Experimental demonstration of the Breit interaction which dominates the angular distribution of x-ray emission in dielectronic recombination.

We report the experimentally determined angular distribution of the [1s2s(2)2p(1/2)](1)→[1s(2)2s(2)](0) transition in dielectronic recombination of Li-like Au. Recently, Fritzsche et al. [Phys. Rev. Lett. 103, 113001 (2009)] predicted that the Breit interaction plays a dominant role in the angular distribution of this transition. However, the predicted phenomenon has not yet been observed experimentally due to technical difficulties in conventional methods. To overcome the difficulties, we combine two different measurements with an electron beam ion trap (EBIT) to obtain the x-ray angular distribution. One is the x-ray measurement at 90° and another is the integral resonant strength measurement through the ion charge abundance in the EBIT. Our measurements agree well with the theoretical prediction and confirm the dominance of the Breit interaction.