Search for direct top squark pair production in final states with one isolated lepton, jets, and missing transverse momentum in $sqrt{s}=8,$TeV $pp$ collisions using 21 fb$^{-1}$ of ATLAS data

A search is presented for direct top squark pair production in final states with one isolated electron or muon, jets, and missing transverse momentum in proton-proton collisions at a centre-of-mass energy of 8 TeV. The analysis is based on 20.7 fb$^{-1}$ of data collected with the ATLAS detector at the LHC. The top squarks are assumed to decay to a top quark and the lightest supersymmetric particle (LSP) or to a bottom quark and a chargino, where the chargino decays to an on- or off-shell $W$ boson and to the LSP. The data are found to be consistent with Standard Model expectations. Assuming both top squarks decay to a top quark and the LSP, top squark masses between 200 and 610 GeV are excluded at 95% confidence level for massless LSPs, and top squark masses around 500 GeV are excluded for LSP masses up to 250 GeV. Assuming both top squarks decay to a bottom quark and the lightest chargino, top squark masses up to 410 GeV are excluded for massless LSPs and an assumed chargino mass of 150 GeV.