If quarks exist, none may ever have been detected. Certainly no isolated quark has been detected. If they do exist, they always occur as pairs – mesons - or triplets – hadrons. For this reason most of the evidence for the existence of quarks is indirect. The evidence includes:
Electron diffraction experiments. High energy, hence short wavelength electrons are fired at a material. The electrons behave like waves (wave particle duality) and are diffracted when they hit the target. The diffraction pattern of the diffracted electrons can be used to estimate the size and shape of particles in the target. The electrons have to be high energy because there exists an inverse relationship between wavelength – which needs to be about the same size of the distances between the particles being investigated – and energy. The results of these experiments indicate that protons have internal structure, and that the electrons are being scattered by particles that mave charges that are a fraction of the charge on an electron. This implies that protons are not fundamental particles but are composed of more fundamental particles, and similarly for neutrons.
Other evidence is less direct. Scientists, in chasing simpler models for particles, found that they could reduce the number of fundamental particles needed to account for the particles we can see if these particles are made up of particles called quarks. These have have charges that are multiples of
the charge on an electron, and must combine in twos and threes to account for the integral multiples of
that we observe particles having.
While much of the evidence for quarks is indirect, the quark model is consistent with many observations. It can explain the decay of neutrons into protons, with an up quark changing into a down quark, and explains the strong nuclear force as being in fact between the quarks that make up particles and not between the particles themselves.