Experimental Tests of QCD at Colliders: Part 2

1. Experimental Tests of QCD at Colliders: Part 2 David Milstead Stockholm University ITEP 2006 Winter School, Moscow. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

2. Part 1 concerned ’clean’ tests of pQCD with jets and structure functions. Part 2 covers the role of single particles and explores non-perturbative QCD. Complementarity: Themes of part 2 David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

3. High pt tracks in gamma-p and hh David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

4. Extraction of photon pdf David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

5. Hadronisation Approaches Generator – string, cluster Power corrections Local parton hadron duality Flavour-dependent fragmentation functions David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

6. Hadronisation Models Implemented in JETSET Implemented in HERWIG String Fragmentation Constant energy between colour Triplet states Density in strong O(1 GeV/fm) Tunneling – exp(pm2q/k) s/u – 0.3 , c/u – 10-11 Strangeness suppression Multi-parameter -JETSET Used in PYTHIA, LEPTO… Cluster fragmentation ’Pre-confinement ideas’ Colourless clusters of partons split and decay into hadrons David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

7. Jetset is highly successful but there is a lot of parameter space. HERWIG tuning parameters Jetset tuning parameters NB – tuning is not a game. Poor MC poor corrections Larger systematic uncertainties. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

8. Our bias… The ’interesting’ hard scale physics accounts for only a small part of the cross-section.. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

9. The Underlying Event Everything but the hard sub-process Can include Initial and final state radition multiple interactions… David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

10. Underlying Event at the Tevatron David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

11. Tuning models for the underlying event David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

12. Central Region (min-bias dNchg/dη ~ 7) Transverse < Nchg > LHC dNchg/dη ~ 30 x 3 dNchg/dη ~ 15 x1.5 Pt (leading jet in GeV) Tevatron Charged Particle Multiplicities at the LHC dNchg/dη at η=0 LHC √s (GeV) • PYTHIA models favour ln2(s); • PHOJET suggests a ln(s) dependence. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

13. Fragmentation Functions Dimensionless functions which describe final state single particle energy distributions in hard scattering processes. Like parton densities, fragmentation functions obey an evolution eqn. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

14. e+e- & ep : Breit Frame DIS event • 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-. Lab Frame Breit Frame PT Breit Frame PL David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

15. Scaling violations in fragmentation functions in e+e- and DIS Universality of fragmentation confirmed. Single particles provide precision probe of QCD David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

16. Extractas from fragmentation functions p+- K+- p,p Convolute NLO with data-derived quark and gluon fragmentation functions to extract as David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

17. Testing the MLLA Assuming gluon coherence MLLA/LPHD describes The evolution of the moments of ln(1/xp) distribution David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

18. Higher moments of ln(1/x) Challenge to pQCD/MLLA David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

19. Is Hadronisation Universal ? How can we tell ? Different environments. Role of the spectator ? Flavour and baryonic composition.. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

20. Precision measurements of identified particles e+e- David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

21. Identified Particles at LEP hh and ep have some catching up to do. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

22. Flavour Composition of the Hadronic Final State P(s)/P(u,d) in hadronisation process. e+e- DIS G. Bocquet et al., Phys.Lett.B366:447-450,1996 Require consistent Definitions/procedures to draw any conlusions David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

23. Baryon Suppression e+e- expectation (theory assumption) gp David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

24. _ bb production Results consistent with LO and NLO David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

25. Fragmentation of heavy quarks David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

26. Charm fragmentation ratios Neutral/Charged D-mesons Strangeness suppression factor Fraction of charged D-mesons in vector state David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

27. KNO Scaling Koba-Nielsen-Olesen + Polyakov Asympotically high energies DIS e+e- David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

28. Differences between quark and gluon jets. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

29. Quark and Gluon Jet multiplicities Search after Colour factors CA/ CF Multiplicities in quark and gluon enriched jets David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

30. Multiplicities in squark and gluino jets Many theories, Split-SUSY (gluino), GMSB (stop) + UED predict stable coloured particles. Challenge of the LHC, ILC to discriminate between competing exotic theories. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

31. QCD Instantons Tunnelling processes between different vacua. Fundamental prediction of Non-Abelian gauge theory. Electroweak instantons Violated lepton+baryon number QCD Instantons violate chirality. Cross-section expected to be significant at low x (30pb) Striking flavour democracy and ’fireball’ topology expected. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

32. Discriminate between instantons and ’QCD’ Instantons QCD Require David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow

33. Summary… Non-perturbative effects play an important role in precision tests of QCD Single particles and jets are complementary probes of QCD Fragmentation universality demonstrated. QCD predicts ’exotic’ instantons – hidden by higher orders and fragmentation. David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow