Abstract

A search is presented for direct top-squark pair production in final states with two leptons (electrons or muons) of opposite charge using 20.3fb-1 of pp collision data at sqrt(s)=8TeV, collected by the ATLAS experiment at the Large Hadron Collider in 2012. No excess over the Standard Model expectation is found. The results are interpreted under the separate assumptions (i) that the top squark decays to a b-quark in addition to an on-shell chargino whose decay occurs via a real or virtual W boson, or (ii) that the top squark decays to a t-quark and the lightest neutralino. A top squark with a mass between 150 GeV and 445 GeV decaying to a b-quark and an on-shell chargino is excluded at 95% confidence level for a top squark mass equal to the chargino mass plus 10 GeV, in the case of a 1 GeV lightest neutralino. Top squarks with masses between 215 (90) GeV and 530 (170) GeV decaying to an on-shell (off-shell) t-quark and a neutralino are excluded at 95% confidence level for a 1 GeV neutralino.

Highlights

  • Background fitThe observed numbers of events in the control regions (CRs) are used to derive SM background estimates in each signal regions (SRs) via a profile likelihood fit [89]

  • A top squark with a mass between 150 GeV and 445 GeV decaying to a b-quark and an on-shell chargino is excluded at 95% confidence level for a top squark mass equal to the chargino mass plus 10 GeV, in the case of a 1 GeV lightest neutralino

  • For the case of a 1 GeV neutralino, a top-squark t1 with a mass between 150 GeV and 445 GeV decaying to a b-quark and a chargino is excluded at 95% confidence level (CL) for a chargino approximately degenerate with the top squark

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Summary

The ATLAS detector

The detector layout consists of inner tracking devices surrounded by a superconducting solenoid, electromagnetic and hadronic calorimeters and a muon spectrometer. The ID is surrounded by a thin superconducting solenoid providing a 2T axial magnetic field and it provides precision tracking of charged particles and vertex reconstruction. The calorimeter system covers the pseudorapidity range |η| < 4.9. In the region |η| < 3.2, high-granularity liquid-argon electromagnetic. The end-cap and forward regions, which cover the range 1.5 < |η| < 4.9, are instrumented with liquid-argon calorimeters for both electromagnetic and hadronic particles. The muon spectrometer surrounds the calorimeters and consists of three large superconducting air-core toroid magnets, each with eight coils, a system of precision tracking chambers (|η| < 2.7) and fast trigger chambers (|η| < 2.4)

Monte Carlo simulations and data samples
Physics object selection
Preselection and event variables
Leptonic mT2 selection
Hadronic mT2 selection
Multivariate analysis
Standard Model background determination
Background fit
Fake and non-prompt lepton background estimation
Leptonic mT2 analysis
Hadronic mT2 analysis
Systematic uncertainties
Background
Results and interpretation
Conclusions
A Generator-level object and event selection
Full Text
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