Quarks have never been observed in isolation or detected as free particles outside of composite particles such as protons and neutrons. This property of quarks, called confinement, is a fundamental aspect of the strong nuclear force, which binds quarks together.
The strong nuclear force is so strong that when you try to separate two quarks, the energy required to do so increases rapidly. As you try to pull them apart, the energy stored in the gluon field between them also increases, and at some point, it becomes more energetically favorable to create a new quark-antiquark pair rather than continue separating the original quarks. This phenomenon is known as color confinement.
Due to confinement, quarks are always found in bound states, forming composite particles known as hadrons, which include protons, neutrons, and various other particles. In particle accelerators and colliders, the experiments involve colliding particles at high energies, but the quarks within these particles remain confined and cannot be isolated or observed as free particles.
Scientists study the properties and interactions of quarks indirectly by observing the behavior of the composite particles they are part of and analyzing the patterns and signatures in the resulting particle collisions. This approach provides valuable insights into the nature of quarks and the strong nuclear force, even though the quarks themselves cannot be directly detected in isolation.