Manifest Möbius invariance of massive tree-level three-point amplitudes in pure spinor superspace

A bstract Using BRST cohomology properties in pure spinor superspace and identities for OPE brackets of non-free fields, we obtain a new compact nested-bracket representation of massive tree-level three-point open-string amplitudes in which Möbius invariance is manifest. Explicit superspace calculations for amplitudes with level-one massive states confirm this finding, and we derive new BRST recurrence relations among three-point numerators to extend the result to arbitrary mass levels. This provides a manifestly Möbius-invariant expression for massive three-point amplitudes in the pure spinor formalism.