arXiv subjects
Publications and source records attributed to A. Schäfer.
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.
We present a new method for reducing the stochastic noise of all-to-all propagators based on stopping the inversion of the propagator before convergence. The method is easy to implement, unbiased and independent of the quark action. Applying this method to the calculation of disconnected loops needed for hadronic structure observables we find savings in computer time of factors of 4-12 depending on the operator inserted in the loop. When combined with a hopping parameter expansion technique we obtain combined gains of up to factors of 30 for some operators.
We systematically compare filtering methods used to extract topological excitations from lattice gauge configurations. We show that there is a strong correlation of the topological charge densities obtained by APE and Stout smearing. Furthermore, a first quantitative analysis of quenched and dynamical configurations reveals a crucial difference of their topological structure: the topological charge density is more fragmented, when dynamical quarks are present. This fact also implies that smearing has to be handled with great care, not to destroy these characteristic structures.
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.
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.
We consider the exclusive diffractive dissociation of a proton into three jets with large transverse momenta in the double-logarithmic approximation of perturbative QCD. This process is sensitive to the proton unintegrated gluon distribution at small x and to the proton light-cone distribution amplitudes. According to our estimates, an observation of such processes in the early runs at LHC is feasible for jet transverse momenta of the order of 5 GeV.
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.
We give an overview of the QPACE project, which is pursuing the development of a massively parallel, scalable supercomputer for LQCD. The machine is a three-dimensional torus of identical processing nodes, based on the PowerXCell 8i processor. The nodes are connected by an FPGA-based, application-optimized network processor attached to the PowerXCell 8i processor. We present a performance analysis of lattice QCD codes on QPACE and corresponding hardware benchmarks.
We explore the possibility to observe hard exclusive three-jet production in early LHC runs, corresponding to diffractive dissociation of the incident proton into three jets with large but compensating transverse momenta. This process is sensitive to the proton unintegrated gluon distribution at small x and to the distribution of the three valence quarks in the proton at small transverse distances. The corresponding cross section is calculated using an approach based on k_t factorization. According to our estimates, observation of hard diffractive three-jet production at LHC is feasible for jet transverse momenta q_perp \sim 5 GeV.
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.
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.
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.
Semi-exclusive and exclusive processes are becoming more and more important in high energy physics since they are excellently suited to study the internal hadronic structure. To analyze such processes the knowledge of the hadron distribution amplitudes, which are universal for different reactions, is essential. Only rather indirect information on these nonperturbative functions can be obtained from measurements. In this work we report on a lattice QCD computation of moments of nucleon distribution amplitudes using suitable three-quark operators. However, these operators have to be renormalized and the mixing is even more complicated than in the continuum. Using the symmetry group of the hypercubic lattice we therefore derive and implement irreducibly transforming three-quark operators, which allow us to control the mixing pattern and will finally lead to quantitative predictions in the MSbar scheme. We present preliminary results for leading-twist and next-to-leading twist nucleon distribution amplitudes based on the QCDSF/UKQCD simulations with 2 flavors of dynamical clover fermions.
We investigate mesons containing charm quarks on fine lattices with a^{-1} \sim 5 GeV. The quenched approximation is employed using the Wilson gauge action at β= 6.6 and nonperturbatively O(a) improved Wilson quarks. We present results for decay constants using various interpolating fields and give preliminary results for form factors of semileptonic decays of D_s mesons to light pseudoscalar mesons.
We present first results for a study with two mass-degenerate dynamical Chirally Improved (CI) fermions on lattices of spatial extent 2.4 fm. The CI Dirac operator obeys the Ginsparg-Wilson condition in good approximation. The pion mass we use is still large O(470 MeV) for the 16**3*32 lattices with lattice spacing of 0.15 fm. The hadron masses are obtained with the variational technique and the results compared with earlier quenched calculations with similar lattice parameters. We find indications for the isovector, scalar meson a0(980) near the experimental value, in contradistinction to quenched results which always produced a mass value compatible with the first excitation a0(1450).
We present our latest results for the excitations of static-light mesons on both quenched and unquenched lattices, where the light quarks are simulated using the chirally improved (CI) lattice Dirac operator.
We compute decay constants of heavy-light mesons in quenched lattice QCD with a lattice spacing of a ~ 0.04 fm using non-perturbatively O(a) improved Wilson fermions and O(a) improved currents. We obtain f_{D_s} = 220(6)(5)(11) MeV, f_D = 206(6)(3)(22) MeV, f_{B_s} = 205(7)(26)(17) MeV and f_B = 190(8)(23)(25) MeV, using the Sommer parameter r_0 = 0.5 fm to set the scale. The first error is statistical, the second systematic and the third from assuming a +-10% uncertainty in the experimental value of r_0. A detailed discussion is given in the text. We also present results for the meson decay constants f_K and f_πand the ρmeson mass.
We compute the electromagnetic form factor of the pion using non-perturbatively O(a) improved Wilson fermions. The calculations are done for a wide range of pion masses and lattice spacings. We check for finite size effects by repeating some of the measurements on smaller lattices. The large number of lattice parameters we use allows us to extrapolate to the physical point. For the square of the charge radius we find =0.441(20) fm^2, in good agreement with experiment.