Abstract
The results of a search for top squark (stop) pair production in final states with one isolated lepton, jets, and missing transverse momentum are reported. The analysis is performed with proton--proton collision data at $\sqrt{s} = 8$ TeV collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of $20$ fb$^{-1}$. The lightest supersymmetric particle (LSP) is taken to be the lightest neutralino which only interacts weakly and is assumed to be stable. The stop decay modes considered are those to a top quark and the LSP as well as to a bottom quark and the lightest chargino, where the chargino decays to the LSP by emitting a $W$ boson. A wide range of scenarios with different mass splittings between the stop, the lightest neutralino and the lightest chargino are considered, including cases where the $W$ bosons or the top quarks are off-shell. Decay modes involving the heavier charginos and neutralinos are addressed using a set of phenomenological models of supersymmetry. No significant excess over the Standard Model prediction is observed. A stop with a mass between $210$ and $640$ GeV decaying directly to a top quark and a massless LSP is excluded at $95$ % confidence level, and in models where the mass of the lightest chargino is twice that of the LSP, stops are excluded at $95$ % confidence level up to a mass of $500$ GeV for an LSP mass in the range of $100$ to $150$ GeV. Stringent exclusion limits are also derived for all other stop decay modes considered, and model-independent upper limits are set on the visible cross-section for processes beyond the Standard Model.
Highlights
The hierarchy problem [1,2,3,4] has gained additional attention with the observation of a new particle consistent with the Standard Model (SM) Higgs boson [5, 6] at the LHC [7]
To account for discrepancies between data and simulated ttevents, the simulated sample is reweighted as a function of the pT of the ttsystem; the weights are based on the ATLAS measurement of the differential ttcross-section at 7 TeV, following the method described in ref
The production of ttin association with vector bosons is calculated at NLO [85, 86], while the production of a single top quark in association with a Z boson is normalised to the LO cross-sections from the generator, because NLO calculations are only available for t-channel production [87]
Summary
The hierarchy problem [1,2,3,4] has gained additional attention with the observation of a new particle consistent with the Standard Model (SM) Higgs boson [5, 6] at the LHC [7]. Effects of the renormalisation group equations are strong for the third generation squarks, usually driving their masses significantly lower than those of the other generations These considerations suggest a light stop which, together with the stringent LHC limits excluding other coloured supersymmetric particles up to masses at the TeV level, motivates dedicated stop searches. SUSY models can violate the conservation of baryon number and lepton number, resulting in a proton lifetime shorter than current experimental limits [25]. This is commonly solved by introducing a multiplicative quantum number called R-parity, which is 1 and −1 for all SM and SUSY particles, respectively. Indirect searches for stops, mediated by gluino pair production, have been reported by the ATLAS [47,48,49,50] and CMS [39, 40, 51,52,53,54,55] collaborations
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