9.1 The gauge group of QED is $U(1)$.
2.1 The first particle accelerator was built in 1930 by Cockcroft and Walton.
7.1 The weak interaction is responsible for $\beta$ decay.
6.2 The quark model explains the hadronic spectrum.
6.1 The quark model predicts the existence of baryons and mesons.
