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Experimental Tests of QCD at Colliders: Part 1

Experimental Tests of QCD at Colliders: Part 1. David Milstead Stockholm University. ITEP 2006 Winter School, Moscow. What do we mean by ”testing QCD” ?. Quantum Chromodynamics is established as the theory of the strong force a S is the least constrained coupling constant of the

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Experimental Tests of QCD at Colliders: Part 1

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  1. Experimental Tests of QCD at Colliders: Part 1 David Milstead Stockholm University ITEP 2006 Winter School, Moscow. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  2. What do we mean by ”testing QCD” ? Quantum Chromodynamics is established as the theory of the strong force aS is the least constrained coupling constant of the fundamental forces. Difficulty calculating beyond NNLO. To test pQCD -> measurements devised to suppress higher orders To develop pQCD -> measurements devised to enhance higher orders David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  3. High precision is vital! David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  4. Precision tests of perturbative QCD using ’clean’ jet, event shapes and structure functions. The capabilities of different collision environments Extraction of as and parton densities. Important theoretical and experimental errors to consider! What have we done and what more can we do ? Are tests limited by experiment/theory/both ? Themes of part 1 Part 2 – particle production + non-perturbative QCD David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  5. gg scattering Collision environments and experiments q q e+e- annihilation Pdf  ISR  FSR  h-h pdf (h)  ISR  FSR  lepton- h Pdf (h, g) ISR  FSR  David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  6. } { Jets, particle production, energy flow, correlations (3) ME-based pQCD Hadronisation – String, cluster, power corrections, LPHD (2) Higher orders via parton showering, leading log resummation (1) Partonic structure into nucleon pdf Understanding Particle Collisions Eg. DIS QCD factorisation to separate out long and short distance interactions David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  7. Proton structure and pdfs Deep-Inelastic Scattering – Reminder! HERA Q2/GeV2 x David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  8. Collinear factorisation and parton densities Factorise long and short range interactions and neglect virtuality of interacting parton. Below a factorization scale mF emissions are absorbed into a universal parton density/distribution function (pdf) DGLAP equations describe evolution of quark and gluon densities with mF David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  9. F2 from HERA-1 Range in x:0.00001-1 Q2: 1-30000 GeV2 Directly sensitive to sum of all quarks and anti-quarks Indirectly sensitive to gluons through scaling violations NLO DGLAP fit pQCD describes the data well over 4 orders of magnitude in Q2 David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  10. Determination of as NLO QCD fits to structure function data provide most precise determination from DIS data. Large theoretical error outsanding . David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  11. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  12. The work of HERA in determining proton structure David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  13. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  14. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  15. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  16. Nucleon Structure with BFKL LO BFKL fit hep-ph/9605389 H.Navelet et al. Difficult to disentangle BFKL/DLGAP/CCFM from F2 David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  17. DGLAP-model underestimates hard emissions in gluon ’ladder’. CCFM model describes data well + heavy quarks in ep,pp But, large uncertainties! David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  18. Structure Functions at the LHC Expand phase space First measurements to include Drell-Yan, multi-jet, prompt photons to determine pdfs. Problem… How do we know SUSY/UED/? Backgrounds. x Require precision from pre-discovery colliders – High x gluon may inhibit discoveries Low x evolution must be well modelled David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  19. T. Carli, (proc. DIS 2004) David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  20. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  21. Lessons for using pdfs Is DGLAP appropriate at low x ? Is your measurement likely to be sensitive to effects from non-ordered emissions ? How do you decide on a pdf uncertainty ? Compare several ? But then one compares only central values. CTEQ prescription (J.Pumplin et al., JHEP0207). Pdfs can be derived at LO, NLO, NNLO. Order of pdf should be matched to scale of matrix element in fixed order program which is used (coming soon). David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  22. Into the Hadronic Final State… Disentangling perturbative QCD from the hadronic final state. Fixed order perturbative QCD calculations for jet cross-sections, leading particles, event shapes, charged particle scaling violations. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  23. An incomplete list of QCD Models for the hadronic Final State • Leading order multi-purpose event generators • QCD Matrix Element + parton showering+ • hadronisation model (string, cluster – lecture 2): • PYTHIA, HERWIG, ARIADNE (CDM), PHOJET…. • Next-to-leading order models calculating specific 2-4 jet • processes. Can be combined with resummed logarithms • - JETRAD, JETVIP, MEPJET….. • See http://www.desy.de/~heramc/mclist.html David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  24. QCD uncertainties- Renormalisation These are lectures about data not QCD calculations… To test QCD we must understand QCD uncertainties. • Renormalisation scale Contribution to cross-section from loops. Divergent as Regularise cross-section with introduction of cut-off – new scale mR P Observable R should be independent of choice of scale – renormalisation group equation David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  25. Renormalisation Scale Uncertainty pQCD calculations contain an assumption for the value of the renormalisation scale – it is arbitrary but related to a hard scale in the event (eg b-mass, jet pt, Q2 ) -Variation of this scale is often the largest theoretical uncertainty. - Scale uncertainty reduces with higher orders A question to ask anyone who shows a QCD calculation is ”what is the scale and what is the scale uncertainty?” David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  26. Scale uncertainty and k-factors K-factors { Choices of renormalisation scale Variation in value of scale (0.5m to 2m) (customary) { K-factors for inclusive jet production in DIS David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  27. NLO – LO cross-section Scale Dependence Scale uncertainity reflects contribution of higher orders. Reduced scale uncertainty for NLO s Dijets in DIS with kt-algorithm David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  28. Jets and Partons Differences between e+e- and hh S. Chekanov, ”jet algorithms A mini-review”, (hep-ph/0211298) David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  29. Gluino pair production at the LHC scale uncertainty Quantification of discovery of any particle with colour requires pQCD calculations David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  30. Jet Physics • ’Reconstruct’ pQCD process with • a jet of hadrons - as • Jet clustering algorithms used to measure • cross-sections. • Requirements: • Infrared and collinear stability • -jet cross-section must not change • if the original parton radiates a soft parton • or splits into 2 collinear partons. • (2)Close correlation with parton direction • (3) Small hadronisation corrections • (3) Small renormalisation scale uncertainty • (4) Suppression of contributions from • beam remnants. jet Hadronisation q q e+ e- David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  31. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  32. Reduce experimental (energy scale) error by studying angular properties David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  33. 4-Jet Rate from ALEPH-LEP Based on NLO+ resummed NLL as=0.1170+-0.0001+-0.0013 David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  34. Jets at HERA Ratio of 3-2 jets – cancel theoretical uncertainties David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  35. The evolution of the strong coupling constant with time! 2005 David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  36. Event Shape Variables Thrust n for TT axis • Thrust: longitudinal momentum sum • Broadening: transverse momentum sum • Measured with n set to the thrust axis, and photon axis • Jet Mass and C parameter: correlations of pairs of particles Infra-red safe and excellent probe of pQCD – use current region of Breit-frame in ep. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  37. DIS event e+e- & ep : Breit Frame Lab Frame • Breit Frame definition: • “Brick wall frame” incoming quark scatters off photon and returns along same axis. • Current region of Breit Frame is analogous to e+e-. • Scale Q in DIS equivalent to sqrt(s) in e+e- Breit Frame PT PL Breit Frame e+e- event David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  38. Event Shapes at LEP E sqrt(s) variables Consistent as from 6 different variables Running David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  39. Used to determine the hadronization corrections Power corrections: an analytical approach • Power correction is used to calculate hadronization corrections for any infrared safe event shape variable, F • Mean event shape variables are sum of perturbative and non-perturbative (power correction) parts • The power correction depends on two parameters, α0 and αs “non-perturbative universal parameter” -(Dokshitzer, Webber Phys. Lett. B 352(1995)451) David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  40. Power corrections in e+e- and DIS Many experients single experiment ep e+e- Consistent picture over full space range. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  41. Simultaneous extraction of as of a0 Reasonably good agreement over many different variables David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  42. Precision tests of perturbative QCD using ’clean’ jet signals and structure functions. No sign of failure of pQCD or new physics The capabilities of different collision environments Complementary precision work at e+e-,hh,lh Extraction of as and parton densities. Both extracted to high precision – high x pdf uncertainties - low x evolution in question Important theoretical and experimental errors to consider! Renormalisation scale uncertainties and energy scale uncertainties are some present limitations What have we done and what more can we do ? Higher orders needed. Optimal calibration. New variables Are tests limited by experiment/theory/both ? Both Summary David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  43. Collision environments and experiments e+e- annihilation Pdf  ISR  FSR  gg scattering Pdf(g) ISR  FSR  h-h pdf (h)  ISR  FSR  lepton- h Pdf (h, g) ISR  FSR  David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  44. Photon Structure at LEP David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

  45. SM or BSM ? Precision limited by experimental and theoretical errors Optimal to determine pdfs at HERA and then test at the Tevatron. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

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