Detection of aσ·EEnergy Shift in Positronium Formed from Polarized Positrons

Abstract

In an experiment designed to set an upper limit on the magnitude of $\sigma ·\mathrm{E}$ energy for electrons and positrons, a persistent effect is found on reversing the direction of the electric field E applied to a gas sample in which positronium is formed from polarized ${\mathrm{Na}}^{22}$ positrons. The positronium decay is studied by the two-photon angular correlation technique, and a 5% effect on the narrow-component coincidence rate is occasioned by reversing the 15-kilovolt/centimeter field. The effect reverses sign when the positron spin-polarization just before capture, $P_z=〈{\sigma }_z〉$, which is at least 0.25, is reversed by changing the source position. Other observations made concurrently suggest that the highly Stark-mixed (excited) levels of positronium in transient existence may be acting as a spin-filter whose action reverses with E. Such behavior would entail mixing between triplet (heavily mixed $S$ and $P$) levels and the adjacent (similarly mixed $S$ and $P$) singlet levels—to which problem quantum electrodynamics has heretofore not been applied.