Quarks are considered real particles rather than virtual particles. Virtual particles are particles that exist as intermediate states in certain interactions or processes in quantum field theory. They are not directly observable and do not persist as stable particles in isolation.
Quarks, on the other hand, are fundamental particles in the Standard Model of particle physics. They are considered real because they are observed as constituents of hadrons (such as protons and neutrons) and can be detected in experiments. Quarks have fractional electric charges and carry color charges, which make them subject to the strong nuclear force mediated by gluons.
However, it is important to note that quarks are confined within hadrons due to a property called color confinement. This means that quarks cannot exist as isolated free particles but are always found in combinations that result in color-neutral objects, such as mesons (quark-antiquark pairs) or baryons (three quarks).
Quark-gluon plasma (QGP), a deconfined state of matter, is an exception where quarks and gluons are temporarily liberated from their confinement within hadrons. QGP is believed to exist at extremely high temperatures and densities, such as those created in high-energy heavy-ion collision experiments.
In summary, quarks are considered real particles due to their presence in observable hadrons and their role in the strong nuclear force. However, their confinement within hadrons and the temporary deconfinement in special conditions like the QGP highlight the fascinating and complex nature of quark behavior.