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QCD at the Tevatron

QCD at the Tevatron. Iain Bertram Lancaster University D Ø Experiment 6 June 2001 - Bristol. Introduction to Tevatron Kinematics DØ – CDF Detectors Jet Cross Sections Jet Structure Photons Run II @ DØ Conclusions

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QCD at the Tevatron

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  1. QCD at the Tevatron Iain Bertram Lancaster University DØ Experiment 6 June 2001 - Bristol

  2. Introduction to Tevatron Kinematics DØ – CDF Detectors Jet Cross Sections Jet Structure Photons Run II @ DØ Conclusions Caveat: This is not the entire Tevatron QCD program – that is impossible in one hour. I have chosen the topics I consider to be most important. Outline Iain A Bertram

  3. Hadron-hadron collisions Photon, W, Z etc. parton distribution Hard scattering Underlying event FSR ISR parton distribution fragmentation Iain A Bertram Jet

  4. Measured Event Variables • In a event the following are measured: Jet, electron 1: ET1, h1, 1 Jet, electron 2: ET2, h2, 2 h = 0 Iain A Bertram

  5. The Detectors (Run I 1993-95) Iain A Bertram

  6. DØ Calorimeter: Run I Full Coverage: || < 4.1 Segmentation: =0.10.1 (=0.050.05 EM shower max) Single Electron Response: 0.15/E + 0.003 Single Hadron Response: 0.5/E + 0.004 Depth:  Iain A Bertram

  7. CH “calorimeter jet” hadrons FH  EM “particle jet” Time “parton jet” Jets at the Tevatron • Mostly Fixed cone-size jets • Add up towers • Iterative process • Jet quantities: • ET, ,  • Correct to Particles • Do not include underlying event. • Model underlying event with Minimum bias event (inelastic scattering) Iain A Bertram

  8. “Typical DØ Dijet Event” ET,1 = 475 GeV, h1 = -0.69, x1=0.66 ET,2 = 472 GeV, h2 = 0.69, x2=0.66 MJJ = 1.18 TeV Q2 = ET,1×ET,2=2.2x105 GeV2 Iain A Bertram

  9. Tevatron x-Q2 Reach Overlaps and extends reach of HERA Iain A Bertram

  10. d2/dET d ET Inclusive Jet Cross Section • How well do we know proton structure (PDFs) ? • Is NLO (s3) QCD “sufficient” ? • Are quarks composite ? pQCD, PDFs, substructure,..? Iain A Bertram

  11. Inclusive Jet Cross Section CDF: hep-ph/0102074 (acc PRD) DØ: PRL:82 2451 (1999) , hep-ex/0012046(acc PRD) CDF: 0.1 < || < 0.7 Good Agreement with NLO QCD Iain A Bertram

  12. NLO pQCD predictions (s3): - Ellis, et al., Phys. Rev. D, 64, (1990) EKS- Aversa, et al., Phys. Rev. Lett., 65, (1990)- Giele, et al., Phys. Rev. Lett., 73, (1994) JETRAD Choices (hep-ph/9801285, EPJ C5, 687, 1998): Renormalization Scale (~10%) PDFs (~20% with ET dependence) Clustering Alg. (~5% with ET dependence) Theory Uncertainties Iain A Bertram

  13. DØ analyzed 0.1 < || < 0.7 to compare with CDF Good Agreement If we calculate 2 between DØ data and fit to CDF and its uncertainties: 2 =30.8 (0.16) Comparisons with CDF Iain A Bertram

  14. DØ Comparisons to NLO Theory • No indication of an excess above 350 GeV. • Good agreement quantitatively as indicated by c2 test: Di and Ti data and theory, Cij covariance matrix. c2 = S (Di-Ti) C-1ij (Dj-Tj) Iain A Bertram

  15. CDF Comparisons to NLO Theory • No indication of an excess above 350 GeV. • Good agreement quantitatively as indicated by c2 test: c2 = c2stat + S2 S2 is a contribution depending on the best fit to the uncertainties Iain A Bertram

  16. Phys.Rev.Lett. 86 1707 (2001) QCDJETRAD d2 dET d (fb/GeV) ET DØ Forward Jets QCDJETRAD Iain A Bertram

  17. Comparison to Theory Closed: CTEQ4HJ Open: CTEQ4M Closed: MRTSg Open: MRST Iain A Bertram

  18. Quantitative Comparison • Discriminates between PDF • Now being used by CTEQ and MRST to restrict gluons to 20% level (see DIS 2001 http://dis2001.bo.infn.it/wg/sfwg.html) Iain A Bertram

  19. What have we learned? • These results extend significantly the kinematic reach of previous studies and are consistentwith pQCD calculations over the large dynamic range accessible (| | < 3). • Once incorporated into revised modern PDFs, these measurements will greatly improve our understanding of the structure of the proton at large x and Q2. • Are gluon distributions at large x enhanced? • factor 20 more data in Run II, starting summer 2001, will extend the reach to higher ET and should make the asymptotic behaviour clearer Iain A Bertram

  20. Snowmass (h , j) (h0, j0) Calorimeter ET R=0.7 Jet Seeds Cone jet KT jet (D=1) KT Jet Algorithm Fixed Cone Algorithm: KT Jet Algorithm: resolution parameter Jet ET Iain A Bertram

  21. KT with D=1.0, equals NLO cross section with Cone R=0.7 Energy difference between KT and cone causes difference in cross section 1-2 GeV Difference caused by Hadronic Showering effects (parton to particle) Underlying Event Showering Difference with theory most at low ET. Jet Cross Section using KT 24 d.o.f. 2=27 (31%) 2=31/24 (31%) 2=27 (31%) Iain A Bertram

  22. Express Inclusive Jet Cross Section as dimensionless quantityas a function of Theory uncertainties could be reduced to 10% Experimental Uncertainties Cancel Ratio of Cross Sections Naive Parton Model = 1 Iain A Bertram

  23. Phys.Rev.Lett.86,2523 (2001);hep-ex/0012046 Data 10-15% below NLO QCD No obvious problem:Interesting! Ratio of Cross Sections Agreement Probability (from c2 test) with CTEQ4M, CTEQ4HJ, MRST, MRSTGU: 25-80% Iain A Bertram

  24. Comparison with CDF • Consistent at high xT, possible discrepancy at low values Iain A Bertram

  25. Different renormalizationscales at the two energies OK, so it’s allowed, but . . . Mangano proposes an O(3 GeV)non-perturbative shift in jet energy losses out of cone underlying event intrinsic kT could be under or overcorrecting the data (or even different between theexperiments — DØ?) Suggested explanations Iain A Bertram

  26. Dijet Mass Spectrum Nevents L DM Dh1 Dh2 Experiments in excellent agreement. Different rapidity ranges and very different analysis techniques. Reasonable agreement with predictions. Iain A Bertram

  27. Dijet Mass Cross Section Ratio s (|h1,2| < 0.5 ) / s ( 0.5 < |h1,2|< 1 ) (s=1800 GeV ) PRL 82, 2457-2462, 1999 Theory uncertainty ~ 6% (m) , 3% (PDF) Systematic Uncertainty ~ 8% NLO QCD in good agreement with data  2.4 TeV (95% confidence level) Iain A Bertram

  28. BFKL BFKL (Balitsky-Fadin-Kuraev-Lipatov) evolution 1/x • In hadron-hadron collisions: PDF DGLAP (Dokshitser-Gribov-Lipatov-Altarelli-Parisi) Q2 DGLAP (as in PYTHIA) gluon radiation strongly ordered BFKL many gluons all ~ same pT One way to realise this situation is jets widely separated in rapidity: the total energy is then much greater than the jet pT scale, and one can have many gluons of comparable pT emitted between the jets Note: BFKL provides a way to resum the contribution of these gluons; it doesn’t predict how many there are, and there is no BFKL event generator yet Iain A Bertram

  29. BFKL • Phys.Rev.Lett. 84, 5722 (2000) • Another attempt to find anobservable which displays BFKLbehaviour: • DØ measurement of 630/1800 GeV cross section ratio at large rapidity separations • use bins such that x and Q2 arethe same in the two datasets(but different ) • What’s going on here? • data behave qualitatively like BFKL (but also like HERWIG) • given that we can’t predict the 630/1800 GeV ratio of inclusive cross sections, how much can we really infer? Iain A Bertram

  30. 1800GeV s 100 200 300 Subjet Multiplicity in Quark & Gluon Jets Method: • Select quark enriched & gluon enriched jet sample • Compare jets at same ( ET , h ) produced at different and assume relative q/g content is known Motivation: • Test of QCD ( Q & G jets are different) • Separate Q jets from G jets (top, Higgs, W+Jets events) Measure the subjet multiplicity in quark and gluon jets s 630GeV qq Contributions of different initial states to the cross section for fixed Jet ET vary with gg qg 100 200 300 Jet ET Jet ET Iain A Bertram

  31. 0.5 Gluon Jets 0.4 Quark Jets 0.3 0.2 0.1 Jet Structure at the Tevatron • Subjets inside jets: perturbative part of fragmentation • DØ compares 630 to 1800 GeV data at same ET and , and infers q and g jet differences DØ Preliminary kT algorithm D=0.5, ycut= 10-3 55 < ET(jet) < 100 GeV |jet| < 0.5 1 2 3 4 5 Subjet Multiplicity DØ Data HERWIG 5.9 Iain A Bertram

  32. Direct Photon Production DØ PRL 84 2786 (2000) CDF Preliminary QCD prediction is NLO Owens et al. Note: ET range probed with photons is lower than with jets Iain A Bertram

  33. Direct Photon Production DØ PRL 84 2786 (2000) CDF Preliminary Low ET excess: Theory QCD NLO Owens et al. Iain A Bertram

  34. kT Smearing??? • Gaussian smearing of the transverse momenta by a few GeV can model the rise of cross section at low ET (hep-ph/9808467) Motivated by observed pT() Iain A Bertram

  35. Alternative Viewpoint Also note that Vogelsang et al. get closer to the observed shape than the Owens prediction with no extra kT (using NLO fragmentation and setting RF) Iain A Bertram

  36. Photons at s = 630 GeV • CDF data, compared with UA2 data show deficit at high ET compared with the theory. • Relation to jets? Iain A Bertram

  37. DØ Photons at 630 GeV 2 Comparison (7 bins) CC: 11.4 -- 12% EC: 4.6 -- 71% First measurement at forward rapidities Iain A Bertram

  38. Slight excess at low XT Insignificant statistically Run II won’t help! Good overall agreement with NLO No significant high ET deficit. No statistics at high ET (> 40 GeV) Ratio of Photons at 630 and 1800 2 Comparison (7 bins) CC: 6.5 -- 49% EC: 3.0 -- 89% Iain A Bertram

  39. Photons: Comments • Situation is complicated and not easily explained. • Low ET kT effects? • High ET deficits at 630 GeV • Lack of statistics in both regions • Challenge for both theory and experiment in Run II. Iain A Bertram

  40. Run II : Coming March 2001 Upgrades to both detectors: Silicon, Tracking (Drift-CDF, Fibres-DØ) Preshowers, Calorimeter Upgrade – CDF, Muon upgrades, new trigger electronics (bunch spacing), C++ OO code development Iain A Bertram

  41. Case Study: Dijet Mass Spectrum Dramatic increase in high pT cross sections Large gains in statistics Iain A Bertram

  42. Looking ahead: Run II Run II: ~100 events ET > 490 GeV ~1K events ET > 400 GeV Run I:16 Events ET> 410 GeV Great reach at high x and Q2, the place to look for new physics! Iain A Bertram

  43. Jet Algorithms • Fermilab Run II workshop • Various species of kT • New Cone Algorithm • Theoretical desires • “infrared safety is not a joke!” • avoid ad hoc parameters like Rsep • Cone algorithm improved by • e.g. by modification of seed choices - midpoints • seedless algorithm? In development. • Experimental desires • sensitivity to noise, pileup, negative energies Iain A Bertram

  44. Requirements for Progress • Quantitative Theoretical Uncertainties • pdf and renormalization scale uncertainties • Full understanding of correlations of experimental uncertainties • Statistical Uncertainties in most cases will be negligible • Underlying event • Require a better understanding and treatment of the surrounding event Iain A Bertram

  45. Status of DØ detector Run commenced March 2001 Two data taking periods completed of a few weeks. Detector Commissioning taking place! Physics data in autumn First results in early 2002 Iain A Bertram

  46. [cm] Firstpp collisions of Run 2 at DØ: April 3, 2001 Antiproton halo • Luminosity counters • timing Luminosity ( coincidence) ~5  1027 cm-2 sec-1 Proton halo Vertex distribution along z of min bias events: Iain A Bertram

  47. Global track finding Track with 5 fiber tracker hits, 5 3D silicon hits Relative alignment of silicon and fiber trackers verified to 40 m level 36  36 Store Run 119679, Event 232931Level 3 (software trigger) Global Tracking Iain A Bertram

  48. Event with 6 tracks pointing to same vertex 36  36 Store Run 119679, Event 231653Level 3 (software trigger) Silicon-only Tracking Iain A Bertram

  49. Vertex finding • Primary vertices reconstructed from tracks in silicon: z z x y y A. Schwartzman Buenos Aires x Iain A Bertram

  50. Tracking in Silicon • 3D event display showing hits and reconstructed track: E. Barberis Y. Kulik  Offline track finding Iain A Bertram

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