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_\pi^2)^{h}, 2k\geq h\geq 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 a` 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_\pi^2)^{k-1}, k\geq 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^2\Lambda^3_{\rm QCD}.

Reducing cutoff effects in maximally twisted lattice QCD 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(pi)(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 a 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(pi)(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 mq > a(2)Lambda(3)(QCD).

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

A first study of numerical Monte Carlo simulations with two quark doublets, a mass-degenerate one and a mass-split one, interpreted as u, d, s and c quarks, is carried out in the framework of the twisted mass Wilson lattice formulation. Tuning the bare parameters of this theory is explored on 123·24 and 163·32 lattices at lattice spacings a≃0.20 fm and a≃0.15 fm, respectively.

/pdf/numerical-simulation-of-qcd-with-u-d-s-and-c-quarks-in-the-1jce8lm062.pdf

Numerical simulation of QCD with u, d, s and c quarks in the twisted-mass Wilson formulation

A first study of numerical Monte Carlo simulations with two quark doublets, a mass-degenerate one and a mass-split one, interpreted as u, d, s and c quarks, is carried out in the framework of the twisted mass Wilson lattice formulation. Tuning the bare parameters of this theory is explored on 12^3x24 and 16^3x32 lattices at lattice spacings a=0.20fm and a=0.15fm, respectively.

/pdf/numerical-simulation-of-qcd-with-u-d-s-and-c-quarks-in-the-4a5bta3gvf.pdf

Twisted mass, overlap and Creutz fermions: Cut-off effects at tree-level of perturbation theory

We compare the behavior of overlap fermions, which are chirally invariant, and of Wilson twisted mass fermions at full twist in the approach to the chiral limit. Our quenched simulations reveal that with both formulations of lattice fermions pion masses of O(250 MeV) can be reached in practical applications. Our comparison is done at a fixed value of the lattice spacing a=0.123 fm. A number of quantities are measured such as hadron masses, pseudoscalar decay constants and quark masses obtained from Ward identities. We also determine the axial vector renormalization constants in the case of overlap fermions.

Going chiral: overlap versus twisted mass fermions

We test the scaling behaviour of Wilson, hypercube, maximally twisted mass and overlap fermion actions in dynamical simulations of the 2-dimensional massive Schwinger model. We also present possibilities to simulate overlap fermions dynamically by replacing the exact overlap operator by an approximate version. This approximation is used either as only the guidance Hamiltonian, keeping the exact overlap operator as the accept/reject Hamiltonian or for both, the guidance and accept/reject Hamiltonian in the Hybrid Monte Carlo algorithm. In the latter case we test whether the approximation error can be corrected for by computing the determinant ratio of the exact and the approximate overlap operators stochastically.

/pdf/testing-fermion-actions-scaling-in-the-schwinger-model-515nkpiazr.pdf

Testing Fermion Actions: Scaling in the Schwinger Model

https://arxiv.org/pdf/hep-lat/0509174.pdf

We compare the behavior of overlap fermions, which are chirally invariant, and of Wilson twisted mass fermions at full twist in the approach to the chiral limit. Our quenched simulations reveal that with both formulations of lattice fermions pion masses of (250 MeV) can be reached in practical applications. Our comparison is done at a fixed value of the lattice spacing a 0.123 fm. A number of quantities are measured such as hadron masses, pseudoscalar decay constants and quark masses obtained from Ward identities. We also determine the axial vector renormalization constants in the case of overlap fermions.

Going chiral: Overlap versus twisted mass fermions

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Going chiral: overlap versus twisted mass fermions

Testing Fermion Actions: Scaling in the Schwinger Model

Testing Fermion Actions: Scaling in the Schwinger Model

Going chiral: Overlap versus twisted mass fermions