Measurement of the positronium 2P fluorescence decay rate

Measurements of positronium (Ps) energy levels and decay rates can be used to test QED theory and search for new physics [1]. Optical and microwave spectroscopy have been used to measure various Ps energy intervals [2-6] and Ps annihilation decay rates have been measured for the ground states [7,8] and the 2S excited state [9]. Here we report a measurement of the fluorescence decay rate of positronium (Ps) atoms in the 2³P level. Ps atoms were optically excited to a Stark-mixed state containing both 2S and 2P components, in which the relative population was controlled via an applied electric field. A larger delayed electric field was then applied to induce rapid quenching and annihilation of the mixed states. By measuring the number of quenched atoms observed for different mixing field strengths we obtained the effective decay rate of the mixed states, from which the 2P decay rate can be obtained. The obtained result is consistent with theory but is not sufficient to test QED. We discuss possible improvements to the methodology that could allow for a more precise measurement.