Top Production with N-jets and with Jet-Vetoes at the CMS Experiment - - PowerPoint PPT Presentation

INSTITUT F ¨

UR EXPERIMENTELLE KERNPHYSIK (IEKP)

Top Production with N-jets and with Jet-Vetoes at the CMS Experiment

Jet Vetoes and Jet Multiplicity Observables at the LHC - Durham - July 2013 Alexis Descroix on behalf of the CMS Collaboration | 17/07/2013

KIT – University of the State of Baden-Wuerttemberg and National Laboratory of the Helmholtz Association

www.kit.edu

Introduction

At the LHC a large fraction of top quark pair events (tt) are produced with additional jets Investigating these processes is very interesting

Test perturbative QCD at top quark energy scale Constrain modeling uncertainties in MC Anomalous tt + jets production can be sign of new physics Background to tt production with Higgs or with other bosons and BSM

This presentation gives an overview of measurements of tt events with jets at CMS:

Measurement of the jet multiplicity in tt events Investigation of the properties of additional jets:

Kinematic properties of additional jets Veto on additional jets Measurement of the additional parton multiplicity

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 2/25

The Compact Muon Solenoid Experiment

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 3/25

Investigated Processes: tt Events

tt produced at LHC mostly with gluons in initial state Top quarks decay almost always into a W boson and a bottom quark tt decay signature depends on decays of both W bosons

Dilepton channel: ee, eµ, and µµ two prompt leptons ✓, two unmeasured neutrinos ✗ Lepton+Jets channel: e+jets and µ+jets

• ne prompt lepton ✓, 4 jets ✗

✓: easy ✗: challenging

Dilepton Channel Lepton+Jets Channel

Similar signature as Drell-Yan (Z/γ∗) production Similar signature as W+jets and QCD multijet

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 4/25

Analyzed Samples

Datasets from 2011 (7 TeV, 5.0 fb−1) and 2012 (8 TeV, 19.6 fb−1) Simulation of standard tt sample with LO generator:

Matrix element from MADGRAPH (tt + 0,1,2,3 jets) Interfaced via MLM with PYTHIA for parton showering Fact./renorm. scale: Q2 = m2

t +

p2

T

Comparison available with NLO tt generators:

POWHEG+PYTHIA MC@NLO+HERWIG

Modeling/correction of dominant backgrounds with data-driven methods: Drell-Yan (dilepton), W+jets and QCD multijet (lepton+jets) Other backgrounds well modeled with MC:

MADGRAPH+PYTHIA (W+jets, and Drell-Yan) POWHEG+PYTHIA (single top) PYTHIA (diboson and QCD multijet)

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 5/25

Event Selection

Dilepton Channel

≥ 2 leptons with opposite charge (pT > 20 GeV)

QCD veto, if mℓℓ < 20 GeV

≥ 2 jets (pT > 30 GeV), one identified as b-jet

ee and µµ case: Emiss

T

> 40 GeV and |mℓℓ − mZ| > 15 GeV

Kinematic reconstruction of tt system

Events 1 10

2

10

3

10

4

10

5

10

6

10

7

10 = 8 TeV s at

• 1

CMS Preliminary, 19.6 fb Dilepton Combined

> 30 GeV

jet T

p Data Signal t t Other t t Single Top W+Jets µ µ ee/ → * γ Z / τ τ → * γ Z / Diboson Jets

2 3 4 5 6 7 8 9

MC

N

data

N 0.5 1 1.5

Lepton+Jets Channel

Only one lepton (pT > 30 GeV) + veto against additional leptons (looser cuts)

≥ 3(4) jets with pT > 35(30) GeV ≥ 2 selected jets identified as b-jets

3 4 5 6 7 8 Events 1 10

2

10

3

10

4

10

5

10

Data t t Single Top W+Jets Z+Jets QCD Multijet Diboson

2 b-tags ≥ 3 jets, ≥ , µ = 7 TeV s ,

• 1

CMS Preliminary 5.0 fb Jets 3 4 5 6 7 8 Data/MC 0.5 1 1.5 ≥ pjet

T

> 35 GeV

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 6/25

MEASUREMENT OF THE DIFFERENTIAL CROSS-SECTION AS A FUNCTION OF THE

NUMBER OF JETS

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 7/25

Correction Back to Particle Level

Subtract background from data → Ni

data − Ni bkg

Migration from particle level to detector level to be corrected Invert migration effects back to particle level with MADGRAPH → Ni

t¯ t

Correction within the visible phase-space

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 8/25

Differential Cross-Section and Theory Comparisons

Calculation of the differential cross-section 1

σt¯

t

dσt¯

t

dNjets

= 1 σt¯

t

Ni

t¯ t

L , with measured cross-section: σt¯

t

Normalization to σt¯

t reduces systematic uncertainty

Combine results and compare to predictions from:

POWHEG+PYTHIA MC@NLO+HERWIG MADGRAPH+PYTHIA with Q2 scale varied to 4 · Q2 and 1/4 · Q2 MADGRAPH+PYTHIA with matrix-element/parton-showering matching threshold varied to 40 and 10 GeV (nominal is 20 GeV)

Systematic uncertainties estimated by repeating the measurement with varied assumptions on sources, most important ones are:

Jet energy uncertainties Modeling uncertainties (Q2 scale, matching threshold, and hadronization uncertainty)

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 9/25

Differential Cross-Section in Lepton+Jets Channel (7 TeV)

1 2 3 4 5

jets

dN

t t

σ d

t t

σ 1

• 2

10

• 1

10

Data (combined) MadGraph+Pythia t t MC@NLO+Herwig t t POWHEG+Pythia t t

=7 TeV s at

• 1

CMS Preliminary, L=5 fb

Jet Multiplicity

3 4 5 6 7 8 ≥

MC/Data 0.5 1 1.5 1 2 3 4 5

jets

dN

t t

σ d

t t

σ 1

• 2

10

• 1

10

Data (combined) MadGraph+Pythia t t scale up

2

MadGraph Q t t scale down

2

MadGraph Q t t MadGraph match threshold up t t MadGraph match threshold down t t

=7 TeV s at

• 1

CMS Preliminary, L=5 fb

Jet Multiplicity

3 4 5 6 7 8 ≥

MC/Data 1 1.5 2

Good agreement of data with predictions from MADGRAPH+PYTHIA and POWHEG+PYTHIA MC@NLO+HERWIG: jet multiplicity lower than data MADGRAPH+PYTHIA: best description for larger Q2/match threshold

pjet

T

> 35 GeV pjet

T

> 35 GeV Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 10/25

Differential Cross-Section in Dilepton Channel (8 TeV)

dJets σ d σ 1

• 3

10

• 2

10

• 1

10 1 10

= 8 TeV s at

• 1

CMS Preliminary, 19.6 fb Dilepton Combined > 30 GeV

jet T

p Data MadGraph+Pythia MC@NLO+Herwig POWHEG+Pythia

Jets

2 3 4 5 6 ≥ Data/MC 0.5 1 1.5

dJets σ d σ 1

• 3

10

• 2

10

• 1

10 1 10

= 8 TeV s at

• 1

CMS Preliminary, 19.6 fb Dilepton Combined > 30 GeV

jet T

p Data MadGraph+Pythia

2

4*Q /4

2

Q Matching up Matching down

Jets

2 3 4 5 6 ≥ Data/MC 0.5 1 1.5

Consistent with results in dilepton and lepton+jets channels at 7 TeV

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 11/25

Differential Cross-Section in Dilepton Channel (8 TeV) with Higher Jet pT Threshold

dJets σ d σ 1

• 4

10

• 3

10

• 2

10

• 1

10 1 10

2

10

= 8 TeV s at

• 1

CMS Preliminary, 19.6 fb Dilepton Combined > 60 GeV

jet T

p Data MadGraph+Pythia MC@NLO+Herwig POWHEG+Pythia

Jets

1 2 3 4 5 6 ≥ Data/MC 0.5 1 1.5

dJets σ d σ 1

• 3

10

• 2

10

• 1

10 1 10

= 8 TeV s at

• 1

CMS Preliminary, 19.6 fb Dilepton Combined > 100 GeV

jet T

p Data MadGraph+Pythia MC@NLO+Herwig POWHEG+Pythia

Jets

1 2 3 4 ≥ Data/MC 0.5 1 1.5

Different behavior of MC@NLO confirmed at higher jet pT

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 12/25

KINEMATICS OF ADDITIONAL JETS

• DILEPTON CHANNEL

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 13/25

Definition of Additional Jets in Dilepton Channel

The kinematic reconstruction of the tt system assigns the jets from the tt decay

Kinematic constraints allow to solve the two-neutrino ambiguity Jet assignment is not straightforward

⇒ choice made with b-jet

identification and neutrino energy spectrum

Study of kinematic properties of additional jets:

Subtract background from data, no correction back to particle level MC distribution scaled with measured tt cross-section

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 14/25

Kinematics of Additional Jets

50 100 150 200 250 300 350 400

Events / binwidth

50 100 150 200 250 300 350 400 =8 TeV s at

• 1

CMS Preliminary, 19.6 fb

Dilepton Combined

Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig (GeV)

T

additional jet p

st

1 50 100 150 200 250 300 350 400 MC/data

0.6 0.8 1 1.2 1.4 50 100 150 200 250 300 350 400

Events / binwidth

20 40 60 80 100 120 140 160 180 200 =8 TeV s at

• 1

CMS Preliminary, 19.6 fb

Dilepton Combined

Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig (GeV)

T

additional jet p

nd

2 50 100 150 200 250 300 350 400 MC/data

0.6 0.8 1 1.2 1.4

• 2
• 1.5
• 1
• 0.5

0.5 1 1.5 2

Events / binwidth

1000 2000 3000 4000 5000 6000 =8 TeV s at

• 1

CMS Preliminary, 19.6 fb

Dilepton Combined

Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig η additional jet

st

1

• 2
• 1.5
• 1
• 0.5

0.5 1 1.5 2 MC/data

0.6 0.8 1 1.2 1.4

• 2
• 1.5
• 1
• 0.5

0.5 1 1.5 2

Events / binwidth

200 400 600 800 1000 1200 1400 1600 1800 2000 =8 TeV s at

• 1

CMS Preliminary, 19.6 fb

Dilepton Combined

Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig η additional jet

nd

2

• 2
• 1.5
• 1
• 0.5

0.5 1 1.5 2 MC/data

0.6 0.8 1 1.2 1.4

Good agreement for first additional jet

η of second

additional jet: MC@NLO less central

1st additional jet pT (GeV) 1st additional jet η 2nd additional jet η 2nd additional jet pT (GeV) Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 15/25

VETO ON ADDITIONAL JETS

• DILEPTON CHANNEL

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 16/25

Definition of Veto on Additional Jets

A veto variable is calculated: gap fraction f(pT) = N(pT )

Ntotal

N(pT): number of selected events without any additional jet fulfilling a kinematic veto on pT Ntotal: total number of selected events Veto on pT of the 1st, the 2nd and the scalar sum of all additional jets

[lepp.cornell.edu]

Threshold on pT of 1st add. jet at 40 GeV → event vetoed Threshold on pT of 1st add. jet at 50 GeV → event counted Veto varied in a wide range of pT Correction back to particle level within visible phase space

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 17/25

f(pT) of First Additional Jet

50 100 150 200 250 300 350 400

Gap fraction

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 =8 TeV s at

• 1

CMS Preliminary, 19.6 fb

Dilepton Combined

Data Syst+Stat error MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data Syst+Stat error MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data Syst+Stat error MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig [GeV]

T

additional jet p

st

1 50 100 150 200 250 300 350 400 Theory/Data

0.9 0.95 1 1.05 1.1

50 100 150 200 250 300 350 400

Gap fraction

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 =8 TeV s at

• 1

CMS Preliminary, 19.6 fb

Dilepton Combined

Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down [GeV]

T

additional jet p

st

1 50 100 150 200 250 300 350 400 Theory/Data

0.9 0.95 1 1.05 1.1

MC@NLO+HERWIG shows better agreement with data than MADGRAPH+PYTHIA and POWHEG+PYTHIA MADGRAPH+PYTHIA Q2 scale down variation decreases agreement Other variations from MADGRAPH+PYTHIA describe data well

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 18/25

f(pT) of Second Additional Jet

40 60 80 100 120 140 160 180 200

Gap fraction

0.8 0.85 0.9 0.95 1 1.05 =8 TeV s at

• 1

CMS Preliminary, 19.6 fb

Dilepton Combined

Data Syst+Stat error MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data Syst+Stat error MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data Syst+Stat error MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig [GeV]

T

additional jet p

nd

2 40 60 80 100 120 140 160 180 200 Theory/Data

0.96 0.98 1 1.02

40 60 80 100 120 140 160 180 200

Gap fraction

0.8 0.85 0.9 0.95 1 1.05 =8 TeV s at

• 1

CMS Preliminary, 19.6 fb

Dilepton Combined

Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down [GeV]

T

additional jet p

nd

2 40 60 80 100 120 140 160 180 200 Theory/Data

0.96 0.98 1 1.02

MC@NLO+HERWIG shows higher gap fraction values than data Better agreement with MADGRAPH+PYTHIA and POWHEG+PYTHIA MADGRAPH+PYTHIA Q2 scale down variation shows too low values

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 19/25

f(HT) of Additional Jets

50 100 150 200 250 300 350 400

Gap fraction

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 =8 TeV s at

• 1

CMS Preliminary, 19.6 fb

Dilepton Combined

Data Syst+Stat error MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data Syst+Stat error MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig Data Syst+Stat error MadGraph+Pythia POWHEG+Pythia MC@NLO+Herwig [GeV]

T

H

50 100 150 200 250 300 350 400

Theory/Data

0.9 0.95 1

50 100 150 200 250 300 350 400

Gap fraction

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 =8 TeV s at

• 1

CMS Preliminary, 19.6 fb

Dilepton Combined

Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down Data Syst+Stat error MadGraph+Pythia

2

MadGraph 4*Q /4

2

MadGraph Q MadGraph matching up MadGraph matching down [GeV]

T

H

50 100 150 200 250 300 350 400

Theory/Data

0.9 0.95 1

MC@NLO+HERWIG predictions above data, MADGRAPH+PYTHIA and POWHEG+PYTHIA predictions below data MADGRAPH+PYTHIA matching up agrees the best with data

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 20/25

MEASUREMENT OF THE DIFFERENTIAL CROSS-SECTION AS A FUNCTION OF THE

NUMBER OF ADDITIONAL PARTONS -

LEPTON+JETS CHANNEL

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 21/25

Definition of Additional Jets - Event Classification

tt MC events are classified using MC information Introduce a cut on distance ∆R between jets and tt decay products:

two b quarks two light quarks prompt lepton

Jets with ∆R > 0.5 count as additional radiated partons

⇒ classification of events in

tt + 0, 1 and ≥ 2 additional partons Extracting rates of these tt classes from data via a template fit of χ, from event reconstruction χ = mrec

W had − mtrue W had

σW had 2 + mrec

thad − mtrue thad

σthad 2 + mrec

tlep − mtrue tlep

σtlep 2

One additional parton

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 22/25

Template Fit Results

χ Events

1 10

2

10

3

10

4

10 5 10 15 5 10 15 5 10 15 20 KS test:0.968 test: 0.364

2

χ

• 1

L dt = 5 fb

∫

CMS preliminary

Data 2011 + 0p t t + 1p t t 2p ≥ + t t Single Top W+Jets Z+Jets QCD Multijet Diboson

+ 4 Jets µ + 5 Jets µ 6 Jets ≥ + µ

Ratio Data/Fit

0.5 1 1.5 5 10 15 5 10 15 5 10 15 20

Simultaneous fit in three jet multiplicity parts, tt classes show different shapes → separation power The fit performs well and checks have proved its stability

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 23/25

Results of Differential Cross-Section with Template Fit

• 0.5

0.5 1 1.5 2 2.5

parton

dN

t t

σ d

t t

σ 1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

+jets) µ Data ( MadGraph+Pythia t t MC@NLO+Herwig t t POWHEG+Pythia t t

=7 TeV s at

• 1

CMS Preliminary, L=5 fb

Additional Partons

1 2 ≥

MC/Data 0.6 0.8 1 1.2

• 0.5

0.5 1 1.5 2 2.5

parton

dN

t t

σ d

t t

σ 1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

+jets) µ Data ( MadGraph+Pythia t t scale up

2

MadGraph Q t t scale down

2

MadGraph Q t t MadGraph match threshold up t t MadGraph match threshold down t t

=7 TeV s at

• 1

CMS Preliminary, L=5 fb

Additional Partons

1 2 ≥

MC/Data 0.8 1 1.2 1.4

Extract results with MADGRAPH prediction for the tt classes Systematic uncertainties evaluated with pseudo-data, same assumed sources Like jet multiplicity results at 7 and 8 TeV:

best agreement with MADGRAPH+PYTHIA and POWHEG+PYTHIA MC@NLO shows discrepancies Visible sensitivity to scale and matching uncertainties

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 24/25

Conclusion and Outlook

Good agreement between all measurements:

Multiplicity of jets and of additional partons Additional jet kinematic Gap fraction of additional jets

MADGRAPH+PYTHIA predictions agree mostly well with data Modeling uncertainty (Q2 and matching) often larger than precision

⇒ could be reduced

MC@NLO+HERWIG produces fewer jets. Comparison with POWHEG+HERWIG required (sample now available) Working towards comparisons with NLO+Parton Showering multileg generators like aMC@NLO and SHERPA

Public results presented today can be found here: https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsTOP TOP-12-018 (lepton+jets channel, 7TeV) TOP-12-023 (dilepton channel, 7TeV) TOP-12-041 (dilepton channel, 8TeV)

Thanks for your attention

Alexis Descroix

Top Production with N-jets and with Jet-Vetoes at CMS

17/07/2013 25/25