6.1 The quark model predicts the existence of baryons and mesons.
9.2 The gauge group of QCD is $SU(3)$.
2.1 The first particle accelerator was built in 1930 by Cockcroft and Walton.
5.1 The quark content of the proton is $uud$.
9.1 The gauge group of QED is $U(1)$.