Abstract
A search for the pair production of top squarks, each with R-parity-violating decays into two Standard Model quarks, is performed using 17.4 fb$^{-1}$ of $\sqrt{s}$ = 8 TeV proton-proton collision data recorded by the ATLAS experiment at the LHC. Each top squark is assumed to decay to a b- and an s-quark, leading to four quarks in the final state. Background discrimination is achieved with the use of b-tagging and selections on the mass and substructure of large-radius jets, providing sensitivity to top squark masses as low as 100 GeV. No evidence of an excess beyond the Standard Model background prediction is observed and top squarks decaying to $\bar{b}\bar{s}$ are excluded for top squark masses in the range $100 \leq m_{\tilde{t}} \leq 315$ GeV at 95% confidence level.
Highlights
Background estimationThe estimation of the dominant SM multijet background in the signal region, including both the expected number of events and the shape of the mjaevtg background spectrum, is performed directly from the data
The bjet-multiplicity mjaevtg shape systematic uncertainty is calculated as the maximum of the bin-by-bin difference of region A2 compared to A0 (figure 6(a)) and C2 compared to C0, σib−jet−multi. syst. = max [|1 − νA2,i/νA0,i|, |1 − νC2,i/νC0,i|], (7.1)
Since POWHEG+PYTHIA Monte Carlo (MC) simulation is used to determine the contribution from ttevents in the signal region and each of the control regions, systematic uncertainties related to the MC simulation of the process itself are included in the total systematic uncertainty for the background estimation
Summary
The ATLAS detector [50, 51] provides nearly full solid angle coverage around the collision point with an inner tracking system (inner detector, or ID) covering the pseudorapidity. Range |η| < 2.5, electromagnetic (EM) and hadronic calorimeters covering |η| < 4.9, and a muon spectrometer covering |η| < 2.7 that provides muon trigger capability up to |η| < 2.4. The EM and hadronic calorimeters are composed of multiple subdetectors spanning |η| ≤ 4.9. The EM barrel calorimeter uses a liquid-argon (LAr) active medium and lead absorbers. In the region |η| < 1.7, the hadronic (Tile) calorimeter is constructed from steel absorber and scintillator tiles and is separated into barrel (|η| < 1.0) and extended-barrel (0.8 < |η| < 1.7) sections. The endcap (1.375 < |η| < 3.2) and forward (3.1 < |η| < 4.9) regions are instrumented with LAr calorimeters for EM as well as hadronic energy measurements. The different parts of the trigger system are referred to as the level-1 trigger, the level-2 trigger, and the event filter [53].
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