Abstract
Results of a search for supersymmetry via direct production of third-generation squarks are reported, using $20.3$ fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = 8$ TeV recorded by the ATLAS experiment at the LHC in 2012. Two different analysis strategies based on monojet-like and $c$-tagged event selections are carried out to optimize the sensitivity for direct top squark pair production in the decay channel to a charm quark and the lightest neutralino ($\tilde{t}_1 \to c + \tilde{\chi}_{1}^{0}$) across the top squark--neutralino mass parameter space. No excess above the Standard Model background expectation is observed. The results are interpreted in the context of direct pair production of top squarks and presented in terms of exclusion limits in the ($m_{\tilde{t}_1}$, $m_{\tilde{\chi}_{1}^{0}}$) parameter space. A top squark of mass up to about 240 GeV is excluded at 95$\%$ confidence level for arbitrary neutralino masses, within the kinematic boundaries. Top squark masses up to 270 GeV are excluded for a neutralino mass of 200 GeV. In a scenario where the top squark and the lightest neutralino are nearly degenerate in mass, top squark masses up to 260 GeV are excluded. The results from the monojet-like analysis are also interpreted in terms of compressed scenarios for top squark pair production in the decay channel $\tilde{t}_1 \to b + ff^{'} + \tilde{\chi}^{0}_{1}$ and sbottom pair production with $\tilde{b}_1 \to b + \tilde{\chi}^{0}_{1}$, leading to a similar exclusion for nearly mass-degenerate third-generation squarks and the lightest neutralino. The results in this paper significantly extend previous results at colliders.
Highlights
Supersymmetry (SUSY) [1,2,3,4,5,6,7,8,9] is a theoretically favored candidate for physics beyond the Standard Model (SM)
Model uncertainties, related to potential differences between W þ jets and Z þ jets final states, affecting the normalization of the dominant Zð→ ννÞ þ jets and the small Z=γÃð→ τþτ−Þ þ jets and Z=γÃð→ eþe−Þ þ jets background contributions, as determined in the Wð→ μνÞþ jets and Wð→ eνÞ þ jets control regions, are studied in detail. This includes uncertainties related to parton distribution functions (PDFs) and renormalization/factorization scale settings, the parton-shower parameters, and the hadronization model used in the Monte Carlo (MC) simulations, and the dependence on the lepton reconstruction and acceptance
A stop mass of about 240 GeV is excluded at 95% confidence level for m~t1 –mχ~01 < 85 GeV, as the maximum mass difference in which the decay mode
Summary
Supersymmetry (SUSY) [1,2,3,4,5,6,7,8,9] is a theoretically favored candidate for physics beyond the Standard Model (SM). In such a scenario, the decay products of the top squark are too soft to be identified in the final state, and the signal selection relies on the presence of an ISR jet. In the case of sbottom pair production, assuming a SUSY particle mass hierarchy such that the sbottom decays exclusively as b~ 1 → b þ χ~01 (see Fig. 1), the expected signal for direct sbottom pair production is characterized by the presence of two energetic jets from the hadronization of the bottom quarks and large missing transverse momentum from the two LSPs in the final state. The monojetlike results are reinterpreted in terms of the search for sbottom pair production with b~ 1 → b þ χ~01 in a compressed scenario (small sbottom-neutralino mass difference) with two soft b jets and an energetic ISR jet in the final state
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