2.3 The $\pi$ meson was discovered in 1947 by Cecil Powell.
7.2 The $W$ and $Z$ bosons mediate the weak interaction.
6.2 The quark model explains the hadronic spectrum.
2.2 The concept of the neutrino was first proposed by Pauli in 1930.
7.1 The weak interaction is responsible for $\beta$ decay.
1.3 The antiparticle of the electron $e^{-}$ is the positron $e^{+}$.