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Short Introduction to QCD

Short Introduction to QCD. Renormalisation Quark Parton Model and its improvement by QCD Factorisation and the Altarelli-Parisi splitting functions Evolution equations: DGLAP and BFKL Determination of the proton parton density Parton shower NLO calculation principles….

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Short Introduction to QCD

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  1. Short Introduction to QCD • Renormalisation • Quark Parton Model and its improvement by QCD • Factorisation and the Altarelli-Parisi splitting functions • Evolution equations: DGLAP and BFKL • Determination of the proton parton density • Parton shower • NLO calculation principles… Deep-inelastic scattering Proton-proton collisions:

  2. physical charge bare charge bare charge screened Principle of Renormalisation - QED Loop momentum can be anything ultraviolet divergence In fact, this integration is rather complicated (see Aitchison&Hey problem 6.13) Geometrical series Result depend on unphysical cut-off M but physically measured coupling contains all orders and must be independent of M

  3. Renormalisation - QED Relation between bare and physical charge has to specified at a particular value of the photon momentum _ Infinities removed at the prise of renormalisation scale, but results depends on arbitrary parameter m Note: physical observable do not depend on m charge experimentalist measures depends on scale “running coupling constant”

  4. Loops in QED and QCD QED:QCD: Low resolution: charge is screened by ee-pairs High resolution : charge is big

  5. The QCD Scale Parameter L “Confinement region”: coupling gets very large Asymptotic freedom: unique to non-abelian theories “soft” “hard” QCD explains confinement of colour and allows calculations of hard hadronic processes via perturbative expansion of coupling !

  6. Naive picture: x 0 1 f(x) 1/3 Gluons f(x) x 0 1 1/3 Quark-Parton Model and Deep-Inelastic Scattering

  7. The Proton Structure Function But number of parton is stable

  8. QPM: QCD improved Parton Model Q2 QCD: Q2 Integral diverges ! Need to introduce artificial regulator.non-perturbative scale where pQCD breaks down

  9. Q2 Factorisation DGLAP*-equation: From iteration Scaling violation of F2 caused by gluon emission ! *DGLAP: Dokshitzer, Gribov, Lipatov, Altarelli, Parisi

  10. Summary DGLAP-Equations DGLAP-equation Altarelli-Parisi splitting functions: Given a parton density f at Q0 the DGLAP evolution predicts f at any Q2 ! DGLAP equation are the basis for parton shower model in MC

  11. The Parton Shower Approximation Hard 22 process calculation has all (external leg) partons on mass shell However, partons can be off-shell for short times (uncertainty principle) close to the hard interaction Outgoing partons radiate softer and softer partons Incoming partons radiate harder and harder partons For more complex reaction often not clear which subdiagram Should be treated as hardest double counting

  12. Scaling Violations Scaling violations Gluon indirectly determined by scaling violations Sensitivity logarithmic

  13. Factorisation scale Determination of the Parton Densities Functions Parton density Most events at LHC In low-x region LHC is gluon-gluon collider !

  14. Scaling violations A part of Wilczek’s comments upon the Nobel Prize announcement … F2 gluon density Scaling F2 LHC is gluon-gluon collider

  15. Calculation of Hadron-Hadron Cross-Section Factorisation Theorem: PDF is universal Once extracted can calculate any cross-section within same theoretical scheme  Diverges for low PT0 Calculation of exact matrix elements: LO, NLO done, NNLO close-by (many processes) (loops, divergences, cancellations between large positive/negative numbers)

  16. Global NLO QCD Analysis Parton densities are from „global“ fits, i.e. from all available data: Recently (2002): PDF with uncertainties using phenomenological analysis Quantifiable uncertainties on PDF and physical predictions Problem: complexity of global analysis as results from many experiments from variety of physical processes with diverse characteristics and errors often mutally not compatible and theoretical uncertainty can not be rigoursly quantified Tung (2004): „PDF-users must be well informed about nature of uncertainties !“

  17. Determination of Parton Density Function Experimental data and errors e.g. DIS structure functions Theoretical framework e.g. NLO DGLAP fit, MS scheme etc. Theoretical assumptions and prejudices e.g. omit certain data, correct for non-perturbative effects (nucleon shadowing etc) LO NLO NNLO

  18. Parton-Parton Luminosities at LHC Example: x2 x1

  19. Example: Single Inclusive Cross-section • - test of pQCD in an energy regime never probed! • - validate our understanding of pQCD at high momentum transfers Rather general string theory toy-model (hep-ph/0111298) Large momentum transfers and small-x ! from PDF from ren+fac scale LHC reach in the first year At very small distances, particles disappear into curled extra-dimensions TeVatron reach ends here At the LHC the statistical uncertainties on the jet cross-section will be small. Main systematic errors ? Theory uncertainty ?

  20. Recursive BFKL equation: BFKL- Evolution Equation

  21. Typical Evolution in an Event DGLAP BFKL leads to strong ordering of transverse momenta diffusion pattern along the ladder

  22. Physical Interpretation of Evolution Equations At low-x probability that parton radiates becomes large Struck parton originates most likely from a cascade initiated by a parton with large longitudinal momentum DGLAP • describes change • of parton densities with • varying spatial resolution • of the probe • leads to strong ordering • of transverse momenta • from photon to proton end BFKL • describes how • high momentum parton • in the proton is dressed • by a cloud of gluons • localised in • fixed transverse spatial • region of the proton • diffusion pattern along • the ladder Non-perturbative region

  23. Quark is reabsorbed Second quark is emitted proton Standard picture Pythia, Herwig Sherpa, Alpgen NLOJET++,MCFM • Parton collinear in proton • DGLAP evolution, i.e. resummation of log Q2-terms ordering in parton virtualities Alternative picture • BFKL evolution equation • - resummation of log (1/x) terms • -unintegrated gluon distribution • -ordering in rapidity, unordered in virtuality • 2) CFFM evolution: • resummation of log Q2-log (1/x) terms • unintegrated parton distribution • angular ordering • Skewed parton distribution Colour dipole showers (?) Cascade Mainly for soft and diffractive processes

  24. Back-up

  25. Renormalisation Group Equation (RGE) RGE ensures that entire Q2 dependence of R comes from running of the strong coupling constant (not shown here)

  26. Example to second order Consider a quantity R:

  27. Reminder: Next-To-Leading-Order calculations Born: First Order: Virtual First-Order: Real Loop diagram infra-red singularities cancel each other (KNL-theorem), if (infra-red safeness) Real and virtual contributions can be regularised by introducing integral in d=4-2e dim. In this case: One can show that for any observable where the NLO prediction is: where:

  28. Subtraction Method Ellis, Ross, Terrano (1981) Let us look at the real contribution: regularised Add and subtract locally a counter-term with same point-wise singular behaviour as R(x): Since By construction this integral is finite Add and subtract counter-term The only divergent term has B&V kinematics and gets cancels against as B/2e-term of virtual contribution  cancellation independent of Observable

  29. Quark Parton Model • Interaction of hadrons due to interaction of partons • Structure of hadron describable by distribution of partons at any time • changes in number and momenta of partons should be small during time they are probed Infinite momentum frame: proton is moving with infinite momentum (all masses can be neglected) flat, frozen, unexpected time dilation: partons frozen (no interaction) they can be treated as ‘free’ during the short time they interact with photon Photon-Proton interaction can be expressed as sum of incoherent scattering from point-like quark !

  30. QPM Quarks interact -> redistribution of momenta QCD

  31. BFKL Toy Model

  32. Relation PDF and cross-section/F2 Parton density function Structure Function/cross-section Physical observable Theoretical construct Model dependent Model independent Definition depends on: 1) order of alpha_s 2) factorisation scheme 3) factorisation scale well defined

  33. LHC gives access: • to high momentum transfers • at relatively low-x • to high-x DIS: o) theoretically well defined o) experimentally clean HERA: o) measure strong coupling and parton densities o) verifiy/falsify DGLAP evolution o) develop techniques to constrain theory uncertainties from LHC data DGLAP evolution

  34. W-Boson Production at LHC Huge statistical samples & clean experimental channel. W and Z production ~105events containing W(pTW>400GeV) ~104events containingZ(pTZ >400 GeV) “Standard candles” at LHC: • Luminosity - detector calibration • - constrain quark and anti-quark densitiesin the proton. • Precision measurements MW etc. …after effort of 10 years a differential NNLO calculation is available ! Scale dependence at y=0: LO: 30% NLO: 6% NNLO: 0.6% No change in shape from NLO->NNLO Precision from theory challenge for experiment ! rapidity

  35. NNLO NLO PDF Impact on W-Boson Cross-section at LHC PDF uncertainties: 2 NNLO sets by MRST Mode=4 gives better description of Tevatron High Et jet data At NLO: PDF uncertainties are absorbed in scale dependence At NNLO: PDF uncertainties are larger ! 1-2% difference visible/measurable at LHC ? rapidity

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