Aldo Antognini

Title

Measurement of the Lamb shift in muonic hydrogen: the proton radius puzzle

Abstract

At the Paul Scherrer Institut, Switzerland, we have measured several 2S?2P transition frequencies in muonic hydrogen and deuterium by means of laser spectroscopy. This results in an order of magnitude improvement on the rms charge radius values of the proton and the deuteron. Additionally the Zemach radii and the deuteron polarizability are also inferred. The new proton radius value is deduced with a relative accuracy of 0.1% but strongly disagrees from CODATA. The origin of this discrepancy is not yet known. It may come from theory of the muonic hydrogen energy levels (used to deduce the new value), or from problems in hydrogen spectroscopy experiments or hydrogen energy level theory (both used to deduce the CODATA value). The decrease of the proton value uncertainty opens the way to compare the hydrogen energy levels predictions with high-precision measurements to an unprecedented accuracy. This will stimulate progress in the understanding of the simplest atom and its related bound-state QED theory. Experimental setup, measurements and results will be presented. Additionally the key issues regarding the observed discrepancy will be given together with the impact of these measurements on bound-state QED test and fundamental constants.