Double charmonium production and an X(3940)

Extending a previous analysis [1] the double charmonium production e⁺e⁻ → J/ψ(c) has been investigated in detail with a data set of 155 fb⁻¹ with the Belle detector. J/ψ production and helicity angular distributions are measured and give important information about the production mechanism. Theoretical predictions for the production cross section are one order of magnitude lower than the measurement and this discrepancy is still not understood. The process e⁺e⁻ → ψ(2S)(c) has been observed for the first time. The measured cross sections are compatible with e⁺e⁻ → J/ψ(c) production.

In a very recent update with a dataset of 285 fb^-1 strong evidence for a new charmonium state at a mass of 3.940 GeV was found.