2.3 The $\pi$ meson was discovered in 1947 by Cecil Powell.
9.1 The gauge group of QED is $U(1)$.
6.2 The quark model explains the hadronic spectrum.
6.1 The quark model predicts the existence of baryons and mesons.
2.2 The concept of the neutrino was first proposed by Pauli in 1930.
1.3 The antiparticle of the electron $e^{-}$ is the positron $e^{+}$.
9.2 The gauge group of QCD is $SU(3)$.
7.1 The weak interaction is responsible for $\beta$ decay.