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What is the experimental evidence that the nucleons are made up of three quarks? What is the point of saying that nucleons are made of quarks when there are also gluons inside it?

What is the experimental evidence that the nucleons are made up of
three quarks?

Some strong pieces of evidence for the quark model of the proton and the neutron, not stated in another answer, are the magnetic moment of the proton and the magnetic moment of the neutron, which are consistent with the quark model and are inconsistent with the magnetic moment that would be predicted by quantum electrodynamics in a point particle model.

What is the point of saying that nucleons are made of quarks when
there are also gluons inside it?

The reasons this is done is called in the field of science communication "lies to children".

Complex topics are often initially taught in a manner the oversimplifies the reality in order to develop key salient points.

Emphasizing the quark composition of the nucleons while ignoring the gluon contribution allows one to explain many key conclusions of the quark-gluon model including:

• The charge of all of the hadrons




• Beta decay




• The list of all possible baryons and of all possible pseudoscalar and vector mesons




• The full list of Standard Model fermions




• The magnetic moment of hadrons




• Deep inelastic scatter of hadrons




• A formula for hadron spin

The existence of gluons as a constituent doesn't have to be explained in detail to reach these results.

Also, while the valence quark content of a hadron is specific to a particular kind of quark, the gluon content of a hadron is not useful for hadron taxonomy.

The process that was first used to resolve the internal structure of the proton and neutron is called deep inelastic scattering. Basically, you hit the target hadron with enough energy that the probing particle's wavelength is short enough to make out the details of the internal structure of the proton or neutron.

You are right to question the idea that a proton is "made up of three quarks" when there are gluons inside as well. The gluons are of course force-carrying particles which mediate the strong force and keep the quarks bound to one another, but because the gluons carry color charge themselves, they couple in strongly non-linear ways. The upshot is that most of the mass of a proton comes from the interaction energy of the gluons.

When I was in university, sitting on my dinosaur, one of my profs mentioned he worked on a neutron polarization system on a particle accelerator at Chalk River.

I had to ask how you polarized a neutral particle with a magnet, still thinking in terms of classical particles and charges. He explained that since there's an internal quark structure, even though the outside looks neutral there's enough asymmetry for you to work with.

It was not until much later that I read how these actually work, there's complexity of course, but the basics are there.