arXiv subjects

Andreas Schafer

Publications and source records attributed to Andreas Schafer.

45 records · Page 3Linked to original sources

arXiv2007

Multiple Parton Scattering in Nuclei: Quark-quark Scattering

Modifications to quark and antiquark fragmentation functions due to quark-quark (antiquark) double scattering in nuclear medium are studied systematically up to order \cal{O}(α_{s}^2)$ in deeply inelastic scattering (DIS) off nuclear targets. At the order $\cal{O}(α_s^2)$, twist-four contributions from quark-quark (antiquark) rescattering also exhibit the Landau-Pomeranchuck-Midgal (LPM) interference feature similar to gluon bremsstrahlung induced by multiple parton scattering. Compared to quark-gluon scattering, the modification, which is dominated by $t$-channel quark-quark (antiquark) scattering, is only smaller by a factor of $C_F/C_A=4/9$ times the ratio of quark and gluon distributions in the medium. Such a modification is not negligible for realistic kinematics and finite medium size. The modifications to quark (antiquark) fragmentation functions from quark-antiquark annihilation processes are shown to be determined by the antiquark (quark) distribution density in the medium. The asymmetry in quark and antiquark distributions in nuclei will lead to different modifications of quark and antiquark fragmentation functions inside a nucleus, which qualitatively explains the experimentally observed flavor dependence of the leading hadron suppression in semi-inclusive DIS off nuclear targets. The quark-antiquark annihilation processes also mix quark and gluon fragmentation functions in the large fractional momentum region, leading to a flavor dependence of jet quenching in heavy-ion collisions.

hep-ph↗

arXiv2007

Quantitative comparison of filtering methods in lattice QCD

We systematically compare filtering methods used to extract topological excitations (like instantons, calorons, monopoles and vortices) from lattice gauge configurations, namely APE-smearing and spectral decompositions based on lattice Dirac and Laplace operators. Each of these techniques introduces ambiguities, which can invalidate the interpretation of the results. We show, however, that all these methods, when handled with care, reveal very similar topological structures. Hence, these common structures are free of ambiguities and faithfully represent infrared degrees of freedom in the QCD vacuum. As an application we discuss an interesting power-law for the clusters of filtered topological charge.

hep-lat↗

arXiv2007

2+1 flavor QCD with the fixed point action in the $ε$-regime

We generated configurations with the approximate fixed-point Dirac operator $D_\mathrm{FP}$ on a $12^4$ lattice with $a \approx 0.13 $fm where the scale was set by $r_0$. The distributions of the low lying eigenvalues in different topological sectors were compared with those of the Random Matrix Theory which leads to a prediction of the chiral condensate.

hep-lat↗

arXiv2007

Photoproduction of Hadron Pairs at Fixed-Target Experiments

We consider the photoproduction of two hadrons in polarized lepton-nucleon collisions in the framework of perturbative QCD at the next-to-leading order accuracy. After illustrating how to obtain the experimentally relevant observables, a phenomenological study of the photoproduction of hadron pairs at high transverse momenta is presented. We show theoretical predictions for the relevant cross sections at COMPASS and HERMES kinematics as well as theoretical uncertainties.

hep-ph↗

arXiv2006

The Decoherence Time in High Energy Heavy Ion Collisions

We calculate the decoherence time of the ground state wave function of a nucleus in a high energy heavy ion collision. We define this time as the decay time of the ratio Tr D^2 / (Tr D)^2 of traces of the density matrix D. We find that this time is smaller or equal to 1/Q_s, where the saturation scale Q_s is defined within the color glass condensate model of parton saturation. Our result supports the notion that the extremely rapid entropy production deduced for the early stage of heavy ion collisions at collider energies is to a large extent caused by the decoherence of the initial-state wave functions.

hep-ph↗

arXiv2005

Center vortices and Dirac eigenmodes in SU(2) lattice gauge theory

We study the interplay between Dirac eigenmodes and center vortices in SU(2) lattice gauge theory. In particular we focus on vortex-removed configurations and compare them to an ensemble of configurations with random changes of the link variables. We show that removing the vortices destroys all zero modes and the near zero modes are no longer coupled to topological structures. The Dirac spectrum for vortex-removed configurations in many respects resembles a free spectrum thus leading to a vanishing chiral condensate. Configurations with random changes leave the topological features of the Dirac eigensystem intact. We finally show that smooth center vortex configurations give rise to zero modes and topological near zero modes.

hep-lat↗

arXiv2003

Quenched spectroscopy with fixed-point and chirally improved fermions

We present results from quenched spectroscopy calculations with the parametrized fixed-point and the chirally improved Dirac operators. Both these operators are approximate solutions of the Ginsparg-Wilson equation and have good chiral properties. This allows us to work at small quark masses and we explore pseudoscalar-mass to vector-mass ratios down to 0.28. We discuss meson and baryon masses, their scaling properties, finite volume effects and compare our results with recent large scale simulations. We find that the size of quenching artifacts of the masses is strongly correlated with their experimentally observed widths and that the gauge and hadronic scales are consistent.

hep-lat↗

arXiv1996

Parton Sum Rules and Improved Scaling Variable

The effect from quark masses and transversal motion on the Gottfried, Bjorken, and Ellis-Jaffe sum rules is examined by using a quark-parton model of nucleon structure functions based on an improved scaling variable. Its use results in corrections to the Gottfried, Bjorken, and Ellis-Jaffe sum rules. We use the Brodsky-Huang-Lepage prescription of light-cone wavefunctions to estimate the size of the corrections. We constrain our choice of parameters by the roughly known higher twist corrections to the Bjorken sum rule and find that the resulting corrections to the Gottfried and Ellis-Jaffe sum rules are relevant, though not large enough to explain the observed sum rule violations.

hep-ph↗

arXiv1996

MC simulation of vector boson production in polarized proton collisions

We present a Monte Carlo (MC) study of $W^{\pm}$ and $Z_{0}$ production of longitudinally polarized proton collisions at a center of mass (CMS) energy of $500\,{\rm GeV}$. All results were obtained with the SPHINX MC code. We consider two different types of polarized parton distributions with large and small gluon polarization. Various spin asymmetries are found to be sensitive to the shape of the polarized gluon distribution between $0.1\stackrel{<}{\sim}x\stackrel{<}{\sim}0.5$. The asymmetries are approximately constant with respect to the transverse momentum carried by the $W^{\pm}$ and $Z_{0}$, {\em except} if the first moment of the gluon polarization is large. In this case some $W^{\pm}$ asymmetries show a significant variation with transverse momentum.

hep-ph↗