arXiv subjects

C. Urbach

Publications and source records attributed to C. Urbach.

At least 73 records · Page 4Linked to original sources

arXiv2007

Dynamical Twisted Mass Fermions with Light Quarks

We present results of dynamical simulations with 2 flavours of degenerate Wilson twisted mass quarks at maximal twist in the range of pseudo scalar masses from 300 to 550 MeV. The simulations are performed at one value of the lattice spacing a \lesssim 0.1 fm. In order to have O(a) improvement and aiming at small residual cutoff effects, the theory is tuned to maximal twist by requiring the vanishing of the untwisted quark mass. Precise results for the pseudo scalar decay constant and the pseudo scalar mass are confronted with chiral perturbation theory predictions and the low energy constants F, \bar{l}_3 and \bar{l}_4 are evaluated with small statistical errors.

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arXiv2006

Iterative methods for overlap and twisted mass fermions

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.

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arXiv2006

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.

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arXiv2006

Numerical simulations with two flavours of twisted-mass Wilson quarks and DBW2 gauge action

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.

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arXiv2005

Nf=2 Lattice QCD and Chiral Perturbation Theory

By employing a twisted mass term, we compare recent results from lattice calculations of Nf=2 dynamical Wilson fermions with Wilson Chiral Perturbation Theory (WChPT). The final goal is to determine some combinations of Gasser-Leutwyler Low Energy Constants (LECs). A wide set of data with different lattice spacings (a ~ 0.2-0.12 fm), different gauge actions (Wilson plaquette, DBW2) and different quark masses (down to the lowest pion mass allowed by lattice artifacts and including negative quark masses) provide a strong check of the applicability of WChPT in this regime and the scaling behaviours in the continuum limit.

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arXiv2005

Parton Distribution Functions with Twisted Mass Fermions

We present a first Wilson twisted mass fermion calculation of the matrix element between pion states of the twist-2 operator, which is related to the the lowest moment < x > of the valence quark parton distribution function in a pion. Using Wilson twisted mass fermions in the quenched approximation 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(a^2) 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.

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arXiv2005

HMC algorithm with multiple time scale integration and mass preconditioning

We describe a new HMC algorithm variant we have recently introduced and extend the published results by preliminary results of a simulation with a pseudo scalar mass value of about 300 MeV. This new run confirms our expectation that simulations with such pseudo scalar mass values become feasible and affordable with our HMC variant. In addition we discuss simulations from hot and cold starts at a pseudo scalar mass value of about 300 MeV, which we performed in order to test for possible meta-stabilities.

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arXiv2005

3-point functions from twisted mass lattice QCD at small quark masses

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.

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arXiv2005

Scaling test of quenched Wilson twisted mass QCD at maximal twist

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.

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arXiv2005

A (P)HMC algorithm for N_f = 2 + 1 + 1 flavours of twisted mass fermions

We present a detailed design of a (P)HMC simulation algorithm for N_f = 2 + 1 + 1 maximally twisted Wilson quark flavours. The algorithm retains even/odd and mass-shift preconditionings combined with multiple Molecular Dynamics time scales for both the light mass degenerate, u and d, quarks and the heavy mass non-degenerate, s and c, quarks. Various non-standard aspects of the algorithm are discussed, among which those connected to the use of a polynomial approximation for the inverse (square root of the squared) Dirac matrix in the s and c quark sector.

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arXiv2005

Dynamical twisted mass fermions

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.

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arXiv2005

Twisted mass fermions: neutral pion masses from disconnected contributions

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.

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arXiv2005

HMC algorithm with multiple time scale integration and mass preconditioning

We present a variant of the HMC algorithm with mass preconditioning (Hasenbusch acceleration) and multiple time scale integration. We have tested this variant for standard Wilson fermions at beta=5.6 and at pion masses ranging from 380 MeV to 680 MeV. We show that in this situation its performance is comparable to the recently proposed HMC variant with domain decomposition as preconditioner. We give an update of the ``Berlin Wall'' figure, comparing the performance of our variant of the HMC algorithm to other published performance data. Advantages of the HMC algorithm with mass preconditioning and multiple time scale integration are that it is straightforward to implement and can be used in combination with a wide variety of lattice Dirac operators.

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arXiv2005

Flavour Breaking Effects of Wilson twisted mass fermions

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.

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arXiv2005

Quenched Scaling of Wilson twisted mass fermions

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.

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arXiv2005

Lattice Spacing Dependence of the First Order Phase Transition for Dynamical Twisted Mass Fermions

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 are used in a range of beta 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.

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arXiv2005

Light quarks with twisted mass fermions

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.

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arXiv2005

The phase structure of lattice QCD with two flavours of Wilson quarks and renormalization group improved gluons

The effect of changing the lattice action for the gluon field on the recently observed [1] 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.

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