To paraphrase Bill Clinton, "It depends on what the definition of 'find' is." The positron was "found" by its particle track in a bubble chamber - an electron bending the wrong way. The neutrino was "found" as a missing track, required to be there by conservation of momentum and energy - corroborated of course in a multitude of interactions.
Then we moved on to the Psi particle with a lifetime of 7e-21 seconds, so don't be lookin' for no tracks. This discovery consisted in a "hump" in the cross section of an interaction as a function of energy - a "resonant energy". OK.
Now where are we? Now we require statistical analysis! A certain collection of data gleaned from millions, or billions ( I don't know ) of interactions must depend, with 99.X% certainty on the existence of this particle, so what was that middle part again?
Like Andy Warhol said, "Death is so abstract."