In the context of the Standard Model of particle physics, protons are indeed composed of three quarks, specifically two up quarks and one down quark. These quarks are bound together by the strong nuclear force, which is mediated by particles called gluons.
Anti-neutrinos, on the other hand, are neutral subatomic particles that belong to the lepton family. They are associated with antiparticles of neutrinos. Neutrinos and anti-neutrinos are generally considered to be elementary particles, meaning they are not composed of smaller particles.
When protons undergo certain types of nuclear interactions or decays, such as beta decay, one of the quarks within the proton can change flavor, transforming into a different quark. In the case of beta decay, a down quark within the proton can convert into an up quark. During this process, a W- boson is involved in the decay, and an anti-neutrino is emitted.
Before the release of the anti-neutrino, it exists as a virtual particle, which means it is not directly detectable. It is involved in the interaction between the quarks within the proton, mediating the decay process. Once the decay occurs and the quark changes flavor, the anti-neutrino becomes a real particle and can be detected or observed as an outgoing particle in the decay process.