Precise measurement of n=2 positronium fine-structure intervals

The n=2 fine-structure transitions in positronium have been measured by observation of increased Lyman-α radiation at microwave induced resonances. We used an accelerator based slow positron beam to produce positronium in the 23S1 excited state inside a waveguide. Our results are ν0=18 499.65±1.20±4.00 MHz, ν1=13 012.42±0.67±1.54 MHz, and ν2=8 624.38±0.54±1.40 MHz for the 23S1→k23P0,1,2 transition frequencies, respectively. The first error is statistical and the second is systematic. This is in agreement with recent bound state QED calculations