Towards a precision test of time dilation at high velocityThis paper was presented at the International Conference on Precision Physics of Simple Atomic Systems, held at University of Windsor, Windsor, Ontario, Canada on 21–26 July 2008.

We report on first measurements towards an Ives–Stilwell test of time dilation at velocities around 0.3c. In Ives–Stilwell type experiments, fast atomic ions containing a well-known transition are used as moving clocks, and time dilation as well as the velocity can be derived from the simultaneous laser-spectroscopic measurements of the Doppler shifts with and against the direction of motion. To accurately measure these Doppler shifts, the Doppler broadening caused by the ions velocity distribution needs to be overcome. We performed first feasibility studies for laser spectroscopy on 7Li+ ions in the 2s3S21 metastable ground state at the Gesellschaft für Schwerinonenforschung (GSI) in Darmstadt. The ions were stored in the Experimental Storage Ring (ESR) at a velocity of 0.338c, and optical–optical double-resonance spectroscopy on a closed Λ-type three level system was performed with two lasers propagating antiparallel to the ions motion. We found that Doppler shift measurements on a narrow subclass of the ions velocity distribution with sufficient signal-to-noise ratio are possible if the ion beam is electron-cooled and bunched. Together with the control of systematic error sources developed in our previous experiments on slower beams, the ESR experiment promises an improvement of previously achieved sensitivities to time dilation by about an order of magnitude.