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A. Schafer

Publications and source records attributed to A. Schafer.

At least 37 records · Page 2Linked to original sources
arXiv2007

Moments of generalized parton distributions and quark angular momentum of the nucleon

The internal structure of hadrons is important for a variety of topics, including the hadron form factors, proton spin and spin asymmetry in polarized proton scattering. For a systematic study generalized parton distributions (GPDs) encode important information on hadron structure in the entire impact parameter space. We report on a computation of nucleon GPDs based on simulations with two dynamical non-perturbatively improved Wilson quarks with pion masses down to 350MeV. We present results for the total angular momentum of quarks with chiral extrapolation based on covariant baryon chiral perturbation theory.

hep-lat↗
arXiv2007

Prospects of Open Charm Production at GSI-FAIR and J-PARC

We present a detailed phenomenological study of the prospects of open charm physics at the future $\bar{p}p$ and $pp$ facilities GSI-FAIR and J-PARC, respectively. In particular, we concentrate on differential cross sections and the charge and longitudinal double-spin asymmetries at next-to-leading order accuracy. Theoretical uncertainties for the proposed observables are estimated by varying the charm quark mass and the renormalization and factorization scales.

hep-ph↗
arXiv2007

Exclusive electroproduction of pion pairs

We investigate electroproduction of pion pairs on the nucleon in the framework of QCD factorization for hard exclusive processes. We extend previous analyses by taking the hard-scattering coefficients at next-to-leading order in $α_s$. The dynamics of the produced pion pair is described by two-pion distribution amplitudes, for which we perform a detailed theoretical and phenomenological analysis. In particular, we obtain constraints on these quantities by comparing our results with measurements of angular observables that are sensitive to the interference between two-pion production in the isoscalar and isovector channels.

hep-ph↗
arXiv2006

Generalized parton distributions for the nucleon in chiral perturbation theory

We complete the analysis of twist-two generalized parton distributions of the nucleon in one-loop order of heavy-baryon chiral perturbation theory. Extending our previous study of the chiral-even isosinglet sector, we give results for chiral-even isotriplet distributions and for the chiral-odd sector. We also calculate the one-loop corrections for the chiral-odd generalized parton distributions of the pion.

hep-ph↗
arXiv2006

Nucleon Form Factors: Probing the Chiral Limit

The electromagnetic form factors provide important hints for the internal structure of the nucleon and continue to be of major interest for experimentalists. For an intermediate range of momentum transfers the form factors can be calculated on the lattice. However, reliability of the results is limited by systematic errors due to the required extrapolation to physical quark masses. Chiral effective field theories predict a rather strong quark mass dependence in a range which was yet unaccessible for lattice simulations. We give an update on recent results from the QCDSF collaboration using gauge configurations with Nf=2, non-perturbatively O(a)-improved Wilson fermions at very small quark masses down to 340 MeV pion mass, where we start to probe the relevant quark mass region.

hep-lat↗
arXiv2006

Hadron-Pair Photoproduction in Longitudinally Polarized Lepton-Nucleon Collisions

We present a detailed phenomenological study of photoproduction of two hadrons, both with high transverse momentum, in longitudinally polarized lepton-nucleon collisions. We consistently include ``direct'' and ``resolved'' photon contributions and examine the sensitivity of the relevant spin asymmetries to the gluon polarization in the nucleon and to the completely unknown parton content of circularly polarized photons. Our results are relevant for the COMPASS and HERMES fixed-target experiments as well as for a possible future polarized lepton-proton collider like eRHIC at BNL. So far, all studies are limited to the lowest order approximation of QCD.

hep-ph↗
arXiv2006

Generalized parton distributions for the pion in chiral perturbation theory

Generalized parton distributions provide a unified parameterization of hadron structure and allow one to combine information from many different observables. Lattice QCD calculations already provide important input to determine these distributions and hold the promise to become even more important in the future. To this end, a reliable extrapolation of lattice calculations to the physical quark and pion masses is needed. We present an analysis for the moments of generalized parton distributions of the pion in one-loop order of chiral perturbation theory.

hep-ph↗
arXiv2005

Nucleon structure from generalized parton distributions in lattice QCD

This talk presents results from the QCDSF-UKQCD collaboration for moments of leading twist generalized parton distributions in two-flavor lattice QCD based on O(a) improved Wilson Fermions. We study helicity independent and helicity flip GPDs with a focus on densities of quarks in the transverse plane.

hep-lat↗
arXiv2005

Soffer Bound and Transverse Spin Densities from Lattice QCD

Generalized transversity distributions encode essential information on the internal structure of hadrons related to transversely polarized quarks. Lattice QCD allows us to compute the lowest moments of these tensor generalized parton distributions. In this talk, we discuss a first lattice study of the Soffer bound and show preliminary results for transverse spin densities of quarks in the nucleon.

hep-ph↗
arXiv2005

Evolution Equation for Generalized Parton Distributions

The extension of the method [arXiv:hep-ph/0503109] for solving the leading order evolution equation for Generalized Parton Distributions (GPDs) is presented. We obtain the solution of the evolution equation both for the flavor nonsinglet quark GPD and singlet quark and gluon GPDs. The properties of the solution and, in particular, the asymptotic form of GPDs in the small x and ξregion are discussed.

hep-ph↗
arXiv2005

Quark helicity flip generalized parton distributions from two-flavor lattice QCD

We present an initiatory study of quark helicity flip generalized parton distributions (GPDs) in n_f=2 lattice QCD, based on clover-improved Wilson fermions for a large number of coupling constants and pion masses. Quark helicity flip GPDs yield essential information on the transverse spin structure of the nucleon. In this work, we show first results on their lowest moments and dipole masses and study the corresponding chiral and continuum extrapolations.

hep-lat↗
arXiv2005

The process p + pbar --> gamma + pi^0 within the handbag approch

We analyse the exclusive channel $p\pb\toγπ^0$, assuming handbag dominance. The soft parts are parametrized in terms of CGLN amplitudes for the $q\qb\toγπ^0$ transition and form factors for the $p\pb\to q\qb$ ones, the latter represent moments of Generalized Distribution Amplitudes. We present a combined fit to Fermilab data from E760 taking simultaneously into account information from other exclusive reactions, especially from $p\pb\to γγ$ data. Overall a nicely consistent picture emerges, such that one can hope, that our theoretical analysis will be reliable also for the kinematics of GSI/FAIR, which hopefully will provide much more precise and complete data. Consequently, data from this facility should improve our knowledge both on the proton-antiproton distribution amplitudes and the pion production mechanism.

hep-ph↗
arXiv2003

Quark Orbital Angular Momentum in the Wandzura-Wilczek Approximation

We show that quark orbital angular momentum is directly related to off-forward correlation functions which include intrinsic transverse momentum corresponding to a derivative with respect to the transverse coordinates. Its possible contribution to scattering processes is therefore of higher twist and vanishes in the forward limit. The relation of OAM to other twist 2 and 3 distributions known in the literature is derived and formalized by an unintegrated sum rule.

hep-ph↗
arXiv2002

Next-to-leading order QCD corrections to high-p_T pion production in longitudinally polarized pp collisions

We present a calculation for single-inclusive large-p_T pion production in longitudinally polarized pp collisions in next-to-leading order QCD. We choose an approach where fully analytical expressions for the underlying partonic hard-scattering cross sections are obtained. We simultaneously rederive the corresponding corrections to unpolarized scattering and confirm the results existing in the literature. Our results allow to calculate the double-spin asymmetry A_LL^pi for this process at next-to-leading order, which will soon be used at BNL-RHIC to measure the polarization of gluons in the nucleon.

hep-ph↗