Search for an Electron Mass Shift inCs133in an Intense Electromagnetic Field

The possibility that the mass of a bound electron changes when placed in an intense electromagnetic field is studied here both theoretically and experimentally. The atomic-beam magnetic-resonance technique was used to examine hyperfine-structure frequency shifts in ${}_{}{}^{133}{}_{}{}^{}\mathrm{Cs}$ that occur when the atom is subjected to an intense, nonresonant radio-frequency magnetic field perpendicular to the static "C" field. A 2921-MHz T${\mathrm{M}}_{010}$ cavity produced the perturbing field and was situated between Ramsey separated oscillatory loops, which induced the resonant transitions of interest. Shifts were observed for six $\Delta m_F=\pm 1$ transitions at field-independent points. No evidence was found for an electron mass shift. Good agreement is found between all observed shifts and those expected from a multilevel Bloch-Siegert effect.