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New x-ray measurements shed light on discrepancy between experiment and QED



Lawrence T. Hudson, C T. Chantler, A.T. Payne, M N. Kinnane, John D. Gillaspy, L F. Smale, Albert Henins, J A. Kimpton, E Takacs


Quantum Electro-Dynamics (QED) is the best tested theory of our physical world, yet significant disagreements have emerged between attempts to apply QED to exotic atomic systems. A recent 15 parts-per-million (ppm) experiment on muonic hydrogen, in which the lepton orbits the nucleus at a distance much less than in conventional atoms, found a major discrepancy with the predictions of QED and independent nuclear size determinations. The cause of this discrepancy remains unknown. Here we find a new area of significant discrepancy at the same level in a different type of exotic atom. We present measurements on highly charged helium-like titanium, in which the orbital radius is reduced by increasing the nuclear charge rather than increasing the lepton mass. We report the first absolute energy measurements of the resonance, inter-combination and forbidden transitions in trapped Ti20+ ions—a system highly sensitive to two-electron QED processes. Our 15 ppm accuracy is able to discriminate between available QED formulations and reveals a pattern of discrepancy of experimental results from the most recent ab initio predictions. In both the muonic and highly charged systems, the sign of the discrepancy is the same, with with the measured transition energy higher than predicted. This may give insight into effective nuclear radii, the Rydberg, the fine-structure constant or unexpected large and unaccounted for higher order QED terms. Resolving these discrepancies is critical to establishing a firm foundation for our understanding of the interaction of light with matter, which forms the basis for all high-technology applications.
Nature Physics


quantum electrodynamics, x-ray spectroscopy


Hudson, L. , Chantler, C. , , A. , Kinnane, M. , Gillaspy, J. , Smale, L. , Henins, A. , Kimpton, J. and Takacs, E. (2014), New x-ray measurements shed light on discrepancy between experiment and QED, Nature Physics, [online], (Accessed February 24, 2024)
Created December 15, 2014, Updated November 10, 2018