Quarks Have Never Been Observed

So why believe in them?

Quarks are a part of the Standard Model of particle physics — a theory that describes the interactions of particles that we observe. The Standard Model is a very “successful” theory (depending on what you mean) and hypothesizes the existence of quarks, which is what makes up baryons like protons and neutrons. Many people believe that since the standard model is regarded as the “ultimate model of reality” by many (naive) physicists, then the fundamental particles that the model relies upon would have been observed long ago. So it comes to a surprise when most people find out that quarks and gluons have never been observed. So why do physicists believe in them?

Classification Nightmare

Throughout the 1930s as technology grew more powerful, new fundamental particles had been discovered such as mesons and pions. So many new fundamental particles had been discovered that it would be a mess to classify them all. This sparked the question: “What if some of these “fundamental” particles weren’t so fundamental after all? What if they are all just different arrangements of the same underlying object?” This gave rise to the Quark Model, a model that arranges particles like protons and neutrons into configurations of smaller particles called quarks. And thus the quark was born — as a convenience tool.