arXiv subjects

Stefan Weinzierl

Publications and source records attributed to Stefan Weinzierl.

175 records · Page 10Linked to original sources

arXiv2002

Nested Sums, Expansion of Transcendental Functions and Multi-Scale Multi-Loop Integrals

Expansion of higher transcendental functions in a small parameter are needed in many areas of science. For certain classes of functions this can be achieved by algebraic means. These algebraic tools are based on nested sums and can be formulated as algorithms suitable for an implementation on a computer. Examples, such as expansions of generalized hypergeometric functions or Appell functions are discussed. As a further application, we give the general solution of a two-loop integral, the so-called C-topology, in terms of multiple nested sums. In addition, we discuss some important properties of nested sums, in particular we show that they satisfy a Hopf algebra.

hep-ph↗

arXiv2001

Steps towards the QCD calculation for e+ e- --> 3 jets at NNLO

High precision analyses of experimental data for e+ e- annihillation, such as determination of jet rates or event shape observables, call for complete next-to-next-to-leading order (NNLO) perturbative QCD predictions. In this talk, we discuss the various ingredients entering the calculation of the NNLO corrections in e+ e- annihillation.

hep-ph↗

arXiv2001

Comparison of Phase Space Slicing and Dipole Subtraction Methods for gamma^* -> Q Qbar

We compare the phase space slicing and dipole subtraction methods in the computation of the inclusive and differential next-to-leading order cross sections for heavy quark production in the simple process gamma^* -> Q Qbar. For the phase space slicing method we study the effects of improvement terms that remove restrictions on the slicing parameter s_min. For the dipole method our comparison is a first check on some of its counterterms involving massive quarks, derived recently. In our comparison we address issues such as numerical accuracy and efficiency.

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arXiv2001

A general algorithm to generate unweighted events for next-to-leading order calculations in electron-positron annihilation

Given a next-to-leading order calculation, we show how to set up a computer program, which generates a sequence of unweighted momentum configurations, each configuration containing either n or n+1 four-vectors, such that for any infrared safe observable the average over these configurations coincides with the NLO calculation up to errors of order y_res. The core of the algorithm is a method to combine real emission and virtual corrections on a point-by-point basis in hard phase space. The algorithm can be implemented on top of existing NLO calculations.

hep-ph↗

arXiv2001

Dipole formalism with heavy fermions

We extend the dipole formalism of Catani and Seymour to QCD processes involving heavy fermions. We give the appropriate subtraction terms together with their integrated counterpart. All calculations are done within dimensional regularization. Our formulae can be used with three variants of dimensional regularization (conventional dimensional regularization, the 't Hooft-Veltman scheme or the four-dimensional scheme).

hep-ph↗

arXiv2000

New approach for calculating heavy-to-light form factors with QCD sum rules on the light-cone

We suggest a new approach for calculating heavy-to-light form factors. The method is based on light cone sum rules (LCSR) and covers the whole kinematical range of momentum transfer. The derivation of the new sum rule uses a suitable combination of double and single dispersion integrals. As an example we give numerical results for the form factor f^+ for the B -> pion transition.

hep-ph↗

arXiv2000

Introduction to Monte Carlo methods

These lectures given to graduate students in high energy physics, provide an introduction to Monte Carlo methods. After an overview of classical numerical quadrature rules, Monte Carlo integration together with variance-reducing techniques is introduced. A short description on the generation of pseudo-random numbers and quasi-random numbers is given. Finally, methods to generate samples according to a specified distribution are discussed. Among others, we outline the Metropolis algorithm and give an overview of existing algorithms for the generation of the phase space of final state particles in high energy collisions.

hep-ph↗

arXiv2000

Calculational techniques (not only) for single top production

A next-to-leading order calculation for single top production including spin-dependent observables requires efficient techniques for the calculation of the relevant loop amplitudes. We discuss the adaption of dimensional regularization, the spinor helicity method and of tensor integral reduction algorithms to these needs.

hep-ph↗

arXiv1999

QCD sum rules on the light cone and B -> pi form factors

The semileptonic decay B -> pi l nu is one of the most important reactions for the determination of the CKM matrix element |V_{ub}|. However, in order to extract |V_{ub}| from data one needs an accurate theoretical calculation of the hadronic matrix element describing the B to pi transition. QCD sum rules, based on operator-product expansion on the light-cone, provide a reliable approach to this aim. QCD corrections and higher twist contributions can be taken systematically into account.

hep-ph↗

arXiv1999

QCD corrections to e+ e- --> 4 jets

We report on the next-to-leading order QCD calculation for e+ e- --> 4 jets. We explain some modern techniques which have been used to calculate the one-loop amplitudes efficiently. We further report on the general purpose numerical program ``Mercutio'', which can be used to calculate any infrared safe four-jet quantity in electron-positron annihilation at next-to-leading order.

hep-ph↗

arXiv1999

Equivariant Dimensional Regularization

The calculation of loop amplitudes with parity violation or spin effects within dimensional regularization needs a consistent definition of gamma5. Also loop calculations in supersymmetric theories need a consistent definition of gamma5. In this paper we develop a new formalism, which allows us to define consistent regularization schemes. We use Grothendieck's K-functor to construct finite-dimensional vectorspaces of non-integer rank. The rank will play the role of the ``4-2eps'' in conventional dimensional regularization. We then define two regularization schemes, one similar to the 't Hooft--Veltman scheme, the other one as a scheme, where all algebra is performed in four dimensions. Lorentz invariance is maintained in both cases. However the structure of the Clifford algebra cannot be preserved. We show that the HV-like scheme and the four-dimensional scheme correspond to two different deformations of the Clifford algebra. It is the purpose of this paper to advocate the four-dimensional scheme for future calculations, since it is easier to use. As a consistency check we performed explicit one-loop calculations of various triangle anomalies in both schemes and we found agreement with Bardeen's results.

hep-ph↗

arXiv1999

Massive gauge bosons from the conservation of topological winding numbers

We consider a U(1) x SU(2) gauge theory on the four-dimensional manifold S^1 x S^3. If we make the assumption that only gauge transformations connected to the identity are allowed, the winding numbers of U(1) around S^1 and of SU(2) around S^3 become topological conserved quantities. We derive the effective theory for non-trivial winding numbers if all distances are small compared to the radii of the spheres. In the non-abelian case the gauge bosons become massive.

hep-ph↗

arXiv1999

QCD Corrections to Four-jet Production and Three-jet Structure in e+ e- annihilation

We report on the general purpose numerical program MERCUTIO, which can be used to calculate any infrared safe four-jet quantity in electron-positron annihilation at next-to-leading order. The program is based on the dipole formalism and uses a remapping of phase-space in order to improve the efficiency of the Monte Carlo integration. Numerical results are given for the four-jet fraction and the D-parameter. These results are compared with already existing ones in the literature and serve as a cross-check. The program can also be used to investigate the internal structure of three-jet events at NLO. We give results for previously uncalculated observables: the jet broadening variable and the softest-jet explanarity.

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