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C. Schappacher

Publications and source records attributed to C. Schappacher.

33 records · Page 2Linked to original sources
arXiv2012

Chargino Decays in the Complex MSSM: A Full One-Loop Analysis

We evaluate two-body decay modes of charginos in the Minimal Supersymmetric Standard Model with complex parameters (cMSSM). Assuming heavy scalar quarks we take into account all decay channels involving charginos, neutralinos, (scalar) leptons, Higgs bosons and Standard Model gauge bosons. The evaluation of the decay widths is based on a full one-loop calculation including hard and soft QED radiation. Special attention is paid to decays involving the Lightest Supersymmetric Particle (LSP), i.e. the lightest neutralino, or a neutral or charged Higgs boson. The higher-order corrections of the chargino decay widths involving the LSP can easily reach a level of about \pm 10%, while the corrections to the decays to Higgs bosons are slightly smaller, translating into corrections of similar size in the respective branching ratios. These corrections are important for the correct interpretation of LSP and Higgs production at the LHC and at a future linear e+e- collider. The results will be implemented into the Fortran code FeynHiggs.

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arXiv2012

Neutralinos from Chargino Decays in the Complex MSSM

We review the evaluation of two-body decay modes of charginos in the Minimal Supersymmetric Standard Model with complex parameters (cMSSM). Assuming heavy scalar quarks we take into account all decay channels involving charginos, neutralinos, (scalar) leptons, Higgs bosons and SM gauge bosons. The evaluation of the decay widths is based on a full one-loop calculation including hard and soft QED radiation. Here we focus on the decays involving the Lightest Supersymmetric Particle (LSP), i.e. the lightest neutralino, or a heavier neutralino and a W boson. The higher-order corrections of the chargino decay widths can easily reach a level of \pm 10%, translating into corrections of similar size in the respective branching ratios. These corrections are important for the correct interpretation of LSP and heavier neutralino production at the LHC and at a future linear e+e- collider.

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arXiv2012

MSSM Higgs Bosons from Stop and Chargino Decays

The Higgs bosons of the MSSM can be produced from the decay of SUSY particles. We review the evalulation of two decay modes in the MSSM with complex parameters (cMSSM). The first type is the decay of the heavy scalar top quark to a lighter scalar quark and a Higgs boson. The second type is the decay of the heavy chargino to a lighter chargino/neutralino and a Higgs boson. The evaluation is based on a full one-loop calculation including hard QED and QCD radiation. We find sizable contributions to many partial decay widths and branching ratios. They are roughly of O(10%) of the tree-level results, but can go up to 30% or higher. These contributions are important for the correct interpretation of scalar top quark decays at a future linear e+e- collider.

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arXiv2011

Gluino Decays in the Complex MSSM: A Full One-Loop Analysis

We evaluate all two-body decay modes of the gluino, in the Minimal Supersymmetric Standard Model with complex parameters (cMSSM). This constitutes an important step in the cascade decays of SUSY particles at the LHC. The evaluation is based on a full one-loop calculation of all two-body decay channels, also including hard QED and QCD radiation. The dependence of the gluino decay to a scalar quark and a quark on the relevant cMSSM parameters is analyzed numerically. We find sizable contributions to the decay widths and branching ratios. They are, roughly of O(5%), but can go up to +-10% or higher, where the pure SUSY QCD contributions alone can give an insufficient approximation to the full one-loop result. Therefore the full corrections are important for the correct interpretation of gluino decays at the LHC. The results will be implemented into the Fortran code FeynHiggs.

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arXiv2010

Consistent Higher-Order Corrections to Stop_i -> Sbottom_j H^+ in the Complex MSSM

We review an analysis of a consistent renormalization of the top and bottom quark/squark sector of the MSSM with complex parameters (cMSSM). Various renormalization schemes are defined, analyzed analytically and tested numerically in the decays Stop_2 -> Sbottom_i H^+/W^+ (i = 1,2). No scheme is found that produces numerically acceptable results over all the cMSSM parameter space, where problems occur mostly already for real parameters. Some numerical examples for Gamma(Stop_2 -> Sbottom_1 H^+) in our preferred scheme, "m_b, A_b DRbar" are shown.

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arXiv2010

Proposals for Bottom Quark/Squark Renormalization in the Complex MSSM

We present a consistent renormalization of the top and bottom quark/squark sector of the MSSM with complex parameters (cMSSM). Various renormalization schemes are defined, analyzed analytically and tested numerically in the decays Stop_2 -> Sbot_i H+/W+ (i = 1,2). No scheme is found that produces numerically acceptable results over all the cMSSM parameter space, where problems occur mostly already for real parameters. Two schemes are identified that show the most robust behavior. A numerical analysis of the four partial stop decay widths is performed in our "preferred" scheme, "m_b, A_b DRbar". The full one-loop corrections to the corresponding partial decay widths are evaluated including hard QED and QCD radiation. We find mostly modest corrections at the one-loop level.

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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.

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arXiv2001

Top Dipole Form Factors and Loop-induced CP-violation in Supersymmetry

The one-loop Minimal Supersymmetric Standard Model (MSSM) contributions to the weak and electromagnetic dipole form factors of the top quark are presented. Far from the Z peak, they are not sufficient to account for all the new physics effects. In the context of the calculation of the process e^+e^- -> t tbar to one loop in the MSSM, we compare the impact on the phenomenology of the CP-violating dipole form factors of the top quark with the contribution from CP-violating box graphs. Some exemplificative observables are analyzed and the relevance of both the contributions is pointed out. The one-loop expressions for the electromagnetic and weak dipole form factors in a general renormalizable theory and the SM and MSSM couplings and conventions are also given.

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arXiv2001

Dipole Form Factors and Loop-induced CP-violation in Supersymmetry

The one-loop Minimal Supersymmetric Standard Model (MSSM) contributions to the weak and electromagnetic dipole form factors of heavy fermions are reviewed. For the $Z$ boson on shell, the weak-magnetic and weak-electric dipole moments of the $τ$ lepton and the $b$ quark can be defined and directly connected to observables. But far from the $Z$ peak, the weak and electromagnetic dipole form factors are not enough to account for all the new physics effects. In the context of the calculation of the process $e^+e^-\to t\bar{t}$ to one loop in the MSSM, we compare the impact on the phenomenology of the CP-violating dipole form factors of the top quark with the contribution from CP-violating box graphs. Some exemplificative observables are analyzed and the relevance of both the contributions is pointed out. The set of tensor integrals employed, the one-loop expressions for the electromagnetic and weak dipole form factors in a general renormalizable theory and the SM and MSSM couplings and conventions are also given.

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arXiv2001

Electroweak Radiative Corrections to Neutral-Current Drell-Yan Processes at Hadron Colliders

We calculate the complete electroweak O(alpha) corrections to pp, pbar p -> l+l- X (l=e, mu) in the Standard Model of electroweak interactions. They comprise weak and photonic virtual one-loop corrections as well as real photon radiation to the parton-level processes q bar q -> gamma,Z -> l+l-. We study in detail the effect of the radiative corrections on the l+l- invariant mass distribution, the cross section in the Z boson resonance region, and on the forward-backward asymmetry, A_FB, at the Fermilab Tevatron and the CERN Large Hadron Collider. The weak corrections are found to increase the Z boson cross section by about 1%, but have little effect on the forward-backward asymmetry in the Z peak region. Threshold effects of the W box diagrams lead to pronounced effects in A_FB at m(l+l-) approx 160 GeV which, however, will be difficult to observe experimentally. At high di-lepton invariant masses, the non-factorizable weak corrections are found to become large.

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arXiv2001

The Implementation of the Minimal Supersymmetric Standard Model in FeynArts and FormCalc

We describe the implementation of the MSSM in the diagram generator FeynArts and the calculational tool FormCalc. This extension allows to perform loop calculations of MSSM processes almost fully automatically. The actual implementation has two aspects: The MSSM Feynman rules are specified in a new model file for FeynArts. The computation of the parameters in the MSSM Lagrangian from the input parameters is realized as a Fortran subroutine in the framework of FormCalc. The model file does not depend on the latter, however, and can be used even if one does not want to continue the calculation with FormCalc. The Feynman rules have been entered in a very generic way to allow e.g. scenarios with complex parameters, and have been tested extensively by reproducing known results for several non-trivial scattering processes.

hep-ph↗
arXiv2000

Top-Quark Production and Decay in the MSSM

We review the features of top-quark decays and loop-induced effects in the production cross section and CP-violating observables of e+e- -> t t-bar which are specific to the R-parity conserving Minimal Supersymmetric Standard Model (MSSM).

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arXiv1999

Supersymmetric one-loop corrections to the process e^+e^- to f fbar

Radiative one-loop corrections to fermion pair production in $e^+e^-$ annihilation are calculated in the Minimal Supersymmetric Standard Model (MSSM). The size of the non-standard corrections is discussed for the process e^+e^- to mu^+mu^- and e^+e^- to hadrons at LEP2 energies and for e^+e^- to t tbar at a high energy e^+e^- collider. The relative difference between the predictions of the MSSM and the Standard Model is typically below 10%.

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