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T. Riemann

Publications and source records attributed to T. Riemann.

At least 55 records · Page 3Linked to original sources
arXiv2005

Master integrals for massive two-loop Bhabha scattering in QED

We present a set of scalar master integrals (MIs) needed for a complete treatment of massive two-loop corrections to Bhabha scattering in QED, including integrals with arbitrary fermionic loops. The status of analytical solutions for the MIs is reviewed and examples of some methods to solve MIs analytically are worked out in more detail. Analytical results for the pole terms in epsilon of so far unknown box MIs with five internal lines are given.

hep-ph↗
arXiv2004

Physics at the CLIC Multi-TeV Linear Collider

This report summarizes a study of the physics potential of the CLIC e+e- linear collider operating at centre-of-mass energies from 1 TeV to 5 TeV with luminosity of the order of 10^35 cm^-2 s^-1. First, the CLIC collider complex is surveyed, with emphasis on aspects related to its physics capabilities, particularly the luminosity and energy, and also possible polarization, γγand e-e- collisions. The next CLIC Test facility, CTF3, and its R&D programme are also reviewed. We then discuss aspects of experimentation at CLIC, including backgrounds and experimental conditions, and present a conceptual detector design used in the physics analyses, most of which use the nominal CLIC centre-of-mass energy of 3 TeV. CLIC contributions to Higgs physics could include completing the profile of a light Higgs boson by measuring rare decays and reconstructing the Higgs potential, or discovering one or more heavy Higgs bosons, or probing CP violation in the Higgs sector. Turning to physics beyond the Standard Model, CLIC might be able to complete the supersymmetric spectrum and make more precise measurements of sparticles detected previously at the LHC or a lower-energy linear e+e- collider: γγcollisions and polarization would be particularly useful for these tasks. CLIC would also have unique capabilities for probing other possible extensions of the Standard Model, such as theories with extra dimensions or new vector resonances, new contact interactions and models with strong WW scattering at high energies. In all the scenarios we have studied, CLIC would provide significant fundamental physics information beyond that available from the LHC and a lower-energy linear e+e- collider, as a result of its unique combination of high energy and experimental precision.

hep-ph↗
arXiv2004

Automatized calculation of 2-fermion production with DIANA and aITALC

The family of two fermion final states is among the cleanest final states at the International Linear Collider (ILC) project. The package aITALC has been developed for a calculation of their production cross sections, and we present here benchmark numerical results in one loop approximation in the electroweak Standard Model. We are using packages like QGRAF, DIANA, FORM, LOOPTOOLS for intermediate steps.

hep-ph↗
arXiv2004

On master integrals for two loop Bhabha scattering

All scalar master integrals (MIs) for massive 2-loop QED Bhabha scattering are identified. The 2- and 3-point MIs have been calculated in terms of harmonic polylogarithms with the differential equation method. The calculation of 4-point MIs is underway. We sketch some alternative methods which help to solve (mainly) singularities of some MIs.

hep-ph↗
arXiv2004

Automated use of DIANA for two-fermion production at colliders

We describe packages for the calculation of radiative corrections to two-fermion production at colliders. The packages use DIANA, and also QGRAF, FORM, Fortran, and further unix/linux tools. The one-loop calculations in the Standard Model are highly automatized with the package aITALC. Further, the automatic determination of all the matrix elements for two-loop corrections to massive Bhabha scattering in QED and the classification of their topologies and prototypes is done with DIANA. A generalization to the Standard Model is straightforward.

hep-ph↗
arXiv2004

Automated calculations for massive fermion production with aITALC

The package aITALC has been developed for the automated calculation of radiative corrections to two-fermion production at $e^+ e^-$ colliders. The package uses Diana, Qgraf, Form, Fortran, FF, LoopTools, and further unix/linux tools. Numerical results are presented for $e^+e^- \to e^+e^-, mu^+mu^-, b \bar{s}, t \bar{c}$.

hep-ph↗
arXiv2004

A complete set of scalar master integrals for massive 2-loop Bhabha scattering: where we are

We define a complete set of scalar master integrals (MIs) for massive 2-loop QED Bhabha scattering. Among others, there are thirty three 2-loop box type MIs. Five of them have been published in (semi-)analytical form, one is determined here, the rest remains to be calculated. Further, the last four so far unknown 2-loop 3-point MIs are identified and also computed here.

hep-ph↗
arXiv2003

Elektroweak one-loop corrections for e^+e^- annihilation into t\bar{t} including hard bremsstrahlung

We present the complete electroweak one-loop corrections to top-pair production at a linear e^+e^- collider in the continuum region. Besides weak and photonic virtual corrections, real hard bremsstrahlung with simple realistic kinematical cuts is included. For the bremsstrahlung we advocate a semi-analytical approach with a high numerical accuracy. The virtual corrections are parametrized through six independent form factors, suitable for Monte-Carlo implementation. Alternatively, our numerical package topfit, a stand-alone code, can be utilized for the calculation of both differential and integrated cross sections as well as forward--backward asymmetries.

hep-ph↗
arXiv2002

Status of Electroweak Corrections to Top Pair Production

We review the status of electroweak radiative corrections to top-pair production at a Linear Collider well above the production threshold. We describe the Fortran package topfit and present numerical results at sqrt(s) = 500 GeV, 1 TeV, and 3 TeV.

hep-ph↗
arXiv2002

Massive two-loop Bhabha scattering -- the factorizable subset

The experimental precision that will be reached at the next generation of colliders makes it indispensable to improve theoretical predictions significantly. Bhabha scattering (e^+ e^- \to e^+ e^-) is one of the prime processes calling for a better theoretical precision, in particular for non-zero electron masses. We present a first subset of the full two-loop calculation, namely the factorizable subset. Our calculation is based on DIANA. We reduce tensor integrals to scalar integrals in shifted (increased) dimensions and additional powers of various propagators, so-called dots-on-lines. Recurrence relations remove those dots-on-lines as well as genuine dots-on-lines (originating from mass renormalization) and reduce the dimension of the integrals to the generic d = 4 - 2 εdimensions. The resulting master integrals have to be expanded to ${\it O}(ε)$ to ensure proper treatment of all finite terms.

hep-ph↗
arXiv2002

Factorizing one-loop contributions to two-loop Bhabha scattering and automatization of Feynman diagram calculations

In higher order calculations a number of new technical problems arise: one needs diagrams in arbitrary dimension in order to obtain their needed $ε$-expansion, zero Gram determinants appear, renormalization produces diagrams with `dots' on the lines, i.e. higher order powers of scalar propagators. All these problems cannot be accessed by the `standard' Passarino-Veltman approach: there is not available what is needed for higher loops. We demonstrate our method of how to solve these problems.

hep-ph↗
arXiv2002

Complete electroweak one-loop radiative corrections to top-pair production at TESLA -- a comparison

Electroweak one-loop radiative corrections to the process e^+ e^- -> t tbar are revisited. Two groups from Karlsruhe and Bielefeld/Zeuthen performed independent calculations of both (virtual and soft) QED contributions and weak virtual corrections. For the angular distribution an agreement of at least eight digits for the weak corrections and of at least seven digits for additional photonic corrections is established.

hep-ph↗
arXiv2002

One-loop corrections to the process e+ e- -> t tbar including hard bremsstrahlung

Radiative corrections to the process e+ e- to t tbar are calculated in one-loop approximation of the Standard Model. There exist results from several groups. This talk provides further comparisons of the complete elektroweak contributions, including hard bremsstrahlung. The excellent final agreement of the different groups allows to continue by working on a code for an event generator for TESLA and an extension to e+ e- to 6 fermions.

hep-ph↗
arXiv2001

Predictions for ${\bar ν} νγ$ Production at LEP

We study predictions for the reaction $e^+e^- \to {\bar ν} ν(n γ)$. The complete one-loop corrections are taken into account and higher order contributions, in particular those for the observed real photons, are added whenever necessary. The event generator \KK MC, a general-purpose Monte Carlo generator for the process $e^+e^- \to \bar f f nγ$ based on the method of exclusive exponentiation, is used as the environment. We extend its applicability to the process $e^+e^- \to \bar ν_l ν_l nγ$, $l=e,μ,τ$, where the observation of at least a single $γ$ is required. The exponentiation is implemented in much the same way as for the s-channel process alone. In particular, all photonic effects present in the case of W exchange, which {\it cannot} be included in the s-channel exponentiation scheme, are calculated to a finite order only. The real hard photon matrix element is calculated up to ${\cal O} ({α^2})$. Leading logarithmic contributions of the two-loop corrections and one-loop photonic corrections accompanying real single photon emission are included. The electroweak corrections are calculated with the DIZET library of the ZFITTER package. Numerical tests and predictions for typical observables are presented.

hep-ph↗
arXiv2001

SMATASY - A program for the model independent description of the Z resonance

SMATASY is an interface for the ZFITTER package and may be used for the model independent description of the Z resonance at LEP 1 and SLC. It allows the determination of the Z mass and width and its resonance shape parameters r and j for cross-sections and their asymmetries. The r describes the peak height and j the interference of the Z resonance with photon exchange in each scattering channel and for $σ_T$, $σ_{FB}$, $σ_{lr}$, $σ_{pol}$ etc. separately. Alternatively, the helicity amplitudes for a given scattering channel may be determined. We compare our formalism with other model independent approaches. The model independent treatment of QED corrections in SMATASY is applicable also far away from the Z peak.

hep-ph↗
arXiv2001

Probing top flavour-changing neutral couplings at TESLA

We present a comprehensive analysis of the sensitivity of the TESLA e+ e- collider to top flavour-changing neutral couplings to the Z boson and photon. We study single top production and top decay processes, and we consider the cases without beam polarization, with only e- polarization and with e- and e+ polarization. We show that the use of the latter substantially enhances the sensitivity to discover or bound these vertices, and for some of the couplings the expected LHC limits could be improved by factors 2-14 for equal running times.

hep-ph↗