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A. Schafer

Publications and source records attributed to A. Schafer.

At least 55 records · Page 3Linked to original sources
arXiv2002

Towards the theory of coherent hard dijet production on hadrons and nuclei

We carry out a detailed calculation of the cross section of pion diffraction dissociation into two jets with large transverse momenta, originating from a hard gluon exchange between the pion constituents. Both the quark and the gluon contribution are considered and in the latter case we present calculations both in covariant and in axial gauges. We find that the standard collinear factorization does not hold in this reaction. The structure of non-factorizable contributions is discussed and the results are compared with the experimental data. Our conclusion is that the existing theoretical uncertainties do not allow, for the time being, for a quantitative extraction of the pion distribution amplitude.

hep-ph↗
arXiv2001

Direct J/Psi hadroproduction in k_\perp-factorization and the color octet mechanism

The hadroproduction of direct J/Psi in the framework of the k_\perp-factorization approach is studied. The color-singlet contribution is essentially larger than in the collinear approach but is still an order of magnitude below the data. The deficit may be well described by the color octet contribution with the value of the matrix element <0|O^{J/Ψ}_8(^3S_1)|0> substantially decreased in comparison with the fits in the collinear factorization. This should lead to a reduction of the large transverse polarization, predicted in the collinear approach.

hep-ph↗
arXiv2001

Towards a solution of the charmonium production controversy: k_t-factorization versus color octet mechanism

The cross section of χ_{cJ} hadroproduction is calculated in the k_t-factorization approach. We find a significant contribution of the χ_{c1} state due to non-applicability of the Landau-Yang theorem because of off-shell gluons. The results are in agreement with data and, in contrast to the collinear factorization, show a dominance of the color singlet part and a strong suppression of the color octet contribution. Our results could therefore lead to a solution of the longstanding controversy between the color singlet model and the color octet mechanism.

hep-ph↗
arXiv2000

A lattice calculation of the nucleon's spin-dependent structure function g_2 revisited

Our previous calculation of the spin-dependent structure function g_2 is revisited. The interest in this structure function is to a great extent motivated by the fact that it receives contributions from twist-two as well as from twist-three operators already in leading order of 1/Q^2 thus offering the unique possibility of directly assessing higher-twist effects. In our former calculation the lattice operators were renormalized perturbatively and mixing with lower-dimensional operators was ignored. However, the twist-three operator which gives rise to the matrix element d_2 mixes non-perturbatively with an operator of lower dimension. Taking this effect into account leads to a considerably smaller value of d_2, which is consistent with the experimental data.

hep-lat↗
arXiv2000

Wandzura-Wilczek approximation for the twist-3 DVCS amplitude

We present a derivation of Wandzura-Wilczek (WW) like relations for skewed parton distributions. It is demonstrated for photon-pion scattering that the skewed twist-3 parton distributions contributing to the DVCS amplitude have discontinuities at the points $x=\pmξ$ in the WW approximation. This may lead to a violation of factorisation for the twist-3 DVCS amplitude with transverse polarization of the virtual photon. We show, however, that the contribution of the divergencies to the scattering of a transversely polarized virtual photon affects DVCS observables only at order $1/Q^2$ and can be neglected at twist-3 accuracy. For the scattering of a longitudinally polarized photon the twist-3 amplitude is free of such divergencies.

hep-ph↗
arXiv2000

Four-quark operators in hadrons

We present calculations of matrix elements of 4-quark operators in the pion and in the nucleon extracted from quenched Monte Carlo simulations at beta = 6.0 using Wilson fermions. These operators are relevant for higher-twist effects. We are particularly careful to avoid mixing with lower-dimensional operators by choosing appropriate flavour structures.

hep-lat↗
arXiv2000

Heavy quark production as sensitive test for an improved description of high energy hadron collisions

QCD dynamics at small quark and gluon momentum fractions or large total energy, which plays a major role for HERA, the Tevatron, RHIC and LHC physics, is still poorly understood. For one of the simplest processes, namely bottom-antibottom production, next-to-leading-order perturbation theory fails. We show that the combination of two recently developed theoretical concepts, the k_perp-factorization and the next-to-leading-logarithmic-approximation BFKL vertex, gives perfect agreement with data. One can therefore hope that these concepts provide a valuable foundation for the description of other high energy processes.

hep-ph↗
arXiv2000

$ΔG(x,μ^2)$ from jet and prompt photon production at RHIC

We compare theoretical results for jet and prompt photon production in polarized pp-collisions at RHIC obained with three different methods: a) the unpolarized event generator PYTHIA combined with hadronic asymmetry weights calculated from leading order formulae, b) the polarized event generator SPHINX which treats the partonic helicity in the hard scattering and the initial state shower explicitly, and c) parton generators which integrate next-to-leading order QCD cross sections using Monte Carlo methods. Method a) requires far less computer time than method b) but we find that it is too imprecise for precision studies. The lacking treatment of partonic polarization in the initial state shower results in relative deviations in the asymmetries of up to 20% relative to SPHINX (method b)). As was to be expected, the event generator predictions for the absolute cross sections differ significantly from the next-to-leading order QCD calculations. But fortunately the asymmetries show a much better agreement: relative deviations for prompt photon production for parameterizations with medium and large $Δg(x,μ^2)$ barely exceed 10%. After fine tuning the parameters of the parton showers the same result can be obtained for jet production.

hep-ph↗
arXiv2000

Hard diffractive electroproduction of two pions

We have calculated the leading order amplitude of hard diffractive electroproduction of two pions in lepton nucleon scattering. At the leading twist level a pion pair can be produced only in an isospin one or zero state. We have shown that isoscalar states are produced dominantly for x_{Bj}>0.3 and with an invariant mass of the two pions close to the threshold (S-wave) and in the f_2 resonance region (D-wave). These isoscalar pion pairs are dominantly produced by two collinear gluons. Comparing the production of charged and neutral pion pairs as a function of x_{Bj} and m_pipi one can get information about the gluonic component of two-pion distribution amplitudes.

hep-ph↗
arXiv1999

Transverse Double-Spin Asymmetries for Muon Pair Production in pp-Collisions

We calculate the rapidity dependence of the transverse double-spin asymmetry for the Drell-Yan process to next-to-leading order in the strong coupling. Input transversity distributions are obtained by saturating the Soffer inequality at a low hadronic mass scale. Results for the polarized BNL-RHIC proton-proton collider and the proposed HERA-N fixed-target experiment are presented, and the influence of the limited muon acceptance of the detectors on measurements of the asymmetry is studied in detail.

hep-ph↗
arXiv1999

The gluon field of a fast moving nucleus and the effective langrangian for QCD at high energy

Starting from the effective lagrangian for QCD at high energy we calculate the lowest perturbative contributions to the potential of a relativistic nucleus and compare our results to those derived by Kovchegov (see Y.V. Kovchegov, Phys. Rev. {\bf D55}, 5445 (1997)). The results differ already at order g^3 which can be traced to the fact that the meaning of the underlying gluon fields is different. (The effective gluon field we use is a gauge invariant object.) Both approaches should therefore be seen as alternatives, the relative merits of which have to be judged by their phenomenological success.

hep-ph↗
arXiv1998

Numerical Solution of the Evolution Equation for Orbital Angular Momentum of Partons in the Nucleon

The evolution of orbital angular momentum distributions within the radiative parton model is studied. We use different scenarios for the helicity weighted parton distributions and consider a broad range of input distributions for orbital angular momentum. In all cases we are lead to the conclusion that the absolute value of the average angular momentum per parton peaks at relatively large $x\approx 0.1$ for perturbatively accessible scales. Furthermore, in all scenarios considered here the average orbital angular momentum per parton is several times larger for gluons than for quarks which favours gluon initiated reactions to measure orbital angular momentum. The large gluon polarization typically obtained in NLO-fits to DIS data is primarily canceled by the gluon orbital angular momentum.

hep-ph↗
arXiv1998

Implications of N=1 supersymmetry for QCD conformal operators

We prove a set of identities for the anomalous dimensions of the quark and gluon conformal operators in the flavour singlet channel in QCD. These relations arise from the graded commutator algebra of the N=1 superconformal group. We evaluate the rotation matrices for the quantities under study from the conventional dimensional regularization to the supersymmetry preserving regularization scheme. Using them we verify the equalities in two-loop approximation employing the results for the NLO anomalous dimensions of the conformal operators in the minimal subtraction scheme derived earlier.

hep-ph↗
arXiv1998

Composite operators in lattice QCD: nonperturbative renormalization

We investigate the nonperturbative renormalization of composite operators in lattice QCD restricting ourselves to operators that are bilinear in the quark fields. These include operators which are relevant to the calculation of moments of hadronic structure functions. The computations are based on Monte Carlo simulations using quenched Wilson fermions.

hep-lat↗
arXiv1998

Nonperturbative Renormalisation of Composite Operators in Lattice QCD

We investigate the nonperturbative renormalisation of composite operators in lattice QCD restricting ourselves to operators that are bilinear in the quark fields. These include operators which are relevant to the calculation of moments of hadronic structure functions. The computations are based on Monte Carlo simulations using quenched Wilson fermions.

hep-lat↗
arXiv1998

Stark quenching for the $1s^22s2p^3P_0$ level in beryllium-like ions and parity-violating effects

In this paper we present some concepts in heavy ion atomic physics for the extraction of parity violating effects. We investigate the effects of the so-called Stark-quenching, i.e., the fast decay of a meta stable state induced by a Stark field, and the superposition of one- and two-photon transitions in beryllium-like heavy ions. It turns out that the discussed theoretical phenomena for heavy ions with few electrons are beyond the scope of present day experimental possibilities because one has to require beam energies of up to 1 TeV/A, laser intensities of up to $10^{17} {\rm W/cm^2}$ and ion currents of up to $10^{11}$ ions per second in beryllium-like uranium. However, especially the superposition of one- and two-photon transitions is a very interesting phenomenon that could provide the germ of an idea to be applied in a more favorable system.

physics.atom-ph↗
arXiv1998

Asymmetry distributions and mass effects in dijet events at a polarized HERA

The asymmetry distributions for several kinematic variables are considered for finding a systematic way to maximize the signal for the extraction of the polarized gluon density. The relevance of mass effects for the corresponding dijet cross section is discussed and the different approximations for including mass effects are compared. We also compare via the programs Pepsi and Mepjet two different Monte Carlo (MC) approaches for simulating the expected signal in the dijet asymmetry at a polarized HERA.

hep-ph↗