Transverse-Momentum Behavior of Muons in $e^{+}{e}^{-}\to e^{+}{e}^{-}{\mu }^{+}{\mu }^{-}$

In connection with a recent measurement performed by Adeva et al. at PETRA, the transverse-momentum behavior of muons in the $e^{+}{e}^{-}\to e^{+}{e}^{-}{\mu }^{+}{\mu }^{-}$ process was studied in a simple and transparent way, with use of the double equivalent-photon approximation. The corresponding distribution is expressed in a scale-invariant form, and good agreement is obtained with the experimental data.