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Light scalar top quarks with a small mass difference with respect to the neutralino mass are of particular cosmological interest. This study uses an Iterative Discriminant Analysis method to optimize the expected selection efficiency at a International Linear Collider (ILC).
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Light scalar top quarks with a small mass difference with respect to the neutralino mass are of particular cosmological interest. This study uses an Iterative Discriminant Analysis method to optimize the expected selection efficiency at a International Linear Collider (ILC). A previous study at sqrt(s)=260 GeV with 50 fb-1 has been extended to sqrt(s)=500 GeV with 500 fb-1, and results from both studies are compared.
The precision determination of scalar top quark properties will play an important role at a future International Linear Collider (ILC). Recent and ongoing studies are discussed for different experimental topologies in the detector. First results are presented for small mass differences between the scalar top and neutralino masses. This corresponds to a small expected visible energy in the detector. An ILC will be a unique accelerator to explore this scenario. In addition to finding the existence of light stop quarks, the precise measurement of their properties is crucial for testing their impact on the dark matter relic abundance and the mechanism of electroweak baryogenesis. Significant sensitivity for mass differences down to 5 GeV are obtained. The simulation is based on a fast and realistic detector simulation. A vertex detector concept of the Linear Collider Flavor Identification (LCFI)collaboration, which studies pixel detectors for heavy quark flavour identification, is implemented in the simulations for c-quark tagging. The study extends simulations for large mass differences (large visible energy) for which aspects of different detector simulations, the vertex detector design, and different methods for the determination of the scalar top mass are discussed. Based on the detailed simulations we study the uncertainties for the dark matter density predictions and their estimated uncertainties from various sources. In the region of parameters where stop-neutralino co-annihilation leads to a value of the relic density consistent with experimental results, as precisely determined by the Wilkinson Microwave Anisotropy Probe (WMAP), the stop-neutralino mass difference is small and the ILC will be able to explore this region efficiently.
We report on detailed Monte Carlo comparisons of selection variables to separate tbH+ signal events from the Standard Model ttbar background using an Iterative Discriminant Analysis (IDA) method. While kinematic differences exist between the two processes whenever m(H+).ne.m(W+), the exploration of the spin difference between the charged Higgs and the W+ gauge boson becomes crucial in the particularly challenging case of near degeneracy of the charged Higgs boson mass with the W+ mass. The TAUOLA package is used to decay the tau leptons emerging from the charged Higgs and W+ boson decays taking the spin difference properly into account. We demonstrate that, even if the individual selection variables have limited discriminant power, the IDA method achieves a significant separation between the expected signal and background. For both Tevatron and LHC energies, the impact of the spin effects and H+ mass on the separation of signal and background has been studied quantitatively. The effect of a hard transverse momentum cut to remove QCD background has been studied and it is found that the spin effects remain important. The separation is expressed in purity versus efficiency curves. The study is performed for charged Higgs boson masses between the W+ mass and near the top mass.
We evaluate all two-body decay modes of the heavy scalar top quark in the Minimal Supersymmetric Standard Model with complex parameters (cMSSM) and no generation mixing. The evaluation is based on a full one-loop calculation of all decay channels, also including hard QED and QCD radiation. The renormalization of the complex parameters is described in detail. The dependence of the heavy scalar top quark decay on the relevant cMSSM parameters is analyzed numerically, including also the decay to Higgs bosons and another scalar quark or to a top quark and the lightest neutralino. 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 the LHC and, if kinematically allowed, at the ILC. The evaluation of the branching ratios of the heavy scalar top quark will be implemented into the Fortran code FeynHiggs.
Scalar top studies at the ILC are reviewed from initial sensitivity studies to a new precision mass determination method.
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