Final state Coulomb interaction and asymmetry of pair production close to threshold in $e^+e^-$ annihilation

TL;DR

This paper studies how Coulomb interactions influence pair production asymmetry near threshold in electron-positron annihilation, revealing a non-vanishing asymmetry due to the d-wave contribution even at zero relative velocity.

Contribution

It demonstrates that Coulomb final state interactions cause a non-zero asymmetry at threshold, challenging previous assumptions that it vanishes at zero velocity.

Findings

Coulomb interaction induces non-zero asymmetry at threshold.

d-wave contribution remains significant near threshold.

Potential observability of the asymmetry despite its small size.

Abstract

We investigate a contribution of the $d$ wave to the cross section of $e^+e^-$ annihilation to the pair of charged leptons or nucleons close to threshold of the process. In contrast to the point of view accepted in literature, due to the Coulomb final state interaction this contribution does not vanish even at zero relative velocity of produced particles. This results in the nonzero asymmetry in angular distribution at threshold. Though value of the asymmetry is small, observation of this effect is not hopeless.