Figure 1: A Feynman diagram represents the decay of a neutral $B$-meson, $B_s^0$, to a pair of muons, ${\mu }^{+}{\mu }^{-}$. The green sphere on the left indicates the meson, which is a bound state of a strange quark and an antibottom quark. These radiate two $W$ bosons and exchange a top quark in the process. The $W$ bosons fuse to a $Z$ boson, which then produces the two muons in the final state.