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The near-threshold $e^+e^- \to \Lambda\bar{\Lambda}$ reaction is studied withthe assumption that the production mechanism is due to a near-$\Lambda\bar{\Lambda}$-threshold bound state. The cross section of $e^+e^- \to\Lambda\bar{\Lambda}$ reaction is parametrized in terms of the electromagneticform factors of $\Lambda$ hyperon, which are obtained with the vector mesondominance model. It is shown that the contribution to the $e^+e^- \to\Lambda\bar{\Lambda}$ reaction from a new narrow state with quantum numbers$J^{PC}=1^{--}$ is dominant for energies very close to threshold. The mass ofthis new state is around 2231 MeV, which is very close to the mass threshold of$\Lambda \bar{\Lambda}$, while its width is just a few MeV. This gives apossible solution to the problem that all previous calculations seriouslyunderestimate the near-threshold total cross section of the $e^+e^- \to\Lambda\bar{\Lambda}$ reaction. We also note that the near-thresholdenhancement can be also reproduced by including these well established vectorresonances $\omega(1420)$, $\omega(1650)$, $\phi(1680)$, or $\phi(2170)$ with aFlatt${\rm\acute{e}}$ form for their total decay width, and a strong couplingto the $\Lambda\bar{\Lambda}$ channel.

chinese physics letters/chinese physics letters

Electromagnetic form factors of $\Lambda$ hyperon in the vector meson  dominance model and a possible explanation of the near-threshold enhancement  of the $e^+e^- \to \Lambda\bar{\Lambda}$ reaction