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

2. 6 June 2001Iain A Bertram2 Outline 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.

3. 6 June 2001Iain A Bertram3 Hadron-hadron collisions fragmentation parton distribution parton distribution Jet Underlying event Photon, W, Z etc. ISR FSR Hard scattering

4. 6 June 2001Iain A Bertram4 Measured Event Variables lIn a event the following are measured: Jet, electron 2: E T 2,  2,  2 Jet, electron 1: E T 1,  1,  1  = 0

5. 6 June 2001Iain A Bertram5 The Detectors (Run I 1993-95) ss1.8 TeVW/Z’s~40k/3k for mass Luminosity2  10 31 ttbar~10-100 events  Ldt~100 pb -1 Higgsnegligible

6. 6 June 2001Iain A Bertram6 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: 

7. 6 June 2001Iain A Bertram7 Jets at the Tevatron lMostly Fixed cone-size jets lAdd up towers lIterative process lJet quantities: èE T, ,  lCorrect to Particles èDo not include underlying event. èModel underlying event with Minimum bias event (inelastic scattering) Time “parton jet” “particle jet” “calorimeter jet” hadrons  CH FH EM

8. 6 June 2001Iain A Bertram8 “Typical DØ Dijet Event” E T,1 = 475 GeV,  1 = -0.69, x 1 =0.66 E T,2 = 472 GeV,  2 = 0.69, x 2 =0.66 M JJ = 1.18 TeV Q 2 = E T,1 ×E T,2 =2.2x10 5 GeV 2

9. 6 June 2001Iain A Bertram9 Tevatron x-Q 2 Reach Overlaps and extends reach of HERA

10. 6 June 2001Iain A Bertram10 Inclusive Jet Cross Section pQCD, PDFs, substructure,..? d 2  /dE T d  ETET l How well do we know proton structure (PDFs) ? l Is NLO (  s 3 ) QCD “sufficient” ? l Are quarks composite ?

11. 6 June 2001Iain A Bertram11 Inclusive Jet Cross Section CDF: 0.1 < |  | < 0.7 Good Agreement with NLO QCD DØ: PRL:82 2451 (1999), hep-ex/0012046 (acc PRD)PRL:82 2451 (1999) hep-ex/0012046 CDF: hep-ph/0102074 (acc PRD)hep-ph/0102074

12. 6 June 2001Iain A Bertram12 Theory Uncertainties lNLO pQCD predictions (  s 3 ): - 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 l Choices (hep-ph/9801285, EPJ C5, 687, 1998): èRenormalization Scale (~10%) è PDFs (~20% with ET dependence) èClustering Alg. (~5% with ET dependence)

13. 6 June 2001Iain A Bertram13 Comparisons with CDF lDØ analyzed 0.1 < |  | < 0.7 to compare with CDF lGood Agreement lIf we calculate  2 between DØ data and fit to CDF and its uncertainties:  2 = 30.8 (0.16)

14. 6 June 2001Iain A Bertram14 lNo indication of an excess above 350 GeV. Good agreement quantitatively as indicated by  2 test: D i and T i data and theory, C ij covariance matrix.  2 =  (D i -T i ) C -1 ij (D j -T j ) DØ Comparisons to NLO Theory |  | < 0.50.1 <|  | < 0.7 CTEQ 3M25.3 (0.39)32.7 (0.11) CTEQ4M20.1 (0.69)26.8 (0.31) CTEQ4HJ16.8 (0.86)22.4 (0.56) MRS(A’)20.4 (0.67)28.5 (0.24) MRST25.3 (0.39)29.6 (0.20)

15. 6 June 2001Iain A Bertram15 lNo indication of an excess above 350 GeV. Good agreement quantitatively as indicated by  2 test:  2 =  2 stat +  S 2 CDF Comparisons to NLO Theory 33 bins0.1 <|  | < 0.7 CTEQ4M63.4 CTEQ4HJ46.8 MRST49.5 S 2 is a contribution depending on the best fit to the uncertainties

16. 6 June 2001Iain A Bertram16 DØ Forward Jets QCD  JETRAD ETET  d 2  dE T d  (fb/GeV) Phys.Rev.Lett. 86 1707 (2001)

17. 6 June 2001Iain A Bertram17 Comparison to Theory Closed: CTEQ4HJ Open: CTEQ4M Closed: MRTSg  Open: MRST

18. 6 June 2001Iain A Bertram18 Quantitative Comparison lDiscriminates between PDF lNow being used by CTEQ and MRST to restrict gluons to 20% level (see DIS 2001 http://dis2001.bo.infn.it/wg/sfwg.html) http://dis2001.bo.infn.it/wg/sfwg.html

19. 6 June 2001Iain A Bertram19 What have we learned? lThese results extend significantly the kinematic reach of previous studies and are consistent with pQCD calculations over the large dynamic range accessible (|  | < 3). lOnce incorporated into revised modern PDFs, these measurements will greatly improve our understanding of the structure of the proton at large x and Q2. lAre gluon distributions at large x enhanced? èfactor 20 more data in Run II, starting summer 2001, will extend the reach to higher ET and should make the asymptotic behaviour clearer

20. 6 June 2001Iain A Bertram20 K T Jet Algorithm R=0.7  0    Jet Seeds Calorimeter E T Snowmass resolution parameter Jet E T K T Jet Algorithm: Cone jet K T jet (D=1) Fixed Cone Algorithm:

21. 6 June 2001Iain A Bertram21 Jet Cross Section using K T l K T with D=1.0, equals NLO cross section with Cone R=0.7 l Energy difference between KT and cone causes difference in cross section l 1-2 GeV Difference caused by èHadronic Showering effects (parton to particle) èUnderlying Event èShowering l Difference with theory most at low E T.  2 =27 (31%)  2 =31/24 (31%)  2 =27 (31%) 24 d.o.f.

22. 6 June 2001Iain A Bertram22 Ratio of Cross Sections lExpress Inclusive Jet Cross Section as dimensionless quantity as a function of lTheory uncertainties could be reduced to 10% lExperimental Uncertainties Cancel Naive Parton Model = 1

23. 6 June 2001Iain A Bertram23 Ratio of Cross Sections l Phys.Rev.Lett.86, 2523 (2001); hep-ex/0012046 Phys.Rev.Lett.86, 2523 (2001); hep-ex/0012046 l Data 10-15% below NLO QCD l No obvious problem: Interesting! Agreement Probability (from   test) with CTEQ4M, CTEQ4HJ, MRST, MRSTGU: 25-80%

24. 6 June 2001Iain A Bertram24 Comparison with CDF lConsistent at high x T, possible discrepancy at low values

25. 6 June 2001Iain A Bertram25 Suggested explanations lDifferent renormalization scales at the two energies èOK, so it’s allowed, but... l Mangano proposes an O(3 GeV) non-perturbative shift in jet energy èlosses out of cone èunderlying event èintrinsic k T ècould be under or over correcting the data (or even different between the experiments — DØ?)

26. 6 June 2001Iain A Bertram26 Dijet Mass Spectrum Experiments in excellent agreement. Different rapidity ranges and very different analysis techniques. Reasonable agreement with predictions. N events L  M    

27. 6 June 2001Iain A Bertram27 Dijet Mass Cross Section Ratio   2.4 TeV (95% confidence level) Systematic Uncertainty ~ 8% Theory uncertainty ~ 6% (m), 3% (PDF) NLO QCD in good agreement with data    < 0.5 ) /  0.5 <    1 ) (  s= 1800 GeV ) PRL 82, 2457-2462, 1999

28. 6 June 2001Iain A Bertram28 BFKL lIn hadron-hadron collisions: Q2Q2 1/x DGLAP (Dokshitser-Gribov-Lipatov-Altarelli-Parisi) BFKL (Balitsky-Fadin-Kuraev-Lipatov) evolution PDF BFKL many gluons all ~ same p T DGLAP (as in PYTHIA) gluon radiation strongly ordered One way to realise this situation is jets widely separated in rapidity: the total energy is then much greater than the jet p T scale, and one can have many gluons of comparable p T 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

29. 6 June 2001Iain A Bertram29 lPhys.Rev.Lett. 84, 5722 (2000) lAnother attempt to find an observable which displays BFKL behaviour: lDØ measurement of 630/1800 GeV cross section ratio at large rapidity separations èuse bins such that x and Q 2 are the same in the two datasets (but different  ) lWhat’s going on here? èdata behave qualitatively like BFKL (but also like HERWIG) ègiven that we can’t predict the 630/1800 GeV ratio of inclusive cross sections, how much can we really infer? BFKL

30. 6 June 2001Iain A Bertram30 Subjet Multiplicity in Quark & Gluon Jets Jet E T 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 Contributions of different initial states to the cross section for fixed Jet E T vary with  1800GeV 100 200 300 gg qg qq Method: Select quark enriched & gluon enriched jet sample Compare jets at same ( E T, h ) produced at different and assume relative q/g content is known  630GeV 100 200 300

31. 6 June 2001Iain A Bertram31 Jet Structure at the Tevatron lSubjets inside jets: perturbative part of fragmentation lDØ compares 630 to 1800 GeV data at same E T and , and infers q and g jet differences Quark Jets Gluon Jets Subjet Multiplicity k T algorithm D=0.5, y cut = 10 -3 55 < E T (jet) < 100 GeV |  jet | < 0.5 DØ Data HERWIG 5.9 DØ Preliminary 12345 0.1 0.2 0.3 0.4 0.5

32. 6 June 2001Iain A Bertram32 Direct Photon Production DØ PRL 84 2786 (2000)CDF Preliminary QCD prediction is NLO Owens et al. Note: E T range probed with photons is lower than with jets

33. 6 June 2001Iain A Bertram33 Direct Photon Production DØ PRL 84 2786 (2000)CDF Preliminary Low ET excess: Theory QCD NLO Owens et al.

34. 6 June 2001Iain A Bertram34 k T Smearing??? lGaussian smearing of the transverse momenta by a few GeV can model the rise of cross section at low E T (hep-ph/9808467) Motivated by observed p T (  ) 

35. 6 June 2001Iain A Bertram35 Alternative Viewpoint Also note that Vogelsang et al. get closer to the observed shape than the Owens prediction with no extra k T (using NLO fragmentation and setting  R  F )

36. 6 June 2001Iain A Bertram36 Photons at  s = 630 GeV lCDF data, compared with UA2 data show deficit at high E T compared with the theory. èRelation to jets?

37. 6 June 2001Iain A Bertram37 DØ Photons at 630 GeV First measurement at forward rapidities  2 Comparison (7 bins) CC: 11.4 -- 12% EC: 4.6 -- 71%

38. 6 June 2001Iain A Bertram38 Ratio of Photons at 630 and 1800 l Slight excess at low XT èInsignificant statistically èRun II won’t help! l Good overall agreement with NLO l No significant high ET deficit. èNo statistics at high ET (> 40 GeV)  2 Comparison (7 bins) CC: 6.5 -- 49% EC: 3.0 -- 89%

39. 6 June 2001Iain A Bertram39 Photons: Comments lSituation is complicated and not easily explained. èLow E T k T effects? èHigh E T deficits at 630 GeV èLack of statistics in both regions lChallenge for both theory and experiment in Run II.

40. 6 June 2001Iain A Bertram40 Run II : Coming March 2001 ss2.0 TeVW/Z’s>1M/>50k Luminosity2  10 32 ttbar>1k events  Ldt2-30fb -1 HiggsPossible 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

41. 6 June 2001Iain A Bertram41 Case Study: Dijet Mass Spectrum Dramatic increase in high p T cross sections Large gains in statistics

42. 6 June 2001Iain A Bertram42 Run II: ~100 events ET > 490 GeV ~1K events ET > 400 GeV Run I: 16 Events E T > 410 GeV Great reach at high x and Q 2, the place to look for new physics! Looking ahead: Run II

43. 6 June 2001Iain A Bertram43 Jet Algorithms lFermilab Run II workshop lVarious species of k T lNew Cone Algorithm lTheoretical desires è“infrared safety is not a joke!” è avoid ad hoc parameters like R sep lCone algorithm improved by èe.g. by modification of seed choices - midpoints èseedless algorithm? In development. lExperimental desires èsensitivity to noise, pileup, negative energies

44. 6 June 2001Iain A Bertram44 Requirements for Progress lQuantitative Theoretical Uncertainties èpdf and renormalization scale uncertainties lFull understanding of correlations of experimental uncertainties èStatistical Uncertainties in most cases will be negligible lUnderlying event èRequire a better understanding and treatment of the surrounding event

45. 6 June 2001Iain A Bertram45 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

46. 6 June 2001Iain A Bertram46 First  pp collisions of Run 2 at DØ: April 3, 2001 lLuminosity counters ètiming Vertex distribution along z of min bias events: [cm] Luminosity ( coincidence) Proton halo ~5  10 27 cm -2 sec -1 Antiproton halo

47. 6 June 2001Iain A Bertram47 Global track finding 36  36 Store Run 119679, Event 232931 Level 3 (software trigger) Global Tracking Track with 5 fiber tracker hits, 5 3D silicon hits Relative alignment of silicon and fiber trackers verified to 40  m level

48. 6 June 2001Iain A Bertram48 Event with 6 tracks pointing to same vertex 36  36 Store Run 119679, Event 231653 Level 3 (software trigger) Silicon-only Tracking

49. 6 June 2001Iain A Bertram49 Vertex finding lPrimary vertices reconstructed from tracks in silicon: x y z y z x A. Schwartzman Buenos Aires

50. 6 June 2001Iain A Bertram50 Tracking in Silicon l3D event display showing hits and reconstructed track:  Offline track finding E. Barberis Y. Kulik

51. 6 June 2001Iain A Bertram51 Silicon Detector North South H1 H4 F1 F11 F12 B1 B4B6

52. 6 June 2001Iain A Bertram52 Tracks in the Silicon + Fiber Tracker lOffline track finding from  pp events: 22 1/p T Since B=0 for this run, real tracks should be found with 1/p T = 0 8 or more points on a track (5 fiber hits, 3 or more Silicon) D. Adams Fermilab Data taken in 36 x 36 store (end April) Plot posted 5/18/01

53. 6 June 2001Iain A Bertram53 Hits in Central Preshower Detector Preshower hits Fiber tracker hits D. Coppage, Kansas

54. 6 June 2001Iain A Bertram54 L1 muon triggered event xy view, looking North zy view, looking East xz view, looking down

55. 6 June 2001Iain A Bertram55 lCalorimeter and forward muon hits in a minimum bias event First  pp collisions of Run 2 at DØ

56. 6 June 2001Iain A Bertram56 The work of many people...

57. 6 June 2001Iain A Bertram57 Closing Remarks lRun II has started: 1 March 2001 BIG Opportunity for QCD lIn most cases QCD predictions work exceptionally well. Exceptions: Low p T processes are problematic low p T jets and photons Underlying Event and Event characteristics