Simplifying and extending the AdS5 x S5 pure spinor formalism

Although the AdS5 x S5 worldsheet action is not quadratic, some features of the pure spinor formalism are simpler in an AdS5 x S5 background than in a flat background. The BRST operator acts geometrically, the left and right-moving pure spinor ghosts can be treated as complex conjugates, the zero mode measure factor is trivial, and the b ghost does not require non-minimal fields. Furthermore, a topological version of the AdS5 x S5 action with the same worldsheet variables and BRST operator can be constructed by gauge-fixing a G/G principal chiral model where G = PSU(2,2|4). This topological model is argued to describe the zero radius limit that is dual to free N = 4 super-Yang-Mills and can also be interpreted as an 'unbroken phase' of superstring theory.