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Recent Results on Jet Physics and a s

Recent Results on Jet Physics and a s. XXI Physics in Collision Conference Seoul, Korea June 28, 2001 Presented by Michael Strauss The University of Oklahoma. Outline. Introduction and Experimental Considerations Jet and Event Characteristics Low E T Multijet Studies

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Recent Results on Jet Physics and a s

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  1. Recent Results on Jet Physics and as XXI Physics in Collision Conference Seoul, Korea June 28, 2001 Presented by Michael Strauss The University of Oklahoma PIC 2001 Michael Strauss The University of Oklahoma

  2. Outline • Introduction and Experimental Considerations • Jet and Event Characteristics • Low ET Multijet Studies • Subjet Multiplicities • Cross Sections • Three-to-Two Jet Ratio • Ratio at Different Center-of-Mass Energies • Inclusive Production • DiJet Production PIC 2001 Michael Strauss The University of Oklahoma

  3. Motivation for Studying Jets d2s dET dh ET Central h region • Investigates pQCD • Compare with current predictions • pQCD is a background to new processes • Investigates parton distribution functions (PDFs) • Initial state for all proton collisions • Investigates physics beyond the Standard Model pdf ? Compositeness ? PIC 2001 Michael Strauss The University of Oklahoma

  4. Developments in Jet Physics calculation of virtual corrections Progress toward NNLO predictions covariance matrices More rigorous treatment of experimental errors More consistent ET calculations between experiments at the Tevatron jet algorithms workshop (with proton initial states) Inclusion of error estimates in the PDFs PIC 2001 Michael Strauss The University of Oklahoma

  5. Cone Definition of Jets R=2 +2 1.3R 2R • Cone DefinitionR=0.7 in h-f • Merging and splitting of jets required if they share energy • Rsep required to compare theoretical predictions to data • (Rsepis the minimum separation of 2 partons to be considered distinct jets) Centroid found with4-vector addition h = -ln[tan(q/2)] PIC 2001 Michael Strauss The University of Oklahoma

  6. KT Definition of Jets • KT Definition • cells/clusters are combined if their relative kT2 is “small”(D=1.0 or 0.5 is a scaling parameter) • Infrared Safe • Same definition for partons, Monte Carlo and data • Allows subjet definitions min(dii, dij) = dij  Merge min(dii, dij) = dii  Jet PIC 2001 Michael Strauss The University of Oklahoma

  7. KT and Cone Algorithm • Use CTEQ4M and Herwig • Match KT jets with cone jets DO Preliminary DO Preliminary 99.9% of Jets have DR<0.5 pT of KT algorithm is slightly higher PIC 2001 Michael Strauss The University of Oklahoma

  8. KT Algorithm and Subjets For subjets, define “large” KT (ycut = 10-3) Increasing ycut PIC 2001 Michael Strauss The University of Oklahoma

  9. Jet Selection Criteria Outer Muon Magnet Inner Muon Hadronic Calorimeter EM Calorimeter Central Tracking jet e g m noise Typical selections on EM fraction, hot cells, missing ET, vertex position, etc. > 97% efficient > 99% pure PIC 2001 Michael Strauss The University of Oklahoma

  10. Jet Energy Corrections Observed Cross Section no distinction between jetsof different kinds • Response functions • Noise and underlying event • “Showering” • Resolutions: Uncertainty on ETEstimated with dijet balancing or simulation d2s dET dh ET d2s dET dh ET Important for cross section measurement PIC 2001 Michael Strauss The University of Oklahoma

  11. Jet and Event Quantities • Low ET Multijet Studies • Subjet Multiplicity PIC 2001 Michael Strauss The University of Oklahoma

  12. D Low ET Multijet events ET of Leading Jet At high-ET, NLO QCD does quite well, but the number of jets at low- ET does not match as well. (Comparison with Pythia) Each jet’s ET>20 GeV. Theory normalized to 2-jet data >40 GeV. Looking also at Jetrad and Herwig PIC 2001 Michael Strauss The University of Oklahoma

  13. D Low ET Multijet events Strong pT ordering in DGLAP shower evolution may suppress “spectator jets” in Pythia BFKL has diffusion in log(pT) (DATA-THEORY)/THEORY PIC 2001 Michael Strauss The University of Oklahoma

  14. D Subjet Multiplicity Using KT Algorithm Monte Carlo • Perturbative and resummed calculations predict that gluon jets have higher subjet multiplicity than quark jets, on average. • Linear Combination: <M> = fgMg + (1-fg) MQ DO Preliminary Mean Jet Multiplicity Gluon Jet Fraction Quark Jet Fraction PIC 2001 Michael Strauss The University of Oklahoma

  15. D Subjet Multiplicity Using KT Algorithm • Assume Mg, MQ independent of √s • Measure M at two √s energies andextract the g and Q components DO Preliminary PIC 2001 Michael Strauss The University of Oklahoma

  16. D Subjet Multiplicity Using KT Algorithm Raw Subjet Multiplicities Extracted Quark and Gluon Mutiplicities DO Preliminary DO Preliminary Higher M  more gluon jets at 1800 GeV PIC 2001 Michael Strauss The University of Oklahoma

  17. D Subjet Multiplicity Using KT Algorithm HERWIGprediction =1.91±0.16(stat) Largest uncertainty comes fromthe gluon fractions in the PDFs Coming soon as a PRD PIC 2001 Michael Strauss The University of Oklahoma

  18. ZEUS Subjet Multiplicity • Comparison at hadron level • Unfolded using Ariadne MC NLO QCD describes data Sensitive to as PIC 2001 Michael Strauss The University of Oklahoma

  19. as from ZEUS Subjets nsub-1   Proportional to as • Major Systematic Errors • Model dependence (2-3%) • Jet energy scale (1-2%) • Major Theoretical Errors • Variation of renormalization scale PIC 2001 Michael Strauss The University of Oklahoma

  20. Cross Sections • Inclusive cross sections • Rapidity dependence • KT central inclusive • R32 • 630/1800 ratio of jet cross sections • Di-Jets • as Conclusions PIC 2001 Michael Strauss The University of Oklahoma

  21. Jet Cross Sections • How well are pdf’s known? • Are quarks composite particles? • What are appropriate scales? • What is the value of as? • Is NLO (as3) sufficient? PIC 2001 Michael Strauss The University of Oklahoma

  22. CTEQ Gluon Distribution Studies • Momentum fraction carried by quarks is very well known from DIS data • Fairly tight constraints on the gluon distribution except at high x • Important for high ET jet production at the Tevatron and direct photon production PIC 2001 Michael Strauss The University of Oklahoma

  23. Experimental Differential Cross Section Theoretical cross section Physics variables are q and x Counting experiment with detector inefficiencies Detector measures ET and h PIC 2001 Michael Strauss The University of Oklahoma

  24. CDF Inclusive Jet Cross Section • 0.1 < |h| < 0.7 • Complete c2 calculation PRD 64, 032001 (2001) PIC 2001 Michael Strauss The University of Oklahoma

  25. x-Q2 Measured Parameter Space Q2 (GeV2 ) x From D Inclusive Cross Section Measurement PIC 2001 Michael Strauss The University of Oklahoma

  26. D Inclusive Jet Cross Section • Five rapidity regions • Largest systematic uncertainty due to jet energy scale • Curves are CTEQ4M d2 dET d (fb/GeV) PRL 86, 1707 (2001) ET (GeV) PIC 2001 Michael Strauss The University of Oklahoma

  27. D Inclusive Jet Cross Section CTEQ4HJ CTEQ4M MRSTg MRST PIC 2001 Michael Strauss The University of Oklahoma

  28. Gluon PDF Conclusions • c2 determined from complete covariance matrix • Best constraint on gluon PDF at high x • Currently being incorporated in new global PDF fits PIC 2001 Michael Strauss The University of Oklahoma

  29. Inclusive Cross Section Using KT Algorithm -0.5 < h < 0.5 D = 1.0 D Preliminary • Predictions IR and UV safe • Merging behavior well-defined for both experiment and theory PIC 2001 Michael Strauss The University of Oklahoma

  30. Comparison with Theory • Normalization differs by 20% or more • No significant deviations of predictions from data • When first 4 data points ignored, probabilities are 60-80% PDF c/dof Prob MRST 1.12 31 MRSTg 1.38 10 MRSTg 1.17 25 CTEQ3M 1.56 4 CTEQ4M 1.30 15 CTEQ4HJ 1.13 29 D Preliminary Strauss The University of Oklahoma PIC 2001 Michael Strauss The University of Oklahoma

  31. CDF as from Inclusive Cross Section • as2X(0)is LO prediction • as3X(0)k1is NLO prediction • X(0) andk1 determined from JETRAD • MS scheme used • Jet cone algorithm used with Rsep = 1.3 • as determined in 33 ETbins ET (GeV) PIC 2001 Michael Strauss The University of Oklahoma Michael Strauss The University of Oklahoma

  32. CDF as from Inclusive Cross Section • Experimental systematic uncertainty • Largest at low ET is underlying event • Largest at high ET is fragmentation and pion response PIC 2001 Michael Strauss The University of Oklahoma Michael Strauss The University of Oklahoma

  33. CDF as from Inclusive Cross Section m scale is the major source of theoretical uncertainty ET/2 < m < 2ET PDF affects as CTEQ4M minimizes c2 Theoretical uncertainties each ~ 5% PIC 2001 Michael Strauss The University of Oklahoma

  34. ZEUS Inclusive Jet Production PIC 2001 Michael Strauss The University of Oklahoma

  35. ZEUS Inclusive Jet Production Measured cross section slightly above NLP pQCD in forward section PIC 2001 Michael Strauss The University of Oklahoma

  36. ZEUS Inclusive Jet Production as Results: Uses various fits of ds/dQ2and ds/dET Full phase-space High-Q2 region (Q2>500 GeV) High-ET region (>14GeVT) PIC 2001 Michael Strauss The University of Oklahoma

  37. R32: Motivation and Method • Study the rate of soft jet emission (20-40 GeV) • QCD multijet production - background to interesting processes • Predict rates at future colliders • Improve understanding of the limitations of pQCD • Identify renormalization sensitivity • Does the introduction of additional scales improve agreement with data ? • Measure the Ratio • with HT • for all jets with • ET > 20, 30, 40 GeV for <3andET > 20 GeV for <2 PIC 2001 Michael Strauss The University of Oklahoma

  38. Inclusive R32 • Features: • Rapid rise HT<200GeV • Levels off at high HT • Interesting: • 70% of high ET jet events have a third jet above 20 GeV • 50% have a third jet above 40 GeV PIC 2001 Michael Strauss The University of Oklahoma

  39. R32 Sensitivity to Renormalization Scale ET>20 GeV, h<2 show greatest sensitivity to scale PIC 2001 Michael Strauss The University of Oklahoma

  40. R32 Results • Jet emission best modeled using the same scale • i.e. the hard scale for all jets • Best scale is that which minimizes 2 for all criteria • R=0.6ETmax, for 20 GeV thresholds • R= HT,.3 for all criteria • Introduction of additional scales unnecessary. ET>20 GeV, h<2 PRL 86, 1955 (2001) PIC 2001 Michael Strauss The University of Oklahoma

  41. D Cross Section Ratio: s(630)/s(1800) vs xT • Ratio of the scale invariant • cross sections: • at different cm energies • ( 630 and 1800 GeV) • Ratio allows substantial reduction • in uncertainties (in theory and experiment). May reveal: • Scaling behavior • Terms beyond LO ( as2 ) s ss = (ET3/2p) (d2s/dETdh) vs XT = ET / (s/ 2 ) ET ss XT QCD 2 s(630)/s(1800) 1 0.0 0.4 xT Naive Parton model PIC 2001 Michael Strauss The University of Oklahoma

  42. D Inclusive Cross Section s = 1800 GeV s = 630 GeV PIC 2001 Michael Strauss The University of Oklahoma

  43. Cross Section Ratio • (630)/(1800)is 10-15% below NLO QCD predictions • Top plot: varying choice of pdf has little effect • Bottom plot: varying R scale is more significant • Better agreement where R different at 630 and 1800 (unattractive alternative !) • Higher order terms will provide more predictive power! Published in PRL 86, 2523 (2001) PIC 2001 Michael Strauss The University of Oklahoma

  44. CDF DiJet Provides precise information about initial state partons Cone of R=0.7 Both Jets: ET>10 GeV Jet 1: 0.1<|h|<0.7 Jet 2: Four h regions 0.1<|h|<0.7 0.7<|h|<1.4 1.4<|h|<2.1 2.1<|h|<3.0 PIC 2001 Michael Strauss The University of Oklahoma

  45. CDF DiJet Cross Section PDF c2/dof MRST 2.68 MRST 3.63 MRST 4.49 CTEQ4M 2.88 CTEQ4HJ 2.43 All < 1% Probability PIC 2001 Michael Strauss The University of Oklahoma

  46. ZEUS DiJet kT algorithm used • ET > 8 GeV (leading) • ET > 5 GeV (other) • -1<h<2 (leading) • 470<Q2<20000 GeV2 Phys Lett B507, 70 (2001) PIC 2001 Michael Strauss The University of Oklahoma

  47. ZEUS DiJet R2+1 parameterized as: R2+1 (MZ) = A1as(MZ) + A1as2(MZ) PIC 2001 Michael Strauss The University of Oklahoma

  48. ZEUS as Summary • Dijets has lowest total error of all Zeus measurements. • All measurements consistent with PDG value of 0.1185±20 PIC 2001 Michael Strauss The University of Oklahoma

  49. Tevatron Run II Run II: Ecm = 1.96 TeV, L 2fb-1 expect: ~100 events ET > 490 GeV and ~1K events ET > 400 GeV Run I: Ecm = 1.8 TeV, L 0.1fb-1 yielded 16 Events ET> 410 GeV Great reach at high x and Q2, A great place to look for new physics! PIC 2001 Michael Strauss The University of Oklahoma

  50. Conclusions from Jet Physics • Growing sophistication in jet physics analysis • Error matrices • New jet algorithms • Better corrections • PDF refinements • Results generally agree with NLO QCD and PDF’s • Cross section measurements will continue to refine PDF’s • as measurements agree with PDG • Low ET physics still require theoretical refinements • Jet physics should continue to provide fruitful developments • High ET region can reveal compositeness and other new physics • Low ET region reveals soft parton distributions in proton • NNLO and other theoretical refinements needed • Results needed for “discovery” measurements PIC 2001 Michael Strauss The University of Oklahoma

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