New precision measurement of hyperfine splitting of positronium

The ground state hyperfine splitting of positronium ΔHFSΔHFS is sensitive to high order corrections of quantum electrodynamics (QED) in bound state. The theoretical prediction and the averaged experimental value for ΔHFSΔHFS have a discrepancy of 15 ppm, which is equivalent to 3.9 standard deviations (s.d.). A new precision measurement which reduces the systematic uncertainty from the positronium thermalization effect was performed, in which the non-thermalization effect was measured to be as large as 10±2 ppm10±2 ppm in a timing window we used. When this effect is taken into account, our new result becomes ΔHFS=203.3942±0.0016(stat.,8.0 ppm)±0.0013(sys.,6.4 ppm) GHz, which favors the QED prediction within 1.2 s.d. and disfavors the previous experimental average by 2.6 s.d.