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A. Schiller

Publications and source records attributed to A. Schiller.

At least 199 records · Page 11Linked to original sources
arXiv1998

Matter near to the Endpoint of the Electroweak Phase Transition

Wave functions and the screening mass spectrum in the 3D SU(2)-Higgs model near to the phase transition line below the endpoint and in the crossover region are calculated. In the crossover region the changing spectrum versus temperature is examined showing the aftermath of the phase transition at lower Higgs mass. Large sets of operators with various extensions are used allowing to identify wave functions in position space.

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

Production of $e^+e^-$ pairs to all orders in $Zα$ for collisions of high-energy muons with heavy nuclei

The $e^+e^-$ pair production in collisions of muon with atoms is considered to all orders in the parameter $Zα$. We obtain energy distribution of $e^+$ and $e^-$ as well as energy loss of muon passing through matter with heavy atoms. The found corrections to the Born contribution do not depend on the details of the target properties except of a simple factor. For the considered example of Pb target the muon energy loss corrections vary from -65 % to -10 % depending on the pair energy.

hep-ph↗
arXiv1998

Embedded Topological Defects in Hot Electroweak Theory: a Lattice Study

We study the properties of Nambu monopoles and Z-vortices in the 3D lattice SU(2) Higgs theory which represents the Standard Model at high temperature. We show that the densities of the Nambu monopoles and the Z-vortices are O(1) in the symmetric phase and generically small in the Higgs phase. Near to the critical Higgs mass and in the vicinity of the phase transition the densities are no more negligible in the broken phase. The percolation probability of the Z-vortex lines is found as a new disorder parameter for this phase transition. We conclude that the transition to the symmetric phase is accompanied by Z-vortex condensation. Simulations comparing elementary and extended vortices and monopoles at different β_G values, aiming to show that the density of vortices and monopoles of fixed physical size might have a well-defined continuum limit, gives encouraging but so far inconclusive results.

hep-lat↗
arXiv1998

Search for a heavy magnetic monopole at the Fermilab Tevatron and CERN LHC

If a heavy Dirac monopole exists, the light to light scattering below the monopole production threshold is enhanced due to the strong coupling of monopoles to photons. This effect could be observable in the collision of virtual photons at proton colliders. At the Tevatron it will be seen as pair production of photons with energies 200--400 GeV and roughly compensated transverse momenta 100--400 GeV/c. This effect could be seen at monopole masses about 1--2.5 TeV at the upgraded Tevatron and 7.4--19 TeV at LHC depending on monopole spin.

hep-ph↗
arXiv1998

$O(a)$ Improvement for Quenched Wilson Fermions

We briefly describe some of our recent results for the mass spectrum and matrix elements using $O(a)$ improved fermions for quenched QCD. Where possible a comparison is made between improved and Wilson fermions.

hep-lat↗
arXiv1997

Perturbative Renormalization of Improved Lattice Operators

We derive bases of improved operators for all bilinear quark currents up to spin two (including the operators measuring the first moment of DIS Structure Functions), and compute their one-loop renormalization constants for arbitrary coefficients of the improvement terms. We have thus control over O(a) corrections, and for a suitable choice of improvement coefficients we are only left with errors of O(a^2).

hep-lat↗
arXiv1997

Local bilinear operators on the lattice and their perturbative renormalisation including O(a) effects

Some basic concepts are discussed to derive renormalisation factors of local lattice operators relevant to deep inelastic structure functions and to other measurable quantities. These $Z$ factors can be used to relate matrix elements measured by lattice techniques to their continuum counterparts. We discuss the $O(a)$ improvement of point and one-link lattice quark operators. Suitable bases of improved operators are derived. Tadpole improvement is applied to get more reliable perturbative results.

hep-lat↗
arXiv1997

Spin Structure Functions from Lattice QCD

We report on new results of the spin dependent structure functions $g_1$ and $h_1$ of the nucleon. An attempt is made to convert the moments, which is what one computes on the lattice, to quark distribution functions.

hep-ph↗
arXiv1997

Helicity amplitudes for the small angle lepton pair production in e^+e^- or μ^+ μ^- collisions

The lepton pair production $e^- e^+ \to e^- e^+ l^- l^+ $ or $μ^- μ^+ \to μ^- μ^+ l^- l^+ $ is studied in the dominant cross section region of scattering angles ${m_j}/{E_j} \stackrel{<}{\sim} θ_j \ll 1 $. An analytical expression is found for all 64 helicity amplitudes of these processes. The accuracy of the obtained formulae is given omitting only terms of the order of ${m_j^2}/{E_j^2}$, $θ_j^2$ and $ θ_j {m_j}/{E_j}$. The result has a compact form convenient both for analytical and numerical calculations of various cross sections in this dominating scattering regime.

hep-ph↗
arXiv1997

Lattice Renormalization of Quark Operators

We have technically improved the non-perturbative renormalization method, proposed by Martinelli et al., by using quark momentum sources and sinks. Composite two-fermion operators up to three derivatives have been measured for Wilson fermions and Sheikholeslami-Wohlert improved fermions in the quenched approximation. The calculations are performed in the Landau gauge on 16^3x32 lattices at beta = 6.0 for 3 kappa values in each case. The improved sources greatly decrease the statistical noise. We extract and discuss here renormalization factors for local operators and moments of the structure functions for Wilson fermions.

hep-lat↗
arXiv1997

New results for non-perturbative order(a) improvement in light hadrons

We have results from light hadron simulations in quenched QCD at beta=6.0 and 6.2 using non-perturbatively improved Sheikholeslami-Wohlert fermions in an effort to remove all order(a) effects. From looking at hadron masses and splittings and the RG-invariant quark masses (where we have one point at beta=5.7) we suggest this is plausible even with the limited data set. The interpretation of the decay constants appears to be less clear.

hep-lat↗
arXiv1997

Non-perturbative improvement and renormalization of lattice operators

The Alpha Collaboration has proposed an optimal value for c_SW in the Sheikholeslami-Wohlert action, chosen to remove O(a) effects. To measure hadronic matrix elements to the same accuracy we need a method of finding O(a) improved operators, and their renormalization constants. We determine the Z factors by a non-perturbative method, measuring the matrix elements for single quark states propagating through gauge fields in the Landau gauge. The data show large effects coming from chiral symmetry breaking. This allows us to find the improvement coefficients too, by requiring that the amount of chiral symmetry breaking agrees with that predicted by the chiral Ward identities.

hep-lat↗
arXiv1997

$O(a)$ Improvement of Nucleon Matrix Elements

We report on preliminary results of a high statistics quenched lattice QCD calculation of nucleon matrix elements within the Symanzik improvement programme. Using the recently determined renormalisation constants from the Alpha Collaboration we present a fully non-pertubative calculation of the forward nucleon axial matrix element with $O(a)$ lattice artifacts completely removed. Runs are made at $β=6.0$ and $β=6.2$, in an attempt to check scaling and $O(a^2)$ effects. We shall also briefly describe results for $ $, the matrix element of a higher derivative operator.

hep-lat↗
arXiv1997

Improved Quenched QCD on Large Lattices - First Results

Continuing our investigations of quenched QCD with improved fermions we have started simulations for lattice size 32^3 x 64 at beta=6.2. We present first results for light hadron masses at kappa=0.13520, 0.13540, and 0.13555. Moreover we compare our initial experiences on the T3E with those for APE/Quadrics systems.

hep-lat↗
arXiv1997

Hot electroweak matter near to the endpoint of the phase transition

The electroweak phase transition is investigated near to its endpoint in the framework of an effective three-dimensional model. We measure the very weak interface tension with the tunneling correlation length method. First results for the mass spectrum and the corresponding wave functions in the symmetric phase are presented.

hep-lat↗