Four-dimensional spectrum generating algebra for the superstring

We derive the spectrum generating algebra for the hybrid string with manifest $\mathcal{N}=1$ super Poincaré symmetry in ${\mathbb{R}}^{3,1}\times {\mathbb{R}}^6$. Our DDF operators establish a one-to-one correspondence with the conventional superstring spectrum, while making its four-dimensional structure manifest. We also discuss part of the antifield spectrum, and introduce a simple realization of the supersymmetry charges involving the ${\mathbb{R}}^6$ directions. As an application, we algebraically compute the helicity partition function. These results provide new tools for analyzing four-dimensional on-shell superspaces and may extend naturally to phenomenologically relevant compactifications.