Thursday, May 19, 2011, 16:00
WHGA/001
F. Kottmann, PSI
Abstract:
The charge radius r of the proton has so far been known from
electron-proton scattering and from hydrogen spectroscopy with a rather low
precision of about 1%. Muonic hydrogen (a proton orbited by a negative muon)
provides an elegant way to improve the uncertainty of r by an order of
magnitude. We have recently performed at PSI the first measurement
of the Lamb shift (energy difference between 2S and 2P states)
in muonic hydrogen, using infrared laser
spectroscopy. Combined with QED calculations we determine r with a relative
precision of 0.08%.
Our new value for r is about 4% smaller and deviates by more than 5 sigma from the values from e-p scattering and H-spectroscopy. After a year of intense theoretical investigations this discrepancy became even more stringent because the more trivial explanations can be excluded. All relevant QED-terms have been recalculated and it was checked that the proton radii deduced from the different experiments are defined consistently. I will give an overview about the present status of the corresponding discussions and some implications for the future.