A Better-Known Electron Mass

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Science  07 Mar 2014:
Vol. 343, Issue 6175, pp. 1058
DOI: 10.1126/science.343.6175.1058-b

The mass of the electron is one of the fundamental constants of nature. It is known to a high precision, but improvements are desirable in order to facilitate continued precision testing of the Standard Model of particle physics. The tiny mass, however, makes direct measurements very challenging. Sturm et al. use an indirect technique, in which an electron is bound to a carbon nucleus in a hydrogen-like configuration. This positively charged ion follows a circular orbit in an external magnetic field at the cyclotron frequency, which is proportional to the local magnetic field B. At the same time, the spin of the electron precesses at a frequency also proportional to B. By measuring the ratio of these two frequencies, one can determine the electron mass, knowing the mass of the ion and the g factor of the bound electron, which is different from its well-known free-space value; to estimate it, the authors used the theory of quantum electrodynamics and related measurements in a silicon system. The measurements were performed in a Penning-trap setup and yielded a relative precision of 3 × 10−11 in the electron mass, a value 13 times smaller than the uncertainty of the current accepted mass obtained by weighted averaging of literature values. It is expected that the improved result will enable future fundamental physics experiments that were previously impossible.

Nature 10.1038/nature13026 (2014).

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