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A. Schäfer

Publications and source records attributed to A. Schäfer.

At least 91 records · Page 5Linked to original sources
arXiv2006

Transverse spin structure of the nucleon from lattice QCD simulations

We present the first calculation in lattice QCD of the lowest two moments of transverse spin densities of quarks in the nucleon. They encode correlations between quark spin and orbital angular momentum. Our dynamical simulations are based on two flavors of clover-improved Wilson fermions and Wilson gluons. We find significant contributions from certain quark helicity flip generalized parton distributions, leading to strongly distorted densities of transversely polarized quarks in the nucleon. In particular, based on our results and recent arguments by Burkardt [Phys. Rev. D 72 (2005) 094020], we predict that the Boer-Mulders-function $h_1^\perp$, describing correlations of transverse quark spin and intrinsic transverse momentum of quarks, is large and negative for both up and down quarks.

hep-lat↗
arXiv2006

Axial coupling constant of the nucleon for two flavours of dynamical quarks in finite and infinite volume

We present data for the axial coupling constant g_A of the nucleon obtained in lattice QCD with two degenerate flavours of dynamical non-perturbatively improved Wilson quarks. The renormalisation is also performed non-perturbatively. For the analysis we give a chiral extrapolation formula for g_A based on the small scale expansion scheme of chiral effective field theory for two degenerate quark flavours. Applying this formalism in a finite volume we derive a formula that allows us to extrapolate our data simultaneously to the infinite volume and to the chiral limit. Using the additional lattice data in finite volume we are able to determine the axial coupling of the nucleon in the chiral limit without imposing the known value at the physical point.

hep-lat↗
arXiv2006

Probing Nucleon Structure on the Lattice

The QCDSF/UKQCD collaboration has an ongoing program to calculate nucleon matrix elements with two flavours of dynamical O(a) improved Wilson fermions. Here we present recent results on the electromagnetic form factors, the quark momentum fraction and the first three moments of the nucleon's spin-averaged and spin-dependent generalised parton distributions, including preliminary results with pion masses as low as 320 MeV.

hep-lat↗
arXiv2006

Deeply virtual Compton scattering beyond next-to-leading order: the flavor singlet case

We study radiative corrections to deeply virtual Compton scattering in the kinematics of HERA collider experiments to next--to--leading and next--to--next--to--leading order. In the latter case the radiative corrections are evaluated in a special scheme that allows us to employ the predictive power of conformal symmetry. As observed before, the size of next--to--leading order corrections strongly depends on the gluonic input, as gluons start to contribute at this order. Beyond next--to--leading order we find, in contrast, that the corrections for an input scale of few GeV^2 are small enough to justify the uses of perturbation theory. For $ξ> 5 10^{-3}$ the modification of the scale dependence is also small. However, with decreasing $ξ$ it becomes moderate or even large, in particular for the phase.

hep-ph↗
arXiv2005

Second moment of the pion's distribution amplitude

We present preliminary results from the QCDSF/UKQCD collaborations for the second moment of the pion's distribution amplitude with two flavours of dynamical fermions. We use nonperturbatively determined renormalisation coefficients to convert our results to the MSbar scheme at 5 GeV^2. Employing a linear chiral extrapolation from our large pion masses >550 MeV, we find =0.281(28), leading to a value of a_2=0.236(82) for the second Gegenbauer moment.

hep-lat↗
arXiv2005

Structure of the Pion from Full Lattice QCD

Moments of generalised parton distributions can be related to off-forward matrix elements of local operators. We calculate a few of the leading twist matrix elements for the pion on the lattice. The simulations are performed using two flavours of dynamical fermions and a range of pion masses from 550 to 1090 MeV. Our lattice spacings and spatial volumes lie in the range 0.07-0.12 fm and (1.6-2.2 fm)^3, respectively. Key features of our investigation are the use of O(a) improved Wilson fermions and non-perturbative renormalisation. We present first results for the two lowest moments of the generalised parton distributions of the pion and compare the pion electromagnetic form factor F_pi to experimental data. Good agreement is found between lattice data and experiment.

hep-lat↗
arXiv2005

Excited meson spectroscopy with chirally improved fermions

We present excited meson masses from quenched calculations using chirally improved (CI) quarks at pion masses down to 350 MeV. The salient features of our analysis are the use of a matrix of correlators from various source and sink operators and a basis which includes quark sources with different spatial widths, thereby improving overlap with states exhibiting radial excitations.

hep-lat↗
arXiv2005

Complex conformal spin partial wave expansion of generalized parton distributions and distribution amplitudes

We introduce a new representation of generalized parton distributions and generalized distribution amplitudes that is based on the partial wave decomposition with respect to the complex collinear conformal spin. This decomposition leads us to a versatile parameterization of these non-perturbative functions in terms of conformal moments, which are measurable for integer value on the lattice. This new representation has several advantages: basic properties and crossing relations are automatically implemented, a rather flexible parameterization is possible, the numerical treatment of evolution is simple and analytic approximation of scattering amplitudes can be given. We demonstrate this for simple examples. In particular, phenomenological considerations indicate that the t-dependence of Mellin moments is governed by Regge trajectories. The new representation is vital to push the analysis of deeply virtual Compton scattering to next-to-next-to-leading order.

hep-ph↗
arXiv2005

Investigation of the Second Moment of the Nucleon's g1 and g2 Structure Functions in Two-Flavor Lattice QCD

The reduced matrix elements a_2 and d_2 are computed in lattice QCD with N_f=2 flavors of light dynamical (sea) quarks. For proton and neutron targets we obtain as our best estimates d_2^(p)=0.004(5) and d_2^(n)=-0.001(3), respectively, in the MSbar scheme at Q^2=5 GeV^2, while for a_2 we find a_2^(p)=0.077(12) and a_2^(n)=-0.005(5), where the errors are purely statistical.

hep-lat↗
arXiv2004

Generalized Parton Distributions in Full Lattice QCD

We present recent results on generalized parton distributions from dynamical lattice QCD calculations. Our set of twelve different combinations of couplings and quark masses allows for a preliminary study of the pion mass dependence of the transverse nucleon structure.

hep-lat↗
arXiv2004

Perturbative Renormalisation of the Second Moment of Generalised Parton Distributions

We calculate the non-forward quark matrix elements of operators with two covariant derivatives needed for the renormalisation of the second moment of generalised parton distributions in one-loop lattice perturbation theory using Wilson fermions. For some representations of the hypercubic group commonly used in simulations we determine the sets of all possible mixing operators. For those representations the one-loop mixing matrices of renormalisation factors are found. Due to non-vanishing contributions of operators with external ordinary derivatives the number of contributing operators increases compared to forward matrix elements.

hep-lat↗
arXiv2004

One-loop renormalisation for the second moment of GPDs with Wilson fermions

We calculate the non-forward quark matrix elements for operators with two covariant derivatives in one-loop lattice perturbation theory using Wilson fermions. These matrix elements are needed in the renormalisation of the second moment of generalised parton distributions measured in lattice QCD. For some commonly used representations of the hypercubic group we determine the sets of all mixing operators and find the matrices of mixing and renormalisation factors.

hep-lat↗
arXiv2003

Generalized Parton Distributions from Lattice QCD

We perform a quenched lattice calculation of the first moment of twist-two generalized parton distribution functions of the proton, and assess the total quark (spin and orbital angular momentum) contribution to the spin of the proton.

hep-ph↗
arXiv2003

Structure of the Nucleon

Generalized parton distributions provide information on the longitudinal and transverse distribution of partons in the fast moving nucleon. Furthermore, they contain information on the spin structure of the nucleon. First results of a lattice study of generalized parton distributions are presented.

hep-ph↗