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Experimental Tests of

Experimental Tests of. QCD. Peter Schleper University of Hamburg. European Conference on High Energy Physics Aachen, July 17, 2003.

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Experimental Tests of

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  1. Experimental Tests of QCD Peter Schleper University of Hamburg European Conference on High Energy Physics Aachen, July 17, 2003 Many thanks to: C.Berger, S.Bethke, A.Bhatti, R.Galik, V.Jain, H.Jung, M.Klein, T.Kluge, M.Martinez, P.Newman, R.Nisius, T.Schoerner, H.Stenzel, A.Tapper, T.Wengler, M.Wobisch, P.Zerwas, …, session speakers and organizers.

  2. Perturbative approach to QCD QCD: SU(3) gauge theory • non abelian, self-interacting gluons, strong ! running coupling • extremely rich phenomenology • short distances: as small  perturbative calculations • factorisation of short (pert.) and long (non-pert.) scales • QCD predictions: • hard inter. • (N)NLO • Scale dependences: • as(Q2), f(x,Q2) A.Bhatti Precision achievable / needed ?

  3. Benchmarks for Precision QCD as determinations: how good ? Zoom into SUSY Standard Model Supersymmetry Alpha_s figure Highest possible precision is vital !

  4. Benchmarks for Precision QCD The high energy frontier: Decade of Hadron colliders: Tevatron,HERA,LHC Higgs production at LHC Comparison NLO / NNLO • Discoveries depend on • input parton distributions • higher order calculations • non-pert. effects • for signal and background

  5. What’s New ? • New Data • LEP: close to final precision • HERA I: close to final High Q2 data • Tevatron: first Run II data • Better Theory • Fixed order: NLO  NNLO • All orders: resummed calculations • factorisation theorems

  6. q e+ Z/g* g e- q Event Shapes at LEP Cleanest measurements for as at LEP: G(Z  hadr.), t decays Event shapes: thrust, jet broadening, 3-jet parameter, C parameter, jet mass New results from LEP1,2, rad events (L3) Energy dependence, corrected for hadronisation with MC NLO resummed calculations

  7. Event Shapes at LEP New LEP average foras Refined treatment of correlated errors Running of as largest error: scale uncert. prel. Still: cleanest measurements for as at LEP: G(Z  hadr.), t decays

  8. Power Corrections at LEP Dokshitzer-Webber ansatz: • a0 = effective as below mI approx. for hadronisation Fit to mean values Fit to shapes as = 0.1207 no consistent as shapes well described

  9. Event Shapes at HERA New: • resummed calculations • fits to both means and shapes (H1) NLO + PC NLO + NLL NLO + NLL + PC Jet Broadening

  10. Event Shapes at HERA Old result without resummation All event shapes well described by NLO + NLL + PC Fit to shapes Resummation + power correction very successful at HERA

  11. Proton Structure many unknowns: uv, dv, us, ds, s, g many processes: DIS,Drell-Yan,Tevatron • Predictions depend on • QCD evolution • PDF’s at low Q2

  12. Quark densities - Deep Inelastic Scattering • Neutral Current: • low Q2: q+q • high Q2: e-p – e+p ~ q-q • Charged Current: • e+p: u+c + (1-y2)(d+s) q+q high sea,g - - - - - Q2 x - q-q x x

  13. Gluon density HERA: Scaling violations, FL ,charm Tevatron: Jets Slope ~ as g(x,Q2) qq qg gg ET

  14. Tevatron Jets CDF cone • Run I: 100 pb-1 • Run II: 200 pb-1 recorded • first prelim. results • CDF: 85 pb**-1, D0: 34 pb**-1 Run I gluon density not constrained at high x D0 KT Jet algorithms: Run I: cone, KT(D0) RUN II: also modified cone algor: midpoint algor.: additional seed allowed between found jets (P recomb. scheme, instead E) differences cone/ KT due to hadr. corrections

  15. Tevatron Run II: D0 Jet-Jet mass midpoint algor. Inclusive jets di-jet mass Calor. Energy scale error dominates PT

  16. Tevatron Run II: CDF incl. cone algor. from Run I Run II / Run I 5% E-scale s(PT,h) PDF error get high x gluon search close to Run I quality

  17. Parton Density fits • QCD fits to parton densities • Fit only inclusive DIS data: theoretically clean (H1,ZEUS,Alekhin) • Global fits: inclusive DIS, DY, Tevatron: jets, W/Z • more constraints (CTEQ,MRST,…) U Zeus/H1: fit to only HERA data ! similar precision as global fits uv g D dv Sea G

  18. Gluon Density: Charm at HERA depends in LO on gluon agrees with ZEUS NLO QCD fit

  19. Gluon Density: FL • NLO: contribution from gluons to FL H1 FL FL HERA III low EP run x x very sensitive: disfavors MRST

  20. Multijet: LEP 4-jet • Aleph 4 jet rate and angular correlations • NLO + resummed (small scale variation) • as = 0.1170+-0.0001(stat)+-0.0013(sys) Colour factors Different error treatment: syst: 0.0022 Very small scale error ! Accidental ?

  21. Photon Structure from LEP Jet production (OPAL) Scale dependence of g-structure from R. Nisius ET Q2 Confirmation of partonic deconvolution of g Albino, Klasen & Soldner Rembold, hep-ph/0205069

  22. L3: g-g at high PT earlier reported: excess in p0 full LEP II data: access to very high PT particles/jets New: p+,p- New: incl.jet data Huge effect: needs confirmation by other experiments

  23. Jets in g-p: HERA g-PDF & NLO describes data full HERA I luminosity Future: high x gluon ~ c.f.Tevatron

  24. as global as from QCD fits Bethke 2002 as from hadr. processses Very impressive success of QCD Limited everywhere by missing higher orders

  25. Higher Order Calculations • State of the art: • data unfolded with LO+PS monte carlo • results compared to NLO (+NLL) + hadr.cor.(LO+PS monte carlo) NNLO: building blocks known/calculated First results for DIS, Drell-Yan, Higgs,.. Within 1 ? year: 3 jets at LEP, 2-jet pp, 2-jet DIS the only way to precise as, sH Drell Yan at NNLO Monte Carlo: (the experimentalists view) Since ~5 years: 22,3,4 processes, LO+PS Needed: NLO + PS for unfolding data ! Better: NLO + NLL + PS the only way to precise data Both need strong support from the community !

  26. Higgs Production at LHC Benchmark test for status of QCD calculations Higher orders Parton densities uncertainty ~ 10 % uncertainty ~ 10 %

  27. Beauty & Charm Production Beauty: Problem for QCD ? • Tevatron: data/theory ~3 • HERA: data/theory ~3 • LEP-gg: data/theory ~ 3 Charm: • Tevatron slightly high • HERA and LEP-gg ~o.k. • 2 (3) scale problem: Mb, PTb, (Q2) • HERA/LEP-gg : Mb ~ PTb small, • Experimentally difficult: S/B ~ 1000 • B as part of gamma structure ?

  28. Charm & Beauty at Tevatron CDF: charm, 5.8 pb-1, but new silicon trigger New D0 silicon tracker LO Nason & Cacciari hep-ph/0306212 first beauty results from Run II theory factor 1.7 low

  29. Beauty at Tevatron Exclusive B Production Previous excess partially explained by: resummation of ln(PT/Mb) data: jet with b theory: b only Update to more recent fragmentation functions Now: data/theory ~ 1.7 Now: data/theory ~ 1.6 Problem partially solved

  30. Beauty at HERA • Exp difficult: low PT jets, S/B ~ 1000 • Comparison data/theorie • Former: extrapolation of data to parton level and full phase space with LO+PS monte carlo • Now: data as is, apply hadronisation corr. to NLO in visible phase space Deep inelastic Now agreement with NLO

  31. Beauty at HERA Photoproduction b b b Data still slightly above NLO, but main effect found: LO+PS differs from NLO invisible part of PS.

  32. Charm & Beauty in gg at LEP New data • L3: full LEP 2 data • Delphi: first analysis (K in RICH) Direct Process Confirmation of previous results Single resolved process • Charm o.k., beauty low • no explanation yet

  33. Parton Dynamics Selection: 2jets, unbalance DGLAP factorisation: integrated over trans. momentum f(x,Q2) expected to fail when Q~KT un-integrated PDFs: f(x,Q2,KT) BFKL,CCFM evolution NLO 3j NLO 2j Important at low –x High parton density

  34. Low – x physics: LEP g*g* scattering: • 2 hard scales • test case for BFKL dynamics L3 OPAL BFKL in LO BFKL in NLO Q2 to 10Q2 for LO, Q2 to 4Q2 for NLO

  35. Diffraction: factorization No colour exchange from Proton Soft QCD ? Hard scattering: Q2 large Factorisation in diffract. PDF and partonic s should follow DGLAP QCD evolution with Q2

  36. Diffraction in NLO QCD NLO QCD fits to incl data Same as in standard QCD fits to DIS PDF(x,Q2) for fixed Proton momentum Large gluon contribution Use diffr. PDF to predict s(jet) , s(charm)

  37. Diffraction in NLO QCD Jets Charm • NLO QCD fit to inclusive data describes jet and charm • successful test of hard scattering factorisation Current state: ~ all hard diffractive processes at HERA are described by NLO QCD

  38. Skewed partons e.g.: DVCS (deeply virtual compton scattering) correlation between initial quarks and gluons f(x1,x2,Q2) Factorisation for large Q2 NLO successful

  39. Spin Parton Distributions proton spin: ½ = ½ ( D uv + D dv +D qsea ) + Dg + Lq + Lg

  40. Spin Parton Distributions NLO QCD fit Next: Gluon distribution • Compass Experiment: • data taking started 2002 • first results on particle • production • gluon density from • g*g  cc X … soon well determined: u, d

  41. Pentaquarks: I LEPS: gn  K- K+ n hep-ex/0301020 g beam up to 2.4 GeV • Mass(K+ n) from K- recoil • background from comparison gn and gp Evidence from 3 experiments: LEPS,DIANA,CLAS • DIANA: K+ n  K0 p hep-ex/0304040 • In Xe bubble chamber • Mass(K+ n) from K- recoil M(K0p) Claim: 4.4 s evidence Claim: 4.6 s evidence

  42. Pentaquarks: 2 CLAS: g d  K+ K- p n • 3 experiments taken together: • Observation of a new state • (although size of effect in each • experiment can be debated) • mass consistent: 1540 + 10 MeV • width smaller than exp. resolution • (10…20 MeV) - Interpretation: Bound system of uudds - • 1.Constituent Quark Model • 2.Chiral symmetry breaking: • Prediction from Diakonov, Petrov,Polyakov: • decuplet of strange 5-quark systems • Mq=1530 MeV , Gq< 15 MeV hep-ph/9703373 Claim 5.8 s observation Related to di-baryon, 4-quark states

  43. Conclusion as: higher orders vital • Now: uncert. ~ 3 % • NNLO: 1-2 % possible Overall: QCD is in excellent shape • Beauty puzzle: • Tevatron and HERA slighty high, but much better with new calculations/observables • Lecture for LEP gg ? • Parton distributions: • pp H: uncert. 10 % • Requires new data from • Tevatron/DY/HERA • L3 gg • Huge excess seen, needs to be confirmed Much progress in resummat., power corr., diffraction, spin, … Ready for LHC ? not quite, but Tevatron RUN II, HERA II, THEORY…

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