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Asymptotics of spin-spin correlators weighted by fermion number measurements with low rapidity threshold in the 2D Ising free-fermion QFT

In the work, we study the averaged number of massive fermions above a low rapidity threshold $Y$, underlying the form-factor expansions of the spin-spin two-point correlators at an Euclidean distance $r$, in the 2D Ising QFT at the free massive fermion point. Despite the on-shell freeness, the spin operators are still far away from being Gaussian, and create particles in the asymptotic states with complicated correlations. We show how the number observables can still be incorporated into the integrable Sinh-Gordon/Painleve-III framework and controlled by linear differential equations with two variables $(r,Y)$. We show how the differential equations and the information of two crucial scaling functions arising in the $r\rightarrow 0$, $e^{Y}r={\cal O}(1)$ scaling limit, can be combined to fully determine the small-$r$ asymptotics of the observables, in the $λ$-extended form. The scaling functions, on the other hand, are analyzed by summing the exponential form-factor expansions directly, generalizing the traditional Ising connecting computations. We show carefully, how the singularities cancel in the physical value limit $λπ\rightarrow 1$ and how the power-corrections that collapse at this value can be resummed. In particular, we show for the physical $λ$-value, the scaling functions are related to an integrated four-point function in the Ising CFT and continue to control the asymptotics of the number-observables in the scaling limit up to ${\cal O}(r^3)$.