We report a new test of quantum electrodynamics (QED) for the w ($1s2p{}^{1}P_1\to 1s^2{}^{1}S_0$) x-ray resonance line transition energy in heliumlike titanium. This measurement is one of few sensitive to two-electron QED contributions. Systematic errors such as Doppler shifts are minimized in our experiment by trapping and stripping Ti atoms in an electron beam ion trap and by applying absolute wavelength standards to calibrate the dispersion function of a curved-crystal spectrometer. We also report a more general systematic discrepancy between QED theory and experiment for the $w$ transition energy in heliumlike ions for $Z>20$. When all of the data available in the literature for $Z=16–92$ are taken into account, the divergence is seen to grow as approximately $Z^3$ with a statistical significance on the coefficient that rises to the level of 5 standard deviations. Our result for titanium alone, 4749.85(7) eV for the $w$ line, deviates from the most recent ab initio prediction by 3 times our experimental uncertainty and by more than 10 times the currently estimated uncertainty in the theoretical prediction.

• Received 11 May 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.153001