A New Method for Determining $F_π$ on the Lattice

We derive the two-point spectral correlation function of the Dirac operator with a specific external source in the $\epsilon$-regime of QCD. This correlation function has a unique and strong dependence on $F_\pi$, and thus provides an novel way to extract $F_\pi$ from lattice simulations. We test the method in a quenched lattice simulation with staggered fermions.