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T. Riemann

Publications and source records attributed to T. Riemann.

At least 73 records · Page 4Linked to original sources
arXiv2000

Charged Lepton Flavour Violation from Massive Neutrinos in Z Decays

Present evidences for neutrino masses and lepton flavour mixings allow to predict, in the Standard Model with light neutrinos, branching rates for the decays Z --> e mu, mu tau, e tau of less than 10^{-54}, while present experimental exclusion limits from LEP 1 are of order 10^{-5}. The GigaZ option of the TESLA Linear Collider project will extend the sensitivity down to about 10^{-8}. We study in a systematic way some minimal extensions of the Standard Model and show that GigaZ might well be sensitive to the rates predicted from these scenarios.

hep-ph↗
arXiv2000

Predictions for Z --> mu tau and Related Reactions

We discuss predictions for the lepton-flavour changing decays Z --> e mu, mu tau, e tau which may be searched for at the Giga-Z option of the Tesla Linear Collider project with Z resonance production rates as high as 10^9. We try to be as model-independent as possible and consider both the Dirac and Majorana mass cases. The Standard Model, if being minimally extended by the inclusion of light neutrino masses with some mixings as observed in neutrino oscillation search experiments, predicts completely negligible rates. With a more general neutrino content quite interesting expectations may be derived.

hep-ph↗
arXiv2000

Radiative corrections to e+e- --> f+f-

The past ten years of physics with e+e- colliding experiments at LEP and SLAC have shown the success of these experiments on not only impressively proving the theoretical predictions of the Standard Model (SM), but also to help provide stringent bounds on physics beyond the SM. With this experience in mind, there appear two equally fascinating opportunities for studying fermion-pair production processes at a future Linear Collider (LC). On the one hand, performing high precision measurements to the SM, for example, when running with high luminosity at the Z boson resonance, could be a quick and feasible enterprise in order to pin down the symmetry breaking mechanism of the electroweak sector through indirectly determining the masses of a light SM or MSSM Higgs boson or supersymmetric particles via virtual corrections. On the other hand, looking for such particles in direct production or other `New Physics' effects at energies between, for example, roughly 500 and 800 GeV will naturally be the main motivation to pursue the challenging endeavor of building and utilizing such a unique facility. These two scenarios for the LC shall be sketched here, with particular emphasis on the semi-analytical program ZFITTER for fermion-pair production in comparison with numerical programs like TOPAZ0, KK2f, and others.

hep-ph↗
arXiv2000

Two-Fermion Production in Electron-Positron Collisions

This report summarizes the results of the two-fermion working group of the LEP2-MC workshop, held at CERN from 1999 to 2000. Recent developments in the theoretical calculations of the two fermion production process in the electron-positron collision at LEP2 center of the mass energies are reported. The Bhabha process and the production of muon, tau, neutrino and quark pairs is covered. On the basis of comparison of various calculations, theoretical uncertainties are estimated and compared with those needed for the final LEP2 data analysis. The subjects for the further studies are identified.

hep-ph↗
arXiv2000

Four-Fermion Production in Electron-Positron Collisions

This report summarises the results of the four-fermion working group of the LEP2-MC workshop, held at CERN from 1999 to 2000. Recent developments in the calculation of four-fermion processes in electron-positron collisions at LEP-2 centre-of-mass energies are presented, concentrating on predictions for four main reactions: W-pair production, visible photons in four-fermion events, single-W production and Z-pair production. Based on a comparison of results derived within different approaches, theoretical uncertainties on these predictions are established.

hep-ph↗
arXiv2000

ZFITTER v.6.21 - A Semi-Analytical Program for Fermion Pair Production in e+e- Annihilation

We describe ZFITTER, a Fortran program based on a semi-analytical approach to fermion pair production in e+e- annihilation at a wide range of centre-of-mass energies, including the PETRA, TRISTAN, LEP1/SLC, and LEP2 energies. A flexible treatment of complete O(alpha) QED corrections and of some higher order contributions is made possible with three calculational chains containing different realistic sets of restrictions in the photon phase space. Numerical integrations are at most one-dimensional. Complete O(alpha) weak loop corrections supplemented by selected higher-order terms may be included. The program calculates Delta r, the Z width, differential cross-sections, total cross-sections, integrated forward-backward asymmetries, left-right asymmetries, and for tau pair production also final-state polarization effects. Various interfaces allow fits to be performed with different sets of free parameters.

hep-ph↗
arXiv1999

The Z Boson Resonance Parameters

The Z line shape is measured at LEP/CERN with an accuracy at the per mille level. Usually it is described in the Standard Model of electroweak interactions with account of quantum corrections. Alternatively, one may attempt an S-matrix based model-independent approach in order to extract quantities like mass and width of the Z boson. I describe the formalism and its application to data.

hep-ph↗
arXiv1999

Predictions of ZFITTER v.6 for fermion-pair production with acollinearity cut

ZFITTER is a Fortran package for the description of fermion-pair production in e+e- annihilation. We report on results of a rederivation of the complete set of analytical O(alpha) formulae for the treatment of photonic corrections to the total cross-section and the integrated forward-backward asymmetry with combined cuts on acollinearity angle, acceptance angle, and minimal energy of the fermions. Numerically, the following changes result in ZFITTER v. 6.11 compared to ZFITTER v. 5.20/21: (i) at the Z resonance -- numerical changes are negligible; (ii) at LEP 1 energies off-resonance -- corrections amount to at most few per mil; (iii) at LEP 2 energies -- corrections amount to one per cent or less. Thus, the predictions for LEP/SLC data remain unchanged within the actual errors.

hep-ph↗
arXiv1999

Hard-Photon Emission in e+e- to bar-f f with Realistic Cuts

We derive compact analytical formulae of the Bonneau-Martin type for the reaction e+e- to bar-f f gamma with cuts on minimal energy and acollinearity of the fermions, where the photons may be emitted both from the initial or final states. Soft-photon exponentiation is also taken into account.

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arXiv1998

Predictions for Fermion-Pair Production at LEP

The status of predictions for fermion-pair production at LEP is summarized with emphasis on LEP2 energies and on the physics interest there. Some numerical comparisons with other programs are performed. We also present first results of a semi-analytical recalculation of photonic corrections with acollinearity cut in the ZFITTER approach.

hep-ph↗
arXiv1998

Radiative Corrections to Deep Inelastic Scattering in the Presence of an Additional Z' at LEP x LHC

The study of ep-scattering at LEP x LHC offers an interesting opportunity to search for additional heavy neutral gauge bosons. We study the influence of radiative corrections, especially of the potentially large QED contributions, to the expected cross sections. These corrections are typically of the same order as the effect searched for or even larger and have to be taken into account properly.

hep-ph↗
arXiv1998

ZZ' Mixing in Presence of Standard Weak Loop Corrections

We derive a method for a common treatment of Z' exchange, QED corrections, and weak loops. It is based on the form factor approach to the description of weak loop corrections to partial Z widths and cross sections. Problems connected with ZZ' mixing are discussed with special care. Our theoretical results are applied to the package ZFITTER. We demonstrate two different ways to the data analysis - one based on an extension of the standard model cross sections, the other on model-independent formulae together with the Z width calculations in presence of a Z'. With the resulting package ZEFIT+ZFITTER, LEP1 data from fermion pair production, including Bhabha scattering, can be analysed on, but also off the Z peak. Further, the code may be used at very high energies, e.g. in the region of a possible future linear e+e- collider.

hep-ph↗
arXiv1998

Analytic approach to the complete set of QED corrections to fermion pair production in e+e- annihilation

We present the convolution integral for fermion pair production in the electroweak standard theory to order O(alpha) including also soft photon exponentiation. The result is complete in the sense that it includes initial and final state radiation and their interference. From the basic result - analytic formulae for the differential cross section - we also derive the corresponding expressions for total cross section and integrated forward-backward asymmetry. The numerical importance of different contributions for the analysis of experiments at LEP/SLC energies is discussed.

hep-ph↗
arXiv1997

Electroweak Working Group Report

The report summarizes the results of the activities of the Working Group on Precision Calculations for the Z Resonance at CERN during 1994.

hep-ph↗
arXiv1997

Semi-analytic predictions for W pair production at 500 GeV

We give a short review on the semi-analytic approach to off-shell W pair production at 500 GeV and above. Numerical calculations are performed with the Fortran program GENTLE v. 2.0 and its upgrade. Recent extensions include: (i) angular distribution for the CC11 process with LLA QED corrections; (ii) anomalous couplings. As an application we study the sensitivity of total cross-section and forward-backward asymmetry to anomalous couplings. A noticeable sensitivity from the asymmetry appears only for a parity-violating anomalous coupling.

hep-ph↗
arXiv1996

GENTLE/4fan v.2.0 - A Program for the Semi-Analytic Calculation of Predictions for the Process e+e- -> 4f

We describe version 2.0 of the Fortran program GENTLE/4fan for the semi-analytic computation of cross-sections and distributions in four-fermion production in e+e- annihilation. GENTLE/4fan covers all charged current and neutral current our-fermion final states with no identical particles, no electrons, and no electron neutrinos in the final state. Initial state radiation representing the most relevant quantum corrections and anomalous triple gauge boson couplings have been included.

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