Muonic hydrogen Lamb shift: the proton radius puzzle

Aldo Antognini
per la Collaborazione CREMA
Max Planck Institut für Quantenoptik, Garching, Germany
ETH, Zürich, Switzerland

ABSTRACT:

At the Paul Scherrer Institute, Switzerland, we have recently measured the 2S – 2P transition frequency (Lamb shift) in muonic hydrogen (μp) 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 r_p = 0.84184(67) fm is 10 times more precise than previously obtained. However, it disagrees by 5 standard deviations from the value extracted from hydrogen spectroscopy and 3 standard deviations 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 unexpected proton shape 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 structure, holding the potential for new insights.

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