Abstract

A search for the supersymmetric partners of the Standard Model bottom and top quarks is presented. The search uses 36.1 fb−1 of pp collision data at sqrt{s}=13 TeV collected by the ATLAS experiment at the Large Hadron Collider. Direct production of pairs of bottom and top squarks ( {overline{b}}_1 and {overline{t}}_1 ) is searched for in final states with b-tagged jets and missing transverse momentum. Distinctive selections are defined with either no charged leptons (electrons or muons) in the final state, or one charged lepton. The zero-lepton selection targets models in which the {overline{b}}_1 is the lightest squark and decays via {overline{b}}_1to b{overline{chi}}_1^0 , where {overline{chi}}_1^0 is the lightest neutralino. The one-lepton final state targets models where bottom or top squarks are produced and can decay into multiple channels, {overline{b}}_1to b{overline{chi}}_1^0 and {overline{b}}_1to t{overline{chi}}_1^{pm } , or {overline{t}}_1to t{overline{chi}}_1^0 and {overline{t}}_1to b{overline{chi}}_1^{pm } , where {overline{chi}}_1^{pm } is the lightest chargino and the mass difference {m}_{{overline{chi}}_1^{pm }}-{m}_{{overline{chi}}_1^0} is set to 1 GeV. No excess above the expected Standard Model background is observed. Exclusion limits at 95% confidence level on the mass of third-generation squarks are derived in various supersymmetry-inspired simplified models.

Highlights

  • Background estimationMonte Carlo simulation is used to estimate the background yield in the signal regions

  • This paper presents a search for the direct pair production of bottom and top squarks decaying into final states with jets, two of them originating from the fragmentation of bquarks (b-jets), and missing transverse momentum

  • Third-generation squarks are searched for in events containing large missing transverse momentum and jets, exactly two of which are identified as b-jets

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Summary

ATLAS detector

The ATLAS detector [21] is a multi-purpose particle physics detector with a forwardbackward symmetric cylindrical geometry and nearly 4π coverage in solid angle. The. The. inner tracking detector consists of pixel and silicon microstrip detectors covering the pseudorapidity region |η| < 2.5, surrounded by a transition radiation tracker which enhances electron identification in the region |η| < 2.0. The inner detector is surrounded by a thin superconducting solenoid providing an axial 2 T magnetic field and by a fine-granularity lead/liquid-argon (LAr) electromagnetic calorimeter covering |η| < 3.2. A steel/scintillator-tile calorimeter provides hadronic coverage in the central pseudorapidity range (|η| < 1.7). The endcap and forward regions (1.5 < |η| < 4.9) of the hadronic calorimeter are made of LAr active layers with either copper or tungsten as the absorber material. Three layers of high-precision tracking chambers provide coverage in the range |η| < 2.7, while dedicated fast chambers allow triggering in the region |η| < 2.4. The ATLAS trigger system consists of a hardware-based level-1 trigger followed by a software-based high-level trigger [23]

Data and simulated event samples
Event reconstruction
Event selection
Discriminating variables
Zero-lepton channel selections
One-lepton channel selections
Background estimation
Background estimation in the zero-lepton signal regions
SFOS 1
Background estimation in the one-lepton signal regions
Validation regions
Systematic uncertainties
Results and interpretation
Conclusion
Full Text
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