Aldo Antognini, Max-Planck Institute for Quantum Optics and Laboratory for Particle Physics, Paul Scherrer Institute.
The proton radius puzzle
Argonne Physics Division Colloquium - 29 Oct 2010
11:00 AM, Building 203 auditorium

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 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 discrepancy is not yet known. It may come from the theory of 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 rom uncalculated or new effects.

The 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 raises new questions in the muonic sector, in bound-state QED and around the proton, holding the potential for new insights.

Argonne Physics Division Colloquium Schedule