arXiv subjects
Publications and source records attributed to W. Schroers.
The lowest three moments of generalized parton distributions are calculated in full QCD and provide new insight into the behavior of nucleon electromagnetic form factors, the origin of the nucleon spin, and the transverse structure of the nucleon.
We present a lattice measurement of the first two moments of the spin-dependent GPD H-tilde(x,xi,t). From these we obtain the axial coupling constant and the second moment of the spin-dependent forward parton distribution. The measurements are done in full QCD using Wilson fermions. In addition, we also present results from a first exploratory study of full QCD using Asqtad sea and domain-wall valence fermions.
Calculations of the magnetic dipole, electric quadrupole and Coulomb quadrupole amplitudes for the transition $γN\to Δ$ are presented both in quenched QCD and with two flavours of degenerate dynamical quarks.
We present results on the ratio of electric quadrupole to magnetic dipole amplitudes, $R_{EM}={\cal G}_{E2}/{\cal G}_{M1}$, for the transition $γN to Δ$ from lattice QCD. We consider both the quenched and the 2-flavor theory.
We present a report on the status of the GRAL project (Going Realistic And Light), which aims at simulating full QCD with two dynamical Wilson quarks below the vector meson decay threshold, m_ps/m_v < 0.5, making use of finite-size-scaling techniques.
The non-normality of Wilson-type lattice Dirac operators has important consequences - the application of the usual concepts from the textbook (hermitian) quantum mechanics should be reconsidered. This includes an appropriate definition of observables and the refinement of computational tools. We show that the truncated singular value expansion is the optimal approximation to the inverse operator D^{-1} and we prove that due to the gamma_5-hermiticity it is equivalent to gamma_5 times the truncated eigenmode expansion of the hermitian Wilson-Dirac operator.
We report about a simulation using three dynamical Wilson quarks and on the progress in going to small quark masses.
As part of a systematic algorithm study, we present first results on a performance comparison between a multibosonic algorithm and the hybrid Monte Carlo algorithm as employed by the SESAM collaboration. The standard Wilson fermion action is used on 32*16^3 lattices at beta=5.5.
We consider the local chirality of near-zero eigenvectors from Wilson-Dirac and clover improved Wilson-Dirac lattice operators as proposed recently by Horváth et al. We studied finer lattices and repaired for the loss of orthogonality due to the non-normality of the Wilson-Dirac matrix. As a result we do see a clear double peak structure on lattices with resolutions higher than 0.1 fm. We found that the lattice artifacts can be considerably reduced by exploiting the biorthogonal system of left and right eigenvectors. We conclude that the dominance of instantons on topological charge fluctuations is not ruled out by local chirality measurements.
We develop and discuss in technical detail an infrared-finite factorization and optimized renormalization scheme for calculating exclusive processes, which enables the inclusion of transverse degrees of freedom without entailing suppression of calculated observables, like form factors. This is achieved by employing an analytic, i.e., infrared stable, running strong coupling $α_{\rm s}(Q^{2})$ which removes the Landau singularity at $Q^{2}=Λ_{\rm QCD}^{2}$ by a minimum power-behaved correction. The ensuing contributions to the cusp anomalous dimension - related to the Sudakov form factor - and to the quark anomalous dimension - which controls evolution - lead to an enhancement at high $Q^{2}$ of the hard part of exclusive amplitudes, calculated in perturbative QCD, while simultaneously improving its scaling behavior. The phenomenological implications of this framework are analyzed by applying it to the pion's electromagnetic form factor, including the NLO contribution to the hard-scattering amplitude, and also to the pion-photon transition at LO. For the pion wave function, an improved ansatz of the Brodsky-Huang-Lepage type is employed, which includes an effective (constituent-like) quark mass, $m_{\rm q}=0.33$ GeV. Predictions for both form factors are presented and compared to the experimental data, applying Brodsky-Lepage-Mackenzie commensurate scale setting. We find that the perturbative hart part prevails at momentum transfers above about 20 GeV ${}^{2}$, while at lower $Q^{2}$-values the pion form factor is dominated by Feynman-type contributions. The theoretical prediction for the $γ^{*}γ\to π^{0}$ form factor indicates that the true pion distribution amplitude may be somewhat broader than the asymptotic one.
We perform a lattice mass analysis in the flavour singlet pseudoscalar channel on the SESAM and TXL full QCD vacuum configurations, with 2 active flavours of dynamical Wilson fermions at beta = 5.6. At our inverse lattice spacing, a^-1 = 2.3 GeV, we retrieve by a chiral extrapolation to the physical light quark masses the value m_eta' = 3.7(+8)(-4) m_pi. A crude extrapolation from (N_f = 3) phenomenology would suggest m_eta' \approx 5.1 m_pi for N_f = 2 QCD. we verify that the mass gap between the singlet state eta' and the pi flavour triplt state is due to gauge configurations with non-trivial topology.
The anomalous dimensions of trilinear-quark operators are calculated at leading twist $t=3$ by diagonalizing the one-gluon exchange kernel of the renormalization-group type evolution equation for the nucleon distribution amplitude. This is done within a symmetrized basis of Appell polynomials of maximum degree $M$ for $M\gg 1$ (up to order 400) by combining analytical and numerical algorithms. The calculated anomalous dimensions form a degenerate system, whose upper envelope shows asymptotically logarithmic behavior.
We calculate electromagnetic pion form factors with an analytic model for $α_{\rm s}(Q^2)$ which is infrared (IR) finite without invoking a ``freezing'' hypothesis. We show that for the asymptotic pion distribution amplitude, $F_{π^{0}γ^{*}γ}$ agrees well with the data, whereas the IR-enhanced hard contribution to $F_π$ and the soft (nonfactorizing) part can jointly account for the data.
We develop and discuss an infrared-finite factorization and optimized renormalization scheme for calculating exclusive processes which enables the inclusion of transverse degrees of freedom without entailing suppression of calculated observables, like form factors. This is achieved by employing an analytic, i.e., infrared stable, effective coupling $α_{s}(Q^{2})$ which removes the Landau singularity at $Q^{2}=Λ_{\rm QCD}^{2}$ by a power-behaved correction. The ensuing contributions to the cusp anomalous dimension, related to the Sudakov form factor, and to the quark anomalous dimension, which controls evolution, lead to enhancement of the hard part of exclusive amplitudes, calculated in perturbative QCD. The phenomenological implications of this framework are analyzed by applying it to the pion's electromagnetic form factor and the pion-photon transition.
We calculate the energy of electroweak sphalerons including one-loop fermionic corrections. This calculation has previously been done by adding the correction to the tree-level sphaleron and interpreting the resulting energy as the 1-loop energy. However, in this paper we calculate the energy by doing a full renormalisation of the parameters on 1-loop-level and redetermining the sphaleron configuration using the full energy functional. When comparing the final result with the tree-level solution we find that the 1-loop calculation will only cause small deviations of the sphaleron energy.