[PAST EVENT] Physics Colloquium
For more than a decade, the rms charge radius of the proton was known to be
0.88fm, with about 1% uncertainty . 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 , making it one
of the biggest discrepancies in the Standard Model.
I will discuss the so-called "proton radius puzzle" , report on more
measurements in muonic atoms , and the result of a new measurement in regular
 P.J. Mohr et al. (CODATA 2006), Rev. Mod. Phys. 80, 633 (2008)
 Pohl et al. (CREMA coll.), Nature 466, 213 (2010)
 Antognini et al.,(CREMA coll.), Science 339, 417 (2013)
 Olive et al. (PDG 2014), Chin. Phys. C40, 090001 (2014)
 Pohl et al., Annu. Rev. Nucl. Part. Sci 63, 175 (2013)
 Pohl et al. (CREMA coll.), Science 353, 669 (2016)