Evaluating the last missing ingredient for the three-loop quark static   potential by differential equations
  We analytically evaluate the three-loop Feynman integral which was the last missing ingredient for the analytical evaluation of the three-loop quark static potential. To evaluate the integral we introduce an auxiliary parameter $y$, which corresponds to the residual energy in some of the HQET propagators. We construct a differential system for 109 master integrals depending on $y$ and fix boundary conditions from the asymptotic behaviour in the limit $y\to \infty$. The original integral is recovered from the limit $y\to 0$. To solve these linear differential equations we try to find an $\epsilon$-form of the differential system. Though this step appears to be, strictly speaking, not possible, we succeed to find an $\epsilon$-form of all irreducible diagonal blocks, which is sufficient for solving the differential system in terms of an $\epsilon$ expansion. We find a solution up to weight six in terms of multiple polylogarithms and obtain an analytical result for the required three-loop Feynman integral by taking the limit $y\to 0$. As a by-product, we obtain analytical results for some Feynman integrals typical for HQET. 
Roman N. Lee and Vladimir A. Smirnov
hep-ph