(I’m moving posts from another site here…)
At the beginning of the XX century, Rutherford sees scattering doesn’t follow Thompson’s formula. Most alpha particles pass straight. Yet, and separated by a significant angular gap, there is again many alpha particles being scattered at high angles, altough less than the number of them passing straight. At around another, even higher angular dispersion value he saw again a profusion of alpha particlea being scattered. And again this amount was smaller than at the previous angular.value of high scattering.
Why the gap in angular dispersion? Why not seeing a continous, though decreasing number of alpha particles for all angular values of measured scattering?
These questions and type of experiments led to the discovery of the nucleus of atoms, of the neitrons and protons as well ad tgat of quarks. It also illuminated a zoo of many different particles (elementary or not).
In doing so, physicists realized the key importance of Symmetry concepts in particle physics.
In these nights of insomnia, I’ll try summarize how Group theory found its way into, and helped in classifying that zoo which in the 50’s was particle physics.