Two-photon laser spectroscopy of antiprotonic helium and the antiproton-electron mass ratio

We report our latest results from our two-photon spectroscopy of antiprotonic helium (a bound state of a helium nucleus, an electron and an antiproton). By measuring three transition frequencies with a precision of 2.3−5×10−9 and comparing these to 3-body QED calculations, we derived the antiproton-electron mass ratio 1,836.1526736(23),which agrees with the proton-electron mass ratio (known to a three-times better precision) within the errors. Since the (anti)proton's atomic mass is known to a better precision than that of the electron, assuming mp = mp this result can be used to determine the electron's atomic mass as me=0.0005485799091(7) u.