Evidence for the Electromagnetic Production ofμMesons

Direct electromagnetic production of pairs of $\mu$ mesons has been investigated. The means of detection chosen consists of searching for the production of single negative $\mu$ mesons peaked sharply in the forward direction. The target chosen was aluminum, bombarded by bremsstrahlung from 575-Mev electrons, and the angular range studied was $10°<\theta <30\mathrm{}°$. The experimental value of the cross section ${\sigma }_{\exp }$ was referred to a theoretical value ${\sigma }_{\mathrm{theor}}$ computed by modifying the conventional formula for the pair production of fermions by photons in a Coulomb field by the introduction of a finite nuclear size. Observations carried out to data give $\frac{{\sigma }_{\exp }}{{\sigma }_{\mathrm{theor}}}=1.93\mathrm{}\pm \mathrm{0.68.}$ The quoted standard error defines the statistical accuracy only and is a measure of the likelihood of existence of the process; in addition, the absolute cross section is uncertain to within a factor of 1.5.