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LO -> NLO -> NNLO -> Experiment

W/Z/ g + Jets at ATLAS Jimmy Proudfoot Argonne National Laboratory On behalf of the ATLAS Collaboration. LO -> NLO -> NNLO -> Experiment. Standard Model Test and Background to New Physics. The ATLAS Calorimeter System. Calorimeter Response. GOAL: s E /E = 50%/√E + 3%.

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LO -> NLO -> NNLO -> Experiment

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  1. W/Z/g + Jets at ATLAS Jimmy ProudfootArgonne National LaboratoryOn behalf of the ATLAS Collaboration LO -> NLO -> NNLO -> Experiment Standard Model Test and Background to New Physics J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  2. The ATLAS Calorimeter System J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  3. Calorimeter Response GOAL: sE/E = 50%/√E + 3% Dead Material Map Tile Sampling Fraction vs h Phi Modulation from Accordian Structure J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  4. Electromagnetic Calorimeter and the Endcap Hadron Calorimeters use Liquid Argon for Ionization Measurement Ionisation signal is sampled: 25ns, 12 bit ADC, 3 gains Physics data: Usually 5 samples (6 samples in TB) Calibration signal: Up to 32 samples (to determine waveform) This technology has 2 specific technical issues: pileup and noise J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  5. Weight Cell/Layer Energies to Optimise Linearity and resolution EM Scale “Simple” weighting scheme using depth dependent weights Other approaches also work well and the merits of the different schemes are being discussed The Ultimate Test will be with DATA Weighted J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  6. W/Z/g + Jets Overview • W+Jets: • Only studied at present as background to other physics (TTbar, Single Top, SUSY • Z + Jets: • SM Test of pQCD in Z+Njets • “Data-Based” calibration of physics backgrounds from Z decays such as Z->nn for SUSY and Z-> BBbar for Higgs , and Ws by inference • Jet energy scale calibration • g + Jets: • Only studied at present in the context of jet energy scale calibration J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  7. Guidance from the Tevatron SM Tests of W+Jets Spectra are sensitive to choice of renormalization and factorization scales J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  8. More Guidance from the TeVatron Backgrounds to Z+Jets do not show the same increase as a function of Jet Pt and the number of jets - at least for a suitable event selection (DR between leptons and jets) BUT - this is hard to establish for the LHC until we have DATA W + Jets Increasing and significant rate of QCD and Top backgrounds to W+Jets Top Background in W+Jets will be even worse at the LHC J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  9. MadEvent: LHC != Scaled TeVatron LHC TeV qg=>Zqg ~0.75stot qg=>Zqg ~0.26stot J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  10. Z+ nJets Use to establish the reliability of our understanding W+Jets LO <=> NLO <=NNLO Basis for determining SM background to searches Bootstrap the light jet energy scale calibration Use to measure the b pdf in the proton Use to establish b jet energy scale J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  11. Standard Model Backgrounds to New PhysicsA “Shopping” List J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  12. Top - Without B-tagging W-BBbar/ W+Jets x-section ratio => the irreducible background J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  13. H production in VBF: mH = 160 GeV Rejection factor ~120 • Tag Jets (WW* Analysis): • Pt > 40 GeV/c, Pt > 20 GeV/c • Dhtags > 3.8 J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  14. H production in VBF: mH = 120 GeV Rejection factor ~70 • Tag Jets (tt Analysis): • Pt > 50 GeV/c, Pt > 30 GeV/c • Dhtags > 4.4 J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  15. Background To SUSY Black: ISAJET Red: PYTHIA • Susy Background: Z(->nn) +jet • Effective Mass distribution for No-Leptons Mode after standard event selection M(g)≈M(q)≈1TeV Susy Atlas meetings T.S.S.Asai U. of Tokyo Event Topology J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  16. Estimating SM Background to SUSY • Assume shape of BG in control region (MT<100 ) is shape of BG in signal region (MT>100 ) → Just need to scale with # events • Assume No SUSY in control region (MT<100) Et J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  17. Calibration side Signal side MT<80GeV MT>80GeV ETmiss>250GeV 1D Method: Use MC Driven by Data Obtain an absolute normalization of W+jets on the calibration side from data by means of (Z+jets, and/or g+jets?) calibration. J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  18. Results for Simple 2D Method TTbar W+Jets (ALPGEN) SUSY No clear SUSY-free region from which to extract the shape of the background which then can be extrapolated into the SUSY region BUT Maybe can use a more sophisticated approach which will probably require a better understanding of the physics of the backgrounds J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  19. B Quark pdf from Z+B bb->Z @ LHC is ~5% of entire Z production Spread of existing pdf’s gives a 10% spread in the prediction of the H cross section J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  20. B-Tagging Life time of a bottom hadron is about t ~ 1.5 ps long enought to permit to a hadron of 30 GeV of energy to do a distance of L ~ 3 mm before decaying BTagging Efficiency 59.5% Purity 60.7% Based on TeVatron and LEP experience expect to determine tagging efficiency from B-enriched samples to a precision of ~5% J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  21. Some MC Tools in Use At ATLAS • AcerMC: Zbb~, tt~, single top, tt~bb~, Wbb~ • Alpgen (+ MLM matching): W+jets, Z+jets, QCD multijets.. • HERWIG: QCD multijets, Drell-Yan, SUSY MadEvent: Z/W+jets... • MC@NLO: tt~, Drell-Yan, boson pair production • Pythia: QCD multijets, B-physics, Higgs production... • Sherpa: W+jets/Z+jets... • WINHAC: W production and decay AND Since we have no DATA for pp at 14 TeV We can only compare Monte Carlo to Monte Carlo ! J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  22. Particle Content of Z/g + Jets Z+Jets Pythia: Large event-to-event fluctuations dominated by low Pt particles - mostly central g+Jets J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  23. Jets In Z + Jets Events Jet Pt >10GeV Large Pt Dependence Large dependence on hadronization and fragmentation model (not a surprise) Many soft Jets Tails and Mean Multiplicity are Sensitive to Underlying Event J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  24. A Monte Carlo Study of Z-Jet Pt Balancing in ATLAS • There are two key issues: • Correlation between the jets • Jet reconstruction efficiency J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  25. Jet Energy Calibration Ave. Pt Balance for all Jets above 1 GeV • Want to decouple as much as possible physics effects from detector effects • => but particle Pt and h cuts alone give a net imbalance J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  26. Another Generator Study Average Pt Balance for Jets above 10 GeV Back-to-Back Cut on Leading Jet relative to Z Back-to-Back Cut on Leading Jet relative to Z AND No Second Jet with Pt>10GeV J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  27. Some Important Z/W + nJets Features for LHC Physics • Compare Z’s to W’s using Monte Carlo to establish level to which Z’s (in data) can be used to establish W backgrounds to new physics and precision with which we understand variables to be used in kinematic cuts e.g.: • (Z/W+{n+1}Jet) / (Z/W+{n}Jet) cross section ratio • Jet Pt for Leading, next-to-leading jet.. • dR between the two leading jets • Jet rapidity • Z + bb (vital for low mass Higgs searches) J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  28. W + n Jets Zee + n Jets Jets in Z events versus Jets in W Events Compare ratio of (N(Jets) >= i)/(N(Jets) = 1) for W + n Jets and Zee + n Jets ALPGEN • Same ratio • (With Some Precision) Next step : compare different MC generators (with new samples) to get an idea of systematic uncertainties on shape and normalization factor. J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  29. Compare Shape of Meff in Z + n Jets vs W + n Jets ALPGEN Considering all Z samples : Z + 0, 1, 2, 3, 4, 5 jets Considering only Z samples : Z + 2, 3, 4, 5 jets as for W 1 <= n jets <= 3, W >= 4 jets, W >= 4 jets, Z 1 <= n jets <= 3, Z Normalization : all shapes a normalize to 1. ! Shapes from W and Z productions are the same (if considering same samples) J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  30. Jet Pt Distributions in Z+Jets ATLFAST Jet Parameterization Leading jet is harder in ALPGEN than in Pythia Z+q/g, although both have same matrix element J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  31. Jet Angular Correlations Z+Jets Larger Angle between two leading jets in ALPGEN than in Pythia J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  32. Summary and Conclusions W’s, Z’s and g’s may be “just” Standard Model Physics BUT They aren’t simple They aren’t dull Even here we may see surprises when we final see pp collisions at 14 TeV J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  33. Backup J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  34. LHC vs Tevatron J.Campbell et al. Phys.Rev.D69:074021,2004 The measurement of Z+b should be more interesting at LHC than at Tevatron: • Signal cross-section larger (x80), and more luminosity • Relative background contribution smaller (x5) J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  35. BTagging All Jets B Jets BTagging Efficiency 59.5% Purity 60.7% Soft Muon Tagging All Muons B Muons Soft MuonTagging Efficiency 7.2% Purity 37.2% J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

  36. A Tale of Two Jets A Monte Carlo Study of Z-Jet Pt Balancing in ATLAS • Z + 1 Jet • No Additional Jet with Et>10GeV • df (Jet,Z)>2.88radians • |h| (Jet)<2.6 • PtZ> 27.5GeV • Z + 2 Jets • No Third Jet with Et>10GeV • df (J1+J2,Z)>2.88radians • |h| (J1 & J2)<2.6 • PtZ> 40.0GeV J. Proudfoot, CTEQ LHC Workshop, MSU, May 2007

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