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Register a new userSearch for heavy Higgs bosons $A/H$ decaying to a top quark pair in $pp$ collisions at $sqrt{s} = 8$ TeV with the ATLAS detector
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ATLAS Collaboration
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A search for heavy pseudoscalar ($A$) and scalar ($H$) Higgs bosons decaying into a top quark pair ($tbar{t}$) has been performed with 20.3 fb$^{-1}$ of proton--proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of $sqrt{s} = 8$ TeV. Interference effects between the signal process and Standard Model $tbar{t}$ production, which are expected to distort the signal shape from a single peak to a peak--dip structure, are taken into account. No significant deviation from the Standard Model prediction is observed in the $tbar{t}$ invariant mass spectrum in final states with an electron or muon, large missing transverse momentum, and at least four jets. The results are interpreted within the context of a type-II two-Higgs-doublet model. Exclusion limits on the signal strength are derived as a function of the mass $m_{A/H}$ and the ratio of the vacuum expectation values of the two Higgs fields, $tanbeta$, for $m_{A/H} > 500$ GeV.
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The ATLAS experiment at the LHC at CERN has analyzed 2010 and 2011 data looking for electroweak production of single top quarks in the lepton+jets and di-lepton final states. The production cross section for the t-channel process is measured to be 76 +41 -21 pb using 156 pb^-1 of 2011 data. A first limit is set on the Wt associated production process using lepton+jets and di-lepton events. The 95% CL upper limit on the Wt production cross section is 158 pb using 35 pb^-1 of 2010 data.
We present a measurement of the charge asymmetry in top-antitop production using data corresponding to an integrated luminosity of $0.70 fb^{-1}$ of proton-proton collisions at $sqrt s = 7$ TeV collected by the ATLAS detector. The top pair events decaying semileptonically (lepton+jets channel) to either an electron or muon, missing transverse energy and at least four jets are selected. The reconstruction of the $tbar{t}$ events was performed using a kinematic likelihood approach. The difference of absolute values of top and antitop rapidities is used to define the charge asymmetry: $A_{C} = (N(|Delta Y|>0) - N(|Delta Y|<0)) / (N(|Delta Y>0) + N(|Delta Y|<0))$. To allow comparisons with theory calculations, a Bayesian unfolding technique is applied to correct the measured $|Delta Y|$ distributions for acceptance and detector effects. The top charge asymmetry in both channels (e and mu) after correction is measured to be: $A_{C} = -0.009 pm 0.023 (stat) pm 0.032 (syst)$ (e+jets channel) and $A_{C} = -0.028 pm 0.019 (stat) pm 0.022 (syst)$ ($mu$+jets channel) giving a combined result of : $A_{C} = -0.024 pm 0.016 (stat) pm 0.023 (syst)$. These results are compatible with the Standard Model predictions of $A_{C}=0.006$.
Measurements of normalized differential cross-sections of top quark pair ($tbar t$) production are presented as a function of the mass, the transverse momentum and the rapidity of the $tbar t$ system in proton-proton collisions at center-of-mass energies of $sqrt{s}$ = 7 TeV and 8 TeV. The dataset corresponds to an integrated luminosity of 4.6 fb$^{-1}$ at 7 TeV and 20.2 fb$^{-1}$ at 8 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Events with top quark pair signatures are selected in the dilepton final state, requiring exactly two charged leptons and at least two jets with at least one of the jets identified as likely to contain a $b$-hadron. The measured distributions are corrected for detector effects and selection efficiency to cross-sections at the parton level. The differential cross-sections are compared with different Monte Carlo generators and theoretical calculations of $tbar t$ production. The results are consistent with the majority of predictions in a wide kinematic range.
The Collider Detector at Fermilab collected a unique sample of jets originating from bottom-quark fragmentation ($b$-jets) by selecting online proton-antiproton ($pbar{p}$) collisions with a vertex displaced from the $pbar{p}$ interaction point, consistent with the decay of a bottom-quark hadron. This data set, collected at a center-of-mass energy of $sqrt{s}=$1.96 TeV, and corresponding to an integrated luminosity of $5.4~rm{fb}^{-1}$, is used to measure the $Z$-boson production cross section times branching ratio into $bbar{b}$. The number of $Zrightarrow bbar{b}$ events is determined by fitting the dijet-mass distribution while constraining the dominant $b$-jet background, originating from QCD multijet events, with data. The result, $sigma(pbar{p} rightarrow Z) times mathcal{B}(Z rightarrow bbar{b})= 1.11pm 0.08(text{stat}) pm 0.14(text{syst})~text{nb}$, is the most precise measurement of this process, and is consistent with the standard-model prediction. The data set is also used to search for Higgs-boson production. No significant signal is expected in our data and the first upper limit on the cross section for the inclusive $pbar p rightarrow Hrightarrow bbar b$ process at $sqrt{s}=$1.96 TeV is set, corresponding to 33 times the expected standard-model cross section, or $sigma = 40.6$ pb, at the 95% confidence level.
We present the results of a search for pair production of a heavy top-like (t) quark decaying to Wq final states using data corresponding to an integrated luminosity of 5.6 fb-1 collected by the CDF II detector in pp collisions at {surd}s = 1.96 TeV. We perform parallel searches for t to Wb and t to Wq (where q is a generic down-type quark) in events containing a lepton and four or more jets. By performing a fit to the two-dimensional distribution of total transverse energy versus reconstructed t quark mass, we set upper limits on the t bar{t} production cross section and exclude a standard model fourth-generation t quark decaying to W b (W q) with mass below 358 (340) GeV/c2 at 95% CL.
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