#14.3fb

A search is presented for production of a heavy up-type quark ( ) together with its antiparticle, assuming a significant branching ratio for subsequent  decay into a Standard Model Higgs boson and a top quark, as predicted by vector-like quark models. The search is based on 14.3 fb of  collisions at  TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton+jets final state, characterised by an isolated electron or muon with moderately high transverse momentum, significant missing transverse momentum, and at least six jets. The search exploits the high total transverse momenta of all final state objects and the high multiplicity of  jets characteristic of signal events with at least one Higgs boson decaying into , to discriminate against the dominant background from top quark pair production. No significant excess of events above the Standard Model expectation is observed, and upper limits are derived for vector-like quarks of various masses in the two-dimensional plane of  versus , where  is the Standard Model Higgs boson, assumed to have a mass of  GeV. Under the branching ratio assumptions corresponding to a weak-isospin doublet (singlet) scenario, a  quark with mass lower than 790 (640) GeV is excluded at the 95\% confidence level.

A search for  bosons is performed with the ATLAS detector at the LHC in proton-proton collisions at a centre-of-mass energy of = 8 TeV, using a dataset corresponding to an integrated luminosity of 14.3 . This analysis searches for  bosons in the  decay channel in final states with electrons or muons, using a multivariate method based on boosted decision trees. The search range covers  -boson masses between 0.5 and 3.0 TeV, with right-handed or left-handed -boson chiralities. No significant deviation from the Standard Model expectation is observed and upper limits are set on the  cross-section times branching ratio and on the -boson effective couplings as a function of the -boson mass, using the CLs procedure. For a left-handed (right-handed)  boson, masses below 1.74 (1.84) TeV are excluded at 95% confidence level.

A search is presented for exotic processes that result in final states containing jets including at least one  jet, sizable missing transverse momentum, and a pair of leptons with the same electric charge. There are several models that predict an enhanced rate of production of such events beyond the expectations of the Standard Model (SM); the ones considered in this note are pair production of chiral  quarks, pair production of vector-like quarks, enhanced four top quark production and production of two positively-charged top quarks. Using a sample of 14.3 fb of  collisions at  TeV recorded by the ATLAS detector at the Large Hadron Collider, with selection criteria optimised for each signal, no significant excess of events over the background expectation is observed. This observation is interpreted as constraining the signal hypotheses, and it is found at 95% confidence level that: the mass of the , assuming 100% branching fraction to , must be  TeV; the mass of a vector-like  () quark, assuming branching ratios to , , and  decay modes consistent with the  or  being a singlet, must be  0.59 (0.54) TeV; the four top production cross section must be  fb in the SM and  fb for production via a contact interaction; the mass of an sgluon must be  TeV; in the context of models with two universal extra dimensions the inverse size of the extra dimensions must be  TeV; and the cross section for production of two positively-charged top quarks must be  fb.

A search for new particles that decay into top quark pairs ( ) is performed with the ATLAS experiment at the LHC using an integrated luminosity of 14 fb of proton-proton collision data collected at center-of-mass energy  TeV. The lepton plus jets final state is used, where the top-pair decays as  with one  boson decaying leptonically and the other hadronically. The  system is reconstructed using both a conventional resolved jet analysis and a large-radius jet substructure analysis. The  invariant mass spectrum is searched for local excesses deviating from the Standard Model prediction. No evidence for a  resonance is found and 95% CL limits on the production rate are determined for massive states in two benchmark models. The upper limits on the cross section times branching ratio of a narrow  boson decaying to top pairs range from 5.3 pb for a resonance mass of 0.5 TeV to 0.08 pb for a mass of 3 TeV. A narrow leptophobic topcolor  boson with a mass below 1.8 TeV is excluded. Upper limits are set on the cross section times branching ratio for a broad color-octet resonance with 15.3% decaying to . These range from 9.6 pb for a mass of 0.5 TeV to 0.152 pb for a mass of 2.5 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.0 TeV.

A search is presented for the production of a new heavy quark with its antiparticle, assuming the new quark has a significant branching ratio to decay into a  boson and a third generation Standard Model quark. In the case of a new charge  quark () the decay targeted is , while for a new charge  quark () the decay targeted is . The search uses a dataset corresponding to  of  collisions at  TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Selected events contain a high transverse momentum  boson candidate reconstructed from a pair of oppositely charged electrons or muons. Additionally, the presence of at least two jets possessing properties consistent with the decay of a  hadron is required, as well as large total transverse momentum of all central jets in the event. No significant excess of events above the Standard Model expectation is observed, and upper limits are derived for vector-like quarks of various masses in a two-dimensional plane of branching ratios. Under branching ratio assumptions corresponding to a weak-isospin singlet scenario, a  () quark with mass lower than 585 (645) GeV is excluded at the 95% confidence level. Under branching ratio assumptions corresponding to a particular weak-isospin doublet scenario, a  () quark with mass lower than 680 (725) GeV is excluded at the 95% confidence level.

A search is presented for production of a heavy top-like quark () together with its antiparticle, assuming a significant branching ratio for subsequent decay into a  boson and a  quark. The search is based on 14.3 fb of  collisions at  TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton+jets final state, characterised by a high-transverse-momentum isolated electron or muon, large missing transverse momentum and at least four jets, with at least one of them  tagged. The analysis strategy relies on the substantial boost of the  bosons in the  signal when  GeV. No significant excess of events above the Standard Model expectation is observed. For a chiral fourth generation quark, and under the assumption of a branching ratio , a mass lower than 740 GeV is excluded at 95% CL. For vector-like  quarks, and under the assumption that only the ,  and  decay modes contribute, 95% CL upper limits are derived for various masses in the two-dimensional plane of  versus , where  is the Standard Model Higgs boson. These limits are significantly improved by combining this search with a previous search for , with .