1. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 1 pQCD  JETS Luca Stanco – INFN Padova LISHEP 2006 Workshop Rio de Janeiro, April 3-7, 2006 on behalf of H1 and ZEUS collaborations at HERA (AND)

2. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 2 more or less… … Single particles production Charged multiplicities Bose-Einstein correlation Neutral, strange, Baryons Charm fragmentation Remnant/Multiple interactions Prompt Photon Resolved-Direct  production Parton Evolutions Color dynamics, Energy flow Multiscale Strong coupling Dijets, Trijets Inclusive Jets production Forward Jets production Heavy Quarks Jets Diffractive Jets … QCD items JETS items AND

3. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 3 QCD: Partons/JETS WHAT ? WHY ? HOW ?

4. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 4 WHY ? at least 3 answers: 1.NOT TRUE it is so well understood.. 2.Strong coupling,  S, measurement 3.Parton Density Functions measurements (see previous talk) From recent HERA-LHC workshop, examples of uncertainties,

5. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 5 Z.Czyczula et al., p T of Higgs (120 GeV) from different Parton-Showers (PS) models and QCD scale choices Uncertainties: up to 170% due to PS (which evolution type ? which ordering ?) up to 100% due to the choice of QCD scale (Q 2, m 2, ŝ etc.)  low-x and forward jets…

6. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 6 HOW ? Outline of the talk: Inclusive Jets in DIS Multi-Jet production in DIS  S measurements Jets in  -production (resolved/direct photon) Gluon PDF Color and sub-jets dynamics Forward Jets

7. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 7 before going into physics, let us look at environment... Results on pQCD  JETS only from HERA-I (up till now)

8. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 8...if nobody else has already shown...

9. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 9 Kinematics of Neutral Current (NC) Deep Inelastic Scattering (DIS)

10. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 10 Processes at HERA Electron beam Proton beam x Bj < 1 x Bj  0 DIFFRACTION x  = 1 Q 2 > 0 Photon* DIS x  < 1 photon RESOLVED  -PRODUCTION x  = 1 Q 2 = 0 photon DIRECT  -PRODUCTION quark/gluon PARTONS (defined by Initial States of HARD Scattering) x Bj << 1 low-x regime

11. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 11 Jets Production in NC Deep Inelastic Scattering

12. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 12 Inclusive JETS in DIS The BREIT frame ! Frame where virtual boson is enterely spacelike At lowest order, quark is back scattered In pratice, high Et jet events must be at least of order  S

13. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 13 JETS observables in the BREIT Frame … more technically

14. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 14 NLO pQCD calculations and interfaced to various PDFs (like CTEQ, MRST) Parton Level cross sections – need to rely on LO MC for hadronisation corrections

15. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 15 Here are the DATA: H1 Inclusive Jet Production in DIS Excellent agreement between DATA and NLO over many orders of magnitude !

16. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 16...and the experimental error is smaller than the theoretical one !

17. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 17 ZEUS Inclusive Jet Production in DIS exp. and theor. errors same scale

18. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 18 measurements at different Q 2 also for ZEUS…

19. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 19 Running and Extraction of  S from Inclusive jets  S vs E t  S vs Q 2 Best result for Q 2 bins with Q 2 >500 GeV 2  uncertainties minimised

20. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 20 and H1 before showing the whole sets of  S measurements, look at the MULTI-JET data sets 

21. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 21 Di-Jet and Tri-Jet PRODUCTION in DIS The  S dependency of their ratio is a sensitive test of pQCD larger errors, but...

22. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 22 3-jet / 2-jet cross section ratio Good agreement for Largest experimental uncertainty: calorimeter energy scale Largest theoretical uncertainty: renormalization and factorization scale H1

23. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 23 room for improvements (HERA-II)  S measurements from 3-jet / 2-jet cross section ratio H1

24. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 24

25. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 25  S running…

26. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 26 JETS in  -production pQCD calculations of jet cross sections … again, another testing ground for QCD

27. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 27 Well observed from the “first days” x  =0.8 cut Photon structure H1 ZEUS

28. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 28 Proton structure More QCD fits from H1 will come ! H1

29. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 29 ZEUS high E t di-jets in  -p Similar situation for E T distributions Consider the enriched sample of direct-  Good agreement with NLO at high ET small experimental uncertainties: → jet energy scale (  1%)  ± 5% on the cross section small theoretical uncertainties * Pioneering paper of H1 in 2001  Perform a simultaneous QCD fit * on  S and Gluon PDF at high x p

30. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 30 Recent results from ZEUS on “old” data are very good, improving the determination of the proton PDF Disadvantages of “ortodox” method (inclusive ep scattering): gluon distribution contributes indirectly Fixed-target DIS: higher twists, heavy target corrections, isospin symmetry assumption With HERA, sufficient sensitivity to determine the proton PDF within a single (ep) experiment. Use of measurements of jet cross section  Directly sensitive to the gluon density with small experimental + theoretical uncertainties Gluon Parton Distribution

31. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 31

32. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 32

33. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 33 Color and sub-jet dynamics Photo-production of three-jet events Direct-  processes provide a clean way to study the effects of the different color configuration:

34. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 34 Angular observables for angular correlations… Comparisons to fixed order calculations based on different symmetry group: The prediction of SU(3) describes reasonably well the data

35. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 35 ZEUS Sub-jet distributions DIS

36. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 36

37. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 37

38. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 38 H1  -p data The jets in the region x  0.75 Consistent with a larger fraction of gluon (broad) jets in resolved- photon events than in direct- photon events

39. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 39 QCD dynamics HERA: DIS at low-x Bj down to 10 -5  large  *p center of mass energy (W) Inclusive F2 measurement not able to discriminate between different QCD approaches  Study of Hadronic Final States (JETS)

40. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 40  FORWARD JETS (proton direction) possibility to study parton dynamics, by looking for effects beyond DGLAP HERA extends x Bj until 10 -5 Caveat: different Monte Carlo models not clear where applicable

41. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 41 LO-DGLAP fails completely ! NLO-DGLAP well below data at low-x Monte Carlo models with extra parton radiation provide an improved description of the data inclusion of a resolved-photon component (RG-DI+RES) parton emissions not ordered in k T (CDM) H1 data in the kinematic region:

42. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 42 ZEUS data less striking… (NLO(  S 2 )-DISENT MC below the data but within theoretical errors)  needs of next NLO ? ( but ZEUS uses  R 2 =Q 2, while H1 takes  R 2 = ) Is it a HERA business ? or HERA phase space too limited to distinguish between different evolution models ?

43. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 43...if nobody else is going to show... Definitively, physics has to be clarified. Also consider Structure Functions results at low-x

44. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 44 Conclusions QCD is a lively field at HERA ! Up till now studies on 1996-2000 DATA Statistical ≤ Theoretical errors HERA-II will lower systematics errors Need THEORY works ! Most precise  S measurement ! Use of Jets for PDF Forward Jet discrepancy pQCD evolution(s) not clear yet low-x Bj regime not clear yet (the improvements of the precision on the gluon density at mid-to-high x has important implications for discovery physics) Need HERA-II for LHC studies

45. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 45 Backup Slides

46. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 46 PENTAQUARKS at HERA  + (1530): still compatibility ZEUS/H1, need HERA-II DATA

47. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 47  C (3100): still compatibility H1/ZEUS, need HERA-II DATA Not seen by ZEUS

48. Luca Stanco - PadovaQCD at HERA, LISHEP 2006 48