Randolf Pohl led a precision measurement of the Lamb shift in muonic hydrogen, an exotic chemical atom in which a muon replaces the electron. The heavier orbiting particle is localized much closer to the proton, making its energy levels especially sensitive to the proton's finite charge distribution.
Measuring the transition and subtracting the other calculated contributions therefore constrains the proton's charge radius. The 2010 result disagreed with then-standard determinations, turning nuclear-size extraction into a consistency test across different experimental and theoretical methods.[ref]
The size of the proton by Randolf Pohl and others. Abstract and spectroscopy method: muonic-hydrogen transition, finite-size sensitivity and comparison with other determinations
New to topics? Read the docs here!