Reducing cutoff effects in maximally twisted LQCD close to the chiral limit

When analyzed in terms of the Symanzik expansion, lattice correlators of multi-local (gauge-invariant) operators with non-trivial continuum limit exhibit in maximally twisted lattice QCD ''infrared divergent'' cutoff effects of the type a 2k/(m π 2) h, 2k h 1 (k,h integers), which tend to become numerically large when the pion mass gets small. We prove that, if the action is O(a) improved la Symanzik or, alternatively, the critical mass counter-term is chosen in some ''optimal'' way, these lattice artifacts are reduced to terms that are at worst of the order a 2(a 2/m π 2) k-1, k 1. This implies that the continuum extrapolation of lattice results is smooth at least down to values of the quark mass, m q, satisfying the order of magnitude inequality m q>a 2Λ 3 QCD. © SISSA 2006.