Chirally improving Wilson fermions I. O(a) improvement

Abstract

We show that it is possible to improve the chiral behaviour and the approach to the continuum limit of correlation functions in lattice QCD with Wilson fermions by taking arithmetic averages of correlators computed in theories regularized with Wilson terms of opposite sign. Improved hadronic masses and matrix elements can be obtained by similarly averaging the corresponding physical quantities separately computed within the two regularizations. To deal with the problems related to the spectrum of the Wilson-Dirac operator, which are particularly worrisome when Wilson and mass terms are such as to give contributions of opposite sign to the real part of the eigenvalues, we propose to use twisted-mass lattice QCD for the actual computation of the quantities taking part to the averages. The choice ±π/2 for the twisting angle is particularly interesting, as O(a) improved estimates of physical quantities can be obtained even without averaging data from lattice formulations with opposite Wilson terms. In all cases little or no extra computing power is necessary, compared to simulations with standard Wilson fermions or twisted-mass lattice QCD.

Kaon mixing beyond the SM from Nf = 2 tmQCD and model independent constraints from the UTA

“…In particular we introduce maximally twisted (Mtm) sea quarks [11] that we take in combination with Osterwalder-Seiler (OS) [38] valence quarks. This strategy has been suggested in ref.…”

“…In section 3, based on the results of this work for the ∆S = 2 B-parameters, we discuss the implications for NP of our updated Unitarity Triangle (UT) analysis [9]. In section 4 we illustrate the main theoretical features of the lattice setup employed in our simulations (twisted mass lattice QCD [10,11]) and we describe the strategy for obtaining accurate numerical estimates of the B-parameters as well as ratios of kaon four-fermion matrix elements. In section 5 we collect our numerical results.…”

Lattice QCD determination of m b , f B and f Bs with twisted mass Wilson fermions

“…The calculation is based on the N f = 2 gauge field configurations generated by the European Twisted Mass (ETM) Collaboration with the tree-level improved Symanzik gauge action [9] and the twisted mass quark action [10] at maximal twist, discussed in detail in [11][12][13][14][15][16][17]. We simulated N f = 2 mass-degenerate dynamical quarks, whose mass is eventually extrapolated to the physical isospin averaged mass of the up and down quarks, m u/d .…”

“…The strange and charm quarks are quenched in the present calculation. In our lattice setup all physical quantities are O(a) improved [16,17], in particular cutoff effects related to the heavy quark mass µ h are of order a 2 µ 2 h . For further details of our simulations we refer to [18], where the same ensembles of gauge configurations were used.…”

“…The factor C stat A (µ * b ,μ h ), defined as provides the matching between the decay constant in QCD for a heavy quark massμ h and in HQET, and the running of the static axial current to the renormalization scale µ * b , and it is known up to N 2 LO in PT [26]. The NLO expression used in the present analysis reads 16) with N f = 2. In order to have better control on the chiral extrapolation, we consider as primary quantities in the present analysis the decay constant f Bs , whose dependence on the light quark mass only occurs through sea effects, and the ratio f Bs /f B which provides a direct determination of the SU(3) breaking effect in the decay constant.…”