Photoproduction of Muon Pairs: ${\rho }^0\to {\mu }^{+}+{\mu }^{-}$, ($e, \mu$) Universality, and ($\rho , \omega$) Phase

Data on photoproduction of muon pairs are reported and compared with quantum electrodynamics. A new average value of $B_{\rho }=\frac{({\rho }^0\to {\mu }^{+}+{\mu }^{-})}{({\rho }^0\to \mathrm{all})}=(8.2\mathrm{}\pm 1.6)\times {10}^{-5\mathrm{}}$ is obtained. Comparison with $B_{\rho }$ for electron pairs provides a valid experimental confirmation, to "distances" of 1.6×${10}^{-14\mathrm{}}$ cm, of ($e, \mu$) universality for vertices involving timelike photons. We interpret the paradoxical agreement of values of $B_{\rho }$ for photoproduction experiments with those for colliding beams in terms of the $\rho$, $\omega$ phase.