  We propose a worldsheet formula for tree-level correlation functions
describing a scalar field with arbitrary mass and quartic self-interaction in
de Sitter space, which is a simple model for inflationary cosmology. The
correlation functions are located on the future boundary of the spacetime and
are Fourier-transformed to momentum space. Our formula is supported on
mass-deformed scattering equations involving conformal generators in momentum
space and reduces to the CHY formula for $\phi^4$ amplitudes in the flat space
limit. Using the global residue theorem, we verify that it reproduces the
Witten diagram expansion at four and six points, and sketch the extension to
$n$ points.
