  Starting with a classical action whose matter variables are a d=10 spacetime
vector $x^m$ and a pure spinor $\lambda^\alpha$, the pure spinor formalism for
the superstring is obtained by gauge-fixing the twistor-like constraint
$\partial x^m (\gamma_m \lambda)_\alpha =0$. The fermionic variables
$\theta^\alpha$ are Faddeev-Popov ghosts coming from this gauge-fixing and
replace the usual (b,c) ghosts coming from gauge-fixing the Virasoro
constraint. After twisting the ghost-number such that $\theta^\alpha$ has
ghost-number zero and $\lambda^\alpha$ has ghost-number one, the BRST
cohomology describes the usual spacetime supersymmetric states of the
superstring.

High Energy Physics - Theory