Some aspects of QCD AT HERA and what we can learn from it

1. Some aspects ofQCD AT HERAand what we can learn from it Thomas Schörner-SadeniusHamburg University Bonn, 11 May 2006

2. OVERVIEW ¶ HERA AND ZEUS Prepared for the unexpected? Data taking, detector development, unexpected problems, trigger … ¶ QCD AT HERA Consequences for LHC? F2, parton distributions, jets, coupling S ¶ PROGRESS AND BIG QUESTIONS What can we learn for the future? Theory, underlying event, parton dynamics etc… ¶ OUTLOOK – An old experiment is still exciting! . – LHC can and should profit from the HERA experience! TSS: QCD at HERA

3. HERA, H1 AND ZEUSep collisions at s = 318 GeV TSS: QCD at HERA

4. Ee = 27.5 GeV k’(e±,) k Lepton (e±) ,Z,W Proton Ep = 920 GeV P ep SCATTERING AT HERA … in lowest order QCD y=1-E’/E:Inelasticity Q2=-q2=-(k-k’)2 Q2:VirtualityResolution~1/Q p=xP x=Q2/2Pq:Proton momentum fraction s = 4EeEp ~ 318 GeV Inclusive measurement: electron E, structure fctns., parton distributions For a given s only two variables are independent: Q2 = s·x·y TSS: QCD at HERA

5. THE ZEUS EXPERIMENT … is running and running … and keeps developping … Microvertex Detectorstarting to profit from it(heavy quarks). 6m Tagger- starts to contribute - Luminosity determination- FL determination Upgraded forwardTracker STT “Working horse” Uranium Calorimeter Experiment needs more and more care-taking ... TSS: QCD at HERA

6. URAN SCINTILLATOR CALORIMETERcompensating, good hadronic energy resolution Signal generation via shower development inuranium and scintillator FCAL modulewith 23 towern with12 PMTs each. • 12000 channels (H1: 45000)- about 240 dead (2004: 360!)- E/E~0.18/E for Electrons- E/E~0.35/E for hadrons • But old cables, connectors, loose connections, detector as dustbin … 23 (32) modulesin F/RCAL (BCAL). TSS: QCD at HERA

7. STT – STRAW TUBE TRACKER… delivers data after long period of cooling problems Temperature model of inner detector without/with additional cooling  “without” stress on solenoid too large (T=230)! End 2005: 72m copper cooling pipes built in (T=60). Temperature behaviour now stable in time Potential for QCD studies, heavy quarks, trigger … TSS: QCD at HERA

8. 6m TAGGER Use for longitudinal structure function FL. – Determination of acceptance of luminosity system – Determination of photoproduction background in deep inelastic scattering  longitudinal structure function FL at high y, tot, jets at high y Aims: E’ [GeV] Acceptance (e–) log10(Q2) – No/hardly any reconstruction and simulation code– No efficiency and acceptance determination Problems: – Acceptance determined “manually” – roughly reproduction of old simulation results Ansatz: TSS: QCD at HERA

9. HERA PERFORMANCEHERA-II programme is a success! Average ZEUS Efficiency about 75% (40-90%). • Roughly 5-fold luminosity wrt. to HERA-I – upgrade aim reached.- integrated luminosity: about 240 pb-1 HERA-II data on tape. - Currently up to 5 pb-1 per week. TSS: QCD at HERA

10. HERA PERFORMANCEReaching 5*1031cm-2s-1!! TSS: QCD at HERA

11. ZEUS SUFFERS FROM BACKGROUNDSThreat for central tracker, problem for DAQ Typical fill situation withproton spikes (green), tripsof central tracker (red). Number of tracker tripsas function of day. HERA cannot always provide good and stable beams. TSS: QCD at HERA

12. FLEXIBLE TRIGGER IS IMPORTANT! What can be learned e.g. for ATLAS? ATLAS Level-1 trigger menu, rates (kHz) ¶ HERA experience 1: Level 1 track and vertex triggers are very important (upstream collisions, beam-wall/gas).¶ No track triggers foreseen at LHC! (but background mainly beam-beam?) ¶ HERA experience 2: Have to be able to react to varying beam conditions (auto-prescale, various trigger setups …). ¶ Dangerous (?) inflexibility at LHC e.g. due to hard-coded muon trajectories (and thus pT)? ¶ Region-of-interest concept and step-wise decision on higher trigger levels clearly advantageous, allowing for more flexibility than at HERA (RoI concept partly implemented in new H1 jet trigger). TSS: QCD at HERA

13. FIRE AT ZEUS !… it never gets boring even with a 15 year old apparatus Since begin of run coordination: once fire close to experiment several evacuation alarms gas alarms, … TSS: QCD at HERA

14. OVERVIEW ¶ HERA AND ZEUS Prepared for the unexpected? Data taking, detector development, unexpected problems, trigger … ¶ QCD AT HERA Consequences for LHC? F2, parton distributions, jets, coupling S, ¶ PROGRESS AND BIG QUESTIONS What can we learn for the future? Theory, underlying event, parton dynamics etc… ¶ OUTLOOK TSS: QCD at HERA

15. QCD IN HERA’S INFANCY13 and more years ago: Structure function F2 e± e± momentum q(Q2 = -q2) q Momentum fraction x Proton (parton distributions fi, charges qi) ¶ First ZEUS publication on structure function F2 in 25nb-1 from 1992. ¶ Extended x range (for given Q2 range) by almost two orders of magnitude! ¶ No extraction of HERA PDFs. Conclusion: F2 rises towards low x! TSS: QCD at HERA

16. THE STRUCTURE FUNCTION F2Precision QCD at HERA ¶ By now about 500 data points from HERA with 2% precision (bulk). ¶ Use of DGLAP theory allows extraction of QCD parameters: ¶ Extractions at HERA so far performed using mostly NLO theory; NNLO theory exists and is in use (large impact)! ¶ Strong coupling from scaling violations dF2/dlnQ2 is one of the most precise determinations of this parameter. QCD coupling structure function gluon density Good description of data is big success! QCD at HERA is precision physics! TSS: QCD at HERA

17. PARTON DISTRIBUTIONSand their uncertainties ¶ Different PDF determinations differ significantly (Differences in data, not in fit!) Q2 = 200 GeV2: uncert. 10%, Q2 = 5 GeV2: uncert. 100%, ¶ Many LHC search channels affected. (ggH)¶ PDF-induced uncertainty on Higgs cross-section 10% from quarks and Gluons each. Sufficient to “verify” essentials of Higgs mechanism (tH~mt)? PDF knowledge (NNLO) important for discoveries and interpretations! TSS: QCD at HERA

18. JET PHYSICS AT HERApQCD, parton distributions, coupling S, … Cross-sections Perturbative QCD, collinear factorization: Series expansion in powers of S; coefficients are con-volutions of PDFs with hard scattering matrix elements. Theory “only” in bext-to-leading order (NLO, partonlevel without parton shower).  often dominated by theo- retical uncertainties! – Investigation of underlying gauge group – Strong coupling.– Parton distributions and their universality – Factorization into soft/hard contributions– Predictions of perturbative QCD(– Independence from type of exchanged boson)(– Parton dynamics in the proton (DGLAP versus BFKL)) Tests Errors often dominated by theory; excellent understanding of jet energy scale (1-3%)! TSS: QCD at HERA

19. JETS IN HERA’S INFANCY13 and more years ago: jets ¶ First measurement of jet production at HERA in DIS (dijets at Q2 > 4 GeV2). ¶ Very limited data sets allow only limited tests of QCD. ¶ Only limited kinematic range accessible (ET < 24 GeV!) ¶ Data compared to leading-order MC model, not fixed-order calculation! ¶ Non-infrared- and collinear safe CONE jet algorithms; analysis in laboratory frame … Conclusion: “Comparison to MC models shows that dijet production due to QCDC and BGFprocesses.” TSS: QCD at HERA

20. JET PHYSICS: QCD TESTS IParticle spins, color factors ¶ QCD: accepted as effective theory of the strong interaction. ¶ But: Do we really see SU(3)C? – non-abelian  3-gluon vertex? – spin-1/2 (1) quarks (gluons)? – color factors? ¶ Tests of spins and color factors (also in e+e-, LEP): gluon prop.(1-|cos*|)-2 quark prop. (1-|cos*|)-1 It seems to be QCD!Strong interaction described by SU(3)C! TSS: QCD at HERA

21. JET PHYSICS: QCD TESTS IICross-sections - examples • QCD in NLO- Analysis in Breit reference frame- kT jet algorithm- multidifferential- extraction of S- … Inclusive jets at lowQ2 values, 5 < Q2 < 100 GeV2 Dijets at high Q2 values, Q2 > 125 GeV2 QCD in NLO desribes a great variety of data.Factorization, perturbative QCD, PDF universality – all fine! TSS: QCD at HERA

22. JETS AT LHCand why they are important ¶ LHC will provide an abundance of (really) hard jets (up to 5 TeV, see cal- culation on the right).¶ Precision of predictions and parameter extractions depends crucially on PDF knowledge! ¶ Current PDF uncertainty up to 50%! ¶ QCD processes are background to (almost) all new physics channels ¶ Improved knowledge of PDFs needed to distinguish SM (multi)jet / multiple interaction events from new physics signatures (SUSY, mini black holes, contact interactions …)! ¶ Also needed: jet energy scale, calibration, higher orders, good jet algorithms … TSS: QCD at HERA

23. JET PHYSICS: QCD TESTS IIIDetermination of strong coupling World(NNLO) S(MZ)=0.1187(20) [PDG]S(MZ)=0.1182(27) [Bethke] HERA(NLO) S(MZ)=0.1186± 0.0011(exp)±0.0050(th) ¶ consistent measurements in many processes.¶ “Running” of coupling described by QCD. ¶ HERA: compatible and competitive results. TSS: QCD at HERA

24. Standard-Modell SUSY IMPORTANCE OF Se.g. for Grand Unification ¶ Only assuming SuperSymmetry (extended particle spectrum) do the three SM couplings unify at about 1016 GeV! 1 = (5/3)·/cos2W 2 = /sinW 2 = S ¶ Behaviour at high scales dictated only by renormalization, particle spectra and starting values at scales accessible today. Tests require best possible knowledge of S! TSS: QCD at HERA

25. OVERVIEW ¶ HERA AND ZEUS Prepared for the unexpected? Data taking, detector development, unexpected problems, trigger … ¶ QCD AT HERA Consequences for LHC? F2, parton distributions, jets, coupling S, ¶ PROGRESS AND BIG QUESTIONS What can we learn for the future? Theory, underlying event, parton dynamics etc… ¶ OUTLOOK TSS: QCD at HERA

26. tWbqq’b3 jets in cone mt [GeV] UE I. Borjanovic et al., hep-ex/0403021 RCluster “UNDERLYING EVENTS” (UE) + multiple interactions (“Multiple Interactions”) UE is sum of– remnant-remnant interaction– soft radiation– multiple collisions Not accessible perturbatively! ¶ UE signature: (Uniform?) energy flow throughout the detector. ¶ UE can be large effect! Importance:– mtop cross checks (right picture).– “all” jet physics – as effect by itself. Tuning of parameters at Tevatron, HERA. TSS: QCD at HERA

27. Teatron tunes (CDF) MODEL TUNINGe.g. to pT sums in “transverse region” Tevatron – Sum of momenta in transverse region – Tuned models describe data approximately. – Extrapolation to LHC clearly fails.– Deeper understanding required. LHC Photoproduction at HERAis effectively hadron-hadron-scattering: HERA p Hadronic charakter of photons is “adjustable”  HERA ideal place for systematic studies! TSS: QCD at HERA

28. Mit UE Ohne UE HERA EXPERIENCEParameter tunings and new ideas (?) Already older analyses (4-jets in photoproduction) showed and quantified need for UE. New 3-jet analysis in photoproduction investigates relevant parameters and initiates discussion with theorists! New collaboration with CMS colleagues. TSS: QCD at HERA

29. EVENT SHAPESwith resummation (NLL) and “power corrections” Power corrections – Alternative description of non-perturbative hadronization. – Depend only on universal parameter 0: Thrustdistribution Determinationof 0, S. Do we really see a clear picture? Do the PCs work in general? Are theoretical errors under-estimated?Can we learn something for hadron-hadron machines? TSS: QCD at HERA

30. SIMULATION FRAMEWORKS Important for efficient data-theory comparisons • Unify access to various MC programs or NLO calculations  simplify comparisons data versus theory Idea Examples • HZTOOL (MC programs for HERA physics)• JetWeb: Web-based access to several MC models. • NLOLIB: NLO calculations for ee, ep, pp. NLOLIB K.Rabbertz and TSS • DISENT, JetViP (Jets in DIS, also DISASTER, MEPJET).• RacoonWW (4f processes in ee)• NLOJET (jet production in pp)• soon: FMNR (photoproduction of heavy quarks in ep) Access to various calculationsdrastically simplified! TSS: QCD at HERA

31. PARTON EVOLUTION and the `forward jet’ question • Since long theory ( NLO,MC programs) fail to describe “forward jets” (low Q2, forward). Possible reasons: - missing higher orders? - missing “resolved” contributions? - “wrong” parton evolution scheme? forwardjet We need higher orders, BFKL calculations, resummed calculations … Relevance for forward/diffractive physics at LHC? Diffractive Higgs production? TSS: QCD at HERA

32. x s s JETS UND PARTON DISTRIBUTIONSAccess to gluon at high x with jets Gluon density xg(x) at high x basically unconstrained (sum rules, Tevatron jets) Use of jet data in ZEUS fitsimproves gluon drastically! today Technicallydemanding! Jet data provide access to gluon at high x (boson-gluon fusion): end 2007? TSS: QCD at HERA

33. NEWS FROM THEORY … at HERA theory slower than experiment … Problem Many QCD measurements theoretically limited (NLO!)- coupling S, jet cross sections, … Progress – Calculations with higher orders here or close: - 2-loop splitting functionen (F2 in NNLO) exist! - NNLO for jets soon? (Problem: Matching of contributions) – Combination of NLO calculations with parton shower algorithms is “in”: - correct description of hard and soft processes in one calculation! - Problem 1: negative weights in parton showers? - Problem 2: avoiding of double-counting? – Resummed calculations e.g. for “event shapes” at HERA Unfortu-nately Much progress in e+e– (“simple”) or pp (“important”).– Much not at all for HERA (or late). – Pity, since progress in ep important also for e.g. LHC (PDFs, parton dynamics, …) TSS: QCD at HERA

34. NEWS FROM THEORY … NNLO, NLO+PS (MC@NLO) R. Thorne, DIS06 NLO+PS– incorporates soft effects (parton shower!) –– improves NNLO substantially– often close to NNLO results! S. Frixione, DIS06 NLO can make big differences (shown here: MRST gluon at Q2 = 20 GeV2)!NLO, NNLO do not agree within errors! (We need this for HERA (hadronisation corrections, theory uncertainties, scales …)) TSS: QCD at HERA

35. SUMMARY AND OUTLOOK HERA is going strong and producing valuable results (– although life is getting a little harder, detector-wise) – QCD: Structure functions, parton distributions, strong coupling – Heavy flavours (MVD!) – Searches (best limits in some parameter space regions,GSMB etc.) – detector is constantly adapted to physics needs and new ideas (FL) – daily life (luckily not entirely) dictated by surprises … Impact of HERA results on future physics (LHC) is large! – Precision of parton distributions and coupling – QCD experience (for example jet algorithms, energy scales, calibration …) – Understanding of QCD as background to new physics phenomena. – … and in scrutinizing potentially important results (high-Q2 events, pentaquarks, isolated leptons, …) The field is moving! – Lots of discussion connecting present (HERA, Tevatron) to future (LHC) (TEV4LHC and HERA-LHC workshops, DIS06 conference, …) – many theoretical initiatives: higher orders, resummation, NLO+PS, etc. – Looking very much forward to surprises from both new and old physics at the LHC. HERA Future Excitement! TSS: QCD at HERA

36. BACKUP TSS: QCD at HERA

37. INCLUSIVE JET CROSS-SECTIONS (DIS)Also double-differentially, comparison to NLO TSS: QCD at HERA

38. EXTRACTION OF STypically done using interpolation Input: Dependence of cross-section on s(MZ) in each bin from NLO pQCD with different input s(MZ) values.for example with MRST or CTEQ4 (3/5 different s values, 0.110 to 0.122). Functional dependence on s(MZ) then approximated by-- Ai, Bi determined in fit. Parame-trisation: Result: Use function to map measured cross-section to value of s(MZ). TSS: QCD at HERA

39. SUMMARY ON HERA S(C. Clasman) Consistency ofall results is animportant check! Nice demon-strating of runningin HERA data. HERA : S(MZ)=0.1186±0.0011(exp.)±0.0050(th.) (only NLO)Bethke: S(MZ)=0.1182±0.0027 (all NNLO) TSS: QCD at HERA

40. s s ACCESS TO PDFs WITH JETS Hope: Improve gx(x,Q2) at high values of x Problem Gluon density xg(x) at high xbasically unknown – constrainedonly by momentum sum rules and Tevatron jets with large ET. Uncertainties are very large: – 15% at x=0.3, – 200% at x=0.5. Idea Jet data provide access to high x gluon via BGF process. Problems … exploiting this feature: … Many … QCDC BGF TSS: QCD at HERA

41. • 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? Problem • Evaluation of NLO jet cross-sections: 8 hours for 50M events.• PDF fit requires O(1000) evaluations  PROBLEM! Assumption • PDFs are approx. flat in small bins of x and f. Flat! Table! Simplification – 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 !!!!! TSS: QCD at HERA

42. THE METHOD WORKSfor 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. TSS: QCD at HERA

43. kT large DGLAP: kT ordering! BFKL: x ordering! x large PARTON EVOLUTION SCHEMES The `forward jet’ question Startingpoint DGLAP approximation resumming terms lnQ2 for parton evolution works very well for most of HERA regime (F2!) Question Breakdown expected at very low x! Can we distinguish the onset of BFKL-like evolution in ln1/x?  “forward jets”. Forwardjets Design phase-space to suppress DGLAP and enhance BFKL: forward region:  > 2 (close to proton) • jet ET2 ~ Q2 (suppressed in DGLAP) • large xjet=Ejet/Eproton (realized in BFKL) All results so far: Problems at low x! But not firm conclusions drawn: -- NNLO terms missing? -- Resolved contribution? -- BFKL evolution scheme? -- pure kinematics? TSS: QCD at HERA

44. x=1“direct” x<1“resolved” FROM PHOTOPRODUCTION TO DIS The transition region and its problems Resolved For ET2>Q2 parton resolvesstructure of photon Aim Quantify amount ofresolved as function of Q2 and ET with R=res/dir. Results Resolved relevant even ifQ2>100 GeV2. Scales?p NLO, MC+PS+res okay! No NLO theory for resolved in DIS!JetViP (Pötter) has problems. TSS: QCD at HERA

45. CFCA CF2 CFTF TFCA COLOR DYNAMICS IN JET EVENTS Testing the underlying gauge group (SU(3)C) like LEP Aim Investigate color dynamics and underlying gauge group using QCD color factors CF, CA, and TF. At LO 3jet Xsection sensitive to various color factor combinations: 23: angle between two lowest-energy jets in 3-jet events! Sensitivity to different contributions hope to test gauge structure! TSS: QCD at HERA

46. DIS: 1015 events p: 2233 events COLOR DYNAMICS IN JET EVENTS in DIS and photoproduction • DIS: 82 pb-1, Q2 > 125 GeV2, ET > 8 / 5 GeV, 3 jets. • Photoproduction: all HERA I (127pb-1), ET > 14 GeV, 3 jets. SU(3) favoured, but U(1)3 not excluded! TSS: QCD at HERA

47. Increasing ycut SUBJET DISTRIBUTIONSin high Q2 DIS – study QCD radiation pattern / jet structure Aim - 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. Method Define subjets by applying k algo to objects of one jet as function of distance measure dcut=ycut·ET2. Chosen here: Jets with two subjets at ycut=0.05 (hadronisation)! Note for experts: Running NLO O(S2) in lab frame  jets with  3partons from fixed-order calculation! Test variables ETsub/ETjet, sub-jet,|sub-jet| and orientation of subjets in - space with respect to proton beam. TSS: QCD at HERA

48. SUBJET DISTRIBUTIONSin high Q2 DIS – study QCD radiation pattern / jet structure Selection – ~80pb-1 from 98-00 – Q2 > 125 GeV2– ET > 14 GeV (Lab) - ratio of subjet ET to jet ET.- 2 entries per jet.- trend towards similar energies All distributions nicely described by NLO QCD within 10% radiation pattern understood / correctly implemented! TSS: QCD at HERA

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

50. INTERJET ENERGY FLOWin 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 TSS: QCD at HERA