Spectral Description of the Spin Ruijsenaars-Schneider System

Abstract: Fix a Weierstrass cubic curve $E$, and an element $\sigma$ in the Jacobian variety $\mathrm{Jac}\, E$ corresponding to the line bundle $\mathcal L_\sigma$. We introduce a space $\mathsf{RS}{\sigma, n}(E, V)$ of pure 1-dimensional sheaves living in $S\sigma = \mathbb{P}(\mathcal O \oplus \mathcal L_\sigma)$ together with framing data at the $0$ and $\infty$ sections $E_0, E_\infty \subset S_\sigma $. For a particular choice of $V$, we show that the space $\mathsf{RS}{\sigma, n}(E, V)$ is isomorphic to a completed phase space for the spin Ruijsenaars-Schneider system, with the Hamiltonian vector fields given by tweaking flows on sheaves at their restrictions to $E_0$ and $E\infty$. We compare this description of the RS system to the description of the Calogero-Moser system in arXiv:math/0603722, and show that the two systems can be assembled into a universal system by introducing a $\sigma \rightarrow 0$ limit to the CM phase space. We also shed some light on the effect of Ruijsenaars' duality between trigonometric CM and rational RS spectral curves coming from the two descriptions in terms of supports of spectral sheaves.