How Big Is the Proton?

Science  25 Jan 2013:
Vol. 339, Issue 6118, pp. 405-406
DOI: 10.1126/science.1233202

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As the simplest possible atom, comprising a single proton and a single electron, hydrogen has played a central role in the development of modern physics. For example, the observation in 1947 of the 2S-2P1/2 energy level splitting, now termed the Lamb shift, stimulated the development of the theory of quantum electrodynamics (QED). Hydrogen and other, more exotic two-body atomic systems continue to provide fertile testing grounds for fundamental physical theories and are still yielding surprises, as demonstrated by the experiments reported by Antognini et al. on page 417 of this issue (1). By using laser spectroscopy to study two transitions in muonic hydrogen (in which the electron is replaced by its heavier cousin, the muon), they determine a value for the root-mean-square charge radius of the proton that differs appreciably from the latest (2010) CODATA recommended value (2).