii are in the limelight of recent and ongoing experimental and theoretical efforts, mainly driven by two anomalies. On the one hand, precise determinations of the form factor ratio with experiments using polarization differ from Rosenbluth-type extractions. The discrepancy is attributed to two-photon exchange contributions to the scattering process. Three modern experiments, at VEPP-3, Jefferson Lab and DESY (OLYMPUS), aim to measure this effect directly. On the other hand, a measurement of the Lamb shift in muonic hydrogen atoms gives a result 10 times more precise, but 7 sigma smaller than determinations from elastic scattering and electronic hydrogen spectroscopy. So far, this discrepancy is not understood. In the talk, I will present the latest results and the state of research on both fronts.