[Show abstract][Hide abstract] ABSTRACT: The motivations for simulating QCD thermodynamics with Wilson fermions and a twisted mass term are introduced. The twisted mass approach provides a natural infrared cutoff and O(a) improvement at maximal twist, and can be extended to finite temperature. Our strategy for exploring the QCD phase diagram at finite temperature in this setup, while taking advantage of the results at T=0, is explained. The first results for the order parameters and susceptibilities on a 16^3 x 8 lattice are presented. All dynamical simulations are carried out on the apeNEXT facility in Rome.
[Show abstract][Hide abstract] ABSTRACT: We present a comparison of a number of iterative solvers of linear systems of equations for obtaining the fermion propagator in lattice QCD. In particular, we consider chirally invariant overlap and chirally improved Wilson (maximally) twisted mass fermions. The comparison of both formulations of lattice QCD is performed at four fixed values of the pion mass between 230MeV and 720MeV. For overlap fermions we address adaptive precision and low mode preconditioning while for twisted mass fermions we discuss even/odd preconditioning. Taking the best available algorithms in each case we find that calculations with the overlap operator are by a factor of 30-120 more expensive than with the twisted mass operator.
[Show abstract][Hide abstract] ABSTRACT: Discretisation errors in two-flavour lattice QCD with Wilson quarks and DBW2 gauge action are investigated by comparing numerical
simulation data at two values of the bare gauge coupling. Both non-zero- and zero-twisted-mass values are considered. The
results, including also data from simulations using the Wilson plaquette gauge action, are compared to next-to-leading order
chiral perturbation theory formulas.
European Physical Journal C 07/2006; 47(2):453-472. · 5.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a quenched Wilson twisted mass fermion calculation of the matrix element between pion states of the twist-2 operator, which is related to the lowest moment 〈x〉 of the valence quark parton distribution function in a pion. Using Wilson twisted mass fermions we demonstrate that 〈x〉 can be computed at small pseudoscalar meson masses down to values of order 250 MeV. We investigate the scaling behaviour of this physically important quantity by applying two definitions of the critical mass and observe a scaling compatible with the expected O(a2) behaviour in both cases. A combined continuum extrapolation allows to obtain reliable results for 〈x〉 at very small pseudoscalar meson masses, which previously could not be explored by lattice QCD simulations.
[Show abstract][Hide abstract] ABSTRACT: We show at the example of the matrix element between pion states of a twist-2, non-singlet operator that Wilson twisted mass fermions allow to compute this phenomenologically relevant quantitiy at small pseudo scalar masses of O(270 MeV). In the quenched approximation, we investigate the scaling behaviour of this observable that is derived from a 3-point function by applying two definitions of the critical mass and find a scaling compatible with the expected O(a^2) behaviour in both cases. A combined continuum extrapolations allows to obtain reliable results at small pion masses, which previously could not be explored by lattice QCD simulations.
[Show abstract][Hide abstract] ABSTRACT: The running of renormalized quark masses is computed in lattice QCD with two flavors of massless O(a) improved Wilson quarks. The regularization and flavor independent factor that relates running quark masses to the renormalization group invariant ones is evaluated in the Schrödinger functional scheme. Using existing data for the scale r0 and the pseudoscalar meson masses, we define a reference quark mass in QCD with two degenerate quark flavors. We then compute the renormalization group invariant reference quark mass at three different lattice spacings. Our estimate for the continuum value is converted to the strange quark mass with the help of chiral perturbation theory.
[Show abstract][Hide abstract] ABSTRACT: We present the results of an extended scaling test of quenched Wilson twisted mass QCD. We fix the twist angle by using two definitions of the critical mass, the first obtained by requiring the vanishing of the pseudoscalar meson mass m_PS for standard Wilson fermions and the second by requiring restoration of parity at non-zero value of the twisted mass mu and subsequently extrapolating to mu=0. Depending on the choice of the critical mass we simulate at values of beta in [5.7,6.45], for a range of pseudoscalar meson masses 250 MeV < m_PS < 1 GeV and we perform the continuum limit for the pseudoscalar meson decay constant f_PS and various hadron masses (vector meson m_V, baryon octet m_oct and baryon decuplet m_dec) at fixed value of r_0 m_PS. For both definitions of the critical mass, lattice artifacts are consistent with O(a) improvement. However, with the second definition, large O(a^2) discretization errors present at small quark mass with the first definition are strongly suppressed. The results in the continuum limit are in very good agreement with those from the Alpha and CP-PACS Collaborations.
[Show abstract][Hide abstract] ABSTRACT: We summarize four contributions about dynamical twisted mass fermions. The resulting report covers results for N_f=2 obtained from three different gauge actions, namely the standard Wilson plaquette gauge action, the DBW2 and the tree-level Symanzik improved gauge action. In addition, first results for N_f=2+1+1 flavours of twisted mass fermions are discussed.
[Show abstract][Hide abstract] ABSTRACT: The non-perturbative running of the quark mass in the Schroedinger functional scheme is computed over a large energy range (covering scales differing by two orders of magnitude). This allows to relate lattice estimates of the running quark mass to the renormalization group invariant mass. The result is used in a preliminary computation of the strange quark mass in the theory with two flavors of non-perturbatively improved Wilson quarks. A more detailed discussion of the calculation can be found in .
[Show abstract][Hide abstract] ABSTRACT: Twisted mass fermions allow light quarks to be explored but with the consequence that there are mass splittings, such as between the neutral and charged pion. Using a direct calculation of the connected neutral pion correlator and stochastic methods to evaluate the disconnected correlations, we determine the neutral pion mass. We explore the dependence on lattice spacing and quark mass in quenched QCD. For dynamical QCD, we determine the sign of the splitting which is linked, via chiral PT, to the nature of the phase transition at small quark mass.
[Show abstract][Hide abstract] ABSTRACT: We study the flavour breaking effects appearing in the Wilson twisted mass formulation of lattice QCD. In this quenched study, we focus on the mass splitting between the neutral and the charged pion, determining the neutral pion mass with a stochastic noise method to evaluate the disconnected contributions. We find that these disconnected contributions are significant. Using the Osterwalder-Seiler interpretation of the connected piece of the neutral pion correlator, we compute the corresponding neutral pion mass to study with more precision the scaling behaviour of the mass splitting.
[Show abstract][Hide abstract] ABSTRACT: We investigate the scaling behaviour of quenched Wilson twisted mass fermions at maximal twist applying two definitions of the critical mass. The first definition uses the vanishing of the pseudoscalar meson mass m_PS while the second employs the vanishing of the PCAC quark mass m_PCAC. We confirm in both cases the expected O(a) improvement. In addition, we show that the PCAC quark mass definition leads to substantially reduced O(a^2) cut-off effects even when the pseudoscalar meson mass m_PS is as small as 270 MeV. At a fixed value of m_PS we perform continuum limits for the vector meson mass m_V and for the pseudoscalar decay constant f_PS and discuss the renormalisation constant Z_V of the vector current. Comment: 20 pages, 5 figures
[Show abstract][Hide abstract] ABSTRACT: Lattice QCD with Wilson fermions generically shows the phenomenon of a first order phase transition. We study the phase structure of lattice QCD using Wilson twisted mass fermions and the Wilson plaquette gauge action in a range of β values where such a first order phase transition is observed. In particular, we investigate the dependence of the first order phase transition on the value of the lattice spacing. Using only data in one phase and neglecting possible problems arising from the phase transition we are able to perform a first scaling test for physical quantities using this action.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: The phase structure of zero temperature twisted mass lattice QCD is investigated. We find strong metastabilities in the plaquette observable in correspondence of which the untwisted quark mass assumes positive or negative values. We provide interpretations of this phenomenon in terms of chiral symmetry breaking and the effective potential model of Sharpe and Singleton.
European Physical Journal C 02/2005; 39(4):421-433. · 5.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The effect of changing the lattice action for the gluon field on the recently observed [F. Farchioni, R. Frezzotti, K. Jansen, I. Montvay, G.C. Rossi, E. Scholz, A. Shindler, N. Ukita, C. Urbach, I. Wetzorke, Eur. Phys. J. C 39, 421 (2005); hep-lat/0406039] first order phase transition near zero quark mass is investigated by replacing the Wilson plaquette action by the DBW2 action. The lattice action for quarks is unchanged: it is in both cases the original Wilson action. It turns out that Wilson fermions with the DBW2 gauge action have a phase structure where the minimal pion mass and the jump of the average plaquette are decreased, when compared to Wilson fermions with Wilson plaquette action at similar values of the lattice spacing. Taking the DBW2 gauge action is advantageous also from the point of view of the computational costs of numerical simulations.
European Physical Journal C 01/2005; 42(1):73-87. · 5.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a scaling test of twisted mass QCD with pure Wilson quarks for a twisting angle of π/2 for a range of pseudoscalar masses 250MeV≲mPS≲1GeV. We have computed the vector meson mass and the pseudoscalar decay constant fPS for different values of β at fixed value of r0mPS. Our results from Wilson twisted mass QCD seems to indicate that the lattice spacing errors are consistent with O(a) improvement, without the need of any improvement terms added, with an increase of the scaling violation going towards the chiral limit.
[Show abstract][Hide abstract] ABSTRACT: Lattice QCD results on finite temperature meson correlations are presented. They have been obtained from a non-perturbatively improved Wilson fermion action on quenched configurations. The observables discussed are spatial as well as temporal correlation functions for light quarks. From those, screening masses and spectral densities have been extracted, the latter through the use of the Maximum Entropy Method. Some emphasis is laid on the discussion of finite volume and finite lattice spacing effects.
[Show abstract][Hide abstract] ABSTRACT: We review recent progress in studying spectral functions for mesonic observables at finite temperatures, by analysis of imaginary time correlators directly calculated on isotropic lattices. Special attention is paid to the lattice artifacts present in such calculations. Comment: Latex, 3 figures uses iopart.cls, talk presented at Hot Quarks 2004, July 18-24, 2004, Taos Valley New Mexico, USA
Journal of Physics G Nuclear and Particle Physics 12/2004; · 5.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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. Comment: 22 pages, 10 figures
Journal of High Energy Physics 10/2004; · 5.62 Impact Factor