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Andreas Schaefer

Publications and source records attributed to Andreas Schaefer.

35 records · Page 2Linked to original sources
arXiv2007

Quark-quark Double Scattering and Modified (Anti-)Quark Fragmentation Functions in Nuclei

Quark-quark double scattering in eA DIS and its contribution to quark and anti-quark fragmentation functions are investigated with the generalized factorization of the relevant twist-four processes in pQCD. It is shown that the resulting modifications to quark and anti-quark fragmentation functions are different. While the numerical size of these effects cannot be determined from pQCD, the structure of our result leads to a number of qualitative predictions for the relative size of the effect for different hadrons. These qualitative predictions agree with the multiplicity ratios for positive and negative hadrons as observed by HERMES.

nucl-th↗
arXiv2005

Extension of the Color Glass Condensate Approach to Diffractive Reactions

We present an evolution equation for the Bjorken x dependence of diffractive dissociation on hadrons and nuclei at high energies. We extend the formulation of Kovchegov and Levin by relaxing the factorization assumption used there. The formulation is based on a technique used by Weigert to describe interjet energy flow. The method can be naturally extended to other exclusive observables.

hep-ph↗
arXiv2005

The Exotic Baryon Theta^+(1540) on the Lattice

We report on a study of the pentaquark Theta^+(1540), using a variety of different interpolating fields. We use Chirally Improved fermions in combination with Jacobi smeared quark sources to improve the signal and get reliable results even for small quark masses. The results of our quenched calculations, which have been done on a 12^3x24 lattice with a lattice spacing of a=0.148 fm, do not provide any evidence for the existence of a Theta^+ with positive parity. We do observe, however, a signal compatible with nucleon-kaon scattering state. For the negative parity the results are inconclusive, due to the potential mixture with nucleon-kaon and N*-kaon scattering states.

hep-lat↗
arXiv2005

Baryon spectroscopy with spatially improved quark sources

We study baryons on the lattice with a special focus on excited states. For that purpose we construct several interpolators which differ in their Dirac structure. These interpolators are built from Jacobi smeared quarks with different widths in order to allow for operators with improved spatial wavefunctions. We compute all cross correlations and use the variational method to determine which combinations of operators have best overlap with ground and excited states. Our approach yields promising results for the spin-1/2 baryons: nucleon, sigma, xi and lambda. For the spin-3/2 baryons, delta and omega, we obtain results which are consistent with results of other groups.

hep-lat↗
arXiv2005

Masses of excited baryons from chirally improved quenched lattice QCD

Whereas ground state spectroscopy for quenched QCD is well understood, it is still a challenge to obtain results for excited hadron states. In our study we present results from a new approach for determining spatially optimized operators for lattice spectroscopy of excited hadrons. In order to be able to approach physical quark masses we work with the chirally improved Dirac operator, i.e., approximate Ginsparg-Wilson fermions. Since these are computationally expensive we restrict ourselves to a few quark sources. We use Jacobi smeared quark sources with different widths and combine them to construct hadron operators with different spatial wave functions. This allows us to identify the Roper state and other excited baryons, also in the strange sector.

nucl-th↗
arXiv2004

Spatially improved operators for excited hadrons on the lattice

We present a new approach for determining spatially optimized operators that can be used for lattice spectroscopy of excited hadrons. Jacobi smeared quark sources with different widths are combined to construct hadron operators with different spatial wave functions. We use the variational method to determine those linear combinations of operators that have optimal overlap with ground and excited states. The details of the new approach are discussed and we demonstrate the power of the method using examples from quenched baryon and meson spectroscopy. In particular we study the Roper state and rho(1450) and discuss some physical implications of our tests.

hep-lat↗
arXiv2004

Excited nucleons with chirally improved fermions

We study positive and negative parity nucleons on the lattice using the chirally improved lattice Dirac operator. Our analysis is based on a set of three operators chi_i with the nucleon quantum numbers but in different representations of the chiral group and with different diquark content. We use a variational method to separate ground state and excited states and determine the mixing coefficients for the optimal nucleon operators in terms of the chi_i. We clearly identify the negative parity resonances N(1535) and N(1650) and their masses agree well with experimental data. The mass of the observed excited positive parity state is too high to be interpreted as the Roper state. Our results for the mixing coefficients indicate that chiral symmetry is important for N(1535) and N(1650) states. We confront our data for the mixing coefficients with quark models and provide insights into the physics of the nucleon system and the nature of strong decays.

hep-ph↗
arXiv2003

Sivers function in a spectator model with axial-vector diquarks

We perform a calculation of the Sivers function in a spectator model of the nucleon, with scalar and axial-vector diquarks. We make use of gluon rescattering to produce the nontrivial phases necessary to generate the Sivers function. The inclusion of axial-vector diquarks enables us to obtain a nonzero Sivers function for down quarks. Using the results of our model, we discuss the phenomenology of transvere single spin asymmetries in pi+, pi-, and pi0 production, which are currently analysed by the HERMES and COMPASS collaborations. We find that the inclusion of axial-vector diquarks substantially reduces the asymmetries.

hep-ph↗
arXiv2002

Chiral symmetry restoration and the Z3 sectors of QCD

Quenched SU(3) lattice gauge theory shows three phase transitions, namely the chiral, the deconfinement and the Z3 phase transition. Knowing whether or not the chiral and the deconfinement phase transition occur at the same temperature for all Z3 sectors could be crucial to understand the underlying microscopic dynamics. We use the existence of a gap in the Dirac spectrum as an order parameter for the restoration of chiral symmetry. We find that the spectral gap opens up at the same critical temperature in all Z3 sectors in contrast to earlier claims in the literature.

hep-lat↗
arXiv2001

Properties of near-zero modes and chiral symmetry breaking

We study localization and chirality properties of eigenvectors of the lattice Dirac operator. In particular we focus on the dependence of our observables on the size of the corresponding eigenvalue, which allows us to study the transition of a near-zero mode into a bulk mode. We analyze ensembles of quenched SU(3) configurations using a Dirac operator which is a systematic expansion in path length of a solution of the Ginsparg-Wilson equation. Our results support the interpretation of the excitations relevant for chiral symmetry breaking as interacting instantons and anti-instantons.

hep-lat↗
arXiv1996

MC-Simulation of the Transverse Double Spin Asymmetry for RHIC

Using {\sc Sphinx tt}, a new MC simulation program for transverse polarized nucleon--nucleon scattering based on {\sc Pythia~5.6}, we calculate the transverse double spin asymmetry $A^{TT}$ in the Drell-Yan process. If one assumes (quite arbitrarily) that the transversity parton distribution $δq(x,Q^2)$ equals the helicity distribution $Δq(x,Q^2)$ at some low $Q_0^2$ scale, the resulting asymmetry is of order 1\%. In this case is $A^{TT}$ would hardly be be measurable with PHENIX at RHIC.

hep-ph↗
arXiv1996

A microscopic semiclassical confining field equation for $U(1)$ lattice gauge theory in 2+1 dimensions

We present a semiclassical nonlinear field equation for the confining field in 2+1--dimensional $U(1)$ lattice gauge theory (compact QED). The equation is derived directly from the underlying microscopic quantum Hamiltonian by means of truncation. Its nonlinearities express the dynamic creation of magnetic monopole currents leading to the confinement of the electric field between two static electric charges. We solve the equation numerically and show that it can be interpreted as a London relation in a dual superconductor.

hep-lat↗
arXiv1994

Proton spin content from skyrmions

It is well known that in lowest order the skyrmion model of the nucleon gives vanishing spin content. With new data indicating a proton spin content $ΔΣ= 0.22\pm 0.14,$ it is an increasing challenge to find ways in which the skyrmion can move away from the null result. We show here that a particular term in the skyrmion lagrangian in SU(3) involving six derivatives of the field can, with plausible parameters, yield a spin content consistent with present experiment.

hep-ph↗
arXiv1994

Sea-quark Content of Nucleons from Proton-induced Drell-Yan Production

We analyse the proton-induced Drell-Yan production in both the flavor asymmetry and isospin breaking explanations for the violation of the Gottfried sum rule. Consequences from three different forms of corrections to the flavor and isospin symmetric parton distributions are examined. It is found that the calculated results are sensitive to the choices of parameters and parton distributions, and to the ways the corrections are introduced. All three forms of corrections could be consistent with the recent Fermilab Experiment E772 data for the ratio of cross section $R=σ_{W}/σ_{IS}$ and for the shape of the differential cross section $m^{3}d^{2}σ/dx_{F}dm$ for $^{2}$H.

hep-ph↗
arXiv1994

Variational description of statistical field theories using Daubechies' wavelets

We investigate the description of statistical field theories using Daubechies' orthonormal compact wavelets on a lattice. A simple variational approach is used to extend mean field theory and make predictions for the fluctuation strengths of wavelet coefficients and thus for the correlation function. The results are compared to Monte Carlo simulations. We find that wavelets provide a reasonable description of critical phenomena with only a small number of variational parameters. This lets us hope for an implementation of the renormalization group in wavelet space.

hep-lat↗