Mass of Helium-4 from the Cyclotron Frequency Ratio ^{4}He^{+}/^{12}C^{3+}.

By measuring the cyclotron frequency ratio of ^{4}He^{+} to ^{12}C^{3+} in a Penning trap the mass of ^{4}He has been determined to be 4.002 603 254 665(36) u. [The corresponding mass of the alpha particle is 4.001 506 179 662 (36) u]. This opens the possibility of deriving the atomic mass of the electron at a relative precision of 1×10^{-11} from proposed measurements of the Landé g-factor of ^{4}He^{+}.