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A. Schaefer

Publications and source records attributed to A. Schaefer.

47 records · Page 3Linked to original sources
arXiv1998

Higher orders and IR renormalon phenomenology in DVCS

We present results for higher order perturbative corrections to Compton scattering in the generalized Bjorken kinematics. The approach we have used is based on the combination of two techniques: conformal operator product expansion on the one side, and resummation of the fermion vacuum insertions with consequent restoration of the full QCD beta-function via the naive nonabelianization assumption, on the other. These are terms which are lost in the former approach. Due to the presence of the infrared renormalon poles in the Borel transform of the resummed amplitude the latter suffers from ambiguities which reflect the asymptotic character of perturbation series. The residues of these IR renormalon poles give an estimate for the size of power corrections in deeply virtual Compton scattering.

hep-ph↗
arXiv1997

On the leading logarithmic evolution of the off-forward distributions

We have found the analytical solution of the LO-evolution equation for off-forward distributions which arise in the processes of deeply virtual Compton scattering or exclusive production of mesons. We present the predictions for their evolution with an input distribution taken from recent bag model calculations.

hep-ph↗
arXiv1996

Prospects for Measuring Delta G from Jets at HERA with Polarized Protons and Electrons

The measurement of the polarized gluon distribution function Delta G(x) from photon-gluon fusion processes in electron-proton deep inelastic scattering producing two jets has been investigated. The study is based on the MEPJET and PEPSI simulation programs. The size of the expected spin asymmetry and corresponding statistical uncertainties for a possible measurement with polarized beams of electrons and protons at HERA have been estimated. The results show that the asymmetry can reach a few percent, and is not washed out by hadronization and higher order processes.

hep-ph↗
arXiv1995

QED in strong fields

We discuss the status of atomic physics in strong field. We focus on the problem of electron-positron lines observed in heavy-ion collisions and on QED effects, calculated in strong Coulomb fields, especially Delbrück-scattering. We discuss the similarities and differences between these effects and channeling respectively beamstrahlung. We investigated the prospects for photon-channeling, calculated channeling from first principles on the basis of the Dirac equation, and determined the rate for electron-positron pair production in the collision of two high-energy particle pulses.

hep-ph↗
arXiv1995

QCD Sum Rule Calculation of Twist-4 Corrections to Bjorken and Ellis-Jaffe Sum Rules

We calculate the twist-4 corrections to the integral of $g_1(x,Q^2)$ in the framework of QCD sum rules using an interpolating nucleon field which contains explicitly a gluonic degree of freedom. This information can be used together with previous calculations of the twist-3 contribution to the second moment of $g_2(x)$ to estimate the higher-twist corrections to the Ellis-Jaffe and Bjorken sum rules. We get $f^{(2)}(proton) = -0.037 \pm 0.006$ and $f^{(2)}(neutron) = -0.013 \pm 0.006$. Numerically our results roughly agree with those obtained by Balitsky, Braun and Kolesnichenko based on a sum rule for a simpler current. Our calculations are far more stable as tested within the sum rule approach but are more sensitive to less well known condensates.

hep-ph↗
arXiv1994

QCD Sum Rule Calculation of Twist-3 Contributions to Polarized Nucleon Structure Functions

Using the framework of QCD sum rules we predict the twist-3 contribution to the second moment of the polarized nucleon structure function $g_2(x)$. As the relevant local operator depends explicitely on the gluon field, we employ a recently studied interpolating nucleon current which contains three quark field and one gluon field operator. Despite the fact that our calculation is based on the analysis of a completely different correlation function, our estimates are consitent with those of Balitsky, Braun and Kolesnichenko who used a three-quark current.

hep-ph↗
arXiv1994

Monte Carlo Simulations for RHIC Spin Physics

Direct photon production in longitudinally polarised proton-proton collisions offers the most direct and unproblematic possibility to determine the polarised gluon distribution of a proton. This information could play a major role for improving our understanding of the nucleon structure and QCD in general. It is hoped that such experiments will be done at RHIC. We present results of detailed Monte Carlo simulations using a code called {\sc SPHINX}. We find that for RHIC energies and large gluon polarisation the Compton graph dominates allowing for a direct test of $Δg$. Triggering on away-side jets with the envisaged jet-criteria should allow to obtain more detailed information on $Δg(x)$. The photon asymmetry resulting from the asymmetry of produced $π^0$'s provides an additional signal, which is complementary to the other two. For small gluon polarisation, i.e. $Δg \le 0.5$ or very soft polarised gluon-distributions the envisaged experiments will require a highly sophisticated simulation and large statistics to extract more than upper bounds for $|Δg(x)|$.}

hep-ph↗
arXiv1993

The connection between single transverse spin asymmetries and the second moment of $g_2$

We point out that the size of the photon single spin asymmetry in high--energy proton proton collisions with one transversely polarized proton can be related to $d^{(2)}$, the twist three contribution to the second moment of $g_2$. Both quantities should be measured in the near future. The first was analysed by Qiu and Sterman, the second was estimated by Balitsky, Braun, and Kolesnichenko. Both experiments measure effectively the strength of the collective gluon field in the nucleon oriented relative to the nucleon spin. The sum rule results suggest that the single spin asymmetry is rather small for the proton, but could be substantial for the neutron.

hep-ph↗
arXiv1993

Wavelets as a variational basis of the XY model

We use Daubechies' orthonormal compact wavelets as a variational basis for the $XY$ model in two and three dimensions. Assuming that the fluctuations of the wavelet coefficients are Gaussian and uncorrelated, minimization of the free energy yields the fluctuation strength of wavelet coefficients at different scales, from which observables can be computed. This model is able to describe the low-temperature phase and makes a prediction about the phase transition temperature.

hep-lat↗