Photoproduction of Muon Pairs: ρ 0 → μ + + μ − , ( e , μ ) Universality, and ( ρ , ω ) Phase

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