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Register a new userSingle top-quark production with the ATLAS detector in pp collisions at sqrt(s)=7TeV
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Reinhard Schwienhorst
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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.
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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 $A_{C}=0.006$.
Single-top production has been studied with the ATLAS detector using 0.7 fb-1 of 2011 data recorded at 7 TeV center-of-mass energy. The measurement of electroweak production of top-quarks allows probes of the Wtb vertex and a direct measurement of the CKM matrix element |V_tb|. It is also expected to be sensitive to new physics such as flavor changing neutral currents or heavy W production. The t-channel cross-section measurements are performed using both a cut-based and neural network approach, while a cut-based selection in the dilepton channel is used to derive a limit on the associated (Wt) production. An observed cross-section of 90 +32 -22 pb (65 +28 -19 pb expected) is obtained for the t-channel, which is consistent with the Standard Model expectation. For the Wt production, an observed limit of < 39.1 pb (40.6 pb expected) is derived, which corresponds to about 2.5 times the Standard Model expectation.
A measurement of the cross-section for pp->Z->e+e- is presented using data at sqrt{s}=7TeV corresponding to an integrated luminosity of 0.94 fb^{-1}. The process is measured within the kinematic acceptance pT>20GeV/c and 2<eta<4.5 for the daughter electrons and dielectron invariant mass in the range 60--120GeV/c^2. The cross-section is determined to be sigma(pp->Z->e+e-)=76.0+-0.8+-2.0+-2.6pb where the first uncertainty is statistical, the second is systematic and the third is the uncertainty in the luminosity. The measurement is performed as a function of Z rapidity and as a function of an angular variable which is closely related to the Z transverse momentum. The results are compared with previous LHCb measurements and with theoretical predictions from QCD.
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.
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.
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