  By analogy with the formula for the massless string disk amplitudes, we
define massive field-theory tree amplitudes and conjecture that the BRST
cohomology structure of pure spinor superspace fixes their form. We give
evidence by deriving the pure spinor superspace expression of the massive
field-theory n-point tree amplitude with one first-level massive and n-1
massless states in two ways: 1) from BRST cohomology arguments in pure spinor
superspace and 2) from the ${\alpha^\prime}^2$ correction to the massless
string amplitudes by inverting the unitarity constraint in superspace.
