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Register a new userScalar Top Quark Studies with Various Visible Energies
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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.
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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.
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).
Scalar top studies at the ILC are reviewed from initial sensitivity studies to a new precision mass determination method.
A precise measurement of the top quark mass, a fundamental parameter of the Standard Model, is among the most important goals of top quark studies at the Large Hadron Collider. Apart from the standard methods, numerous new observables and reconstruction techniques are employed to improve the overall precision and to provide different sensitivities to various systematic uncertainties. Recently, the normalised inverse invariant mass distribution of the $tbar{t}$ system and the leading extra jet not coming from the top quark decays has been proposed for the $pp to tbar{t}j$ production process, denoted as ${cal R}(m_t^{pole},rho_s)$. In this paper, a thorough study of different theoretical predictions for this observable, however, with top quark decays included, is carried out. We focus on fixed order NLO QCD calculations for the di-lepton top quark decay channel at the LHC with $sqrt{s}=13$ TeV. First, the impact on the extraction of $m_t$ is investigated and afterwards the associated uncertainties are quantified. In one approach we include all interferences, off-shell effects and non-resonant backgrounds. This is contrasted with a different approach with top quark decays in the narrow width approximation. In the latter case, two cases are employed: NLO QCD corrections to the $ppto tbar{t}j$ production process with leading order decays and the more sophisticated case with QCD corrections and jet radiation present also in top quark decays. The top quark mass sensitivity of ${cal R}(m_t^{pole},rho_s)$ is investigated and compared to other observables: the invariant mass of the top anti-top pair, the minimal invariant mass of the $b$-jet and a charged lepton as well as the total transverse momentum of the $tbar{t}j$ system.
Effective field theory (EFT) approaches are widely used at the LHC, such that it is important to study their validity, and ease of matching to specific new physics models. In this paper, we consider an extension of the SM in which a top quark couples to a new heavy scalar. We find the dimension six operators generated by this theory at low energy, and match the EFT to the full theory up to NLO precision in the simplified model coupling. We then examine the range of validity of the EFT description in top pair production, finding excellent validity even if the scalar mass is only slightly above LHC energies, provided NLO corrections are included. In the absence of the latter, the LO EFT overestimates kinematic distributions, such that over-optimistic constraints on BSM contributions are obtained. We next examine the constraints on the EFT and full models that are expected to be obtained from both top pair and four top production at the LHC, finding for low scalar masses that both processes show similar exclusion power. However, for larger masses, estimated LHC uncertainties push constraints into the non-perturbative regime, where the full model is difficult to analyse, and thus not perturbatively matchable to the EFT. This highlights the necessity to improve uncertainties of SM hypotheses in top final states.
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