In 2010 the proton charge radius was extracted for the first time from muonic hydrogen, a bound state of a muon and a proton. The value obtained was five standard deviations away from the regular hydrogen extraction. Taken at face value, this might be an indication of a new force in nature coupling to muons, but not to electrons. It also forces us to reexamine our understanding of the structure of the proton.
This "proton radius puzzle" can be viewed as the latest chapter in a 100+ year old adventure: how hydrogen spectroscopy keeps pushing the boundaries of science. I will briefly review this history and describe an ongoing theoretical research effort to address this puzzle. In particular, I will present a reevaluation of the proton structure effects, correcting 40 years of such calculations, and the development of new effective field theory tools.
The College of Arts