arXiv subjects

M. Della Morte

Publications and source records attributed to M. Della Morte.

At least 37 records · Page 2Linked to original sources

arXiv2012

The nucleon axial charge from lattice QCD with controlled errors

We report on our calculation of the nucleon axial charge gA in QCD with two flavours of dynamical quarks. A detailed investigation of systematic errors is performed, with a particular focus on contributions from excited states to three-point correlation functions. The use of summed operator insertions allows for a much better control over such contamination. After performing a chiral extrapolation to the physical pion mass, we find gA=1.223 +/- 0.063 (stat) +0.035 -0.060 (syst), in good agreement with the experimental value.

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arXiv2011

Form factors in lattice QCD

Lattice simulations of QCD have produced precise estimates for the masses of the lowest-lying hadrons which show excellent agreement with experiment. By contrast, lattice results for the vector and axial vector form factors of the nucleon show significant deviations from their experimental determination. We present results from our ongoing project to compute a variety of form factors with control over all systematic uncertainties. In the case of the pion electromagnetic form factor we employ partially twisted boundary conditions to extract the pion charge radius directly from the linear slope of the form factor near vanishing momentum transfer. In the nucleon sector we focus specifically on the possible contamination from contributions of higher excited states. We argue that summed correlation functions offer the possibility of eliminating this source of systematic error. As an illustration of the method we discuss our results for the axial charge, gA, of the nucleon.

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arXiv2010

Wilson fermions at fine lattice spacings: scale setting, pion form factors and (g-2)_mu

We present an update on our on-going project to compute hadronic observables for Nf=2 flavours of O(a) improved Wilson fermions at small lattice spacings. The procedure to determine the lattice scale via the mass of the Omega baryon is described. Furthermore we present preliminary results for the pion form factor computed using twisted boundary conditions, and report on the implementation of a novel approach to determine the contribution of the hadronic vacuum polarisation to the anomalous magnetic moment of the muon.

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arXiv2009

Mesonic and baryonic correlation functions at fine lattice spacings

We report on our on-going project to compute mesonic and baryonic two- and three-point correlation functions in simulations using Nf=2 flavours of O(a) improved Wilson quarks and the Wilson plaquette action. We present performance figures for the DD-HMC algorithm on commodity cluster hardware and discuss the issue of critical slowing down, which is particularly pronounced for the topological charge. The effectiveness of stochastic noise sources and Jacobi smearing are investigated. Our preliminary results obtained at three quark masses on 96x48^3 at beta=5.5 imply that the lattice spacing is about 0.06 fm, while the smallest pion mass in the current runs is around 360 MeV, which corresponds to m_pi*L=5.3.

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arXiv2008

Efficient use of the Generalized Eigenvalue Problem

We analyze the systematic errors made when using the generalized eigenvalue problem to extract energies and matrix elements in lattice gauge theory. Effective theories such as HQET are also discussed. Numerical results are shown for the extraction of ground-state and excited B-meson masses and the ground-state decay constant in the static approximation.

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arXiv2007

Preparing for N_f=2 simulations at small lattice spacings

We discuss some large effects of dynamical fermions. One is a cutoff effect, others concern the contribution of multi-pion states to correlation functions and are expected to survive the continuum limit. We then turn to the preparation for simulations at small lattice spacings which we are planning down to around a=0.04fm in order to understand the size of O(a^2)-effects of the standard O(a)-improved theory. The dependence of the lattice spacing on the bare coupling is determined through the Schr"odinger functional renormalized coupling.

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arXiv2007

Non-perturbative improvement of the axial current with three dynamical flavors and the Iwasaki gauge action

We perform a non-perturbative determination of the improvement coefficient c_A to remove O(a) discretization errors in the axial vector current in three-flavor lattice QCD with the Iwasaki gauge action and the standard O$(a)$-improved Wilson quark action. An improvement condition with a good sensitivity to c_A is imposed at constant physics. Combining our results with the perturbative expansion, c_A is now known rather precisely for 1/a \gtrsim 1.6 GeV.

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arXiv2005

Heavy quark propagators with improved precision using domain decomposition

We show that, in four dimensions, the quark propagator is affected by round-off errors for large values of the quark mass am and the time extent T/a even when double precision arithmetics is used to compute it. We introduce a definition of the solver residual which is sensitive to the problem and apply a Schwarz alternating procedure to compute the propagator in a number of sub-domains in the time direction. The effectiveness of the method is demonstrated in a numerical computation of the free one-dimensional Dirac propagator.

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arXiv2005

Lattice HQET with exponentially improved statistical precision

We introduce an alternative discretization for static quarks on the lattice retaining the O(a) improvement properties of the Eichten-Hill action. In this formulation, statistical fluctuations are reduced by a factor which grows exponentially with Euclidean time, x_0. For the first time, B-meson correlation functions are computed with good statistical precision in the static approximation for x_0>1 fm. At lattice spacings a \approx 0.1 fm, a \approx 0.08 fm and a \approx 0.07 fm the B_s-meson decay constant is determined in static and quenched approximations. A correction due to the finite mass of the b-quark is estimated by combining these static results with a recent determination of F_Ds.

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arXiv2004

Cutoff-effects in the spectrum of dynamical Wilson fermions

We investigate the low-lying eigenvalues of the improved Wilson-Dirac operator in the Schroedinger functional with two dynamical quark flavors. At a lattice spacing of approximately 0.1 fm we find more very small eigenvalues than in the quenched case. These cause problems with HMC-type algorithms and in the evaluation of fermionic correlation functions. Through a simulation at a finer lattice spacing we are able to establish their nature as cutoff-effects

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arXiv2004

The locality problem for two tastes of staggered fermions

We address the locality problem arising in simulations, which take the square root of the staggered fermion determinant as a Boltzmann weight to reduce the number of dynamical quark tastes from four to two. We study analytically and numerically the square root of the staggered fermion operator as a candidate to define a two taste theory from first principles. Although it has the correct weight, this operator is non-local in the continuum limit. Our work serves as a warning that fundamental properties of field theories might be violated when employing blindly the square root trick. The question, whether a local operator reproducing the square root of the staggered fermion determinant exists, is left open.

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arXiv2004

Locality with staggered fermions

We address the locality problem arising in simulations, which take the square root of the staggered fermion determinant as a Boltzmann weight to reduce the number of dynamical quark tastes. A definition of such a theory necessitates an underlying local fermion operator with the same determinant and the corresponding Green's functions to establish causality and unitarity. We illustrate this point by studying analytically and numerically the square root of the staggered fermion operator. Although it has the correct weight, this operator is non-local in the continuum limit. Our work serves as a warning that fundamental properties of field theories might be violated when employing blindly the square root trick. The question, whether a local operator reproducing the square root of the staggered fermion determinant exists, is left open.

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arXiv2003

Large cutoff effects of dynamical Wilson fermions

We present and discuss results for cutoff effects in the PCAC masses and the mass dependence of r_0 for full QCD and various fermion actions. Our discussion of how one computes mass dependences - here of r_0 - is also relevant for comparisons with chiral perturbation theory.

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arXiv2003

Exploring two non-perturbative definitions of $c_A$

We present two determinations of the coefficient $c_A$ in quenched QCD, needed to build the $O(a)$ improved axial current. The first condition used is the requirement that the PCAC quark mass, as a function of $x_0$, stays flat for a non-trivial spatial phase for the fermions in the Schrödinger functional. The second condition is that the PCAC relation for the ground-state and the first excited state at finite L give the same quark mass. Our results confirm previous findings that in the quenched theory the intrinsic $O(a)$ ambiguity of $c_A$ gets relevant around $β\simeq6.0$.

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arXiv2003

Towards a precision computation of f_Bs in quenched QCD

We present a computation of the decay constant f_Bs in quenched QCD. Our strategy is to combine new precise data from the static approximation with an interpolation of the decay constant around the charm quark mass region. This computation is the first step in demonstrating the feasability of a strategy for f_B in full QCD. The continuum limits in the static theory and at finite mass are taken separately and will be further improved.

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arXiv2003

Static quarks with improved statistical precision

We present a numerical study for different discretisations of the static action, concerning cut-off effects and the growth of statistical errors with Euclidean time. An error reduction by an order of magnitude can be obtained with respect to the Eichten-Hill action, for time separations beyond 1.3 fm, keeping discretization errors small. The best actions lead to a big improvement on the precision of the quark mass Mb and F_Bs in the static approximation.

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arXiv2003

Simulating the Schroedinger functional with two pseudo-fermions: algorithmic study and the running mass

We present an algorithmic study for the simulation of two massless flavors of O(a) improved Wilson quarks with Schroedinger functional boundary conditions. The algorithm used is Hybrid Monte Carlo with two pseudo-fermion fields as proposed by M. Hasenbusch. A gain in CPU cost of a factor two is reached when compared to one pseudo-fermion field due to the larger possible step-size. This study is integrated in the ALPHA project for the computation of the running of the renormalized quark mass. We include an update on these physics results.

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arXiv2003

Simulating the Schroedinger functional with two pseudo-fermions

We report on simulations with two flavors of O(a) improved degenerate Wilson fermions with Schroedinger functional boundary conditions. The algorithm which is used is Hybrid Monte Carlo with two pseudo-fermion fields as proposed by M. Hasenbusch. We investigate the numerical precision and sensitivity to reversibility violations of this algorithm. A gain of a factor two in CPU cost is reached compared with one pseudo-fermion field due to the larger possible step-size.

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