The scalar pion form factor in two-flavor lattice QCD

Abstract

We calculate the scalar form factor of the pion using two dynamical flavors of non-perturbatively $\mathcal{O}(a)$-improved Wilson fermions, including both the connected and the disconnected contribution to the relevant correlation functions. We employ the calculation of all-to-all propagators using stochastic sources and a generalized hopping parameter expansion. From the form factor data at vanishing momentum transfer, $Q^2=0$, and two non-vanishing $Q^2$ we obtain an estimate for the scalar radius ${⟨r^2⟩}_{{}_{\mathrm{S}}}^{\pi }$ of the pion at one value of the lattice spacing and for five different pion masses. Using Chiral Perturbation Theory at next-to-leading order, we find ${⟨r^2⟩}_{{}_{\mathrm{S}}}^{\pi }=0.635\pm 0.016$ fm${}^2$ at the physical pion mass (statistical error only). This is in good agreement with the phenomenological estimate from $\pi \pi$-scattering. The inclusion of the disconnected contribution is essential for achieving this level of agreement.

Comments: 15 pages, 10 pdf figures, uses revtex4-1; version to appear in PRD