Spin(7) and generalized SO(8) instantons in eight dimensions

We present a simple compact formula for a topologically nontrivial map $S^7 \to Spin(7)$ associated with the fiber bundle $Spin(7) \stackrel{G_2}{\to} S^7$. The homotopy group $\pi_7[Spin(7)] = \mathbb{Z}$ brings about the topologically nontrivial 8-dimensional gauge field configurations that belong to the algebra $spin(7)$. The instantons are special such configurations that minimize the functional $\int {\rm Tr} \{F\wedge F \wedge \star(F \wedge F)\}$ and satisfy non-linear self-duality conditions, $F \wedge F \ =\ \pm \star (F\wedge F)$. $Spin(7) \subset SO(8)$, and $Spin(7)$ instantons represent simultaneously $SO(8)$ instantons of a new type. The relevant homotopy is $\pi_7[SO(8)] = \mathbb{Z} \times \mathbb{Z}$, which implies the existence of two different topological charges. This also holds for all groups $SO(4n)$ with integer $n$. We present explicit expressions for two topological charges and calculate their values for the conventional 4-dimensional and 8-dimensional instantons and also for the 8-dimensional instantons of the new type. Similar constructions for other algebras in different dimensions are briefly discussed.