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G. Schierholz

Publications and source records attributed to G. Schierholz.

At least 127 records · Page 7Linked to original sources
arXiv2009

The spin structure of the pion

We present the first calculation of the transverse spin structure of the pion in lattice QCD. Our simulations are based on two flavors of non-perturbatively improved Wilson fermions, with pion masses as low as 400 MeV in volumes up to (2.1 fm)^3 and lattice spacings below 0.1 fm. We find a characteristic asymmetry in the spatial distribution of transversely polarized quarks. This asymmetry is very similar in magnitude to the analogous asymmetry we previously obtained for quarks in the nucleon. Our results support the hypothesis that all Boer-Mulders functions are alike.

hep-lat↗
arXiv2009

Non-perturbative improvement of stout-smeared three flavour clover fermions

We discuss a 3-flavour lattice QCD action with clover improvement in which the fermion matrix has single level stout smearing for the hopping terms together with unsmeared links for the clover term. With the (tree-level) Symanzik improved gluon action this constitutes the Stout Link Non-perturbative Clover or SLiNC action. To cancel O(a) terms the clover term coefficient has to be tuned. We present here results of a non-perturbative determination of this coefficient using the Schroedinger functional and as a by-product a determination of the critical hopping parameter. Comparisons of the results are made with lowest order perturbation theory.

hep-lat↗
arXiv2009

Electroproduction of the N*(1535) resonance at large momentum transfer

We report on the first lattice calculation of light-cone distribution amplitudes of the N*(1535) resonance, which are used to calculate the transition form factors at large momentum transfers using light-cone sum rules. In the region Q^2 > 2 GeV^2, where the light-cone expansion is expected to converge, the results appear to be in good agreement with the experimental data.

hep-ph↗
arXiv2008

Nucleon distribution amplitudes and proton decay matrix elements on the lattice

Baryon distribution amplitudes (DAs) are crucial for the theory of hard exclusive reactions. We present a calculation of the first few moments of the leading-twist nucleon DA within lattice QCD. In addition we deal with the normalization of the next-to-leading (twist-four) DAs. The matrix elements determining the latter quantities are also responsible for proton decay in Grand Unified Theories. Our lattice evaluation makes use of gauge field configurations generated with two flavors of clover fermions. The relevant operators are renormalized nonperturbatively with the final results given in the MSbar scheme. We find that the deviation of the leading-twist nucleon DA from its asymptotic form is less pronounced than sometimes claimed in the literature.

hep-lat↗
arXiv2008

Clover improvement for stout-smeared 2+1 flavour SLiNC fermions: non-perturbative results

We discuss an action in which the fermion matrix has single level stout smearing for the hopping terms together with unsmeared links for the clover term. With the (tree level) Symanzik improved gluon action this constitutes the Stout Link Non-perturbative Clover or SLiNC action. To cancel O(a) terms the clover coefficient, csw, has to be tuned. We present here preliminary results of a non-perturbative determination of csw using the Schrodinger functional and as a by-product also a determination of the critical hopping parameter. A determination of the renormalisation constant for the local vector current is also given. Comparisons of the results are made with lowest order perturbation theory results.

hep-lat↗
arXiv2008

Non-perturbative renormalization of three-quark operators

High luminosity accelerators have greatly increased the interest in semi-exclusive and exclusive reactions involving nucleons. The relevant theoretical information is contained in the nucleon wavefunction and can be parametrized by moments of the nucleon distribution amplitudes, which in turn are linked to matrix elements of local three-quark operators. These can be calculated from first principles in lattice QCD. Defining an RI-MOM renormalization scheme, we renormalize three-quark operators corresponding to low moments non-perturbatively and take special care of the operator mixing. After performing a scheme matching and a conversion of the renormalization scale we quote our final results in the MSbar scheme at mu=2 GeV.

hep-lat↗
arXiv2008

Clover improvement for stout-smeared 2+1 flavour SLiNC fermions: perturbative results

For the Stout Link Non-perturbative Clover (SLiNC) action we determine in one-loop lattice perturbation theory the critical hopping parameter $κ_c$ and the clover parameter $c_{SW}$ which is needed for $\mathcal{O}(a)$ improvement. Performing this calculation off-shell we are also able to compute the non gauge invariant quark field improvement coefficient $c_{NGI}$. Additionally, we present first results for the renormalization factors of the scalar, pseudoscalar, vector and axial vector currents. We discuss mean field improvement for the SLiNC action.

hep-lat↗
arXiv2008

Nucleon structure in terms of OPE with non-perturbative Wilson coefficients

Lattice calculations could boost our understanding of Deep Inelastic Scattering by evaluating moments of the Nucleon Structure Functions. To this end we study the product of electromagnetic currents between quark states. The Operator Product Expansion (OPE) decomposes it into matrix elements of local operators (depending on the quark momenta) and Wilson coefficients (as functions of the larger photon momenta). For consistency with the matrix elements, we evaluate a set of Wilson coefficients non-perturbatively, based on propagators for numerous momentum sources, on a 24^3 x 48 lattice. The use of overlap quarks suppresses unwanted operator mixing and lattice artifacts. Results for the leading Wilson coefficients are extracted by means of Singular Value Decomposition.

hep-lat↗
arXiv2008

The electric dipole moment of the nucleon from simulations at imaginary vacuum angle theta

We compute the electric dipole moment of proton and neutron from lattice QCD simulations with N_f=2 flavors of dynamical quarks at imaginary vacuum angle theta. The calculation proceeds via the CP odd form factor F_3. A novel feature of our calculation is that we use partially twisted boundary conditions to extract F_3 at zero momentum transfer. As a byproduct, we test the QCD vacuum at nonvanishing theta.

hep-lat↗
arXiv2008

Perturbative determination of $c_{SW}$ for plaquette and Symanzik gauge action and stout link clover fermions

Using plaquette and Symanzik improved gauge action and stout link clover fermions we determine the improvement coefficient $c_{SW}$ in one-loop lattice perturbation theory from the off-shell quark-quark-gluon three-point function. In addition, we compute the coefficients needed for the most general form of quark field improvement and present the one-loop result for the critical hopping parameter $κ_c$. We discuss mean field improvement for $c_{SW}$ and $κ_c$ and the choice of the mean field coupling for the actions we have considered.

hep-lat↗
arXiv2008

Renormalisation of Composite Operators in Lattice Perturbation Theory with Clover Fermions: Non-forward Matrix Elements

We consider the renormalisation of lattice QCD operators with one and two covariant derivatives related to the first and second moments of generalised parton distributions and meson distribution amplitudes. Employing the clover fermion action we calculate their non-forward quark matrix elements in one-loop lattice perturbation theory. For some representations of the hypercubic group commonly used in simulations we determine the sets of all possible mixing operators and compute the matrices of renormalisation factors in one-loop approximation. We describe how tadpole improvement is applied to the results.

hep-lat↗
arXiv2008

Vacuum structure revealed by over-improved stout-link smearing compared with the overlap analysis for quenched QCD

A detailed comparison is made between the topological structure of quenched QCD as revealed by the recently proposed over-improved stout-link smearing in conjunction with an improved gluonic definition of the topological density on one hand and a similar analysis made possible by the overlap-fermionic topological charge density both with and without variable ultraviolet cutoff $λ_{cut}$. The matching is twofold, provided by fitting the density-density two-point functions on one hand and by a point-by-point fitting of the topological densities according to the two methods. We point out the similar cluster structure of the topological density for moderate smearing and $200 \mathrm{MeV} < λ_{cut} < 600 \mathrm{MeV}$, respectively. We demonstrate the relation of the gluonic topological density for extensive smearing to the location of the overlap zero modes and the lowest overlap non-zero mode as found for the unsmeared configurations.

hep-lat↗
arXiv2007

The operator product expansion on the lattice

We investigate the Operator Product Expansion (OPE) on the lattice by directly measuring the product (where J is the vector current) and comparing it with the expectation values of bilinear operators. This will determine the Wilson coefficients in the OPE from lattice data, and so give an alternative to the conventional methods of renormalising lattice structure function calculations. It could also give us access to higher twist quantities such as the longitudinal structure function F_L = F_2 - 2 x F_1. We use overlap fermions because of their improved chiral properties, which reduces the number of possible operator mixing coefficients.

hep-lat↗
arXiv2007

Probing the chiral limit with clover fermions II: The baryon sector

Algorithmic progress in recent years made it possible to simulate QCD with Nf=2 flavours of O(a)-improved Wilson fermions at very light quark masses. We present the current results for baryon spectrum states, the nucleon axial coupling and the lowest moment of unpolarised nucleon structure functions. Special emphasis is given to a comparison of our calculations with results from chiral effective theories.

hep-lat↗
arXiv2007

Distribution Amplitudes of Vector Mesons

Results are presented for the lowest moment of the distribution amplitude for the K-star vector meson. Both longitudinal and transverse moments are investigated. We use two flavours of O(a) improved Wilson fermions, together with a non-perturbative renormalisation of the matrix element.

hep-lat↗
arXiv2007

Finite temperature Lattice QCD with two flavors of improved Wilson fermions

We present results of finite temperature QCD from N_f=2 flavors of non-perturbatively improved Wilson fermions on lattices with N_t=8, 10 and 12. The transition temperature T_c is determined for the first time at lattice spacings as low as a = 0.08 fm, albeit at pion masses 1.3 < r_0 m_π. We furthermore compute the screening masses in various color channels at T>T_c.

hep-lat↗