Speaker
Description
Summary
We have calculated the asymmetry in angular distribution of pair production in $e^+e^-$ annihilation taking into account the Coulomb FSI. It is shown that the asymmetry does not vanish in the limit $\beta \rightarrow 0$. The origin of this anomaly is the singularity of the "dressed" Dirac form factors ${\cal F}_1$ and ${\cal F}_2$ at $\beta\rightarrow 0$, see Eq. (\ref{calf1f2}).
The corresponding electromagnetic Sachs form factors $G_E$ and $G_M$ are not singular, but $G_E \ne G_M$ at $\beta\rightarrow 0$. The effect is non-perturbative , since in this limit $\alpha/\beta \gg 1$. This very nonzero difference $G_E-G_M$ at $\beta\rightarrow 0$ provides nonzero asymmetry at threshold. Although the value of the asymmetry is small for electromagnetic Coulomb interaction ($\sim \alpha^2/24)$, the asymmetry for heavy quark pair production, where the Coulomb-like strong interaction must be considered, can be noticeable.
