Abstract
The results of a search for the direct pair production of top squarks, the supersymmetric partner of the top quark, in final states with one isolated electron or muon, several energetic jets, and missing transverse momentum are reported. The analysis also targets spin-0 mediator models, where the mediator decays into a pair of dark-matter particles and is produced in association with a pair of top quarks. The search uses data from proton-proton collisions delivered by the Large Hadron Collider in 2015 and 2016 at a centre-of-mass energy of sqrt{s}=13 TeV and recorded by the ATLAS detector, corresponding to an integrated luminosity of 36 fb−1. A wide range of signal scenarios with different mass-splittings between the top squark, the lightest neutralino and possible intermediate supersymmetric particles are considered, including cases where the W bosons or the top quarks produced in the decay chain are off-shell. No significant excess over the Standard Model prediction is observed. The null results are used to set exclusion limits at 95% confidence level in several supersymmetry benchmark models. For pair-produced top-squarks decaying into top quarks, top-squark masses up to 940 GeV are excluded. Stringent exclusion limits are also derived for all other considered top-squark decay scenarios. For the spin-0 mediator models, upper limits are set on the visible cross-section.
Highlights
The hierarchy problem [1,2,3,4] has gained additional attention with the observation of a particle consistent with the Standard Model (SM) Higgs boson [5, 6] at the Large Hadron Collider (LHC) [7]
This paper presents a search for direct t1 pair production in final states with exactly one isolated charged lepton from the decay of either a real or a virtual W boson
Collinearity of each t1 with both of its decay products is assumed. This results in the transverse-momentum vector of the neutrino from the leptonic W -boson decay being calculable by subtracting the momenta of the lightest supersymmetric particle (LSP) from pmTiss, when assuming a specific mass ratio α: pT(να) = (1 − α)pmTiss − αpT(tIhSaRd + blep + ), where να represents the neutrino four-vector for a given value of α, blep is the b-jet candidate assigned to the semileptonic top-quark candidate and is the charged lepton
Summary
The hierarchy problem [1,2,3,4] has gained additional attention with the observation of a particle consistent with the Standard Model (SM) Higgs boson [5, 6] at the Large Hadron Collider (LHC) [7]. Effects of the renormalisation group equations are strong for the third-generation squarks, usually driving their masses to values significantly lower than those of the other generations These considerations suggest a light stop2 [25, 26] which, together with the stringent LHC limits excluding other coloured supersymmetric particles with masses below the TeV level, motivates dedicated stop searches. The charginos χ±1,2 and neutralinos χ01,2,3,4 are the mass eigenstates formed from the linear superposition of the charged and neutral SUSY partners of the Higgs and electroweak gauge bosons (higgsinos, winos and binos). They are referred to in the following as electroweakinos. Searches for spin-0 mediators decaying into a pair of DM particles and produced in association with heavy-flavour quarks have been reported with zero or two leptons in the final state by the ATLAS collaboration [58], and by the CMS collaboration [41, 59]
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