9.1 The gauge group of QED is $U(1)$.
5.1 The quark content of the proton is $uud$.
2.3 The $\pi$ meson was discovered in 1947 by Cecil Powell.
7.2 The $W$ and $Z$ bosons mediate the weak interaction.
2.1 The first particle accelerator was built in 1930 by Cockcroft and Walton.
2.2 The concept of the neutrino was first proposed by Pauli in 1930.