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Top Quark Pair Production Cross Section

Top Quark Pair Production Cross Section. Andrea Bangert, ATLAS-D Workshop, Zeuthen, 19.09.2007. Contributors. Ludwig-Maximilian’s University Marion Lambacher Hadronic channel Cut-based analysis Raphael Mameghani Semileptonic, Dileptonic channels Ratio of cross sections

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Top Quark Pair Production Cross Section

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  1. Top Quark Pair Production Cross Section Andrea Bangert, ATLAS-D Workshop, Zeuthen, 19.09.2007

  2. Contributors • Ludwig-Maximilian’s University • Marion Lambacher Hadronic channel Cut-based analysis • Raphael Mameghani Semileptonic, Dileptonic channels Ratio of cross sections • University of Dortmund • Moritz Bunse Semileptonic channel • Reiner Klingenberg Double-differential cross section • Max Planck Institute • Andrea Bangert Semileptonic channel • Sophio Pataraia Cut-based analysis • University of Bonn • Markus Cristinziani Dileptonic channel • Duc Bao Ta Maximum Likelihood fit

  3. Hadronic Channel (LMU) • Goal: Selection of large sample of fully hadronic ttbar events during first year of LHC. • LO analysis, reconstruction performed using Atlfast. • TTbar events were produced with Pythia (LO). • QCD background events with 3,4,5 and 6+ final state partons generated with Alpgen (LO). Pythia provided parton shower, MLM matching. • Detector simulation, b-tagging and jet reconstruction performed by Atlfast. • Exclusive jet reconstruction from calorimeter cells using kT algorithm. • Studied pileup at generator level.

  4. Hadronic Channel: Event Selection Cuts: Cut 0: |ηjets| < 3 Cut 1: Njets == 6 Cut 2: pT(j1, j2, j3, j4, j5, j6) > (115, 90, 70, 55, 40, 30) GeV Cut 3:ΣpT(j) > 140 GeV Cut 4: 75 GeV < m(jj) < 150 GeV Cut 5: 165 < m(jjb) < 400 GeV Cut 6:ΣpT(jjb) > 250 GeV Cut 7: Aplanarity > 0.1 Cut 8: double b-tag (optional)

  5. Hadronic Channel: Results • Cut analysis without b-tag: S/B = 1/16 • With double b-tag: S/B = 1/1 • Remaining statistics without b-tag: • 3300 signal events in first year of LHC. • With double b-tag: 1000 signal events. Jet Eta • Study of pileup: • Pileup produces many forward jets. • Central portion of detector less prone to effect of pileup. • ATLAS Internal Note describing QCD multijet background has been produced: • ATL-PHYS-INT-2007-007, • “Generation of QCD Multijet Background Events”, • M. Lambacher, O. Biebel, F. Fiedler

  6. Semileptonic Channel (Dortmund) • Goal: Double differential cross section. • Determine phase space resolved selection efficiencies ε(pT, η). • Efficiencies for t→jjb and t→lνb will be analyzed separately. • Errors in measurement of kinematic variables must be minimized. • Improvement and validation of reconstruction algorithms. • High statistics available in particular regions of phase space. Kinematics of true top (t→jjb) Kinematics of reconstructed top (t→jjb) TTbar sample mc12 5200. Tops generated by MC@NLO; reconstructed in Athena 12.0.6; Topview 12.13.

  7. Preliminary Results (Dortmund) t→jjb Ntrue(pT, η) Nreconstructed (pT, η)

  8. Semileptonic Channel (MPI) Goal: Perform cut-based analysis on first 100 pb-1 data. Estimation of systematic uncertainties is underway. • csc11 5200, reconstruction in Athena 11.0.42, L = 973 pb-1 • Statistical uncertainty on efficiency: δε = √(ε (1-ε) / Ni) • Tendency for kT, D=0.4 to allow selection of more events than Cone4. • Kinematic characteristics of jets depend upon jet reconstruction algorithm. • → Selection efficiencies may depend on jet algorithm. • → Measured value for cross section may depend on jet algorithm. Selection Efficiencies

  9. Semileptonic Channel (MPI) • I and II represent two statistically independent “data” samples. • L = 97 pb-1 • From MC: σtt · Γtt→lνbjjb = 248 pb • δεMC << δNdata • Only statistical uncertainty due to δNdata is shown. • Uncertainty even with L ~ 100 pb-1 will be dominated by systematics, including: • Resolution of jet momentum scale. • Performance of jet reconstruction algorithm. • Analysis has been implemented in Athena 12.0.6. Code is available: • http://atlas-sw.cern.ch/cgi-bin/viewcvs-atlas.cgi/groups/MPP/TopQuarkAnalysis L = 97 pb-1 σtt · Γtt→lνbjjb (l = e, μ)

  10. Dileptonic Channel (Bonn) • Goal: use maximum likelihood fit to determine excess of signal over background → cross section measurement. • Compare cut-based and likelihood analyses. • Preselection Cuts: • 2 isolated leptons of opposite charge, pT > 20 GeV • 2 jets, pT > 20 GeV, no b-tagging • |η| < 2.5 for all visible objects • ee and μμ : MET > 35 GeV, veto Z mass peak • eμ: MET > 20 GeV Dilepton mass Maximum Likelihood fit: S(x) is signal distribution, B(x) background distribution. Ntotal = Ns + NBG, G(x) = NsS(x) + NBGB(x) L = - Σ ln [G(x)] + Ntotal  Ns and NBG

  11. Results of Preselection (Bonn) Jet Multiplicity

  12. Discriminant Variables (Bonn) | ∆φ(jet1, MET) | Discriminant variables: |∆φ(lepton1, lepton2)| |Ση(lepton)| |∆φ(lepton1, MET)| |∆φ(lepton2, MET)| |∆φ(jet1, MET)| signal background S + B | Ση(lepton) |

  13. Cross Section Ratio (LMU) • Ratio of dileptonic to semileptonic cross sections: • Goal:Estimate achievable precision for R during the early phase of LHC. • ATLAS TDR: statistical precision (Atlfast, double b-tag, L=10 fb-1) ∆R / R ~ 0.5 % Cut Flow, semileptonic ttbar Cut Flow, dileptonic ttbar Cuts still being optimized Commissioning Analysis cuts [Please refer to backup slides for cuts]

  14. Summary • Four institutes are currently involved in the preparation of cross section analyses. • All ttbar decay channels are covered. • Cut-based analyses being prepared in hadronic and semileptonic channels (LMU and MPI). • Double differential cross section analysis underway in semileptonic channel (Dortmund). • Analysis in dileptonic channel exploits maximum likelihood method (Bonn). • Ratio of cross sections in dileptonic and semileptonic channels is under investigation (LMU). • We are looking forward to the advent of data in 2008.

  15. Backup Slides

  16. Hadronic channel (LMU) • Generation of b-quarks: • In NJets mode Alpgen can produce u, d, c, and s quarks. • QCD background samples thus produced include only b quarks from gluon splitting. • In order to analyze properties related to b-tagging samples with b-quarks are necessary. • These were produced from 4-vectors by replacing light quark pairs from parton shower with b-quark pairs. Original kinematics were not modified. • No quark-antiquark pairs originating during hard process were modified. Only pairs originating in the parton shower were replaced. • Procedure was applied to 4-jet, 5-jet and 6-jet QCD multijet events.

  17. Njets N b-jets pT(j1) ΣpT(j)

  18. Hadronic Channel:Calculation of invariant W and top masses • Consider four lowest-pT jets to be W decay products. Two highest-pT jets represent b-jets. • Calculate invariant mass for each possible dijet pair. • mjj = √[(E1 + E2)2-(px1+px2)2-(py1+py2)2-(pz1+pz2)2] • Calculate χ2 for each pair of dijet combinations. • Χ2 = (mj3j4 - mW)2 + (mj5j6 - mW)2 • Selecting the minimum χ2 delivers two dijet masses which represent reconstructed W bosons. • Form two possible combinations of dijet pairs with remaining two jets. • Calculate trijet masses. • Calculate χ2 for each pair of trijet masses. • Selecting minimum χ2 delivers two trijet masses which represent reconstructed top quarks.

  19. Hadronic Channel (LMU) • Sphericity tensor S • Sij = Σkpikpjk / Σpk2 • S has eigenvalues e1, e2, e3 • Aplanarity A = 3 e2 e3 / 2

  20. Hadronic Channel (LMU): Pileup • Pile-up events generated using Pythia 6.2 for luminosity per bunch crossing of LBC=0.25 mb-1. • ttbar events with pileup produce many jets in forward calorimeter. • Conclusions: pileup at generator level mostly negligible. Number of pile-up events in all hadronic ttbar events.

  21. Semileptonic Channel (Dortmund) • Phase space: (-3.0 < η < 3.0), (0 < pT < 250 GeV) • Divide phase space into 375 bins of size (0.2 x 20 GeV). • Expect ~ 20,000 ttbar events per bin during several months at low luminosity at LHC. • Compare to ~ 100 ttbar events / bin at Tevatron. Measurement of double differential cross section requires knowledge of phase space resolved efficiencies ε(pT, η).

  22. Semileptonic Channel (Dortmund) • Sample: • csc 5200, MC@NLO, dileptonic and semileptonic ttbar events • AODs produced in Athena 12.0.6 with 1mm bug fix • 116250 events • Commissioning Cuts: • 3 jets with pT > 40 GeV • 4th jet with pT > 20GeV • lepton (µ, e) with pT > 20GeV • missing ET > 20GeV • leptons and jets within |η|<2.5 • Reconstruction: • TopView v. 12.13 default reconstruction: • No b-tagging information • Select trijet combination with maximal pT to represent t→jjb

  23. Semileptonic Channel (Dortmund) • Goal: study accuracy of reconstruction of t→jjb • Topview v. 12.13, no b-tagging, trijet combination with maximal pT represents t→jjb True pT vs. reconstructed pT <(true pT – reco pT)> vs. rec pT <(true pT – reco pT)> vs. reco η

  24. Semileptonic Channel (MPI) • Event Sample: • Semileptonic and Dileptonic ttbar events: • csc11 #5200, MC@NLO / Herwig • σ = 461 pb, mt = 175 GeV • Used ~10% of sample as “data” • Used ~90% as “Monte Carlo” • Ldata = 97 pb-1, Ndata ~ 45,000 • LMC = 970 pb-1, NMC ~ 450,000 • Hadronic ttbar events: • csc11 #5204, MC@NLO / Herwig • σ = 369 pb, L = 77 pb-1, N ~ 30,000 • All samples were reconstructed using Athena 11.0.42. • Selection cuts: • Exactly one e or μ: • E(e)∆R=0.45<6 GeV, E(μ)∆R=0.2<1 GeV • pT(l) > 20 GeV, |η| < 2.5, for electrons (isEM==0) • At least four jets: • |η| < 2.5, ∆R(j,e) > 0.4 • For three leading jets pT(j) > 40 GeV • pT(j4) > 20 GeV or pT(j4) > 40 GeV • no b-tagging was required • MET > 20 GeV

  25. Dileptonic Channel (Bonn) • Signal:dileptonic events with e,μ • mc12 sample 5200 with 1mm bug fix • 600,000 events • TTbar background: • semileptonic ttbar and dileptonic ttbar with τ lepton • mc12 sample 5200 with 1mm bug fix • Z→ll: • Pythia, 850,000 events • WW, WZ, ZZ: • Herwig, 150,000 events • All samples processed with Topview 12.13

  26. Dileptonic Channel (Bonn) • Preselection Cuts: • 2 isolated leptons of opposite charge, pT > 20 GeV • At least two jets • no b-tagging • |η| < 2.5 for all visible objects • Electrons removed if 1.35 < |η| < 1.65 • Muons removed if |η| < 0.1 or 1.0 < |η| < 1.3 • ee: • MET > 35 GeV • Veto 85 GeV<mll<95 GeV • pT(j1)>35 GeV, pT(j2)>25 GeV • eμ: • MET > 20 GeV • pT(j1)>30 GeV, pT(j2)>25 GeV • μμ: • MET > 35 GeV • veto 85 GeV<mll<95 GeV • pT(j1)>30 GeV, pT(j2)>25 GeV

  27. |Ση(lepton)| |∆φ(jet1, MET)| Discriminant Variables Discriminant variables: |∆φ(lepton1, lepton2)|, |Ση(lepton)| |∆φ(lepton1, MET)| , |∆φ(lepton2, MET)| |∆φ(jet1, MET)| Dileptonic signal Z→μμ BG ∆φ(l1, l2) Z→ll WW/WZ/ZZ ttbar signal + BG ∆φ(l1, MET) ∆φ(l2, MET) Signal,BG

  28. Cross Section Ratio (LMU) • Selection Cuts, Semileptonic Channel: • Exactly one lepton, pT > 20 GeV, |η| < 2.5 • 3 jets with pT > 40 GeV, fourth with pT > 20 GeV, |η| < 2.5 • MET > 20 GeV • In trijet combination with maximal pT (t → jjb) must be at least one dijet pair with |mjj - mW| < 10 GeV • mtotal < 900 GeV • |cos θ*| < 0.7 for three leading jets • Selection Cuts, Dileptonic Channel: • Exactly two leptons • Leptons must have opposite charges • pT(j1) > 55 GeV, pT(j2) > 40 GeV • Veto Z mass peak: 85 GeV < mll < 95 GeV • MET > 25 GeV • pT(l1) > 30 GeV, pT(l2) > 15 GeV • ∆R(l,j) > 0.2

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