To help answer this, scientists may use something like a vault full of safe-deposit boxes. The valuables they hold will be plates of lead and photographic film waiting for a neutrino to make a near-impossible collision with the nucleus of a lead atom and leave a debris trail in the film that betrays its presence. In the 1950s and '60s scientists faced a bewildering array of particles coming from particle accelerators as they pushed to ever higher energies. Order was offered in the 1960s when several scientists proposed what is now called the Standard Model.
Leptons are their own fundamental particles.
Like quarks, leptons are believed to be fundamental particles with no underlying structure. Pauli suggested that the missing energy was carried away by a particle that could not then be detected. They stayed invisible until 1956 when detector methods had advanced. By 1962, scientists had identified the electron and muon neutrinos. Even though only one neutrino in several million is ever detected, scientists expected to snare a few dozen out of the countless numbers flooding to Earth from the fusion reactor inside our sun.