A measurement of the cross section of top quark pair production in proton-proton collisions at a center-of-mass of 7TeV at the LHC using events with an isolated μ and a τ lepton decaying hadronically is reported. In addition to a μ and a τ lepton, large missing transverse energy and two or more jets are required. At least one of the jets must be identified as originating from a b quark. To identify τ leptons, the analysis uses a multivariate technique based on boosted decision trees. A data sample collected by ATLAS corresponding to an integrated luminosity of 1.08 fb−1 yields σt¯t = 142 ± 21(stat.) ±20 16 (syst.) ± 5 (lumi.) pb

Searcy, J.

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DOI 10.1393/ncc/i2012-11260-3Colloquia: TOP2011IL NUOVO CIMENTO Vol. 35 C, N. 3 Maggio-Giugno 2012Measurement of the top-quark pair production cross section in ppcollisions at√s = 7TeV in μ + τ final states with ATLASJ. Searcy on behalf of the ATLAS CollaborationBrookhaven National Laboratory (BNL), University of Oregon - Eugene, USAricevuto l’ 1 Marzo 2012pubblicato online il 6 Giugno 2012Summary. — A measurement of the cross section of top quark pair productionin proton-proton collisions at a center-of-mass of 7TeV at the LHC using eventswith an isolated μ and a τ lepton decaying hadronically is reported. In additionto a μ and a τ lepton, large missing transverse energy and two or more jets arerequired. At least one of the jets must be identified as originating from a b quark.To identify τ leptons, the analysis uses a multivariate technique based on boosteddecision trees. A data sample collected by ATLAS corresponding to an integratedluminosity of 1.08 fb−1 yields σtt¯ = 142± 21(stat.)±2016 (syst.)± 5 (lumi.) pb.PACS 14.65.Ha – Top quarks.PACS 14.60.Fg – Taus.1. – IntroductionMeasuring the top-quark pair (tt¯) production cross section (σtt¯) using a channel withtau leptons (τ) is of great interest because it can open a window to physics beyondthe Standard Model (SM). In the SM, the top-quark decays nearly 100% of the timeinto a W -boson and a b-quark, and tt¯ pairs are identified by either the hadronic decays(W → qq¯′) or leptonic decays (W → ν) of the W -bosons and the presence of additionaljets.The W → τν branching ratio has been measured with high precision, and it is in verygood agreement with the SM expectations. However, the best previous measurement ofσ(tt¯) × BR(t → τν + b) has an uncertainty of 25% [1]. If a charged Higgs boson (H±)exists, as required by the minimal supersymmetric standard model (MSSM) [2], andits mass is lower than the top-quark mass minus the b-quark mass, the top quark canhave a substantial branching ratio to H± + b. For large values of the MSSM parametertanβ the charged Higgs decays mainly to τν and can increase significantly the top quarkbranching ratio to final states with τs. The much larger cross section for tt¯ production atthe LHC provides an opportunity to measure the BR(t → τνb) with higher precision, andthus increases the sensitivity to H± or other processes that could enhance this branchingc© CERN on behalf of the ATLAS Collaboration under CC BY-NC 3.0 233234 J. SEARCY on behalf of the ATLAS COLLABORATIONratio [3,4]. CMS has recently measured this cross section at the LHC with an uncertaintyof 24% using a μ+τ data sample of 1.09 fb−1 integrated luminosity [5]. These proceedingssummarize the measurement presented in ATLAS-CONF-2011-119 [6].After application of kinematic preselection criteria described in sect. 2 and identifi-cation of a jet as a b-quark jet (b-tagging), the dominant background to the tt¯ → μ + τchannel with the τ decaying hadronically is the tt¯ → μ+ jets channel in which a jet fakesa hadronic tau decay. Therefore, the most powerful remaining discriminator betweensignal and background is τ identification (TauID). The TauID employed in this analysisis a multivariate discriminator built using a boosted decision tree (BDT) algorithm [7].The number of τs in a sample is extracted by fitting the shape of the BDT discriminantusing background and signal templates.The method relies on a background model for jets misidentified as τ ’s derived fromdata and a signal model derived from Monte Carlo (MC). This analysis uses 1.08 fb−1 ofdata recorded between April and June 2011.2. – Event selectionThe μ+ τ analysis starts by requiring events selected online by a single-muon triggerwith a pT threshold of 18GeV. The event selection was optimized using Monte Carloand requires: 1) an event to have a primary vertex with at least five tracks, 2) eventsare discarded if any jet with pT > 20GeV fails jet-quality selections, 3) one and only oneisolated muon and no identified electrons, 4) at least one loose τ candidate (defined in [6]),5) at least two jets reconstructed with the anti-kt algorithm [8] (with distance parameterR = 0.4) having pT > 25GeV not overlapping with a τ candidate, 6) EmissT > 30GeV,7) HT =∑ |ET| > 200GeV, and 8) at least one jet identified as a b-jet (≥ 1 b-tag). Thedata are split into two categories: τ1 for one track candidates, and τ3 for candidates withtwo or three tracks. This selection when applied to a 1.08 fb−1 data sample yields 1593 τ1events and 4086 τ3 events. These samples are further divided by the charge of the τ andμ into events with opposite-sign charges (OS) and events with same-sign charges (SS).3. – Analysis based on fits of BDTj distributionsThe final background normalization and signal measurements are established throughfitting templates to data distributions of the BDTj for the τ candidates. The backgroundtemplates are built from data using both OS and SS W → μν+jets samples. The signalBDTj template is derived from real τs in tt¯ and Z → ττ Monte Carlo simulations.Because the contribution from electrons cannot be distinguished by BDTj , it is treatedas part of the signal. The contribution from electrons in tt¯ → μe+ jets is also derivedfrom the Monte Carlo simulation and added to the signal BDTj template. A χ2 fit to theopposite-sign minus same-sign (OS−SS) distribution is made with three free parametersto set the normalization of each template(1) (a ·OS− b · SS) + c · signal,where OS, SS and signal are the templates of W+jets (OS), W+jets (SS) and signalevents, and a, b and c are the number of W+jets (OS), W+jets (SS) and signal events.The fits are performed separately for τ1 and τ3 candidates (fig. 1).Figure 2 shows the OS − SS distribution of the number of jets for b-tagged eventssatisfying BDTj < 0.7, which is expected to be dominated by tt¯ → μ+ jets, and BDTj >0.7, which is expected to have a large contribution from tt¯ → μ + τ . As expected, theMEASUREMENT OF THE TOP-QUARK PAIR PRODUCTION CROSS SECTION ETC. 235jBDT0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Events-20020406080100120140160180 BG from fit(338) Signal from fit 30)±(163 ATLAS Preliminary(a)-1 L dt = 1.08fb∫ Data1τjBDT0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Events050100150200250 BG from fit(911) Signal from fit 20)±(62 ATLAS Preliminary(b)-1 L dt = 1.08fb∫ Data3τFig. 1. – The OS−SS BDTj distributions for (a) τ1, (b) τ3 candidates. The normalization of eachtemplate is derived from a fit to the data and are shown as the blue (signal), red (background)and black (total) lines. Red hatched bands are the statistical uncertainties of the backgroundtemplate.multiplicity of jets peaks at four when BDTj < 0.7 and three when BDTj > 0.7 (the τis counted as a jet).4. – Results and conclusionsAfter subtracting the small contributions from channels other than tt¯ → μ + τ , thecross section is determined using measured luminosity and acceptance calculated fromMC, including systematics described in [6]. The cross section is measured to beσtt¯ = 142± 21 (stat.)±2016 (syst.)± 5 (lumi.) pb,in good agreement with the Standard Model prediction (164±1116 pb) [9].Jet Multiplicity0 1 2 3 4 5 6 7 8 9 10OS - SS Events0100200300400500600700OS - SS Events < 0.7jBDTATLAS Preliminary-1 L dt = 1.08 fb∫Datab+bhadτμ→ttST, dibosonZ + jetW + jetbll+b→ttbl+jjb→ttuncertainty(a)Jet Multiplicity0 1 2 3 4 5 6 7 8 9 10OS - SS Events020406080100120140OS - SS Events > 0.7jBDTATLAS Preliminary-1 L dt = 1.08 fb∫Datab+bhadτμ→ttST, dibosonZ + jetW + jetbll+b→ttbl+jjb→ttuncertainty(b)Fig. 2. – Number of jets distributions for OS−SS events after the b-tagging selection. The solidcircles indicate data and the histograms indicate the expected signal and backgrounds from MC.The normalization of the expected signal and the backgrounds are based on the fit result. Thefraction of each background is estimated from MC. (a) BDTj < 0.7, (b) BDTj > 0.7.236 J. SEARCY on behalf of the ATLAS COLLABORATIONThe production cross section for tt¯ production in pp collisions at 7TeV has beenmeasured in the μ + τ channel in which the τ decays hadronically. A boosted decisiontree multivariate technique is used to obtain the number of signal events by fitting theopposite sign minus same sign BDTj distributions with signal and background templates.The measured cross section is in good agreement with the SM predictions and othermeasurements.REFERENCES[1] DØ Collaboration (Abazov V. M. et al.), Phys. Rev. D, 80 (2009) 071102(R).[2] Kane G. L., Kolda C., Roszkowski L. and Wells J. D., Phys. Rev. D, 49 (1994) 6173.[3] Han T. and Magro M. B., Phys. Lett. B, 476 (2000) 79.[4] Yue C., Zong H. and Liu L., Mod. Phys. Lett. A, 18 (2003) 2187.[5] CMS Collaboration, First measurement of the top quark pair production in the dileptonchannel with tau leptons in the final state in pp collisions at√s = 7TeV, CMS-PAS-TOP-11-06.[6] ATLAS Collaboration, Measurement of the top quark pair production cross section inpp collisions at√s = 7TeV in μ + τ final states with ATLAS, ATLAS-CONF-2011-119.https://cdsweb.cern.ch/record/1376411[7] Freund Y. and Schapire R. E., in Machine Learning: Proceedings of the ThirteenthInternational Conference, edited by Saitta L. (Morgan Kaufmann, San Francisco) 1996,p. 148; Roe B. P., Yang H.-J., Zhu J., Liu Y., Stancu I. and McGregor G., Nucl.Instrum. Methods Phys. Res. A, 543 (2005) 577.[8] Cacciari M., Salam G. P. and Soyez G., JHEP, 0804 (2008) 063.[9] Aliev M. et al., Comput. Phys. Commun., 182 (2011) 1034, arXiv:1007.1327.

Measurement of the top-quark pair production cross section in pp collisions at √s = 7TeV in μ + τ final states with ATLAS

A measurement of the cross section of top quark pair production in proton$-$proton collisions at a center-of-mass of 7 TeV at the LHC using events with an isolated $mu$ and a $ au$ lepton decaying hadronically is reported. In addition to a $mu$ and a $ au$ lepton, large missing transverse energy and two or more jets are required. At least one of the jets must be identified as originating from a $b$ quark. To identify $ au$ leptons, the analysis uses a multivariate technique based on boosted decision trees. A data sample collected by ATLAS corresponding to an integrated luminosity of 1.08 ${ m fb}^{-1}$ yields [ sigma_{tar{t}} = 142pm 21 stat pm^{20}_{16}syst pm 5 ( ext{lumi.}) ext{pb.} 

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Measurement of the top quark pair production cross section in pp collisions at $sqrt{s}$=7 TeV in $\mu$ + $\tau$ final states with ATLAS

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Measurement of the top-quark pair production cross section in pp collisions at √s = 7TeV in μ + τ final states with ATLAS

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