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[PAST EVENT] Physics Colloquium
Abstract:
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 the result of a new measurement in regular
atomic hydrogen.
[1] P.J. Mohr et al. (CODATA 2006), Rev. Mod. Phys. 80, 633 (2008)
[2] Pohl et al. (CREMA coll.), Nature 466, 213 (2010)
[3] Antognini et al.,(CREMA coll.), Science 339, 417 (2013)
[4] Olive et al. (PDG 2014), Chin. Phys. C40, 090001 (2014)
[5] Pohl et al., Annu. Rev. Nucl. Part. Sci 63, 175 (2013)
[6] Pohl et al. (CREMA coll.), Science 353, 669 (2016)
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