Aldo Antognini, Max Planck Institut fur Quantenoptik and ETH,"The proton radius puzzle"

Thu, Oct 28, 2010, 4:30 pm to 6:00 pm
McDonnell A02
At the Paul Scherrer Institute, Switzerland, we have recently measured the 2S-2P transition frequency (Lamb shift) in muonic hydrogen with 15 ppm precision by means of laser spectroscopy. From this measurement we have determined the rms charge radius of the proton. The new proton radius value rp = 0.84184(67) fm is 10 times more precise than previously obtained. However, it disagrees by 5 standard deviations from the values extracted from hydrogen spectroscopy and from electron-proton scattering data. The origin of the discrepancies is not yet known. It may come from theory of the muonic hydrogen energy levels (used to deduce our new value), from problems in hydrogen spectroscopy experiments or hydrogen energy level theory (both used to deduce the "H" value), from inconsistent definitions of the proton radius in the various systems, or it occurs from uncalculated or new effects. Experimental setup, measurements, results and future perspectives will be presented. Additionally the key issues and excluded explanations regarding the observed discrepancy will be discussed. The discrepancy simply raises new questions in the muonic sector, in bound-state QED and around the proton, holding the potential for new insights.