Measurement of the cross section for W -boson production in - - PowerPoint PPT Presentation

Measurement of the cross section for W -boson production in association with jets in p¯ p collisions at √s =1.96 TeV

Alessandra Baas

Universit¨ at Heidelberg, Kirchhoff-Institut f¨ ur Physik Journal Club 29.10.2012

Fermilab and Tevatron

◮ Fermi National Accelerator Laboratory (Fermilab) is a US

national laboratory.

◮ Specialized on high energy physics (Tevatron) and neutrino

physics (MiniBooNE, SciBooNE, MINOS).

◮ Tevatron was a p¯

p collider, with beam energies of ∼1 TeV.

◮ Its circumference was 6.28 km. ◮ It was run from 1983 to 2011. ◮ The two main experiments were CDF and DØ. ◮ One of the main discoveries was the top quark discovery in

1995.

Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 2 / 23

Collider Detector at Fermilab II

Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 3 / 23

Motivation for: W → eν+ ≥ n-jet (n = 1, . . . , 4) as function of the E jet

T

• f the nth leading jet?

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Motivation

Why measure the W+jet(s) production cross section?

◮ Many physics signals have also vector boson (W,Z) plus jet(s)

signals, such as t¯ t, single top or Higgs boson production.

◮ V+jet(s) produced via QCD is large background to these

processes and hence good knowledge is crucial.

◮ Stringent test of the power of perturbative QCD predictions.

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W +jet Production

Some leading order Feynam diagrams for W +jet production.

W g q,q q,q q,q q,q g q,q W W q,q g q,q q,q q,q

More jets in the final state due to initial and final state radiation.

Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 6 / 23

W +2jets Production

Does this diagrams also count for W +2jets production? (From single top production last week.)

W g q,q q,q q,q q,q q,q q,q W q,q q,q q,q q,q q,q W W q,q q,q W W q,q q,q

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W Selection and Jet Reconstruction

W → eν Selection: Jet Reconstruction:

◮ Online: EM calo cluster with

ET > 18 GeV.

◮ Offline: Standard electron

identification cuts plus E ele

T

>20 GeV.

◮ Neutrino enriched sample:

E miss

T

> 30 GeV.

◮ Reduce background

contamination: mW

T > 20 GeV.

◮ Z boson veto: reject Z → ee

events.

◮ Jets are reconstructed via

JETCLU (cone algorithm) with R =

• φ2 + η2 = 0.4.

◮ Apply JES correction. ◮ E jet

T

> 20 GeV.

◮ |η| < 2.0 . ◮ Jets are rejected if the lie

within ∆R = 0.52 of the electron.

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W Background?

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W Background

◮ Leptonic:

◮ Contains real electrons and/or neutrinos including decays of

W → τν, Z → ee, WW , W γ and t¯ t.

◮ Monte Carlo event sample with the W selection applied but

without E miss

T

requirement.

◮ Multi-jet:

◮ One or more jets are incorrectly reconstructed as electrons and

have mis-measured energy, leading to large E miss

T

.

◮ Modeled by an event sample from the same dataset by

requiring that at least two electron criteria fail, but leave kinematic cuts unchanged.

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Background Estimation

◮ Background enriched samples for each W + ≥ n-jet sample by

removing E miss

T

> 30 GeV.

◮ E miss T

spectrum for leptonic, multi-jet and signal histograms are fitted.

◮ From these distributions determine the fraction for

normalization of ET distribution.

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E miss

T

Distributions

FIG.1: The results of fitting the signal and background E miss

T

template distributions to the data in the W + ≥ 1-jet sample before the final E miss

T

cut is applied.

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E miss

T

Distributions

Systematical Uncertainty on Background:

◮ Low E jet

T : 15 % independent on jet multiplicity.

◮ High E jet

T : 50% in 1-jet sample, 20% in 4-jet sample.

◮ Low jet multiplicities are limited due to statistics. ◮ high jet multiplicities are dominated by 12% uncertainty on top pair

production cross section.

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Event yield correction?

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Event Yield Correction

Corrections for:

◮ W → eν selection efficiency, about (60±3)%. ◮ Online trigger efficiency to accept high ET electrons, about 96.2±0.6%. ◮ Calorimeter jet energy resolution on the measured x-section (to form

particle level W → eν+ ≥ n-jet x-section).

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Total Systematic Uncertainty on the Cross Section

◮ Increases with increasing ET (5%-20%). ◮ Dominated by 3% uncertainty on the JES correction.

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Measured Differential Cross Sections

◮ Statistical Uncertainty: on

event yield of each bin.

◮ Systematical Uncertainty:

contributions added in quadrature (on bkg. estimation, acceptance correction and jet energy measurement).

◮ Additionally 5.8%

uncertainty on the integrated luminosity.

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Measured Total Cross Sections

Total cross section σn = σ(W → eν+ ≥ n-jet; E nth−jet

T

>25 GeV)

◮ σ1 = (53.5 ± 0.6 ± 4.6 ± 3.1) pb. ◮ σ2 = (6.8 ± 0.2 ± 1.0 ± 0.4 )b. ◮ σ3 = (0.84 ± 0.10 ± 0.21 ± 0.05) pb. ◮ σ4 = (0.074 ± 0.039 ± 0.035 ± 0.004) pb. ◮ p¯

p → W × B(W → eν) (restricted phase space): σ0 = (798 ± 2 ± 40 ± 46) pb. σn = (value) ± (stat.) ± (syst.) ± (lum.) pb.

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Theoretical Predictions

◮ Leading order (LO) plus modeling initial and final state partons and

hadronization (through perturbative parton shower): MLM and SMPR.

◮ Next-to-leading order predictions for W + 1 and W + 2 jets: MCFM.

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Cross Section Ratios I

◮ Differences between the measured cross sections and SMPR/MLM reflect

the LO nature of these determinations (upper plot).

◮ Good agreement in the lower plot.

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Cross Section Ratios II

W + 1 jet: W + 2 jets:

◮ SMPR prediction is better than MLM prediction.

⇒ probably due to missing underlying event model in MLM.

◮ Overall good agreement between data and MCFM predictions

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Cross Section Ratios III

W + 3 jets:

Alessandra Baas Cross section for W +jets production. Journal Club 29.10.2012 22 / 23

Bibliography

Talk based on a paper of the same name by CDF: arXiv: 0711.4044v2 [hep-ex]

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