Shrinking the proton
Prof. Dr. Randolf Pohl (MPI für Quantenoptik, Garching /JGU Mainz)

For more than a decade, the rms charge radius of the proton was known to be 0.88fm, with about 1% uncertainty [1]. Two methods, elastic electron scattering and precision laser spectroscopy of atomic hydrogen, yielded consistent values.

In 2010, our result from laser spectroscopy of the exotic "muonic hydrogen" atom yielded a 4% smaller value, 0.84 fm, with an uncertainty of less than 0.1% [2,3]. In muonic hydrogen, a negative muon orbits a proton with a 200 times smaller Bohr orbit than in regular hydrogen, which increases the sensitivity of muonic hydrogen to the proton charge radius by 200^3 ~ 10 million! Since 2010, the discrepancy increased to more than 7 sigmas [4], making it one of the biggest discrepancies in the Standard Model.

I will discuss the so-called "proton radius puzzle" [5], report on more measurements in muonic atoms [6], and a new measurement in regular atomic hydrogen.