Bastian Knippschild

Nucleon axial charge in lattice QCD with controlled errors

We report on our calculation of the nucleon axial charge gA in QCD with two flavors 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 ¼ 1:223 � 0:063ðstatÞ þ0:035 � 0:060 ðsystÞ, in good agreement with the experimental value.

First physics results at the physical pion mass from N f = 2 Wilson twisted mass fermions at maximal twist

We present physics results from simulations of QCD using Nf 1⁄4 2 dynamical Wilson twisted mass fermions at the physical value of the pion mass. These simulations are enabled by the addition of the clover term to the twisted mass quark action. We show evidence that compared to previous simulations without this term, the pion mass splitting due to isospin breaking is almost completely eliminated. Using this new action, we compute the masses and decay constants of pseudoscalar mesons involving the dynamical up and down as well as valence strange and charm quarks at one value of the lattice spacing, a ≈ 0.09 fm. Further, we determine renormalized quark masses as well as their scale-independent ratios, in excellent agreement with other lattice determinations in the continuum limit. In the baryon sector, we show that the nucleon mass is compatible with its physical value and that the masses of the Δ baryons do not show any sign of isospin breaking. Finally, we compute the electron, muon and tau lepton anomalous magnetic moments and show the results to be consistent with extrapolations of older ETMC data to the continuum and physical pion mass limits. We mostly find remarkably good agreement with phenomenology, even though we cannot take the continuum and thermodynamic limits.

Scale setting via the Omega baryon mass

We present the first results of an ongoing effort to determine the lattice scale on the N_f=2 CLS lattice ensembles via the mass of the \Omega\ baryon. Results from different methods are compared, and various sources of systematic uncertainty are discussed.

Hadron-hadron interactions from N f = 2 + 1 + 1 lattice QCD: isospin-2 ππ scattering length

A bstractWe present results for the I = 2 ππ scattering length using Nf = 2 + 1 + 1 twisted mass lattice QCD for three values of the lattice spacing and a range of pion mass values. Due to the use of Laplacian Heaviside smearing our statistical errors are reduced compared to previous lattice studies. A detailed investigation of systematic effects such as discretisation effects, volume effects, and pollution of excited and thermal states is performed. After extrapolation to the physical point using chiral perturbation theory at NLO we obtain Mπa0 = − 0.0442(2)stat(− 0+ 4)sys.

Systematic errors in extracting nucleon properties from lattice QCD

Form factors of the nucleon have been extracted from experiment with high precision. However, lattice calculations have failed so far to reproduce the observed dependence of form factors on the momentum transfer. We have embarked on a program to thoroughly investigate systematic effects in lattice calculation of the required three-point correlation functions. Here we focus on the possible contamination from higher excited states and present a method which is designed to suppress them. Its effectiveness is tested for several baryonic matrix elements, different lattice sizes and pion masses.

Higgs-Yukawa model in chirally-invariant lattice field theory

Nonperturbative numerical lattice studies of the Higgs-Yukawa sector of the standard model with exact chiral symmetry are reviewed. In particular, we discuss bounds on the Higgs boson mass at the standard model top quark mass and in the presence of heavy fermions. We present a comprehensive study of the phase structure of the theory at weak and very strong values of the Yukawa coupling as well as at nonzero temperature.

A lattice study of a chirally invariant Higgs–Yukawa model including a higher dimensional Φ6-term

Abstract We discuss the non-thermal phase structure of a chirally invariant Higgs–Yukawa model on the lattice in the presence of a higher dimensional Φ 6 -term. For the exploration of the phase diagram we use analytical, lattice perturbative calculations of the constraint effective potential as well as numerical simulations. We also present first results of the effects of the Φ 6 -term on the lower Higgs boson mass bounds.

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.

A high-statistics study of the nucleon EM form factors, axial charge and quark momentum fraction

We present updated results for the nucleon axial charge and electromagnetic (EM) form factors, which include a significant increase in statistics for all ensembles (up to 4000 measurements), as well as the addition of ensembles with pion masses down to

Excited state systematics in extracting nucleon electromagnetic form factors

m_\pi\sim195