A search for annihilation asymmetries in positron-chiral molecule collisions

The Vester–Ulbricht hypothesis suggests that parity violation in weak interactions, manifested in collisions between β radiation and primordial chiral organic molecules, introduced a small enantiomeric asymmetry that was amplified to yield the homochirality observed in present-day biological systems. We report an experiment searching for such effects in which chiral organic molecular samples are irradiated with positrons from a Na-22 source and the resulting gamma spectra from opposite enantiomers are compared. The measurement is most sensitive to two-photon channels arising from positronium formation (and decay) or direct positron–electron annihilation at 511 keV. Targets studied were glucose, leucine, cystine, and achiral succinic acid. From these measurements, we place an upper limit of 10^-4 on any enantiomer-dependent asymmetry in the two-photon annihilation rate.