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

2. CTEQ07 Michigan May 2007 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

3. CTEQ07 Michigan May 2007 Tevatron  LHC Q 2 (GeV)  tot

4. CTEQ07 Michigan May 2007 Low pt physics

5. CTEQ07 Michigan May 2007 Minimum bias and Underlying Event: LHC predictions Tevatron ● CDF 1.8 TeV PYTHIA6.214 - tuned PYTHIA predictions dN/dη (η=0) N ch jet- p t =20GeV 1.8TeV (pp)4.12.3 14TeV (pp)7.0 increase~x1.8~x3 ~80% ~200% LHC prediction Tevatron PYTHIA6.214 - tuned ● CDF 1.8 TeV LHC MB only UE includes radiation and small impact parameter bias dN/d  in minimum bias events Minimum bias = inelastic pp interaction Underlying event = hadronic environment not part of the hard scatter P t leading jet (GeV) Particle density

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

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

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

9. CTEQ07 Michigan May 2007 Resolving hard and soft components “Leading Jet” “Back-to-Back” 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

10. CTEQ07 Michigan May 2007 Simulation of underlying event MC tools for simulation of underlying event –PYTHIA (UE+min bias) –Herwig + Jimmy (UE only, p t -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

11. CTEQ07 Michigan May 2007 PDFs

12. CTEQ07 Michigan May 2007 Impact of PDF uncertainty on new physics  Similarly PDF uncertainties limits the sensitivity in inclusive xsect to BSM physics  Extra-dimensions affect the di-jet cross section through the running of  s. 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 2XD 4XD 6XD SM Mc= 2 TeV uncertainties PDF Mc= 2 TeV Mc= 6 TeV

13. CTEQ07 Michigan May 2007 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

14. CTEQ07 Michigan May 2007 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%

15. CTEQ07 Michigan May 2007 Theoretical Errors 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). 5%-10% scale error. From changing scale µ r =µ f from 0.5pT jet to 2.0pT jet

16. CTEQ07 Michigan May 2007 Experimental Errors 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  Used NLOJET. 10% JES 5% JES 1% JES

17. CTEQ07 Michigan May 2007 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: x x Gluon uncertainty Reducing JES from 3% to 1% A very good control (1%) of the Jet Energy Scale is needed in order to constrain PDFs using collider data.

18. CTEQ07 Michigan May 2007

19.  At the LHC we will have dominantly sea-sea parton interactions at low-x  And at Q 2 ~M 2 W/Z the sea is driven by the gluon by the flavour blind g ->qq  gluon is far less precisely determined for all x values _ The uncertainties on the W and Z rapidity distributions are dominated by the uncertainties along gluon PDF dominated eigenvectors Measurement of W and Z rapidity distributions can increase our knowledge of the gluon PDF  key to using W,Z as luminosity monitor Improving low-x gluon using rapidity distribution in W-decay   2.5

20. CTEQ07 Michigan May 2007 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) W and Z Rapidity Distributions for different PDFs CTEQ6.1M MRST02 ZEUS-S Analytic calculations: Error bands are the full PDF Uncertainties GOAL: syst. exp. error ~3-5% CTEQ6.1M

21. CTEQ07 Michigan May 2007 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: ZEUS-PDF BEFORE including W data e + CTEQ6.1 pseudo-data 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 NB: in ZEUS-PDF fit the e ± Normalisation is left free => no assumption on Luminosity measurement ZEUS-PDF AFTER including W data e + CTEQ6.1 pseudo-data 

22. CTEQ07 Michigan May 2007 Multijets

23. CTEQ07 Michigan May 2007 Azimuthal dijet decorrelation Early measurement to benchmark generators particularly parton showers/higher orders 2     dijet        dijet   2    dijet  =   dijet  ~ 2  

24. CTEQ07 Michigan May 2007 Reconstructed di-jet azimuthal decorrelations Selecting di-jet events: 300 < E T MAX < 600 GeV Cone jet algorithm (R=0.7) N jets = 2, | η jet | < 0.5, E T jet #2 > 80 GeV, Two analysis regions: 600 < E T MAX < 1200 GeV J5 J6

25. CTEQ07 Michigan May 2007 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 Spectra include tt

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

27. CTEQ07 Michigan May 2007 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

28. CTEQ07 Michigan May 2007 Tracking in MB events Acceptance limited in rapidity and pt Rapidity coverage –Tracking covers |  |<2.5 p T problem –Need to extrapolate by ~x2  Need to understand low pt charge track reconstruction 1000 events  dN ch /d  dN ch /dp T Black = Generated (Pythia6.2) Blue = TrkTrack: iPatRec Red = TrkTrack: xKalman Reconstruct tracks with: 1) pT>500MeV 1) pT>500MeV 2) |d 0 | < 1mm 2) |d 0 | < 1mm 3) # B-layer hits >= 1 3) # B-layer hits >= 1 4) # precision hits >= 8 4) # precision hits >= 8 p T (MeV)

29. CTEQ07 Michigan May 2007 Minimum bias studies: Charged particle density at  = 0 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 Why? soft physics, pile-up at higher luminosities, calibration of experiment

30. CTEQ07 Michigan May 2007 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 Use MB multiplicity distributions to tune fluctuations in number of events abrupt smooth Uniform gaussian d-gaussian

31. CTEQ07 Michigan May 2007 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-min 2.5  3.25GeV

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

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

34. CTEQ07 Michigan May 2007 ModelParameter Simple MSTP(82)=1 PARP(81)=1.9 Complex MSTP(82)=4 PARP(82)=1.9 Tuned MSTP(82)=4 PARP(82)=1.8 PARP(84)=0.5 ModelCJV (eff)LEPACC (eff)All Vetoes (eff) Simple0.943 ± 0.0030.610 ±0.0010.084 ±0.001 Complex0.885 ± 0.0030.575 ±0.0010.076 ±0.001 Tuned0.915 ± 0.0030.589 ±0.0010.080 ±0.001 Uncertainty at the level of ~6% on CJV Pt>20GeV and ~3% on lepton Giving a total uncertaintly in the range ~8%

35. CTEQ07 Michigan May 2007 Effect of UE on lepton efficiency Vary pt-min by 3  Determine from data Good muons –Barrel  7GeV –Endcap 1.1 9GeV Isolation –  Pt for charged tracks excluding  s with Pt>0.8GeV and  R<0.3 around  in  -  space S.Abdullin et al (CMS) Les Houches 05 Lepton isolation in H->4  ProcessEvent eff Default Event eff –3  Event eff +3  H  4  MH=150GeV 0.775 ± 0.0040.707 ± 0.0050.812 ± 0.004 ZZ background0.780 ± 0.0040.721 ± 0.0050.838 ± 0.004 4 random  Z-inclusive 0.762 ± 0.0070.706 ± 0.0070.821 ±0.006

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