arXiv subjects

G. Schierholz

Publications and source records attributed to G. Schierholz.

At least 109 records · Page 6Linked to original sources

arXiv2011

Baryon Axial Charges and Momentum Fractions with N_f=2+1 Dynamical Fermions

We report on recent results of the QCDSF/UKQCD Collaboration on investigations of baryon structure using configurations generated with N_f=2+1 dynamical flavours of O(a) improved Wilson fermions. With the strange quark mass as an additional dynamical degree of freedom in our simulations we avoid the need for a partially quenched approximation when investigating the properties of particles containing a strange quark, e.g. the hyperons. In particular, we will focus on the nucleon and hyperon axial charges and quark momentum fractions.

hep-lat↗

arXiv2011

Hyperon Form Factors from N_f=2+1 QCD

We present results from the QCDSF/UKQCD collaboration for the electromagnetic and semi-leptonic form factors for the hyperons. The simulations are performed on our new ensembles generated with 2+1 flavours of dynamical O(a)-improved Wilson fermions. A unique feature of these configurations is that the quark masses are tuned so that the singlet quark mass is held fixed at its physical value. We use 5 such choices of the individual quark masses on 24^3x48 lattices with a lattice spacing of about 0.078 fm.

hep-lat↗

arXiv2011

Nucleon form factors and structure functions

We give an update on our ongoing efforts to compute the nucleon's form factors and moments of structure functions using Nf=2 flavours of non-perturbatively improved Clover fermions. We focus on new results obtained on gauge configurations where the pseudo-scalar meson mass is in the range of 170-270 MeV. We will compare our results with various estimates obtained from chiral effective theories since we have some overlap with the quark mass region where results from such theories are believed to be applicable.

hep-lat↗

arXiv2010

Flavour symmetry breaking and tuning the strange quark mass for 2+1 quark flavours

QCD lattice simulations with 2+1 flavours typically start at rather large up-down and strange quark masses and extrapolate first the strange quark mass to its physical value and then the up-down quark mass. An alternative method of tuning the quark masses is discussed here in which the singlet quark mass is kept fixed, which ensures that the kaon always has mass less than the physical kaon mass. Using group theory the possible quark mass polynomials for a Taylor expansion about the flavour symmetric line are found, which enables highly constrained fits to be used in the extrapolation of hadrons to the physical pion mass. Numerical results confirm the usefulness of this expansion and an extrapolation to the physical pion mass gives hadron mass values to within a few percent of their experimental values.

hep-lat↗

arXiv2010

Nucleon and $N^* (1535)$ Distribution Amplitudes

The QCDSF collaboration has investigated the distribution amplitudes and wavefunction normalization constants of the nucleon and its parity partner, the $N^* (1535)$. We report on recent progress in the calculation of these quantities on configurations with two dynamical flavors of $\mathcal{O}(a)$-improved Wilson fermions. New data at pion masses of approximately 270 MeV helps in significantly reducing errors in the extrapolation to the physical point.

hep-lat↗

arXiv2010

Very high order lattice perturbation theory for Wilson loops

We calculate perturbative Wilson loops of various sizes up to loop order $n=20$ at different lattice sizes for pure plaquette and tree-level improved Symanzik gauge theories using the technique of Numerical Stochastic Perturbation Theory. This allows us to investigate the behavior of the perturbative series at high orders. We observe differences in the behavior of perturbative coefficients as a function of the loop order. Up to $n=20$ we do not see evidence for the often assumed factorial growth of the coefficients. Based on the observed behavior we sum this series in a model with hypergeometric functions. Alternatively we estimate the series in boosted perturbation theory. Subtracting the estimated perturbative series for the average plaquette from the non-perturbative Monte Carlo result we estimate the gluon condensate.

hep-lat↗

arXiv2010

Renormalisation of composite operators in lattice QCD: perturbative versus nonperturbative

The perturbative and nonperturbative renormalisation of quark-antiquark operators in lattice QCD with two flavours of clover fermions is investigated within the research programme of the QCDSF collaboration. Operators with up to three derivatives are considered. The nonperturbative results based on the RI-MOM scheme are compared with estimates from one- and two-loop lattice perturbation theory.

hep-lat↗

arXiv2010

Probing the finite temperature phase transition with Nf=2 nonperturbatively improved Wilson fermions

The critical temperature and the nature of the QCD finite temperature phase transition are determined for Nf=2 dynamical flavors of nonperturbatively improved Wilson fermions. The calculations are performed on large lattices with temporal extents Nt=12, 10 and 8, and lattice spacings down to a = 0.075 fm. We find the deconfinement and chiral phasetransitions to take place at the same temperature. Our results are in broad agreement with a second order phase transition in the chiral limit. The critical temperature at the physical quark mass is found to be Tc = 174(3)(6) MeV.

hep-lat↗

arXiv2010

Exploring the Nucleon Structure from First Principles of QCD

Quantum Chromodynamics (QCD) is generally assumed to be the fundamental theory underlying nuclear physics. In recent years there is progress towards investigating the nucleon structure from first principles of QCD. Although this structure is best revealed in Deep Inelastic Scattering, a consistent analysis has to be performed in a fully non-perturbative scheme. The only known method for this purpose are lattice simulations. We first sketch the ideas of Monte Carlo simulations in lattice gauge theory. Then we comment in particular on the issues of chiral symmetry and operator mixing. Finally we present our results for the Bjorken variable of a single quark, and for the second Nachtmann moment of the nucleon structure functions.

hep-lat↗

arXiv2010

Pion in a Box

The residual mass of the pion in a finite spatial box at vanishing quark masses, the mass gap, is computed with two flavors of dynamical clover fermions. The result is compared with predictions of chiral perturbation theory in the delta regime.

hep-lat↗

arXiv2010

Tuning the strange quark mass in lattice simulations

QCD lattice simulations with 2+1 flavours typically start at rather large up-down and strange quark masses and extrapolate first the strange quark mass to its physical value and then the up-down quark mass. An alternative method of tuning the quark masses is discussed here in which the singlet quark mass is kept fixed, which ensures that the kaon always has mass less than the physical kaon mass. It can also take into account the different renormalisations (for singlet and non-singlet quark masses) occurring for non-chirally invariant lattice fermions and so allows a smooth extrapolation to the physical quark masses. This procedure enables a wide range of quark masses to be probed, including the case with a heavy up-down quark mass and light strange quark mass. Results show the correct order for the baryon octet and decuplet spectrum and an extrapolation to the physical pion mass gives mass values to within a few percent of their experimental values.

hep-lat↗

arXiv2010

Topological Structure of the QCD Vacuum Revealed by Overlap Fermions

Overlap fermions preserve a remnant of chiral symmetry on the lattice. They are a powerful tool to investigate the topological structure of the vacuum of Yang-Mills theory and full QCD. Recent results concerning the localization of topological charge and the localization and local chirality of the overlap eigenmodes are reported. The charge distribution is radically different, if a spectral cut-off for the Dirac eigenmodes is applied. The density q(x) is changing from the scale-a charge density (with full lattice resolution) to the ultraviolet filtered charge density. The scale-a density, computed on the Linux cluster of LRZ, has a singular, sign-coherent global structure of co-dimension 1 first described by the Kentucky group. We stress, however, the cluster properties of the UV filtered topological density resembling the instanton picture. The spectral cut-off can be mapped to a bosonic smearing procedure. The UV filtered field strength reveals a high degree of (anti)selfduality at "hot spots" of the action. The fermionic eigenmodes show a high degree of local chirality. The lowest modes are seen to be localized in low-dimensional space-time regions.

hep-lat↗

arXiv2009

Lattice Investigations of Nucleon Structure at Light Quark Masses

Lattice simulations of hadronic structure are now reaching a level where they are able to not only complement, but also provide guidance to current and forthcoming experimental programmes at, e.g. Jefferson Lab, COMPASS/CERN and FAIR/GSI. By considering new simulations at low quark masses and on large volumes, we review the recent progress that has been made in this exciting area by the QCDSF/UKQCD collaboration. In particular, results obtained close to the physical point for several quantities, including electromagnetic form factors and moments of ordinary parton distribution functions, show some indication of approaching their phenomenological values.

hep-lat↗

arXiv2009

Quark structure from the lattice Operator Product Expansion

We have reported elsewhere in this conference on our continuing project to determine non-perturbative Wilson coefficients on the lattice, as a step towards a completely non-perturbative determination of the nucleon structure. In this talk we discuss how these Wilson coefficients can be used to extract Nachtmann moments of structure functions, using the case of off-shell Landau-gauge quarks as a first simple example. This work is done using overlap fermions, because their improved chiral properties reduce the difficulties due to operator mixing.

hep-lat↗

arXiv2009

Semileptonic form factors D to pi, K and B to pi, K from a fine lattice

We extract the form factors relevant for semileptonic decays of D and B mesons from a relativistic computation on a fine lattice in the quenched approximation. The lattice spacing is a=0.04 fm (corresponding to a^{-1}=4.97 GeV), which allows us to run very close to the physical B meson mass, and to reduce the systematic errors associated with the extrapolation in terms of a heavy quark expansion. For decays of D and D_s mesons, our results for the physical form factors at q^2=0 are as follows: f_+^{D to pi}(0)= 0.74(6)(4), f_+^{D to K}(0)= 0.78(5)(4) and f_+^{D_s to K}(0)=0.68(4)(3). Similarly, for B and B_s we find: f_+^{B to pi}(0)=0.27(7)(5), f_+^{B to K}(0)=0.32(6)(6) and f_+^{B_s to K}(0)=0.23(5)(4). We compare our results with other quenched and unquenched lattice calculations, as well as with light-cone sum rule predictions, finding good agreement.

hep-lat↗

arXiv2009

Wilson loops in very high order lattice perturbation theory

We calculate Wilson loops of various sizes up to loop order $n=20$ for lattice sizes of $L^4 (L=4, 6, 8, 12)$ using the technique of Numerical Stochastic Perturbation Theory in quenched QCD. This allows to investigate the behaviour of the perturbative series at high orders. We discuss three models to estimate the perturbative series: a renormalon inspired fit, a heuristic fit based on an assumed power-law singularity and boosted perturbation theory. We have found differences in the behavior of the perturbative series for smaller and larger Wilson loops at moderate $n$. A factorial growth of the coefficients could not be confirmed up to $n=20$. From Monte Carlo measured plaquette data and our perturbative result we estimate a value of the gluon condensate $<\fracαπGG>$.

hep-lat↗

arXiv2009

Results from 2+1 flavours of SLiNC fermions

QCD results are presented for a 2+1 flavour fermion clover action (which we call the SLiNC action). A method of tuning the quark masses to their physical values is discussed. In this method the singlet quark mass is kept fixed, which solves the problem of different renormalisations (for singlet and non-singlet quark masses) occuring for non-chirally invariant lattice fermions. This procedure enables a wide range of quark masses to be probed, including the case with a heavy up-down quark mass and light strange quark mass. Preliminary results show the correct splittings for the baryon (octet and) decuplet spectrum.

hep-lat↗

arXiv2009

A Non-Perturbative Operator Product Expansion

Nucleon structure functions can be observed in Deep Inelastic Scattering experiments, but it is an outstanding challenge to confront them with fully non-perturbative QCD results. For this purpose we investigate the product of electromagnetic currents (with large photon momenta) between quark states (of low momenta). By means of an Operator Product Expansion the structure function can be decomposed into matrix elements of local operators, and Wilson coefficients. For consistency both have to be computed non-perturbatively. Here we present precision results for a set of Wilson coefficients. They are evaluated from propagators for numerous quark momenta on the lattice, where the use of chiral fermions suppresses undesired operator mixing. This over-determines the Wilson coefficients, but reliable results can be extracted by means of a Singular Value Decomposition.

hep-lat↗