The positive orthogonal Grassmannian

Abstract: The Pl\"ucker positive region $\mathrm{OGr}+(k,2k)$ of the orthogonal Grassmannian emerged as the positive geometry behind the ABJM scattering amplitudes. In this paper we initiate the study of the positive orthogonal Grassmannian $\mathrm{OGr}+(k,n)$ for general values of $k,n$. We determine the boundary structure of the quadric $\mathrm{OGr}+(1,n)$ in $\mathbb{P}^{n-1}{+}$ and show that it is a positive geometry. We show that $\mathrm{OGr}+(k,2k+1)$ is isomorphic to $\mathrm{OGr}+(k+1, 2k+2)$ and connect its combinatorial structure to matchings on $[2k+2]$. Finally, we show that in the case $n>2k+1$, the \emph{positroid cells} of $\mathrm{Gr}+(k,n)$ do not induce a CW cell decomposition of $\mathrm{OGr}+(k,n)$.