A flag version of Beilinson-Drinfeld Grassmannian for surfaces

TL;DR

This paper generalizes the Beilinson-Drinfeld Grassmannian from curves to surfaces using flags of subschemes, establishing formal gluing, simplicial structures, and factorization properties for a new derived geometric object.

Contribution

It introduces a flag-based extension of the Beilinson-Drinfeld Grassmannian for surfaces, including formal gluing results, a simplicial flag structure, and factorization properties.

Findings

Defined a simplicial flag object with 2-Segal property

Constructed a derived Grassmannian for surfaces with factorization

Extended properties of curve Grassmannians to surface case

Abstract

In this paper we define and study a generalization of the Belinson-Drinfeld Grassmannian to the case where the curve is replaced by a smooth projective surface $X$, and the trivialization data are given on loci suitably associated to a nonlinear flag of closed subschemes. In order to do this, we first establish some general formal gluing results for moduli of almost perfect complexes, perfect complexes and torsors. We then construct a simplicial object $Fl_X$ of flags of closed subschemes of a smooth projective surface $X$, naturally associated to the operation of taking union of flags. We prove that this simplicial object has the 2-Segal property. For an affine complex algebraic group $G$, we finally define a derived, flag analog $Gr_X$ of the Beilinson-Drinfeld Grassmannian of $G$-bundles on the surface $X$, and show that most of the properties of the Beilinson-Drinfeld Grassmannian for curves can be extended to our flag generalization. In particular, we prove a factorization formula, the existence of a canonical flat connection, and define a chiral product on suitable sheaves on $Fl_X$ and on $Gr_X$. We also sketch the construction of actions of flags analogs of the loop group and of the positive loop group on $Gr_X$. To fixed ``large'' flags on $X$, we associate ``exotic'' derived structures on the classical stack of $G$-bundles on $X$. Analogs of the flag Grassmannian for other Perf-local stacks (replacing the stack of $G$-bundles) are briefly considered, and flag factorization is proved for them, too.