Are quarks really considered fundamental particles?
Only 6 months ago, I would have said that the evidence is convincing that quarks are the smallest known particles, but the Fermilab ‘CDF’ team has uncovered too many ‘jet’ events during high energy proton-antiproton collisions. They concluded that this is the signal for some kind of particle or particles inside the quark. Other experimenters are not as convinced. The neighboring ‘DZERO’ team at the same facility has looked at 5 times as much data and finds that, overall, the statistics are consistent with the standard quark model with no internal structure at the energies in question. There are theoretical models for quarks having sub-particles called ‘preons’ or ‘rishons’, but these only become detectable at thousands of times the energy to which the DZERO and CDF instruments at Fermilab are sensitive. One possibility for the anomalous jets is that some new particle may have been temporarily created which some dub the Z-prime; a heavier second cousin to the Z-zero particle which helps
