The picture that baryon consist of three quarks and mesons consist of a quark,anti-quark pair is inaccurate.However this simple picture is enough to explain many properties of the hadrons as mentioned in the answer here What is the experimental evidence that the nucleons are made up of three quarks? A more accurate picture is that baryons are made up of three valence quarks plus a sea of quarks,anti-quarks and gluons.It is the sea of quark,anti quark pairs that determine their masses.What is the experimental evidence of the presence of sea quarks ?experimentally how can we prove the existence of sea quarks as opposed to valence quarks?
First of all physics is not about proofs. It is about mathematical models that are fitted to data, and the same models make predictions for new experiments. If the predictions come true, the model is validated.
To start with there was the parton model, before quarks as such were accepted as a standard model. When deep inelastic scattering data on protons showed that there was a hard core
In particle physics, the parton model is a model of hadrons, such as protons and neutrons, proposed by Richard Feynman. It is useful for interpreting the cascades of radiation (a parton shower) produced from QCD processes and interactions in high-energy particle collisions.
The parton model was proposed by Richard Feynman in 1969, used originally for analysis of high-energy collisions. It was applied to electron/proton deep inelastic scattering by Bjorken and Paschos. Later, with the experimental observation of Bjorken scaling, the validation of the quark model, and the confirmation of asymptotic freedom in quantum chromodynamics, partons were matched to quarks and gluons. The parton model remains a justifiable approximation at high energies, and others have extended the theory over the years.
It was recognized that partons describe the same objects now more commonly referred to as quarks and gluons.
This happened when the quark model with the basic SU(3)xSU(2)xU(1) symmetries became the standard model for particle physics.
Parton distribution functions can be extracted from the data by using elaborate QCD calculation models. This laborious method is explained in this link.
So this type of plot is the experimental evidence, because it is a fit to data, and in addition it is predictive of other data.
Well in a baryon there are three quarks that are not virtual. Then there is the quark and the gluon field which constantly creates quarks and gluons and those are destroyed and created again. For a pion(neutral) there is not even these valence quarks. There is only the quark field. The charged pions are a little different and they can not have virtual quarks only. They have vitual quarks and real quarks(to make sure the charge is all perfect).