Decay of a J/ψ to leptons
Plot the mass distribution of a J/ψ which decays to two leptons:<
![\[J/\psi \longrightarrow e^+ e^-\quad\text{or}\quad J/ψ \longrightarrow \mu^+ \mu^-\]](https://belle2.ijs.si/public/wp-content/ql-cache/quicklatex.com-a50af58d7cf95b025558021eca82e448_l3.png "Rendered by QuickLaTeX.com")
You will find a peak at a mass of J/ψ at 3.10 
The probability for the production of a J/ψ is very small, so you will have to process at least 100.000 events.