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Benchmark QCD Measurements and Tools at ATLAS

Benchmark QCD Measurements and Tools at ATLAS. Craig Buttar University of Glasgow. Outline. Going from the Tevatron to the LHC Pdfs Total xsect vs 2  2 xsect Multijets  sudakov form factors

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Benchmark QCD Measurements and Tools at ATLAS

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  1. Benchmark QCD Measurements and Tools at ATLAS Craig Buttar University of Glasgow CTEQ07 Michigan May 2007

  2. Outline • Going from the Tevatron to the LHC • Pdfs • Total xsect vs 2  2 xsect • Multijets  sudakov form factors • These considerations define measurements that are needed to define the hadronic environment at the LHC • Measurements of PDFs with inclusive jet xsect • Underlying event and minimum bias • Multijet rates • Implications for physics • PDFs identifying new physics • Multijets  MET? • Underlying event  Higgs; different processes CTEQ07 Michigan May 2007

  3. Tevatron  LHC stot Q2 (GeV) CTEQ07 Michigan May 2007

  4. Low pt physics CTEQ07 Michigan May 2007

  5. LHC prediction Tevatron Minimum bias and Underlying Event: LHC predictions dN/dh in minimum bias events Particle density LHC PYTHIA6.214 - tuned Tevatron ● CDF 1.8 TeV ● CDF 1.8 TeV ~80% ~200% Pt leading jet (GeV) PYTHIA6.214 - tuned Minimum bias = inelastic pp interaction Underlying event = hadronic environment not part of the hard scatter UE includes radiation and small impact parameter bias MB only CTEQ07 Michigan May 2007

  6. High PT scatter Beam remnants ISR The underlying event LHC PYTHIA6.214 - tuned PHOJET1.12 Transverse < Nchg > x 3 x1.5 • Extrapolation of UE to LHC is unknown • Depends on • Multiple interactions • Radiation • PDFs • Striing formation • Lepton isolation • Top • Jet energy • VBF CTEQ07 Michigan May 2007

  7. Reconstructing the underlying event Njets > 1, |ηjet| < 2.5, ETjet >10 GeV, |ηtrack | < 2.5, pTtrack > 1.0 GeV/c Ratio <NTrackReco>/<NTrackMC> CDF Run 1 underlying event analysis Phys. Rev. D, 65 092002 (2002) CTEQ07 Michigan May 2007 Leading jet ET (GeV)

  8. Underlying event for different processes R.Field • The underlying event for electroweak processes needs to be studied • Critical for Higgs search in VBF CTEQ07 Michigan May 2007

  9. “Leading Jet” “Back-to-Back” Resolving hard and soft components • TransMAX and transMIN sensitive to radiation and soft UE respectively • Back-to-back sample suppresses radiation • difference between tranMAX region and transMIN in leading jet and b-2-b jet sample CTEQ07 Michigan May 2007

  10. Simulation of underlying event • MC tools for simulation of underlying event • PYTHIA (UE+min bias) • Herwig + Jimmy (UE only, pt-cut) • PHOJET (Min bias and UE) • All give a reasonable description of Tevatron data with tuning • Energy extrapolation is essentially a free parameter and uncertain  data required • SHERPA also has simulation of underlying event but has been studied less CTEQ07 Michigan May 2007

  11. PDFs CTEQ07 Michigan May 2007

  12. SM 2XD 4XD 6XD Impact of PDF uncertainty on new physics • Extra-dimensions affect the di-jet cross section through the running of as. Parameterised by number of extra dimensions D and compactification scale Mc. • PDF uncertainties (mainly due to high-x gluon) reduce sensitivity to compactification scale from ~5 TeV to 2 TeV Mc= 2 TeV Mc= 6 TeV Mc= 2 TeV PDF uncertainties • Similarly PDF uncertainties limits the sensitivity in inclusive xsect to BSM physics CTEQ07 Michigan May 2007

  13. Measure high-x gluon pdfs from inclusive jet cross-section • Measure inclusive xsect to get high-x gluons • Measure in different rapidity bins • New physics vs pdf • Theoretical uncertainties in QCD calculation • Scale dependence • Experimental errors • Jet energy scale CTEQ07 Michigan May 2007

  14. Uncertainty due to PDF High pT PDF errors dominated by the high x-gluon. Estimates below from CTEQ6.1 error sets 29 and 30 compared to best fit (NLOJET). At 1TeV in central region error is 10-15% CTEQ07 Michigan May 2007

  15. Theoretical Errors From changing scale µr=µf from 0.5pT jet to 2.0pT jet 5%-10% scale error. • NLO QCD cross-sections can be calculated to compare with the experimental results. • There are errors on the theoretical prediction due to PDFs and the finite order of the calculation (renormalisation and factorisation scales). CTEQ07 Michigan May 2007

  16. Experimental Errors Used NLOJET. 10% JES 5% JES 1% JES For a jet with pT=1TeV: 1% error on jet energy -> 6% on  5% error on jet energy -> 30% on  10% error on jet energy -> 70% on  CTEQ07 Michigan May 2007

  17. Analysis – Constraining High x-Gluon • Effect of adding simulated ATLAS collider data to gluon uncertainty in a global PDF fit (NLOGRID) Fits by Claire Gwenlan: Reducing JES from 3% to 1% Gluon uncertainty x x • A very good control (1%) of the Jet Energy Scale is needed in order to constrain PDFs using collider data. CTEQ07 Michigan May 2007

  18. CTEQ07 Michigan May 2007

  19. At the LHC we will have dominantly sea-sea parton interactions at low-x • And at Q2~M2W/Z the sea is driven by the gluon • by the flavour blind g ->qq • gluon is far less precisely determined for all x values _ Improving low-x gluon using rapidity distribution in W-decay The uncertainties on the W and Z rapidity distributions are dominated by the uncertainties along gluon PDF dominated eigenvectors |h|<2.5 Measurement of W and Z rapidity distributions can increase our knowledge of the gluon PDF  key to using W,Z as luminosity monitor CTEQ07 Michigan May 2007

  20. GOAL: syst. exp. error ~3-5% W and Z Rapidity Distributions for different PDFs Analytic calculations: Error bands are the full PDF Uncertainties CTEQ6.1M MRST02 ZEUS-S At y=0 the total W PDF uncertainty is ~ ±5.2% from ZEUS-S ~ ±3.6% from MRST01E ~ ±8.7% from CTEQ6.1M ZEUS to MRST01 central value difference ~5% ZEUS to CTEQ6.1 central value difference ~3.5% (From LHAPDF eigenvectors) CTEQ6.1M CTEQ07 Michigan May 2007

  21. ZEUS-PDFBEFORE including W data ZEUS-PDFAFTER including W data e+CTEQ6.1 pseudo-data e+CTEQ6.1 pseudo-data |h| low-x gluon shape parameter λ, xg(x) ~ x –λ: BEFORE λ = -0.199 ± 0.046 AFTER λ = -0.181 ± 0.030 35% error reduction PDF constraining potential of ATLAS Effect of including the ATLAS W Rapidity “pseudo-data” in global PDF Fits: how much can we reduce the PDF errors when LHC is up and running? Simulate real experimental conditions: Generate 1M “data” sample with CTEQ6.1 PDF through ATLFAST detector simulation and then include this pseudo-data (with imposed 4% error) in the global ZEUS PDF fit (with Det.->Gen. level correction). Central value of ZEUS-PDF prediction shifts and uncertainty is reduced: |h| NB: in ZEUS-PDF fit the e± Normalisation is left free => no assumption on Luminosity measurement CTEQ07 Michigan May 2007

  22. Multijets CTEQ07 Michigan May 2007

  23. Dfdijet= p 2p/3Dfdijetp Dfdijet~2p/3 p/2Dfdijet2p/3 Azimuthal dijet decorrelation Early measurement to benchmark generators particularly parton showers/higher orders CTEQ07 Michigan May 2007

  24. Reconstructed di-jet azimuthal decorrelations Selecting di-jet events: J5 Cone jet algorithm (R=0.7) Njets = 2, |ηjet| < 0.5, ETjet #2 > 80 GeV, Two analysis regions: 300 < ETMAX < 600 GeV 600 < ETMAX < 1200 GeV J6 CTEQ07 Michigan May 2007

  25. Spectra include tt Multijets in top events • MC@NLO and ALPGEN agree for hardest jet • HERWIG fails at high pt • Significant number of events have 3 additional jets there is a discrepency between MC@NLO and HERWIG vs ALPGEN • Key that such multijet spectra are measured • Possible with early high energy running CTEQ07 Michigan May 2007

  26. Effect of multijets on inclusive SUSY studies CTEQ07 Michigan May 2007

  27. Summary and conclusions • QCD benchmarks (inc low-pt processes) • Minimum bias and underlying event • Related, need to understand for many analyses • Baseline for HI, minijets evolution from low pt • Do this before high lumi • PDFs • New regime in PDFs • Need to measure for precision SM and high-pt physics • Multijets • Measure azimuthal decorrelations to validate simulations • Many more jets in events tt6J+nJ • Need to understand multiplicities CTEQ07 Michigan May 2007

  28. 1000 events dNch/d h Black = Generated (Pythia6.2) Blue = TrkTrack: iPatRec Red = TrkTrack: xKalman dNch/dpT Reconstruct tracks with: 1) pT>500MeV 2) |d0| < 1mm 3) # B-layer hits >= 1 4) # precision hits >= 8 pT (MeV) Tracking in MB events • Acceptance limited in rapidity and pt • Rapidity coverage • Tracking covers |h|<2.5 • pT problem • Need to extrapolate by ~x2 • Need to understand low pt charge track reconstruction CTEQ07 Michigan May 2007

  29. Minimum bias studies: Charged particle density at  = 0 Why? soft physics, pile-up at higher luminosities, calibration of experiment LHC? Large uncertainties in predicted particle density in minimum bias events ~x2  Measurement with central tracker at level of ~10% with ~10k events – first data CTEQ07 Michigan May 2007

  30. Use MB multiplicity distributions to tune fluctuations in number of events d-gaussian smooth Uniform abrupt gaussian Compare abrupt and smooth pt-cut-off: Abrupt cut-off generates a Poisson distribution with too few multi-parton interactions in a single event Compare matter distributions: uniform, gaussian, double gaussian  Use double gaussian CTEQ07 Michigan May 2007

  31. Herwig+Jimmy • Jimmy is multi-parton interaction model similar to PYTHIA • Main parameter is pt-min • Only for hard UE cannot model low-pt ie MB difficult to get energy dependence • Matter distribution is determined from em form factor • Gives a good description of CDF data with increased pt-min2.53.25GeV CTEQ07 Michigan May 2007

  32. Herwig+Jimmy • Extrapolation to LHC predicts much larger UE c.f. underlying event • No energy dependent pt-min • Which is correct? LHC Transverse < Nchg > x5 x4 x3 Pt (leading jet in GeV) Tevatron CTEQ07 Michigan May 2007

  33. Tagging jet W Z/W H Z/W W Tagging jet VBF Signal (HWWll) • forward tagging jets • correlated isolated leptons • low hadronic activity in central region • central Higgs production Important discovery channel For Higgs in mass range 120-200GeV CTEQ07 Michigan May 2007

  34. Uncertainty at the level of ~6% on CJV Pt>20GeV and ~3% on lepton Giving a total uncertaintly in the range ~8% CTEQ07 Michigan May 2007

  35. Lepton isolation in H->4m S.Abdullin et al (CMS) Les Houches 05 • Effect of UE on lepton efficiency • Vary pt-min by 3s • Determine from data • Good muons • Barrel h<1.1 Pt>7GeV • Endcap 1.1<h<2.4 P>9GeV • Isolation • SPt for charged tracks excluding ms with Pt>0.8GeV and DR<0.3 around m in h-f space CTEQ07 Michigan May 2007

  36. Extract effect of UE from data • Use inclusive Z-sample, high statistics • Similar dependence to ZZ sample but small systematic shift m m UE m m random CTEQ07 Michigan May 2007

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