J/ψ production from electromagnetic fragmentation in Z0 decay

The rate for Z0→J/ψ+l+l- is surprisingly large with about one event for every million Z0 decays. The reason for this is that there is a fragmentation contribution that is not suppressed by a factor of Mψ2/MZ2. In the fragmentation limit MZ→ with Eψ/MZ fixed, the differential decay rate for Z0→J/ψ+l+l- factors into electromagnetic decay rates and universal fragmentation functions. The fragmentation functions for lepton fragmentation and photon fragmentation into J/ψ are calculated to the lowest order in α. The fragmentation approximation to the rate is shown to match the full calculation for Eψ greater than about 3Mψ.