Electromagnetic Form Factors of \varLambda Hyperon in the Vector Meson Dominance Model and a Possible Explanation of the Near-Threshold Enhancement of the e^+e^- \to \varLambda\bar\varLambda Reaction

The near-threshold e^+e^- \to \varLambda\bar\varLambda reaction is studied with the assumption that the production mechanism is due to a near-\varLambda \bar\varLambda-threshold bound state. The cross section of the e^+e^- \to \varLambda\bar\varLambda reaction is parameterized in terms of the electromagnetic form factors of \varLambda hyperon, which are obtained with the vector meson dominance model. It is shown that the contribution to the e^+e^- \to \varLambda\bar\varLambda 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 \varLambda \bar\varLambda, while its width is just a few MeV. This gives a possible solution to the problem that all previous calculations seriously underestimated the near-threshold total cross section of the e^+e^- \to \varLambda\bar\varLambda reaction. We also note that the near-threshold enhancement can also be reproduced by including these well established vector resonances \omega(1420), \omega(1650), \phi(1680), or \phi(2170) with a Flatté form for their total decay width, and a strong coupling to the \varLambda\bar\varLambda channel.