1. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 1 QCD results from HERA Wesley H. Smith U. Wisconsin - Madison University of Chicago Seminar March 3, 2008 Topics: Parton Density Functions & Structure Functions (F 2, F L ) Jets & Measurements of  S Multijets, Inclusive Jets Prompt Photons Diffraction

2. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 2 HERA ep Collider at DESY Beam energies Proton energies 820 GeV (1992 – 1997) 920 GeV (1998 – 2007) 460, 575 GeV (2007) Facilitated F L Measurement 27.5 GeV e - or e + 300 – 318 GeV CMS Energy 220 bunches Not all filled Crossings every 96 ns Typical beam currents Proton: 100 mA Electron: 40 mA Instantaneous luminosity ~5 x 10 31 cm -2 s -1 H1 HERMES HERA-B H1 ZEUS 2 collider experiments  ZEUS and H1 2 fixed target experiments  HERMES and HERA-b

3. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 3 HERA I: 1992-2000 ~180 pb-1/experiment delivered. (mostly e + ) HERA II: 2002-2007 ~580 pb-1/experiment delivered. (e + and e - ) Upgrade: 2001-2002 For HERA II: - Luminosity ~  3 (low-  insertion) - Long. polarized leptons - Some running at lower proton energy: 460 and 575 GeV HERA I & II Integrated Luminosity HERA II e - HERA II e + HERA I LER/MER

4. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 4 Complete 4π detector Tracking: - central tracking detector - Silicon μ-Vtx (operate in a B field of 1.43 T) Calorimeters: - uranium-scintillator (CAL) σ(E)/E=0.18/√E [emc] σ(E)/E=0.35/√E [had] - instrumented-iron (BAC) Muon chambers 18 countries, ~400 members The ZEUS Detector e+e+ p

5. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 5 Physics at HERA 920 GeV 27.5 GeV

6. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 6 cross section  F 2  quark charge density Gluon from scaling violation QCD evolution DGLAP analysis jets, Heavy Quarks: direct sensitivity to gluon density QCD dynamics HERA, nucleon structure & QCD

7. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 7 H1+ZEUS combined NC cross sections Goal 1: gain in precision Goal 2: understanding the systematic error of each measurement through the difference (cross-calibration!) QCD explains data towards very low Q 2 (down to 2.7 GeV 2 ) No large difference between H1 and ZEUS on F 2

8. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 8 smaller x resolution ( Q2 ) Incr. Lines are pQCD fits to parton distributions. HERA measurements & pQCD

9. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 9 To LO: F 2 Scaling Violatons

10. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 10 HERA Parton Densities QCD evolution Zeus and H1 DGLAP analyses of HERA-I (130 pb -1 ) and fixed target) data:  H1 2000 PDFs  ZEUS-S PDFs HERA PDFs essential for LHC

11. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 11 Gluon Densities from high-p T partons in the hadronic final state High-p T jets & Heavy Quarks: mostly produced by gluons in the initial state Higher-order processes may change the cross sections significantly The theory should describe the data, if we use hadronic final state data for extracting gluons HQ

12. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 12 Jet Finding

13. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 13 H1 Jets in high-Q 2 DIS Inclusive and dijet cross sections well described by NLO for Q 2 > 100 GeV 2 : ready to be used for QCD studies  s QCD fit for parton densities ss

14. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 14 Jets can probe the gluon Jet measurements are consistent with NLO QCD fits from F 2 Can be used to further constrain the gluon and/or  s More on  s later Jets & Gluons from ZEUS

15. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 15 Jets at HERA constrain Gluon Distribution Fit uses HERA incl and jets data HERA-I inclusive cross sects Inclusive jet cross sects in NC DIS Dijet cross sects in photo-production Data from one Experiment only: Syst unc. well understood No fixed target unc. Valence quarks from high-Q 2 NC and CC cross sections Jet data help to constrain the gluon and  s : Reduction of gluon unc. by ~ factor 2 for 0.01 < x < 0.4 Precise determination of  s

16. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 16 High E T Neutral Current Dijets

17. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 17 Extracting  s using HERA data

18. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 18 ZEUS 2007 Extraction of  s (Q 2 ) - 1

19. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 19 ZEUS 2007 Extraction of  s (Q 2 ) - 2

20. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 20 H1 2007 Extraction of  s (Q 2 ) - 1

21. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 21 H1 2007 Extraction of  s (Q 2 ) - 2

22. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 22  s from jets Combined H1+ZEUS  s using high-Q 2 inclusive jets Running from HERA data alone Experimental error small  s (m Z ) = 0.1198  0.0019(exp.)  0.0026 (th.)

23. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 23 S. Bethke hep-ex/0606035 HERA measurement (green triangles)  s measurements at HERA as precise as those from LEP

24. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 24  s from scaling violation  s from the QCD fit F 2 + jet: consistent with the world average F 2 alone prefers to have lower  s – like in the DIS data from fixed target An indication of the physics beyond DGLAP?

25. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 25 Directly connected to gluon distribution dominant at low-x Predictions are very uncertain Martin,Stirling,Thorne Measurement of F L at HERA Need to measure cross sections at same x,Q 2 and different y. To change y=Q 2 /xs, HERA changes s by lowering E p to 460, 575 GeV

26. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 26 Prospects for F L from HERA-II F L is significant at high-y (  y 2 ) Determined from y- dependence of cross section at same (x,Q 2 ) points → Need data from different CM energies Data with low CM energy taken in spring 2007 14 pb  1 @ Ep = 460 GeV 7 pb  1 @ Ep = 575 GeV Points: simulation of ZEUS with projected precision

27. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 27 Technical issues in measuring F L High-y: energy of scattered electron is low Difficult to separate from photoproduction background hadrons (especially photons from  0 ) mimic scattered e H1: charge asymmetry measured by BST (backward silicon tracker) ZEUS: subtraction by tagged photoproduction sample E/p Signal (scattered e + ) and background Background (photon conversion etc.) H1 ZEUS

28. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 28 High-y cross sections @ nominal energy Demonstrates technical feasibility of F L measurement

29. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 29 F L Expectations: exp.uncertainty vs. theo. prediction F L gives an independent information on the low-x (low-Q 2 ) parton evolution cross-check of gluons extracted from the DGLAP analysis F L shape also depending on the scheme of pQCD calculations F L (x, Q 2 ) from proceedings DIS2004 conf. by Robert Thorne overlaid: H1 simulation of projected uncertainty in F L from the talk in EPS2007, by N. Raicievic (H1)

30. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 30 HERA II 04-05 HERA II 04-05 HERA I HERA Charged Current Scattering: sensitive to u, d valence at high Q 2 more e - data: 16 pb -1  122 pb -1 Central values unchanged Uncertainty on u V reduced

31. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 31 ➔ Add all e + p (and e - p) data, correct for residual pol.  ➔ Add to the knowledge of valence quarks at lower x e±e±  -Z interference flips sign when e +  e -, H1 & ZEUS combined data HERA I + II Neutral Currents vs. lepton charge: xF 3 e±e±

32. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 32  Demonstrates absence of RH charged currents  M(W R ) > ~180-208 GeV with current precision Electroweak tests: Charged current vs P e Polarized electron-proton scattering at HERA

33. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 33 ~ Parity violating ➔ First observation of parity viol. in NC e ± p data at R < 10 -18 m ➔ Use prelim. H1 and ZEUS e ± p data from 2003-2005 terms H1 & ZEUS combined data Electroweak tests: Neutral currents vs P e e±e± e±e±

34. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 34 Heavy flavor production in DIS Quark pair production – dominated by Boson-gluon fusion process High sensitivity to gluons Multi-scale process (Q 2, p T (HQ), m HQ ) Charm production (right): Description by NLO: ~ OK

35. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 35 F 2 cc : Charm contribution to DIS  Description fairly good Give us a confidence that gluon from the QCD fit is trustful F 2 cc is not yet included in the QCD fit for the gluon pdf Theoretical treatment in the QCD fit is non-trivial More HERA-II data to be analysed, to give insight to models, pdfs

36. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 36 Beauty in photoproduction & DIS Photoproduction: some excess at low-p T ? Yet to see if the NLO is good enough DIS: large variety between models HERA-II data to be analysed, for precision

37. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 37  test/constrain gluon density or  obtain virtual charm content (charm PDF) of proton (Q 2 >> m c 2 ) c massive c massless charm contribution to F 2

38. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 38 first measurement of F 2 bb first NNLO calculation data in agreement with NLO and NNLO checks b PDF for LHC: Beauty contribution to F 2 Beauty contribution to F 2 HERA LHC, e.g. H,Z Q 2 >> m b 2

39. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 39 Evolution & Resummation  Expect more energetic jets in forward region (LEPTO, DISENT) (~ARIADNE) (CASCADE)

40. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 40 ZEUS Forward Jets

41. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 41 H1 forward jets LO is suppressed by kinematics, NLO is a factor of 2 too low, Cascade somewhat better

42. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 42 Multijets in photoproduction: 3-jet & 4-jet events from ZEUS Test pQCD at higher order of  S NLO calculations for O(  S 2 ): 3 jets 4 jets measure O(  S 3 ) Test of MC models (LO+PS) & Multiple Parton interactions MPIs and Multi-jet HFS will be abundant at the LHC Test of parton showers (LLA) used to simulate multi-jet states in (LO ME+PS) Monte Carlos. Noteworthy: 7.5  more lumi than older 3-jet photoproduction results. 3-jets studied in more inclusive phase-space region. X  obs < 1  (resolved) X  obs = 1  (direct)

43. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 43 ZEUS 3-jet photoproduction

44. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 44 ZEUS 4-jet photoproduction

45. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 45 three jet events “NLO” “LO” four jets: or need unordered gluon radiation? O(  s 3 ) (“NLO”) much better than O(  s 2 ) (“LO”) but not perfect need NNLO ? NLO: not available! => use LO+PS Color Dipole Model OK (DJANGOH: CDM: k T unordered gluons) DGLAP parton shower fails (Rapgap: k T ordered parton showers & resolved photon component) Study parton dynamics with DIS multijets at HERA

46. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 46 Multijets in Charged Current

47. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 47 Minijet Production in DIS

48. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 48 DIS Event Shapes Use power corrections to correct for non-perturbative effects in infrared and collinear safe event shape variable, F: Used to determine the hadronization corrections Valid for event shape means and differential distributions Power correction: Independent of any fragmentation assumptions Universal “non-perturbative parameter” * – (Dokshitzer, Webber, phys. Lett. B 352(1995)451) - Collinear & infrared safe combination of sphericity eigenvalues

49. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 49  S &  0 from DIS event shapes Extracted values of  S are in good agreement with world average(shown as yellow band)

50. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 50 Inclusive Prompt  at HERA: effective quark to photon fragmentation fn. Prediction for prompt  in ep DIS: Gehrmann et al. (hep-ph/0601073,0604030) LO(  3 ) calculation with lepton radiation (LL), quark radiation (QQ - incl. non-perturb. frag. function based on ALEPH data) & interference (QL): QQ LL LL dominates at small  QQ dominates at large  Reasonable description of data Q 2 > 4 GeV 2 Q 2 > 30 GeV 2 LL QQ  ETET

51. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 51 Photoproduced Prompt  + Jet Sensitive to k t unintegrated parton densities in the proton, resolved  E T Jet  Jet ETET  Photon Kinematics Jet Kinematics KZ23.31 pb FGH23.52 pb LZ30.73 pb PYTHIA19.98 pb HERWIG13.54 pb Underestimate data: KZ: GRV p &  SFs FGH: MRST01 p & AFG02  SFs Both below data for Low E t jet & deviate from data at low E t  LZ: K t -factorization & unintegrated q/g densities Better in forward jet region & for E t  &low   also seen by H1: hep-ex/0407018 5.0 ≤  t  ≤ 16.0 GeV -0.7 ≤   ≤ 1.1 0.2 ≤ Y JB ≤ 0.8 Q 2 <1 GeV 2 6.0 ≤ E t jet ≤ 17.0 GeV -1.6 ≤  jet ≤ 2.4

52. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 52 Color Non-Singlet and Singlet Exchange in Photoproduction Color Non-Singlet Exchange: Final state partons color connected Space between final state partons filled with final state particles No Gap between jets Color Singlet Exchange: Final state partons not color connected Space btw final state partons empty Rapidity Gap between jets--measure E T Gap Color Non-Singlet ExchangeColor Singlet Exchange Jet gap proton remnant jet 1 jet 2 Resolved E T Gap  color-flow (gap from fluct.)  gap proton remnant W ,Z,  * *shorthand for gluon ladder

53. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 53 Evidence for Color Singlet Inclusive dijet gap cross section vs. E T Gap for Herwig (x 3.3) & Pythia (x 1.8) with & w/o color- singlet exchange. MCs w/o color-singlet do not describe data in two lowest E T Gap bins. Color-singlet needed for best fit to data is ~3-4% of total inclusive dijet gap cross section for E T Gap from 0 - 12 GeV Indicates variation in gap probability (i.e. survival) from Tevatron-like low x  to pointlike DIS at x  = 1. Fraction of events with less than 1 GeV in gap between jets vs. fraction of photon energy participating in hard scatter.

54. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 54 Diffractive DIS & Factorization x IP - momentum fraction of proton carried by color singlet exchange z IP - momentum fraction of color singlet carried by parton entering hard sub-process  - momentum fraction of color singlet carried by struck quark Assumption: proton vertex factorization: shape of diffractive PDFs independent of x IP and t (needed?)

55. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 55 DPDFs from Inclusive Diff DIS ~70% gluons Integrated over z Fit DPDFs (  & Q 2 dependences) using NLO QCD.

56. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 56 Inclusive DPDFs at high z Fit B: Cg = 0: Gluon constant at Q 0 2 g  qq q  qg

57. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 57 Obtain better constraint on high z gluon Jet g Combined fit of F 2 D and dijet data Jet Dijets Add Diff. Dijets to DPDF fit ➔ Both datasets well described by combined fit LHC Caveat: Gap survival (reinter- actions)

58. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 58 Test factorization: diffr. charm  DIS: D* & H1 Lifetime method data consistent with NLO QCD  Photoprod: D* data consistent with NLO QCD within scale error DIS & Direct Photo. Resolved Photo. H1: hep-ex/0610076 x  =1 dominates 0 < x  < 1 small fraction (~10%)

59. Wesley Smith, U. Wisconsin, March 3, 2008 University of Chicago, QCD Results from HERA - 59 QCD @ HERA Summary Jet data constrains gluon & PDFs at high-x &  S Final HERA PDFs should improve by 50% from inclusive measurements, jets, cross-sections, NNLO fits, etc. Improved measurements of heavy flavor PDFs QCD evolution has been studied at lower x Region over which DGLAP works is extended, but may need BFKL Better understanding of extrapolation of PDFs to the LHC Multijets measure higher order pQCD processes Need theoretical calculations to higher order Multiple Parton Interactions are important NLO QCD describes new jet data & prompt  well. Diffractive DIS factorizes into DPDF + hard scatter  Consistent with NLO QCD