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

2. 2 Contributors Ludwig-Maximilian’s University Marion LambacherHadronic channel Cut-based analysis Raphael MameghaniSemileptonic, 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 CristinzianiDileptonic channel Duc Bao Ta Maximum Likelihood fit

3. 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 k T algorithm. Studied pileup at generator level.

4. 4 Hadronic Channel: Event Selection Cuts: Cut 0:|η jets | < 3 Cut 1:Njets == 6 Cut 2:p T (j 1, j 2, j 3, j 4, j 5, j 6 ) > (115, 90, 70, 55, 40, 30) GeV Cut 3:Σp T (j) > 140 GeV Cut 4:75 GeV < m(jj) < 150 GeV Cut 5:165 < m(jjb) < 400 GeV Cut 6:Σp T (jjb) > 250 GeV Cut 7:Aplanarity > 0.1 Cut 8:double b-tag (optional)

5. 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. 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 Jet Eta

6. 6 Semileptonic Channel (Dortmund) Goal: Double differential cross section. Determine phase space resolved selection efficiencies ε(p T, 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. TTbar sample mc12 5200. Tops generated by MC@NLO; reconstructed in Athena 12.0.6; Topview 12.13. Kinematics of true top (t→jjb)Kinematics of reconstructed top (t→jjb)

7. 7 Preliminary Results (Dortmund) t→jjb N reconstructed (p T, η)N true (p T, η)

8. 8 Semileptonic Channel (MPI) csc11 5200, reconstruction in Athena 11.0.42, L = 973 pb -1 Statistical uncertainty on efficiency: δε = √(ε (1-ε) / N i ) Tendency for k T, 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. channelk T, D=0.4Cone4 Semileptonic with e 17.55 ± 0.11 %16.83 ± 0.11% Semileptonic with μ 25.13 ± 0.12 %24.41 ± 0.12 % Semileptonic with τ 3.68 ± 0.05 % 3.53 ± 0.05 % Dileptonic 7.73 ± 0.09 % 7.17 ± 0.09 % Selection Efficiencies Goal: Perform cut-based analysis on first 100 pb -1 data. Estimation of systematic uncertainties is underway.

9. 9 Semileptonic Channel (MPI) I and II represent two statistically independent “data” samples. L = 97 pb -1 From MC: σ tt · Γ tt→lνbjjb = 248 pb δε MC << δN data Only statistical uncertainty due to δN data 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 σ tt · Γ tt→lνbjjb (l = e, μ) L = 97 pb -1

10. 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, p T > 20 GeV 2 jets, p T > 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. N total = N s + N BG, G(x) = N s S(x) + N BG B(x) L = - Σ ln [G(x)] + N total  N s and N BG

11. 11 Results of Preselection (Bonn) channelSignal efficiency ttbar l+j efficiency Z→ll efficiency Diboson efficiency S/BS/√(S+B) ee 1.94%0.02%0.01%0.04% 6.71 ± 0.6112.43 ± 0.05 eμeμ 5.76%0.06%ε < 0.01%0.13%13.01 ± 0.6022.13 ± 0.03 μμ 2.75%0.02%0.01%0.05% 5.52 ± 0.6314.60 ± 0.04 Jet Multiplicity

12. 12 Discriminant Variables (Bonn) Discriminant variables: |∆φ(lepton 1, lepton 2 )| |Ση(lepton)| |∆φ(lepton 1, MET)| |∆φ(lepton 2, MET)| |∆φ(jet 1, MET)| | Ση(lepton) | signal background S + B

13. 13 Cross Section Ratio (LMU) 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 % Cuts still being optimized Cut Flow, semileptonic ttbarCut Flow, dileptonic ttbar Ratio of dileptonic to semileptonic cross sections: Commissioning Analysis cuts [Please refer to backup slides for cuts]

14. 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. 15 Backup Slides

16. 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. 17 NjetsN b-jets p T (j 1 )Σp T (j)

18. 18 Hadronic Channel: Calculation of invariant W and top masses Consider four lowest-p T jets to be W decay products. Two highest-p T jets represent b-jets. Calculate invariant mass for each possible dijet pair. m jj = √[(E 1 + E 2 ) 2 -(p x1 +p x2 ) 2 -(p y1 +p y2 ) 2 -(p z1 +p z2 ) 2 ] Calculate χ 2 for each pair of dijet combinations. Χ 2 = (m j3j4 - m W ) 2 + (m j5j6 - m W ) 2 Selecting the minimum χ 2 delivers two dijet masses which represent reconstructed W bosons. Form two possible combinations of dijet pairs with remaining two jets. Calculate trijet masses. Calculate χ 2 for each pair of trijet masses. Selecting minimum χ 2 delivers two trijet masses which represent reconstructed top quarks.

19. 19 Hadronic Channel (LMU) Sphericity tensor S S ij = Σ k p i k p j k / Σp k 2 S has eigenvalues e 1, e 2, e 3 Aplanarity A = 3 e 2 e 3 / 2

20. 20 Hadronic Channel (LMU): Pileup Pile-up events generated using Pythia 6.2 for luminosity per bunch crossing of L BC =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. 21 Semileptonic Channel (Dortmund) Phase space: (-3.0 < η < 3.0), (0 < p T < 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(p T, η).

22. 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 p T > 40 GeV 4th jet with p T > 20GeV lepton (µ, e) with p T > 20GeV missing E T > 20GeV leptons and jets within |η|<2.5 Reconstruction: TopView v. 12.13 default reconstruction: No b-tagging information Select trijet combination with maximal p T to represent t→jjb

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

24. 24 Semileptonic Channel (MPI) Event Sample: Semileptonic and Dileptonic ttbar events: csc11 #5200, MC@NLO / Herwig σ = 461 pb, m t = 175 GeV Used ~10% of sample as “data” Used ~90% as “Monte Carlo” L data = 97 pb -1, N data ~ 45,000 L MC = 970 pb -1, N MC ~ 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 p T (l) > 20 GeV, |η| < 2.5, for electrons (isEM==0) At least four jets: |η| 0.4 For three leading jets p T (j) > 40 GeV p T (j 4 ) > 20 GeV or p T (j 4 ) > 40 GeV no b-tagging was required MET > 20 GeV

25. 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. 26 Dileptonic Channel (Bonn) Preselection Cuts: 2 isolated leptons of opposite charge, p T > 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<m ll <95 GeV p T (j 1 )>35 GeV, p T (j 2 )>25 GeV eμ: MET > 20 GeV p T (j 1 )>30 GeV, p T (j 2 )>25 GeV μμ: MET > 35 GeV veto 85 GeV<m ll <95 GeV p T (j 1 )>30 GeV, p T (j 2 )>25 GeV

27. 27 Discriminant Variables ∆ φ(l 1, l 2 ) ∆ φ(l 1, MET) ∆ φ(l 2, MET) Z→ll WW/WZ/ZZ ttbar signal + BG Dileptonic signal Z→μμ BG Discriminant variables: |∆φ(lepton 1, lepton 2 )|, |Ση(lepton)| |∆φ(lepton 1, MET)|, |∆φ(lepton 2, MET)| |∆φ(jet 1, MET)| |Ση(lepton)| Signal, BG

28. 28 Cross Section Ratio (LMU) Selection Cuts, Semileptonic Channel: –Exactly one lepton, p T > 20 GeV, |η| < 2.5 –3 jets with p T > 40 GeV, fourth with p T > 20 GeV, |η| < 2.5 –MET > 20 GeV –In trijet combination with maximal p T (t → jjb) must be at least one dijet pair with |m jj - m W | < 10 GeV –m total < 900 GeV –|cos θ*| < 0.7 for three leading jets Selection Cuts, Dileptonic Channel: –Exactly two leptons –Leptons must have opposite charges –p T (j 1 ) > 55 GeV, p T (j 2 ) > 40 GeV –Veto Z mass peak: 85 GeV < m ll < 95 GeV –MET > 25 GeV –p T (l 1 ) > 30 GeV, p T (l 2 ) > 15 GeV –∆R(l,j) > 0.2