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PDF Uncertainties on W+Jets

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Presentation on theme: "PDF Uncertainties on W+Jets"— Presentation transcript:

1 PDF Uncertainties on W+Jets
Maria Fiascaris University of Oxford In collaboration with Alessandro Tricoli and Amanda Cooper-Sarkar ATLAS UK SM Meeting, Sept 24th 2007

2 Overview Aim: W+jets events: main background to SUSY processes
Evaluate PDF uncertainties and compare systematic experimental uncertainties: which ones greater? Focus on the cross section as a function of cumulative jet multiplicity Used PDF Reweighting Technique In this talk: Sample: SM Sample (Loose cuts) (Previously: SUSY Sample - Tight Event Filter cuts) Test PDF Reweighting (SUSY Sample) PDF uncertainties (SM Sample) for Et and η distributions of electrons and jets Jet multiplicities Comparison with Jet Energy Scale Uncertainty (SUSY samples) Kinematic Region in x, Q2 for W+N partons Sept 24th 2007 ATLAS UK SM Meeting

3 Samples W(→ e nu)+ jets csc11 data, generated with Alpgen+Herwig
PDF set: CTEQ6LL SM Dataset #Partons Incl/exc #events σ (pb) 6101 exc 41240 1760 6102 1 49240 1650 6103 2 46202 587 6104 3 43741 177 6105 4 31989 49.3 6106 5 inc 10489 18.0 SM Sample: Event Filter Cuts Electrons: N>=1 with Pt >10GeV, -2.7 < η < 2.7 Jets: N>=1 with Pt >20GeV, -2.5 < η < 2.5 SUSY Sample: Event Filter Cuts N ≥ 4 with Pt> 40GeV Leading Jet: Pt > 80GeV MissEt > 80GeV SUSY Dataset #Partons Incl/exc #events σ (pb) 5223 2 exc 2090 504 5224 3 5853 122 5225 4 6249 28.4 5226 5 inc 5802 6.1 Sept 24th 2007 ATLAS UK SM Meeting

4 Electron & Jet Offline Selection
Electron Selection isEM=0 + track-cluster matching Crack removal (|η| between ) Et > 25 GeV Jet Selection (Jet Algorithm KT06) Electron-jet overlap (ΔR<0.4) MissEt Cut: MissEt > 25 GeV Min. Jet Pt Cut: Jet Pt > 20 GeV (SUSY Sample: 40GeV) Sept 24th 2007 ATLAS UK SM Meeting

5 PDF Reweighting Technique
Use PDF Reweighting to avoid many Monte Carlo generations Generate a MC sample with PDF set 1 and weight every event to PDF 2: where the incoming partons carry momentum fractions xA and xB , Q is the scale MC sample :CTEQ6LL(LO) PDF error sets available: CTEQ6M (NLO) Need to reweight: CTEQ6LL→ CTEQ6M central+ 40 error sets Asymmetric Errors calculated as: (Xi+, Xi- are the upper/lower error sets for eigenvector i, X0 is the central value ): Accuracy for reweighting LO→NLO ? e n jet xA xB Sept 24th 2007 ATLAS UK SM Meeting

6 Test of PDF Reweighting
Accuracy of Reweighting technique tested on SUSY samples (Alpgen+Herwig with tight filter cuts): Generate 2 samples of W+3 partons with different PDFs (CTEQ6LL, MRST2001NLO) Requirements: High statistics (~500,000 events per sample) Generation consistent with SUSY sample Compare electrons/jets distributions: MRST2001NLO generated directly MRST2001NLO weighted from CTE6LL Large weights (LO→NLO): MRST2001NLO/CTEQ6LL Et w (Gev) y w Accuracy better than 1% after kinematic cuts (SUSY) → Sufficiently accurate to study PDF uncertainties Cuts : |eta|,|y| < 2.5 Etparton > 40 GeV EtW > 40 GeV Etele > 25 GeV Sept 24th 2007 ATLAS UK SM Meeting

7 PDF Uncert. On SM Sample: Electrons (Detector)
CTEQ6M central CTEQ6M UncertPlus CTEQ6M UncertMinus No Cuts MissEt Cut Et(GeV) Pt Eta PDF Uncertainties increase: - at High Pt - in Central η region Sept 24th 2007 ATLAS UK SM Meeting

8 Jets: Generator PDF Uncertainties below 5%
CTEQ6M central CTEQ6M UncertPlus CTEQ6M UncertMinus No Cuts MinJetEt Cut Pt Eta Pt Eta PDF Uncertainties below 5% Asymmetry in Eta after cuts ? Sept 24th 2007 ATLAS UK SM Meeting

9 Jets: Detector (MinJetEt Cut)
CTEQ6M central CTEQ6M UncertPlus CTEQ6M UncertMinus ? Pt Eta NJets Uncertainties within 5% Pt: Approximately flat Eta: increase in central region + edges NJets: minimum between 4 -6 jets Sept 24th 2007 ATLAS UK SM Meeting

10 Jets: Cumulative Multiplicity
Δ+ PDF (%) Δ- PDF (%) W+ ≥1 jets 3.5 5.0 W+ ≥2 jets 3.0 4.3 W+ ≥3 jets 2.7 3.7 W+ ≥4 jets 3.3 W+ ≥5 jets 3.1 W+ ≥6 jets 3.6 4.9 Cross Section and Cumulative Jet Multiplicity: number of W→e nu events with at least n jets PDF Uncertainties remain within 5% Minimum at N=3-4 Sept 24th 2007 ATLAS UK SM Meeting

11 Previous Results (SUSY samples)
CTEQ6M central CTEQ6M UncertPlus CTEQ6M UncertMinus ≥N jets Cross Section Uncertainty (PDF) Uncertainty (%) Events Previous results from SUSY samples Compare PDF Uncertainties with Experimental Systematic Uncert: Jet Energy Scale Uncertainty (Δ JS) with 10% (5%) jet energy miscalibration ±10% ±5% Uncertainty Cross Section Uncertainty (jet energy miscalibration) Thanks to Alessandro Tricoli ≥N jets Sept 24th 2007 ATLAS UK SM Meeting

12 Comparison with Jet Scale Uncert.
Multipl Δ+ PDF (%) Δ- PDF (%) Δ+ JS (%) Δ- JS (%) W+ ≥1 jets 3.2 2.9 10.7 (5.2) W+ ≥2 jets 10.2 (5.1) W+ ≥3 jets 3.3 5.8 (3.6) 9.0 (4.0) W+ ≥4 jets 5.0 3.9 14.7 (7.8) 15.6 (7.0) W+ ≥5 jets 5.9 4.8 20.8 (9.5) 20.5 (10.7) W+ ≥6 jets 7.3 22.2 (10.4) 23.6 (11.9) PDF Uncertainty Jet Scale Uncertainty (Δ JS) with 10% (5%) jet energy miscal. 2.9% < Δ PDF < 7.3% 5.8% < Δ JS (10%)< 23.6% 3.6% < Δ JS (5 %) < 11.9% PDF Uncert < Expt. Syst. Uncert This was obtained with SUSY sample. Need to the same with SM Sample. Sept 24th 2007 ATLAS UK SM Meeting

13 Back to SM Samples: Kinematic Regions
W+ 0 partons W+ 1 p W+ 3 p W+ 5 p x, Q2 plane High N partons → higher x, Q2 Sept 24th 2007 ATLAS UK SM Meeting

14 Kinematic Region :W+3partons
Q2 vs Pt jet Log (x) vs Pt jet Higher Pt, higher x Gluon unc. Kinematic region: < x < 1 104 < Q2 < 2x105 Correlation of x, Q2 with Pt Leading Jet High Pt → higher x region ( ) more uniform over Q2 range Gluon PDF with high uncertainty dominates Up and down quarks important as x increases Sept 24th 2007 ATLAS UK SM Meeting

15 Conclusions Used Reweighting technique to evaluate PDF Uncertainties for W+N partons reweighting from LO to NLO accurate to better than 1% in measurable kinematic regions PDF Uncertainties on SM sample of W+jets (loose filter cuts) Weighted CTEQ6L→CTEQ6M central + 40 error sets PDF Uncertainties below 5% Larger at very low/high Et and central η region PDF Unc. on Jet multiplicity has minimum at N=4-6 Comparison between PDF and Jet energy scale uncertainty on cumulative jet multiplicity (SUSY Sample): PDF uncert < 10%, lower than jet energy scale uncertainty Kinematic Region covered by events W+ N partons Further Studies Rerun the analysis on new samples (problems with duplicated events) Compare PDF Uncert. and Jet Scale Uncert. on SM sample Can use jet multiplicity measurement to improve our knowledge on PDF ? Look at ratios, eg: σ (W+3 jets) / σ (W+2 jets) Sept 24th 2007 ATLAS UK SM Meeting

16 EXTRAS

17 Electrons: Generator (no cuts)
Sept 24th 2007 ATLAS UK SM Meeting

18 Electrons: Generator (cuts)
Sept 24th 2007 ATLAS UK SM Meeting

19 Electrons (Generator) : Eta distrib.→ anomaly in sample W+0 partons
Sept 24th 2007 ATLAS UK SM Meeting

20 Look at electrons / positrons separately
Here plotted all samples (0,1,2,3,4,5 partons) merged Anomaly more significant for positrons? Sept 24th 2007 ATLAS UK SM Meeting

21 Compare W+0 partons, W+1parton
No Cuts MissEt Cut MinJetEt Cut W+0 W+1 Sept 24th 2007 ATLAS UK SM Meeting

22 Compare W+3 partons, W+5parton
No Cuts MissEt Cut MinJetEt Cut Problem from low multiplicty sample (W+1, 2, 3 but NOT W+0) ! Jet Pt Cut has an effect Sept 24th 2007 ATLAS UK SM Meeting

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