The pion mass in finite volume

We determine the relative pion mass shift \(M_\pi(L)/M_\pi-1\) due to the finite spatial extent L of the box by means of two-flavor chiral perturbation theory and the one-particle Lüscher formula. We use as input the expression for the infinite volume \(\pi \pi\) forward scattering amplitude up to next-to-next-to-leading order and can therefore control the convergence of the chiral series. A comparison to the full leading order chiral expression for the pion mass in finite volume allows us to check the size of subleading terms in the large-L expansion.