Friday 23 September, 11.00h, at Nikhef in H331.
Speaker: prof. dr. Kjeld Eikema (Vrije Universiteit Amsterdam).
Title: “The Proton Radius Puzzle – precision spectroscopy at extremes”.
Abstract:
What is the charge radius of the proton? Before 2010 this was a simple question. There was a value based on electron scattering, and one on precision spectroscopy of atomic hydrogen, and both agreed. The method via spectroscopy is possible given the finite size effects of the proton on the optical transitions in the atom. This can be measured due to the incredible precision that can now be achieved on the hydrogen 1S-2S transition (nearly 15 decimals), combined with an accurate Rydberg constant, and the assumption that quantum-electrodynamic effects (QED) in hydrogen are now well enough understood and calculated. However, in 2010 a paper was published in Nature about precision spectroscopy in muonic hydrogen (where the electron is replaced with the 200 times heavier muon; this experiment was performed at PSI in Switzerland). The results could be interpreted as a proton radius which was 4% smaller (5 sigma) than the previously accepted CODATA value. In 2013 the finding was confirmed with additional muonic hydrogen spectroscopy. This has encouraged a lot of theoretical speculation and new experiments to solve the puzzle. But despite much effort, the situation is still very confusing. In the talk an overview will be given of the current status of the Proton Radius Puzzle, and of an experiment now in preparation at the LaserLaB of the VU Amsterdam to investigate the puzzle with precision spectroscopy in the extreme ultraviolet on a single He+ ion.