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

The near-threshold $e^+e^- \to Λ\barΛ$ reaction is studied with the assumption that the production mechanism is due to a near-$Λ\barΛ$-threshold bound state. The cross section of $e^+e^- \to Λ\barΛ$ reaction is parametrized in terms of the electromagnetic form factors of $Λ$ hyperon, which are obtained with the vector meson dominance model. It is shown that the contribution to the $e^+e^- \to Λ\barΛ$ reaction from a new narrow state with quantum numbers $J^{PC}=1^{--}$ is dominant for energies very close to threshold. The mass of this new state is around 2231 MeV, which is very close to the mass threshold of $Λ\barΛ$, while its width is just a few MeV. This gives a possible solution to the problem that all previous calculations seriously underestimate the near-threshold total cross section of the $e^+e^- \to Λ\barΛ$ reaction. We also note that the near-threshold enhancement can be also reproduced by including these well established vector resonances $ω(1420)$, $ω(1650)$, $ϕ(1680)$, or $ϕ(2170)$ with a Flatt${\rm\acute{e}}$ form for their total decay width, and a strong coupling to the $Λ\barΛ$ channel.

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