Fourth Generation Leptons and Muon $g-2$

We consider the contributions to $g_{\mu }-2$ from fourth generation heavy neutral and charged leptons, $N$ and $E$, at the one-loop level. Diagrammatically, there are two types of contributions: boson-boson-$N$, and $E$-$E$-boson in the loop diagram. In general, the effect from $N$ is suppressed by off-diagonal lepton mixing matrix elements. For $E$, we consider flavor changing neutral couplings arising from various New Physics models, which are stringently constrained by $\mu \to e\gamma$. We assess how the existence of a fourth generation would affect these New Physics models.