Page "Strong interaction" Paragraph 12

The elementary quark and gluon particles affected are unobservable directly, but instead emerge as jets of newly created hadrons, whenever energy is deposited into a quark-quark bond, as when a quark in a proton is struck by a very fast quark ( in an impacting proton ) during a particle accelerator experiment.

However, quark-gluon plasmas have been observed.