High energy particles - the science behind

All the particles we interact with have high-energy, unstable cousins. It's often said that advances in science aren't met with "eureka!" but rather with "that's funny," but this actually happened in fundamental physics! If you charge up an electroscope — where two conducting metal leaves are connected to another conductor — both leaves will gain the same electric charge, and repel one another as a result. If you place that electroscope in a vacuum, the leaves shouldn't discharge, but over time, they do. The best idea we had for this discharge was that there were high-energy particles hitting Earth from outer space, cosmic rays, and the products of these collisions were discharging the electroscope.

In 1912, Victor Hess conducted balloon-borne experiments to search for these high-energy cosmic particles, discovering them immediately in great abundance and becoming the father of cosmic rays. By constructing a detection chamber with a magnetic field in them, you could measure both the velocity and charge-to-mass ratio based on how the particle’s track curves. Protons, electrons, and even the first particles of antimatter were detected via this method, but the biggest surprise came in 1933, when Paul Kunze, working with cosmic rays, discovered a track from a particle that was just like the electron... except hundreds of times heavier!

The muon, with a lifetime of just 2.2 microseconds, was later experimentally confirmed and detected by Carl Anderson and his student, Seth Neddermeyer, using a cloud chamber on the ground. When the physicist I.I. Rabi, himself a Nobel Laureate for the discovery of nuclear magnetic resonance, learned of the muon's existence, he famously quipped, "Who ordered that?" It was later discovered that both composite particles (like the proton and neutron) and fundamental ones (quarks, electrons, and neutrinos) all have multiple generations of heavier relatives, with the muon being the first "generation 2" particle ever discovered.