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QCD results from colliders

2007 Aspen Winter Conference Wesley H. Smith U. Wisconsin - Madison January 11, 2007 Outline: Parton Density Functions & Structure Functions Jets & Measurements of  S Multijets, Inclusive Jets, W + Jets, Dijets Prompt Photons Diffraction This talk is available on:

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QCD results from colliders

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  1. 2007 Aspen Winter Conference Wesley H. Smith U. Wisconsin - Madison January 11, 2007 Outline: Parton Density Functions & Structure Functions Jets & Measurements of S Multijets, Inclusive Jets, W + Jets, Dijets Prompt Photons Diffraction This talk is available on: http://www.hep.wisc.edu/wsmith/files/exp_qcd_smith_aspen07.pdf QCD results from colliders

  2. 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

  3. Jets at HERA constrain gluon distribution • First fit using 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-Q2 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

  4. Measurement of FL at HERA Martin,Stirling,Thorne Directly connected to the gluon distribution dominant at low-x Predictions are still very uncertain • Need to measure cross sections at same x,Q2 and different y. • To change y=Q2/xs, HERA changes s by lowering Ep to 460 GeV • The price in integrated luminosity is a factor of 410 pb-1 requires 3 months

  5. HERA Charged Current Scattering:sensitive to u, d valence at high Q2 more e- data:16 pb-1 122 pb-1 Centralvaluesunchanged Uncertainty on uV reduced HERA II 04-05 HERA I HERA-I: 130 pb-1, HERA-II: 350 pb-1 with e polarization so far -- stops June 30.

  6. Neutral Currents vs.lepton charge: xF3 e± HERA I + II e± g-Z interference flips sign when e+  e- • Add all e+p (and e-p) data, correct for residual pol. , H1 & ZEUS combined data • Add to the knowledge of valence quarks at lower x

  7. Electroweak tests:Charged current vs Pe  Demonstrates absence of RH charged currents  M(WR) > ~180-208 GeV with current precision Polarized electron-proton scattering at HERA

  8. Electroweak tests:Neutral currents vs Pe e± e± • Use prelim. H1 and ZEUS e±p data from 2003-2005 ~ Parity violating terms H1 & ZEUS combined data • First observation of parity viol. in NC e±p data at R < 10-18 m

  9. High pT jets at the Tevatron In 2005: CDF published both central cone and kT jets with 400pb-1 Now: new preliminary results with full rapidity coverage for 1fb-1 Midpoint Searchcone Algorithm kT Algorithm

  10. Tevatron Inclusive Jets: cone & kT Midpoint Searchcone Algorithm kT Algorithm Data are well-described by NLO pQCD Experimental Uncertainties: Smaller than PDF Uncertainties!! (only shown for kT Algorithm)

  11. Inclusive Jet Cross Section DØ preliminary (800pb-1) Compare to NLO pQCD + 2-loop Threshold Corrections (Kidonakis, Owens)  Sensitive to PDFs!! -- esp. Gluon!

  12. New DIS Jet data: gluons & S Jet energy scale uncertainty of 1% These measurements will provide additional constraints for the gluon distribution at middle x; and give a measurement of S

  13. charm contribution to F2 c massive  test/constrain gluon density or c massless (Q2 >> mc2)  obtain virtual charm content (charm PDF) of proton

  14. Beauty contribution to F2 first measurement of F2bb first NNLO calculation data in agreement with NLO and NNLO checks b PDF for LHC: HERA LHC, e.g. H,Z Q2 >> mb2

  15. Probe b PDF with Z+b-jet at Tevatron b b b b CDF Z+b-jet/Z+jet : CDF: 2.4+-0.7(stat) +-0.5(sys) % D0: 2.1+-0.4(stat) +0.3-0.2(sys) % NLO: 1.8+-0.4 % (CTEQ6) OK

  16. can Tevatron results be used for LHC? • Are PDFs measured thus far applicable? • Does DGLAP QCD evolution work sufficiently to extrapolate? • Need to take care of low-x effects: • saturation • multiple interactions • BFKL vs. DGLAP •  study at HERA LHC Tevatron HERA

  17. Evolution & Resummation • Expect more energetic jets in forward region (LEPTO, DISENT) (~ARIADNE) (CASCADE)

  18. ZEUS Forward Jets

  19. H1 forward jets • LO is suppressed by kinematics, • NLO is a factor of 2 too low, Cascade somewhat better

  20. Multijets in photoproduction:3-jet & 4-jet events from ZEUS • Test pQCD at higher order of S • NLO calculations for O(S2): 3 jets • 4 jets measure O(S3) • 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) usedto simulate multi-jet states in(LO ME+PS) Monte Carlos. • What’s new: • 7.5  more lumi than existing 3-jet photoproduction results. • 3-jets studied in more inclusivephase-space region. • No published 4-jet photoproduction results by ZEUS or H1 yet. Xobs < 1(resolved) Xobs = 1(direct)

  21. ZEUS 3-jet photoproduction

  22. ZEUS 4-jet photoproduction

  23. Study parton dynamics with DIS multijets at HERA O(as3) (“NLO”) much better than O(as2) (“LO”) but not perfect three jet events “NLO” “LO” need NNLO ? NLO: not available! => use LO+PS Color Dipole Model OK (DJANGOH: CDM: kTunordered gluons) DGLAP parton shower fails (Rapgap: kT ordered parton showers & resolved photon component) four jets: or need unordered gluon radiation?

  24. W+n Jets W+2 Jets CDF preliminary (320pb-1) • Cross Section for Restricted W Phase Space •  Avoid Model-Dependent Acceptance Corr. Shape-Comparison with LO Alpgen+PYTHIA: Reasonable Agreement!

  25. Z+n Jets • Smaller  than (W+Jets) but cleaner exp. signature D0 preliminary (340pb-1) Ratios to total inclusive Zee (normalized) (normalized at n=1) Data are described by: MCFM: NLO for Z+jet, Z+2-jet Madgraph: ME+PS (Z+ ≤ 3 parton) tree-level + PYTHIA parton shower ALPGEN (LO ME) (pT Dependence)

  26. Dijet Azimuthal Decorrelation • Compare with theory: • LO has Limitation >2/3 • & Divergence towards  • NLO is very good – down to /2 • & better towards  • … still: resummation needed

  27. Dijet Azimuthal Decorrelation • Compare with theory: • HERWIG is perfect “out-the-box” • PYTHIA too low in tail but can tune • SHERPA (ME+PS+CKKW match) good • ALPGEN (LO ME) looks good – but low efficiency  large stat. flucts.

  28. 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 - Collinear & infrared safe combination of sphericity eigenvalues 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)

  29. S & 0from DIS event shapes Extracted values of S are in good agreement with world average(shown as yellow band)

  30. DIS Jets, Event Shapes & S Precise measurement of S at HERA, error dominated by theory Running measured from jets and event shapes in a single experiment New world measurement incl. HERA Jets: S = 0.1189  0.0010 (hep-ex/0606035)

  31. Isolated Photon Cross Sect. Direct photons directly probe hard scattering dynamics: DØ hep-ex/0511054, submitted to PRL |y|<0.9 L = 326 pb-1 pTg (GeV) pTg (GeV) data/theory (NLO: JETPHOX): good agreement over 23<pT<300GeV •  PDF sensitivity requires: • Reduced exp. uncertainties – dominated by purity uncertainty • Improved theory (resummation / NNLO)

  32. DIPHOX: with and w/o NNLO gg-diagram Di-Photon Cross Section CDF Collab., Phys. Rev. Lett. 95, 022003, 2005. (207pb-1) • Pseudorapidity < 0.9 • Photon pT> 13 & 14 GeV • DIPHOX: - NLO prompt di-photons - NLO fragmentation (1 or 2 g) - NNLO gggg diagram • ResBos: - NLO prompt di-photons • - LO fragmentation contribution - Resummed initial state gluon radiation (important for qT) • PYTHIA (increased by factor 2) Mgg (GeV/c2)

  33. Additional measurement for Df (gamma-gamma) < p/2 (open markers) compared to DIPHOX Di-Photon Cross Section • NLO fragmentation contribution • - only in DIPHOX •  at high qT, low Df, low mass • Resummed initial-state gluon radiation – only in ResBos  at low qT Important: Combined Calculation with NLO Fragmentation & Initial State Resummation • Statistical fluctuations: much larger data set soon - 1 fb-1 Dfgg(rad)

  34. 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 withlepton radiation (LL), quark radiation (QQ - incl. non-perturb. frag. functionbased on ALEPH data) & interference (QL): ET Q2 > 30 GeV2 Q2 > 4 GeV2 QQ LL LL dominates at small  QQ dominates at large  Reasonable description of data   LL QQ QQ LL

  35. Photoproduced Prompt  + Jet Sensitive to kt unintegrated parton densities in the proton, resolved  • KZ: GRV p &  SFs • FGH:MRST01 p & AFG02  SFs • Both below data for Low Etjet & deviate from data at low EtgLZ: Kt-factorization & unintegrated q/g densities • Better in forward jet region & for Etg &low hg ETJet ETg • 0.2 ≤ YJB ≤ 0.8 • Q2 <1 GeV2 Photon Kinematics Jet Kinematics hg • 5.0 ≤ Etg ≤ 16.0 GeV-0.7 ≤ hg ≤ 1.1 • 6.0 ≤ Etjet ≤ 17.0 GeV • -1.6 ≤ hjet ≤ 2.4 hJet Underestimate data: also seen by H1:hep-ex/0407018

  36. Resolved Resolved Color Non-Singlet and Singlet Exchange in Photoproduction Color Non-Singlet Exchange Color Singlet Exchange Jet Jet * W,Z,  gap  color-flow (gap from fluct.) Jet Jet proton remnant *shorthand for gluon ladder proton remnant jet 1 • Color Non-Singlet Exchange: • Final state partons color connected • Space between final state partonsfilled 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 ETGap ETGap jet 2 gap

  37. Evidence for Color Singlet • Inclusive dijet gap cross section vs. ETGap 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 ETGap bins. • Color-singlet needed for best fit to data is ~3-4% of total inclusive dijet gap cross section for ETGap from 0 - 12 GeV • Fraction of events with less than 1 GeV in gap between jets vs. fraction of photon energy participating in hard scatter. • Indicates variation in gap probability (i.e. survival) from Tevatron-like low x to pointlike DIS at x = 1.

  38. Diffractive DIS & Factorization Assumption: proton vertex factorization: shape of diffractive PDFs independent of xIP and t (needed?) xIP- momentum fraction of proton carried by color singlet exchange zIP- momentum fraction of color singlet carried by parton entering hard sub-process - momentum fraction of color singlet carried by struck quark

  39. DPDFs from Inclusive Diff DIS Fit DPDFs ( & Q2 dependences) using NLO QCD. ~70% gluons Integrated over z

  40. Inclusive DPDFs at high z gqq qqg Fit B: Cg = 0: Gluon constant at Q02

  41. Add Diff. Dijets to DPDF fit Jet Combined fit of F2D and dijet data Dijets Jet g Obtain better constraint onhigh z gluon • Both datasets well described by combined fit LHC Caveat:Gapsurvival (reinter-actions)

  42. 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=1dominates 0 < x< 1smallfraction (~10%) H1: hep-ex/0610076

  43. Inclusive Diffraction at CDF • Use high pT jets to measure Diffractive Structure Function,FjjD  = Pparton/ PPomeron  = Ppomeron/ Pproton • Slope of d/dt indep. of Q2 in SD dijets: t = ( Pi - Pf )2 No Q2 dependence (100 < Q2 < 10000 GeV2) Diffractive SF ~ proton SF

  44. QCD Mediated Dijet Production • select DPE dijets: • examine the dijet mass fraction Rjj • Strategy: ExclusiveQCD: DPEMC ExHuME InclusiveQCD: POMWIG Similar to central exclusive Higgs production ExHuMEbased on Khoze, Martin, Ryskin calculation used for Higgs Similar for DPEMC Study tail

  45. Experimental QCD 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

  46. Acknowledgements • Many thanks are due to the following for suggestions, comments & material used: • Michele Arneodo, Olaf Behnke, Gregorio Bernardi, Duncan Brown, Brendon Casey, Mary Convery, Massimo Corradi, Sridhara Dasu, Elisabetta Gallo, Max Klein,Regis Lefevre,Vivian O’Dell, Frank Petriello, Sascha Savin

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