The Radius of the Proton: Size Does Matter

Understand

The measurement by Pohl et al.

• [1] of the 2S_1/2^F=1 to 2P_3/2^F=2 transition in muonic hydrogen and the subsequent analysis has led to a conclusion that the rms charge radius of the proton differs from the accepted (CODATA [2]) value by approximately 4%, leading to a 4.9 s.d.
• discrepancy.
• We investigate the muonic hydrogen spectrum relevant to this transition using bound-state QED with Dirac wave-functions and comment on the extent to which the perturbation-theory analysis which leads to the above conclusion can be confirmed.