tt Differential Cross Section Measurement at√s=7 TeV with the CMS Detector

DESY-CMS Top Group

Maria Aldaya, Ivan Asin, Wolf Behrenhoff,Dirk Dammann, Tyler Dorland, Achim Geiser,David J. Fischer, Benjamin Lutz, Markus Marienfeld,Andreas Meyer, Sebastian Naumann-Emme

DPG

Göttingen, 28/2/2012

Ivan Asin | ttbar Differential Cross Section Measurement | 28/02/2012 | Page 2

Outline

> Event Selection

> Kin. Reconstruction and DY contribution determination

> Systematic Uncertainties

>Results and Conclusion

>Messung differentieller Top-Quark Wirkungsquerschnitte bei 7 TeV mit dem CMS-Detektor (Wolf Behrenhoff)

Introduction and Motivation

Control Plots

Cross Section Measurement

Ivan Asin | ttbar Differential Cross Section Measurement | 28/02/2012 | Page 3

Introduction and Motivation

> tt differential cross section measurement

1σ

d σ

dX

X = lepton, lepton pair, top and top pair properties: p

T,

η, y, m

>Dileptonic decay channel:

ee, eμ, μμ and combined

>Motivation:

top quark production cross section and properties are important test of SM.

LHC has high tt production rate

Ivan Asin | ttbar Differential Cross Section Measurement | 28/02/2012 | Page 4

Event Selection

> ee and µµ channels:

Mll<76 GeV or Mll >106 GeV

ET

Miss.>30GeV

> Event Kin. reconstruction

>Dilepton triggers

> Leptons:

Isolated

High pT

Opposite sign

Mll>12 GeV

> 2 high pT jets, 1 b-tag

Ivan Asin | ttbar Differential Cross Section Measurement | 28/02/2012 | Page 5

Cross Section Measurement

>Nidata

-NiBG

= #signal events in bin i

> L = total integrated luminosity

> ΔiX = i-th bin width for variable X

> εi = detector efficiencies and acceptances corrections

> σ = Total visible phase space cross section

CMS Work in Progress

Ivan Asin | ttbar Differential Cross Section Measurement | 28/02/2012 | Page 6

Kinematic Reconstruction

> Kinematic reconstruction of top quark properties

> Input: leptons, jets and ET

Miss. Info.

> Syst. underconstrained: 2 neutrinos

>Constraints

MW

= 80.4 GeV

px, y

(ν1)+px, y

(ν2) = Ex, y

Miss.

mt = m

t = fixed

> Vary mt from 100-300 GeV in 1 GeV

steps

> Best solution: E(ν) reco vs MC

BeforeKin. Reco.

AfterKin. Reco.

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Systematic Uncertainties (Kinematic Reco.)

> εη almost flat

> ε(pT) large trend → calculate SF → Kin. Reco. Variation

CMS Work in Progress CMS Work in Progress

√SF (l1)×SF (l2)

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Drell-Yan Contribution

> In ee and μμ channels DY contribution estimated from data

> DY background scaled so #DY evt. inside Z peak are equal in data and simulation:

SF = NDYdata

/NDYMC

> Noutll, data

= Rllout/in (N

llin-0.5 · N

eμin · k

ll)

Nllin

Neμin

> Kll: e vs. μ id. correction

> Rllout/in: Nout

DYMC/Nin

DYMC

Mll [GeV] M

eμ [GeV]

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Systematic Uncertainties (Drell-Yan)

Standard DY Variation: +50%

Ivan Asin | ttbar Differential Cross Section Measurement | 28/02/2012 | Page 10

Experimental

> DY: ± 50% normalization

> BG: ± 30% normalization

> JES: from CMS Jet Group

> JER: ± 10%, ± 15%, ± 20% for |η|<1.5, 1.5<|η|<2.0 and |η|>2.0

> PU: evt. multiplicity ±0.6 event

> B-tagging: from CMS BTV Group

> Kin. Reco.: diff. between standard and

Systematic Uncertainties

Model

> Q2 scale: varied up and down by factor 2

> Top mass: mtstd. = 172.5 GeV

varied by ± 6 GeV, error divided by 5

> Hadronisation: Difference between POWHEG+PYTHIA and POWHEG+HERWIG20

√SF (l1)×SF (l2)

>Uncorrelated systematic errors ⇒ Summed in quadrature

> Vary 1 property of analysis → Repeat analysis → Cross Section

> Syst. Uncertainty: Diff. between standard and varied measurement

Ivan Asin | ttbar Differential Cross Section Measurement | 28/02/2012 | Page 11

Systematic UncertaintiesC

MS

Work in P

rogress

CM

S W

ork in Progress

Combined

Combined

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Final Results

Mll

pT

tt

yt and t

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Conclusions

> First tt system differential cross section measurement in pp collisions at √s = 7 TeV is presented.

> The measurement is preformed as function of

Leptons: pT, η

Lepton pair: pT, η, m

Top quarks: pT, y

Top quark Pair: pT, y, m

> The results are found to agree with the SM predictions

> The systematic uncertainties

Extensively studied and found under control

Similar size contributions from statistical, experimental and model uncertainties

> Further details and more plots in CMS PAS-TOP-11-013

http://cdsweb.cern.ch/record/1422425?ln=en