Wilson chiral perturbation theory for dynamical twisted mass fermions vs lattice data-A case study

We compute the low lying eigenvalues of the Hermitian Dirac operator in lattice QCD with N-f = 2 + 1+ 1 twisted mass fermions. We discuss whether these eigenvalues are in the epsilon-regime or the p-regime of Wilson chiral perturbation theory (chi PT) for twisted mass fermions. Reaching the deep epsilon-regime is practically unfeasible with presently typical simulation parameters, but still the few lowest eigenvalues of the employed ensemble evince some characteristic epsilon-regime features. With this conclusion in mind, we develop a fitting strategy to extract two low energy constants from analytical epsilon-regime predictions at a fixed index. Thus, we obtain results for the chiral condensate and the low energy constant W-8. We also discuss how to improve both the theoretical calculation and the lattice computation. (C) 2018 Elsevier B.V. All rights reserved.