The size of the proton

The proton is the primary building block of the visible Universe, but many of its properties-such as its charge radius and its anomalous magneticmoment-are not well understood. The root-meansquare charge radius, r(p), has been determined with an accuracy of 2 per cent (at best) by electron-proton scattering experiments(1,2). The present most accurate value of r(p) (with an uncertainty of 1 per cent) is given by the CODATA compilation of physical constants(3). This value is based mainly on precision spectroscopy of atomic hydrogen(4-7) and calculations of bound-state quantum electrodynamics (QED; refs 8, 9). The accuracy of r(p) as deduced from electron-proton scattering limits the testing of bound-state QED in atomic hydrogen as well as the determination of the Rydberg constant (currently the most accurately measured fundamental physical constant 3). An attractive means to improve the accuracy in the measurement of r(p) is provided by muonic hydrogen (a proton orbited by a negative muon); its much smaller Bohr radius compared to ordinary atomic hydrogen causes enhancement of effects related to the finite size of the proton. In particular, the Lamb shift 10 (the energy difference between the 2S(1/2) and 2P(1/2) states) is affected by as much as 2 per cent. Here we use pulsed laser spectroscopy to measure a muonic Lamb shift of 49,881.88(76) GHz. On the basis of present calculations(11-15) of fine and hyperfine splittings and QED terms, we find r(p) = 0.84184(67) fm, which differs by 5.0 standard deviations from the CODATA value(3) of 0.8768(69) fm. Our result implies that either the Rydberg constant has to be shifted by 2110 kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.