We present the single-top production results carried out with 0.70 fb−1 of integrated luminosity collected by the ATLAS detector during the 2011 LHC run at 7TeV center of mass. The t-channel production cross section in the

lepton+jets final state and the Wt production cross section in the dilepton final state are measured. An upper limit on the s-channel production cross section at 95% CL is set

Alvarez Gonzalez, B.

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DOI 10.1393/ncc/i2012-11246-1Colloquia: TOP2011IL NUOVO CIMENTO Vol. 35 C, N. 3 Maggio-Giugno 2012Measurement of the single top quark cross section in 0.70 fb−1of pp collisions with the ATLAS detectorB. Alvarez Gonzalez on behalf of the ATLAS CollaborationDepartment of Physics and Astronomy, Michigan State University3218 Biomedical and Physical Sciences Building, East Lansing, Michigan 48824-2320 USAricevuto l’ 1 Marzo 2012pubblicato online il 4 Giugno 2012Summary. — We present the single-top production results carried out with0.70 fb−1 of integrated luminosity collected by the ATLAS detector during the 2011LHC run at 7TeV center of mass. The t-channel production cross section in thelepton+jets final state and the Wt production cross section in the dilepton finalstate are measured. An upper limit on the s-channel production cross section at95% CL is set.PACS 14.65.Ha – Top quarks.The ATLAS detector [1] is a multi-porpuse particle physics detector at the LargeHadron Collider (LHC) at CERN. It is optimized to record information coming fromproton-proton collisions happening in the center of the detector. Single top-quark pro-duction proceeds through three different mechanisms, resulting in distinct final states.These three mechanisms are presented in fig. 1: the t-channel exchange of a W boson [2],the associated production of a top quark and a W boson (Wt-channel) [3], and the s-channel production [4]. The main contribution comes from t-channel with a productioncross section of 64.57+3.32−2.62 pb, compared the production cross sections of Wt-channel of15.74+1.34−1.36 pb and the s-channel 4.63+0.29−0.27 pb all corresponding a 7TeV.For the t-channel analysis, events are selected with one isolated lepton (electron ormuon) with pT > 25GeV, missing transverse energy (E/T > 25GeV), and two or threejets with pT > 25GeV and |ηjet| < 4.5, one of them identified as a b-jet. Jets containingb-quarks are tagged by reconstructing secondary vertices from the tracks associated witheach jet and requiring a minimum signed decay length significance of the secondaryvertices relative to the primary vertex. The chosen working point corresponds to a b-tagging efficiency of 50% and a light quark jet rejection factor of around 270, as measuredin simulated top pair events.For the s-channel analysis, the selection is very similar to the t-channel selectionexcept that only two jets were selected, the requirement on the η of the jets is muchc© CERN on behalf of the ATLAS Collaboration under CC BY-NC 3.0 201202 B. ALVAREZ GONZALEZ on behalf of the ATLAS COLLABORATIONFig. 1. – Leading-order diagrams corresponding to the three single top production mechanisms:t-channel (left), Wt-channel (middle), and s-channel (right).tighter (|ηjet| < 2.5), and both jets could be identified as a b-jet. The same b-taggingworking point is used in this case.The background contribution for both analyses consists of multijets, W+jets andtop quark pair events, with smaller contributions from Z+jets and diboson events. Wenormalize the electroweak Z+jets, WW , WZ and ZZ, the tt¯ and the Wt and t-channelsingle top background contributions to their theory predictions and the shape of thedistributions is taken from the Monte Carlo (MC) simulated events. Since the multijetsbackground is difficult to model precisely, its contribution is reduced with a dedicatedtriangular cut, mT (W )(1)> 60GeV − E/T , where mT (W ) is the lepton-E/T transversemass. The multijets background normalization is estimated using a binned likelihood fitto the E/T distribution. The shape of the multijets background in this fit is taken from thejet-electron sample, which requires a jet with a similar detector signature as an electron.This jet must have pT > 25GeV, the same acceptance in η as the signal electron and80–95% of its energy deposited in the electromagnetic section of the calorimeter. Thejet must also contain at least four tracks, thus reducing the contribution from convertedphotons. The fit is performed separately in the pretag, 1-tag and 2-tag samples, afterapplying all selection cuts, including the triangular cut, but leaving out the E/T cut.The result of the likelihood fit in the muon channel is shown in fig. 2. A systematicuncertainty of 50% is assigned to the multijets normalization. The distributions for theW+jets background are taken from Monte Carlo samples, while the overall normalizationand the flavor composition are derived from data. The number of W+jets events in thedata is taken as the difference between the observed data counts and the number of eventsestimated for non-W+jets backgrounds in each jet multiplicity bin. The overall W+jetsnormalization factor is the ratio of the number of W+jets events in the data over thenumber of W+jets events in MC. The flavor composition of the W+jets background isestimated from samples obtained from: the 1-jet 1-tag sample and the 2-jet pretag and2-jet 1-tag samples. Scale factors for each of the individual W+jets flavor samples areobtained such that the W+jets event yields in the two-jet samples agree with the dataW+jets event yields. Systematic uncertainties and their correlations are included in theevaluation of these scale factors.Events used for the Wt dilepton analysis contain two isolated leptons (electrons ormuons) with pT > 25GeV, missing transverse energy, E/T > 50GeV, and at least one jet(1) Defined as√2plT pνT (1− cos(φl − φν)), with l and ν the lepton and the neutrino, respec-tively, coming from W decay, where the measured missing ET vector provides the neutrinoinformation.MEASUREMENT OF THE SINGLE TOP QUARK CROSS SECTION IN 0.70 fb−1 203 [GeV]missTE0 50 100Candidate Events / 5 GeV02004002 jets 1 tag muon @ 7 TeV-1 Preliminary                0.70 fb ATLAStopW+jetsZ+jets, dibosonMultijetsATLAS dataFig. 2. – EmissT distribution for the 2-jet tag sample in the muon channel.with pT > 25GeV. Some other requirements has been included to optimize the signalregion like a Z mass window cut: |ml1,l2−mZ | > 10GeV or a Z → ττ rejection (Z → ττveto: Δφ(l1, EmissT )+Δφ(l2, EmissT ) > 2.5). The analysis uses three lepton categories twoelectrons (ee), two muons (μμ) and one electron and one muon (eμ), the categories areexclusive samples. Z mass veto is only applied to the ee and μμ samples, while Z → ττveto is applied to all three final states. The background model consists of top quark pair,drell-yan, diboson, and multijets events as shown on the jet multiplicity distribution forall lepton categories combined in fig. 3.Finally, the three results quoted on this paper are estimated using a cut-based ap-proach and an integrated luminosity of 0.70 fb−1. We measure the t-channel productioncross section obtaining a result of σt = 90 ± 9(stat)+31−20(syst) pb [5]. This result is con-sistent with the Standard Model (SM) expectation. We also perform a t-channel crosssection measurement using a neural network [6]; result which is as well consistent withthe SM expectation. For the cut-based analysis the b-tagging uncertainty is the largestsystematic uncertainty.The Wt dilepton cross section is measured obtaining a result of σWt = 14.4+5.3−5.1(stat)+9.79.4 (syst) pb [7], that corresponds with a 1.2σ significance. The uncertainties related to0 1 2 3 4 5 6 7 8 9Number of Events0100200300400500600Dilepton Combined DataWtFake leptons)+jetsμμ ee/→Z()+jetsττ →Z(DibosonttATLAS Preliminary-1 L dt = 0.70 fb∫Number of JetsFig. 3. – Jet multiplicity distribution for the preselected sample with all channels combined.204 B. ALVAREZ GONZALEZ on behalf of the ATLAS COLLABORATIONjet energy scale, jet energy reconstruction efficiency and jet energy resolution are thelargest systematic uncertainties in this measurement.On the s-channel analysis, after the final selection, 296 data events are observedcompared with a prediction of 285. The predicted signal purity is 6%. We set an upperlimit on the s-channel production cross section of 26.5 pb at 95% CL [8], five times higherthan the SM prediction. The main source of systematic uncertainties is the data andMC statistics.In summary, work is already in progress to update these results including new ap-proaches and the data collected by the ATLAS detector during the 2011 LHC run. Theseanalyses will also benefit from improving the systematic uncertainties since they are allsystematics dominated. These results are already competitive with the Tevatron [9, 10]results.REFERENCES[1] The ATLAS Collaboration, JINST, 3 (2008) S08003.[2] Kidonakis Nikolaos, Phys. Rev. D, 81 (2010) 054028.[3] Kidonakis Nikolaos, Phys. Rev. D, 83 (2011) 091503.[4] Kidonakis Nikolaos, Phys. Rev. D, 82 (2010) 054018.[5] The ATLAS Collaboration, https://cdsweb.cern.ch/record/1357892/files/ATL-COM-PHYS-2011-695.pdf (2011).[6] The ATLAS Collaboration, https://cdsweb.cern.ch/record/1357883/files/ATL-COM-PHYS-2011-688.pdf (2011).[7] The ATLAS Collaboration, https://cdsweb.cern.ch/record/1357842/files/ATL-COM-PHYS-2011-682.pdf (2011).[8] The ATLAS Collaboration, https://cdsweb.cern.ch/record/1367753/files/ATL-COM-PHYS-2011-928.pdf (2011).[9] The D0 Collaboration, arXiv:1105.2788 (2011).[10] The CDF Collaboration, Phys. Rev. D, 82 (2010) 112005.

Measurement of the single top quark cross section in 0.70 fb−1 of pp collisions with the ATLAS detector

We present the single-top production results carried out with the integrated luminosity collected by the ATLAS detector during the 2011 LHC run

Alvarez Gonzalez, B

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Measurement of the single top quark cross section in 0.70/fb of pp Collisions with the ATLAS detector

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Measurement of the single top quark cross section in 0.70 fb−1 of pp collisions with the ATLAS detector

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