1. JETS AND THE HADRONIC FINAL STATE AT ZEUS Thomas Schörner-Sadenius Hamburg University Heidelberg 23 January 2006

2. Heidelberg, 23 January 2006TSS: Jets at ZEUS2 OVERVIEW ¶ INTRODUCTION ¶ BASIC JET PHYSICS AT ZEUS ¶ MORE DETAILED STUDIES ¶ CONCLUSIONS AND OUTLOOK I will not cover specific resonances, strange particles, pentaquarks, baryon production properties etc.

3. Heidelberg, 23 January 2006TSS: Jets at ZEUS3 OVERVIEW ¶ INTRODUCTION DESY, HERA and the ZEUS Experiment Current Data Taking (Jet) Physics: Introduction and Motivation ¶ BASIC JET PHYSICS AT ZEUS ¶ MORE DETAILED STUDIES ¶ CONCLUSIONS AND OUTLOOK

4. Heidelberg, 23 January 2006TSS: Jets at ZEUS4 DESY, HERA UND ZEUS The only ep collider world-wide!

5. Heidelberg, 23 January 2006TSS: Jets at ZEUS5 THE ZEUS EXPERIMENT … still going strong, even with new components! But a lot of care-taking necessary to keep it going stably. Microvertex tracker MVD installed 2001. S tarting to profit from it. 6m tagger starts to contribute to: - lumi measurement - F L determination Freshly equipped FDET (STT): Inclusive,heavy flavour, HFS measurements?

6. Heidelberg, 23 January 2006TSS: Jets at ZEUS6 EVENTS IN ZEUS … our event display

7. Heidelberg, 23 January 2006TSS: Jets at ZEUS7 URANIUM-SCINTILLATOR CALORIMETER Compensating, good hadronic energy resolution An FCAL module with 23 towers with 12 PMTs each. 23 (32) modules in F/RCAL (BCAL). Signal generation via shower develop- ment in scintillator and uranium. - ~12000 channels (H1: ~45000) - About 300 dead/disabled (now 200). -  E /E~0.18/  E for electrons (H1: 0.12) -  E /E~0.35/  E for hadrons (H1: 0.50)

8. Heidelberg, 23 January 2006TSS: Jets at ZEUS8 HERA PERFORMANCE HERA II programme is a success! - Roughly increase of 5 in integrated luminosity – upgrade aim reached. - now about 150pb -1 on tape from HERA II. ZEUS overall efficiency is about 75%; ranging from 40-90%.

9. Heidelberg, 23 January 2006TSS: Jets at ZEUS9 HERA PERFORMANCE Approaching 5*10 31 cm -2 s -1 !!

10. Heidelberg, 23 January 2006TSS: Jets at ZEUS10 ZEUS SUFFERS FROM BACKGROUNDS Threat to central tracker, problem for data taking Typical fill situation with proton beam spikes (dark green) and central chamber trips (red). Number of CTD trips versus day of running. HERA not always successful in providing good and stable running conditions.

11. Heidelberg, 23 January 2006TSS: Jets at ZEUS11

12. Heidelberg, 23 January 2006TSS: Jets at ZEUS12 BASICS OF ep SCATTERING AT HERA … in lowest order ,Z,W k Lepton (e ± ) Proton P k’(e ±, ) p=xP Q 2 =-q 2 =-(k-k’) 2 y=1-E’/E: Inelasticity Q 2 : resolution ~1/Q For a given  s only two independent variables: Q 2 = s·x·y x=Q 2 /2Pq: proton momentum fraction E e = 27.5 GeV E p = 920 GeV  s =  4E e E p ~ 318 GeV

13. Heidelberg, 23 January 2006TSS: Jets at ZEUS13 JET PHYSICS: BASICS I A variety of interesting processes Leading-order QCD, O(  s ). NLO QCD, O(  s 2 ) Suppressed with increasing Q 2 ss ss QPM QCDC BGF Real/virtual corrections “Resolved” `boring’ Direct access to coupling and gluon Interesting! Difficult!

14. Heidelberg, 23 January 2006TSS: Jets at ZEUS14 JET PHYSICS: MOTIVATION pQCD, PDF universality, measurements of PDFs,  S Jet cross- sections Access to strong coupling. Access to PDFs and test of their universality. Tests of factorisation in soft and hard parts of the inter- action. Test of pQCD predictions. Is jet physics independent of type of exchanged boson? Many options … in pQCD assuming collinear factorisation:

15. Heidelberg, 23 January 2006TSS: Jets at ZEUS15 JET PHYSICS: BASICS II Algorithms, reference frames, theory, uncertainties, EFOs Jet algos / correction Almost exclusively longitudinally invariant k T in the inclusive mode (in Breit frame for DIS). Applied to energy flow objects or calorimeter objects. Current uncertainty on hadronic energy scale: 1-3% Correction for dead material etc. in individual analysis. Theory DGLAP-based NLO calculations for 1/2/3 jets in DIS (2 jets in photoproduction). Data correction for detector/QED/hadronisation effects done with LO MC programs. Consistency? Looking forward to NLO+PS programs (MC@NLO). Uncertain- ties Mostly now dominated by theoretical uncertainties. - Only NLO theory  estimate effect by varying renormalization scale. - PDF uncertainties not negligible. Uncertainty on coupling? jet energy scale 1-3%  10-15% in Xsection. Statistics is not the critical item. Progress in systematics?

16. Heidelberg, 23 January 2006TSS: Jets at ZEUS16 OVERVIEW ¶ INTRODUCTION ¶ BASIC JET PHYSICS AT ZEUS (HERA I) Tests of perturbative QCD with jets Determinations of the strong coupling with jets Contributions to PDF determinations from jets ¶ MORE DETAILED STUDIES ¶ CONCLUSIONS AND OUTLOOK

17. Heidelberg, 23 January 2006TSS: Jets at ZEUS17 INCLUSIVE JET CROSS-SECTIONS (DIS) `Simple’ measurement – take PDFs/  S as given Tests – factorisation – pQCD, – PDF universality Selection – ~80pb -1 from 98-00 – 125 8 GeV (Breit)

18. Heidelberg, 23 January 2006TSS: Jets at ZEUS18 INCLUSIVE JET CROSS-SECTIONS (DIS) Also double-differentially, comparison to NLO

19. Heidelberg, 23 January 2006TSS: Jets at ZEUS19 DI / TRIJETS IN DIS Testing higher orders in pQCD Selection – ~80pb -1 from 98-00 – E T > 5 GeV (Breit) – 10 25 GeV In ratio of dijets to trijets some uncertainties cancel out (lumi, partly energy scales, etc.) Nice possibility to extract  S !

20. Heidelberg, 23 January 2006TSS: Jets at ZEUS20 DIJETS IN PHOTOPRODUCTION In principle access to photon PDF. Selection: – 39pb -1 from 1996/97. – E T > 14 (11) GeV – separation power between different photon PDFs! Highest sensitivity at low E T  subtraction of underlying event – but great model dependence! Compare to NLO calculations and exploit sensitivity to  structure via x  (gluon!) Aim: Problem: We need a dedicated study of the underlying event at HERA!

21. Heidelberg, 23 January 2006TSS: Jets at ZEUS21 EXTRACTION OF  S Typically done using interpolation Dependence of cross-section on  s (M Z ) in each bin from NLO pQCD with different input  s (M Z ) values. for example with MRST or CTEQ4 (3/5 different  s values, 0.110 to 0.122). Use function to map measured cross-section to value of  s (M Z ). Functional dependence on  s (M Z ) then approximated by -- A i, B i determined in fit. Input: Parame- trisation: Result:

22. Heidelberg, 23 January 2006TSS: Jets at ZEUS22  S FROM INCLUSIVE, 2, 3 JETS Theory starting to dominate measurement World average:  s (M Z ) = 0.1187  0.0020 H1 inclusive jets:  s (M Z ) = 0.1197  0.0016(exp)  0.0047(theo) ZEUS inclusive jets:  s (M Z ) = 0.1196  0.0025(exp)  0.0023(theo) ZEUS R 3/2 :  s (M Z ) = 0.1179  0.0013(stat)  0.0037(exp)  0.0055(theo)

23. Heidelberg, 23 January 2006TSS: Jets at ZEUS23 SUMMARY ON HERA  S (C. Clasman) HERA :  S (M Z )=0.1186±0.0011(exp.)±0.0050(th.) (only NLO) Bethke:  S (M Z )=0.1182±0.0027(all NNLO) Nice demon- strating of running in HERA data. Consistency of all results is an important check!

24. Heidelberg, 23 January 2006TSS: Jets at ZEUS24 ACCESS TO PDFs WITH JETS Hope: Improve gx(x,Q 2 ) at high values of x ss ss Uncertainties are very large: – 15% at x=0.3, – 200% at x=0.5. Problem Gluon density xg(x) at high x basically unknown – constrained only by momentum sum rules and Tevatron jets with large E T. Idea Jet data provide access to high x gluon via BGF process. Problems … exploiting this feature: … Many … QCDCBGF

25. Heidelberg, 23 January 2006TSS: Jets at ZEUS25 ACCESS TO PDFs WITH JETS Reminder: Thesis M. Wobisch (2000), xg(x) from jets

26. Heidelberg, 23 January 2006TSS: Jets at ZEUS26 – from integration of PDF and hard scattering matrix element – to multiplication of constant PDF and tabulated and summation over all bins of x and  f.  0.01s for NLO !!!!! Divide phase-space in small x-  f bins. Remove `constant’ PDF bit from integration in each bin, integrate the in each bin for once and for good and store the integrated values in ASCII table. METHOD How to get jets NLO in <1s? Assumption PDFs are approx. flat in small bins of x and  f. Problem Evaluation of NLO jet cross-sections: 8 hours for 50M events. PDF fit requires O(1000) evaluations  PROBLEM! Simplification Flat! Table!

27. Heidelberg, 23 January 2006TSS: Jets at ZEUS27 THE METHOD WORKS for DIS, photoproduction, heavy flavours, Tevatron … Comparison of normal NLO calculation with NLO cross- sections calculated with the formulae on the slide before. ZEUS inclusive jets. D0 inclusive jets.

28. Heidelberg, 23 January 2006TSS: Jets at ZEUS28 INPUT DATA in addition to F 2 Photo- production dijets High-Q 2 inclusive jets

29. Heidelberg, 23 January 2006TSS: Jets at ZEUS29 RESULTS Reduction in gluon uncertainty, effect on coupling F 2 + jets F 2 alone Simultaneous fit to both  S and PDFs. Fractional gluon uncertainty.

30. Heidelberg, 23 January 2006TSS: Jets at ZEUS30 EFFECT ON STRONG COUPLING Very much constrained by jets data Strong correlation between  s and the gluon density for in- clusive F 2 data  large increase in gluon uncertainty when  s free! Jet cross sections directly sensitive to  s via  *g  qqbar (coupled to gluon) and via  *q  qg (NOT coupled to gluon)   s NOT as strongly correlated to gluon F 2 + jets F 2 + alone

31. Heidelberg, 23 January 2006TSS: Jets at ZEUS31 OVERVIEW ¶ INTRODUCTION ¶ BASIC JET PHYSICS AT ZEUS ¶ MORE DETAILED STUDIES Parton evolution in the proton Transition from photoproduction to DIS More selected topics from jet physics Event-shapes and charged multiplicities ¶ CONCLUSIONS AND OUTLOOK

32. Heidelberg, 23 January 2006TSS: Jets at ZEUS32 PARTON EVOLUTION SCHEMES The `forward jet’ question DGLAP approximation resumming terms lnQ 2 for parton evolution works very well for most of HERA regime (F 2 !) DGLAP: k T ordering! BFKL: x ordering! k T large x large forward region:  > 2 (close to proton)  jet E T 2 ~ Q 2 (suppressed in DGLAP)  large x jet =E jet /E proton (realized in BFKL) Breakdown expected at very low x! Can we distinguish the onset of BFKL-like evolution in ln1/x?  “forward jets”. Starting point Question Forward jets Design phase-space to suppress DGLAP and enhance BFKL: All results so far: Problems at low x! But not firm conclusions drawn: -- NNLO terms missing? -- Resolved contribution? -- BFKL evolution scheme? -- pure kinematics?

33. Heidelberg, 23 January 2006TSS: Jets at ZEUS33 PARTON EVOLUTION SCHEMES The `forward jet’ question 17 - Tighter cuts than H1  same statistics from 72pb -1.  R =Q. - 20 < Q 2 < 100 GeV 2.  only 1997 data, 13.7pb -1, 5 5 GeV  All models below the data at low x; CDM and MC `dir+res’ best  non-k t -ordered contribution mimicking BFKL, or higher orders?  First distinction between dir+res MC and CDM if require additional dijet system: only CDM works. H1 New ZEUS No new experimental conclusions. Theory (Sassot et al.): Kinematics!

34. Heidelberg, 23 January 2006TSS: Jets at ZEUS34 FROM PHOTOPRODUCTION TO DIS The transition region and its problems x  =1 “direct” x  <1 “resolved” Resolved relevant even if Q 2 >100 GeV 2. Scales?  p NLO, MC+PS+res okay! Quantify amount of resolved as function of Q 2 and E T with R=  res /  dir. For E T 2 >Q 2 parton resolves structure of photon Resolved Aim Results No NLO theory for resolved in DIS! JetViP (Pötter) has problems.

35. Heidelberg, 23 January 2006TSS: Jets at ZEUS35 COLOR DYNAMICS IN JET EVENTS Testing the underlying gauge group (SU(3) C ) like LEP  23 : angle between two lowest- energy jets in 3-jet events! Sensitivity to different contributions  hope to test gauge structure! Investigate color dynamics and underlying gauge group using QCD color factors C F, C A, and T F. At LO 3jet Xsection sensitive to various color factor combinations: CF2CF2 CFCACFCA CFTFCFTF TFCATFCA Aim

36. Heidelberg, 23 January 2006TSS: Jets at ZEUS36 COLOR DYNAMICS IN JET EVENTS in DIS and photoproduction SU(3) favoured, but U(1) 3 not excluded! DIS: 1015 events  p: 2233 events  DIS: 82 pb -1, Q 2 > 125 GeV 2, E T > 8 / 5 GeV, 3 jets.  Photoproduction: all HERA I (127pb -1 ), E T > 14 GeV, 3 jets.

37. Heidelberg, 23 January 2006TSS: Jets at ZEUS37 SUBJET DISTRIBUTIONS in high Q 2 DIS – study QCD radiation pattern / jet structure - Study QCD radiation pattern using LL MC models and fixed order QCD calculations. - Use high-ET jets to minimise fragmentation etc. effetcs  test structure of radiaton implemented in NLO pQCD. Test variables E T sub /E T jet,  sub -  jet,|  sub -  jet | and orientation of subjets in  -  space with respect to proton beam. Increasing y cut Aim Define subjets by applying k  algo to objects of one jet as function of distance measure d cut =y cut ·E T 2. Chosen here: Jets with two subjets at y cut =0.05 (hadronisation)! Method Note for experts: Running NLO O(  S 2 ) in lab frame  jets with  3partons from fixed-order calculation!

38. Heidelberg, 23 January 2006TSS: Jets at ZEUS38 SUBJET DISTRIBUTIONS in high Q 2 DIS – study QCD radiation pattern / jet structure All distributions nicely described by NLO QCD within 10%  radiation pattern understood / correctly implemented! Selection – ~80pb -1 from 98-00 – Q 2 > 125 GeV 2 – E T > 14 GeV (Lab) - ratio of subjet E T to jet E T. - 2 entries per jet. - trend towards similar energies

39. Heidelberg, 23 January 2006TSS: Jets at ZEUS39 INTERJET ENERGY FLOW in photoproduction dijet events with large rapidity gaps Normally lots of activity between two jets  color connection. Events with little activity between jets: study color singlet exchange in pQCD regime (high E T !) pomeron gap Idea Compare to MC models with(out) color singlet (CS) contribution: -- PYTHIA: high-t-  exchange (MPI); -- Herwig: BFKL-Pomeron (JIMMY) added to dir+res. Globally 3-4% CS exchange needed to account for small E t gap cross- section. Amount depends on rapidity separation of jets (up to 50%). Result

40. Heidelberg, 23 January 2006TSS: Jets at ZEUS40 INTERJET ENERGY FLOW in photoproduction dijet events with large rapidity gaps Fraction of events with rapidity gap energy below cut value as function of gap width. For low cut values data and CS contribution level out at higher values of gap width! – non-CS contributions fall off exponentially

41. Heidelberg, 23 January 2006TSS: Jets at ZEUS41 EVENT SHAPES: MEANS as a means to test power corrections (in high Q 2 >80 GeV 2 ) Idea Alternative way to describe non- pert. hadronisation corrections to event shapes. PC depend on one universal parameter  0. Example: Thrust (long. collimation) PC approach works for means … … but fit to  S and  0 difficult.

42. Heidelberg, 23 January 2006TSS: Jets at ZEUS42 EVENT SHAPES: DIFFERENTIALLY Theory including resummation (NLL) NLO+PC+NLL On differential distributions effect is only lateral shift: Differential distributions Extractions of  0 and  S. New ZEUS analysis: No consistent extraction of the parameters from the different event shape variables, in contrast to H1 and the LEP experiments. Do PC not work at all? Just underestimation/neglection of theoretical uncertainties like matching scheme, scale variations etc.?

43. Heidelberg, 23 January 2006TSS: Jets at ZEUS43 CHARGED MULTIPLICITIES At LEP, Tevatron and HERA Expectation: – The current region in the Breit reference frame looks like a e + e - hemisphere. – LEP and Tevatron find the same dependence of multiplicity on available energy. – What does HERA observe? Naively: Q~  s ee. Analysis: – 39pb -1 from 96/97. – Q 2 > 25 GeV 2, – tracks with p T > 150 MeV – use 2E curr instead of Q. – Multiply mean charged multiplicity by 2 for HERA comparison with LEP. Result: – HERA exhibits same energy dependence as LEP and Tevatron. Mean charged multiplicity.

44. Heidelberg, 23 January 2006TSS: Jets at ZEUS44 CONCLUSION AND OUTLOOK HERA is a great machine for QCD! HERA ZEUS – Aim of lumi upgrade reached: 5-times higher lumi than HERA I – Hoping to get 700pb -1 on tape before shutdown in 2007. – Experiments and machine getting a bit old … need lot of caretaking. QCD@ HERA – HERA is an ideal laboratory which allows to adjust the parameters with which one wants to investigate QCD. – HERA contributing significantly to - tests of pQCD (factorisation, PDF universality). - determinations of QCD parameters (PDFs,  S ). - tests of theories / models (PC, BFKL, …) Future – More statistics will allow for higher precision and thus better insight into QCD in few places.. – Theoretical progress might improve our understanding of some problematic aspects of QCD (NLO@MC). – Hopefully more insight into difficult subjects like parton evolution, DIS-photoproduction transition, …. And input for LHC!

45. Heidelberg, 23 January 2006TSS: Jets at ZEUS45 ADVERTISEMENT : NLOLIB A common framework work for fixed-order calculations Purpose Unify access to various fixed-order calculations for ep, pp, ee. Thereby simplify data comparison to various predictions. Authors Formerly T. Hadig and K. Rabbertz (formerly H1). Currently K. Rabbertz and TSS. T. Toll starting to contribute with FMNR. Programs DISENT and JetViP for ep DIS jet production. Problems with DISASTER++ and MEPJET. RacoonWW for 4f processes in e+e-. NLOJET for jet production in pp collisions almost there. FMNR for photoproduction heavy flavour jets in ep: Work has just begun. Getting it: www.desy.de/~nlolib or one of the authors. HERA-LHC proceedings contribution.

46. Heidelberg, 23 January 2006TSS: Jets at ZEUS46 NLOLIB – Results Implementation of DISENT and JetViP

47. Heidelberg, 23 January 2006TSS: Jets at ZEUS47 BACKUP

48. Heidelberg, 23 January 2006TSS: Jets at ZEUS48 URANIUM-SCINTILLATOR CALORIMETER provides good control on beam, timing … Momentum distribution. F/B/RCAL timing. Imbalance between left and right PMT for each EMC FCAL cell.

49. Heidelberg, 23 January 2006TSS: Jets at ZEUS49 ENERGY FLOW OBJECTS IN ZEUS Combining tracks and calorimeter information Idea Make best use of both detectors calorimeter at high momenta, tracker at low momenta. Result Resolution in p T,h and (E-p z ) h. improves by 20%. Method Combine cells to ‘cone islands’. Match tracks to islands. Decide which information to use.

50. Heidelberg, 23 January 2006TSS: Jets at ZEUS50 JET ENERGY CORRECTIONS to account for mis/non-measurements Idea Detector effects lead to mis- measurements of jet energies: Use MC simulation to estimate the effect and derive corrections. Method Fit straight lines to detector-versus-hadron distributions (differentially in  !, possibly several bins in E T ). derive slope b and offset m and correct E T : offset ~1,slope ~0.8. offset ~0.1,slope ~0.98.

51. Heidelberg, 23 January 2006TSS: Jets at ZEUS51 EVENT CORRECTIONS to account for detector/QED/Z 0 effects Data detektor akzeptance, efficiency Comparison NLO QED running, radiation Z 0 exchange hadroni- sation

52. Heidelberg, 23 January 2006TSS: Jets at ZEUS52 INCLUSIVE JET CROSS-SECTIONS (DIS) 1998-2000 versus 1996/97 – As expected increase from 96/97 to 98-00 at high E T and Q 2. – Change described by NLO  everything consistent.

53. Heidelberg, 23 January 2006TSS: Jets at ZEUS53 TRIJETS IN DIS Testing higher orders in pQCD Selection – ~80pb -1 from 98-00 E T > 5 GeV (Breit) – 10 25 GeV

54. Heidelberg, 23 January 2006TSS: Jets at ZEUS54 TRI- AND DIJETS IN DIS in the same phase-space. Idea: In ratio of dijets to trijets some uncertainties cancel out (lumi, partly energy scales, etc.) Theory: Nice possibility to extract the strong coupling with rather small uncertainties!

55. Heidelberg, 23 January 2006TSS: Jets at ZEUS55  S FROM INCLUSIVE JETS Theory starting to dominate measurement World average:  s (M Z ) = 0.1187  0.0020 H1 inclusive jets:  s (M Z ) = 0.1197  0.0016(exp)  0.0047(theo) ZEUS inclusive jets:  s (M Z ) = 0.1196  0.0025(exp)  0.0023(theo)

56. Heidelberg, 23 January 2006TSS: Jets at ZEUS56  S FROM THE RATIO R 3/2. Cancellation of some uncertainties ZEUS R 3/2 :  s (M Z ) = 0.1179  0.0013(stat)  0.0037(exp)  0.0055(theo)

57. Heidelberg, 23 January 2006TSS: Jets at ZEUS57 SUMMARY ON  S FROM ZEUS (C. Clasman)

58. Heidelberg, 23 January 2006TSS: Jets at ZEUS58 RESULTS Massive reduction in gluon density uncertainty

59. Heidelberg, 23 January 2006TSS: Jets at ZEUS59 EFFECT ON STRONG COUPLING Very much constrained by jets data

60. Heidelberg, 23 January 2006TSS: Jets at ZEUS60 PHOTON PDF FROM DIJETS … just to point out the problems …  Low E T jets (6 GeV) are sensitive to the gluon content in the photon.  Large contributions from underlying events  subtraction procedure.  Size of correction depends on model!  large uncertainties!  Gluon density in the photon as extracted from H1 dijet  p data.  Agreement with other determinations; very large errors.