Branching Ratios for the Decay of Vector Mesons into Lepton Pairs

Abstract

We have studied the decays into lepton pairs of vector mesons ($\rho ,\phi ,\omega$) produced by a beam of $3-\frac{\mathrm{BeV}}{c{\pi }^{-}}$ mesons incident on a liquid-${\mathrm{H}}_2$ target at the Brookhaven AGS. The experiment utilized scintillation counters and spark chambers as well as a lead-scintillator "shower chamber" constructed for the identification of electrons and rejection of background. A Monte Carlo technique was used to predict the background and determine the number of real events. For the muon-pair mode, the results are upper limits, with the branching ratio $\frac{(\rho \to \mu \overline{\mu })}{(\rho \to \mathrm{all}\mathrm{modes})}<14\mathrm{}\times {10}^{-4\mathrm{}}$ being the best limit determined. For the electron-pair decays we obtain $\frac{(\rho \to e\overline{e})}{(\rho \to \mathrm{all}\mathrm{modes})}=\left({{0.65}_{-0.5\mathrm{}}}^{+1.1\mathrm{}}\right)\times {10}^{-4\mathrm{}}$ and $\left(\frac{\omega \to e\overline{e}}{\omega \to \mathrm{all}\mathrm{modes}}\right)=\left({{1.0}_{-0.75\mathrm{}}}^{+1.7\mathrm{}}\right)\times {10}^{-4\mathrm{}}$ for an assumed $\phi -\omega$ mixing angle of 38°. Because of insufficient knowledge of the production process, we tentatively quote for the $\phi$ meson a result ${\sigma }_{\phi }\times \frac{(\phi \to e\overline{e})}{(\phi \to \mathrm{all}\mathrm{modes})}={{0.31}_{-0.21\mathrm{}}}^{+.\mathrm{}34\mathrm{}}$ μb. The results are compared with other experiments.