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Testing the SUSY-QCD Yukawa coupling in a combined LHC/ILC analysis

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 Added by Ayres Freitas
 Publication date 2006
  fields
and research's language is English




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In order to establish supersymmetry (SUSY) at future colliders, the identity of gauge couplings and the corresponding Yukawa couplings between gauginos, sfermions and fermions needs to be verified. Here a first phenomenological study for determining the Yukawa coupling of the SUSY-QCD sector is presented, using a method which combines information from LHC and ILC.



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Supersymmetry predicts that gauge couplings are equal to the corresponding gaugino-sfermion-fermion Yukawa couplings. This prediction can be tested for the QCD sector of the MSSM by studying the processes eplus+eminus -> squark+antisquark+gluon and eplus+eminus -> squark+antiquark+gluino at a future linear collider. We present results for these processes at next-to-leading order in alpha_s in the framework of the MSSM. We find sizable SUSY-QCD corrections. The renormalization scale dependence is significantly reduced at next-to-leading order.
Combined analyses at the Large Hadron Collider and at the International Linear Collider are important to unravel a difficult region of supersymmetry that is characterized by scalar SUSY particles with masses around 2 TeV. Precision measurements of masses, cross sections and forward-backward asymmetries allow to determine the fundamental supersymmetric parameters even if only a small part of the spectrum is accessible. Mass constraints for the heavy particles can be derived.
We present a case study for the synergy of combined LHC and LC analyses in Susy searches in which simultaneous running of both machines is very important. In this study only light non-coloured Susy particles are accessible at a Linear Collider with an initial energy of $sqrt{s}=500$ GeV. Nevertheless the precise analysis at the LC leads to an accurate Susy parameter determination and prediction of heavy Susy particles. Providing these LC results as input for the LHC analyses could be crucial for the identification of signals resulting in a direct measurement of the heavy neutralinos. The interplay of the LHC and LC will thus provide an important consistency test of the underlying model.
Measurement of the top-Yukawa coupling is important to understand the fermion mass generation mechanism and dynamics of electroweak symmetry breaking. We discuss the top quark anomalous couplings which can be described by higher dimensional operators. We investigate the process $e^-e^+ to W^-W^+ ubar u to t bar t ubar u$ to study the contribution of the anomalous top-Higgs coupling to the cross section. The effect of the dimension-six top-Higgs interaction on the cross section can be a few hundred percent greater than the SM prediction. Such a large effect can be measured at the International Linear Collider.
Simplified models have become a widely used and important tool to cover the more diverse phenomenology beyond constrained SUSY models. However, they come with a substantial number of caveats themselves, and great care needs to be taken when drawing conclusions from limits based on the simplified approach. To illustrate this issue with a concrete example, we examine the applicability of simplified model results to a series of full SUSY model points which all feature a small stau-LSP mass difference, and are compatible with electroweak and flavor precision observables as well as current LHC results. Various channels have been studied using the Snowmass Combined LHC detector implementation in the Delphes simulation package, as well as the Letter of Intent or Technical Design Report simulations of the ILD detector concept at the ILC. We investigated both the LHC and ILC capabilities for discovery, separation and identification of all parts of the spectrum. While parts of the spectrum would be discovered at the LHC, there is substantial room for further discoveries and property determination at the ILC.
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