NNLO unquenched calculation of the b quark mass

By combining the first unquenched lattice computation of the B-meson binding energy with the recently calculated two-loop contribution to the lattice HQET mass, we determine the $\overline{MS}$ b-quark mass, ${\bar{m}}_b({\bar{m}}_b)$ at the NNLO. We find $\bar{m}{b}(\bar{m}{b}) = (4.26\pm 0.03 \pm 0.04 \pm 0.05) {\rm GeV}$. The inclusion of the two-loop effects is one of the steps necessary to extract ${\bar{m}}_b({\bar{m}}_b)$ with a precision better than ${\cal O}(\Lambda_{QCD})$, which is the uncertainty due to the presence of an IR renormalon singularity in the perturbative series of the residual mass. Our results have been obtained on a sample of 60 lattices of size ${24}^3\times 40$ at $\beta =5.6$, using the unquenched Wilson action with two degenerate sea quarks. The quark propagators have been computed using the unquenched links generated by the T$\chi$L Collaboration.