Light quarks with twisted mass fermions
K. Jansen, M. Papinutto, A. Shindler, C. Urbach, I. Wetzorke
TL;DR
This paper analyzes Wilson twisted mass fermions in the quenched regime and examines how the definition of the critical mass affects automatic $O(a)$-improvement. It advocates a PCAC-based, nonzero-twist method to locate the critical parameter by determining $\kappa_c(\mu a)$ at fixed $\mu a$ where the PCAC mass vanishes and then extrapolating to $\mu a = 0$, aiming to minimize $O(a^2)$ lattice artefacts. The approach enables reliable simulations at small quark masses (down to $m_\pi \simeq 250$ MeV) while keeping discretization effects under control, and it provides explicit formulas for the action, correlators, and observables at maximal twist. These results lay the groundwork for improved chiral behavior and more accurate lattice QCD computations using twisted-mass fermions, with further scaling analyses to be reported separately.
Abstract
We investigate Wilson twisted mass fermions in the quenched approximation using different definitions of the critical bare quark mass m_c to realize maximal twist and, correspondingly, automatic O(a) improvement for physical observables. A particular definition of m_c is given by extrapolating the value of m_c obtained from the PCAC relation at non-vanishing bare twisted quark mass mu to mu=0. Employing this improved definition of the critical mass the Wilson twisted mass formulation provides the possibility to perform reliable simulations down to very small quark masses with correspondingly small pion masses of m_pi \simeq 250 MeV, while keeping the cutoff effects of O(a^2) under control.