In the Lagrangian of self-dual Yang-Mills theory, an interaction term is added where an axion like field is coupled with a Chern-Simons term to cancel an anomaly in the twistor space. The interaction term is given by
$\mathcal{L}_{int} = \int \rho \textrm{d} CS(A)$
where $\rho$ is the axion field, d$CS(A)=Tr(F(A)^2)$ and $CS(A)$ is the Chern-Simons term which is given by
$CS(A)=Tr(A\wedge dA + \frac{2}{3}A\wedge A\wedge A)$
I want to write the interaction term above in spinor notations. If someone throws some light on it, it will be helpful.