On the motive of the nested Quot scheme of points on a curve

Let $C$ be a smooth curve over an algebraically closed field $\mathbf{k}$, and let $E$ be a locally free sheaf of rank $r$. We compute, for every $d>0$, the generating function of the motives $[$\mathrm{Q}\mathrm{u}\mathrm{o}\mathrm{t}$C(E,\boldsymbol{n} )] \in K_0($\mathrm{V}\mathrm{a}\mathrm{r}${$\mathbf{k}$})$,varying$\boldsymbol{n} = (0\leq n_1\leq\cdots\leq n_d)$,where$$\mathrm{Q}\mathrm{u}\mathrm{o}\mathrm{t}$_C(E,\boldsymbol{n} )$ is the nested Quot scheme of points, parametrising $0$-dimensional subsequent quotients $E \twoheadrightarrow T_d \twoheadrightarrow \cdots \twoheadrightarrow T_1$offixedlength$n_i = \chi(T_i)$. The resulting series, obtained by exploiting the Bialynicki-Birula decomposition, factors into a product of shifted motivic zeta functions of $C$. In particular, it is a rational function.