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−→ΛΛ¯ Reaction

Abstract

The near-threshold e + e − → Λ Λ ¯ reaction is studied with the assumption that the production mechanism is due to a near- Λ Λ ¯ -threshold bound state. The cross section of the e + e − → Λ Λ ¯ reaction is parameterized 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 − → Λ Λ ¯ reaction from a new narrow state with quantum numbers JPC = 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 Λ ¯ , 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 − → Λ Λ ¯ reaction. We also note that the near-threshold enhancement can also be reproduced by including these well established vector resonances ω(1420), ω(1650), ϕ(1680), or ϕ(2170) with a Flatté form for their total decay width, and a strong coupling to the Λ Λ ¯ channel.