Top Quark Pair Production Cross Section

Andrea Bangert, ATLAS-D Workshop, Zeuthen, 19.09.2007

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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

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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.

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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)

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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

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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.

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)

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Preliminary Results (Dortmund)

t→jjb

Nreconstructed (pT, η)Ntrue(pT, η)

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Semileptonic Channel (MPI)

• 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.

channel kT, D=0.4 Cone4

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.

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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

σtt · Γtt→lνbjjb (l = e, μ)

L = 97 pb-1

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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

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Results of Preselection (Bonn)

channel Signal

efficiency

ttbar l+j

efficiency

Z→ll

efficiency

Diboson

efficiency

S/B S/√(S+B)

ee 1.94% 0.02% 0.01% 0.04% 6.71 ± 0.61 12.43 ± 0.05

eμ 5.76% 0.06% ε < 0.01% 0.13% 13.01 ± 0.60 22.13 ± 0.03

μμ 2.75% 0.02% 0.01% 0.05% 5.52 ± 0.63 14.60 ± 0.04

Jet Multiplicity

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Discriminant Variables (Bonn)

Discriminant variables:|∆φ(lepton1, lepton2)| |Ση(lepton)||∆φ(lepton1, MET)| |∆φ(lepton2, MET)| |∆φ(jet1, MET)|

| ∆φ(jet1, MET) |

| Ση(lepton) |

signal

background

S + B

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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 ttbar Cut Flow, dileptonic ttbar

• Ratio of dileptonic to semileptonic cross sections:

Commissioning Analysis cuts [Please refer to backup slides for cuts]

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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.

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Backup Slides

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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.

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Njets N b-jets

pT(j1) ΣpT(j)

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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.

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Hadronic Channel (LMU)

• Sphericity tensor S

• Sij = Σkpikpj

k / Σpk2

• S has eigenvalues e1, e2, e3

• Aplanarity A = 3 e2 e3 / 2

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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.

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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).

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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 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

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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

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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

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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

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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

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Discriminant Variables

∆φ(l1, l2)

∆φ(l1, MET) ∆φ(l2, MET)

Z→ll

WW/WZ/ZZ

ttbar signal + BG

Dileptonic signal

Z→μμ BG

Discriminant variables:|∆φ(lepton1, lepton2)|, |Ση(lepton)||∆φ(lepton1, MET)| , |∆φ(lepton2, MET)| |∆φ(jet1, MET)|

|∆φ(jet1, MET)|

|Ση(lepton)|

Signal, BG

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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