Published on September 27, 2007
Experimental Tests of QCD at Colliders: Part 2: Experimental Tests of QCD at Colliders: Part 2 David Milstead Stockholm University ITEP 2006 Winter School, Moscow. Themes of part 2: Themes of part 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: High pt tracks in gamma-p and hh: High pt tracks in gamma-p and hh Extraction of photon pdf: Extraction of photon pdf Hadronisation Approaches: Hadronisation Approaches Generator – string, cluster Power corrections Local parton hadron duality Flavour-dependent fragmentation functions Slide6: 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 Hadronisation Models Implemented in JETSET Implemented in HERWIG Slide7: Jetset tuning parameters HERWIG tuning parameters Jetset is highly successful but there is a lot of parameter space. NB – tuning is not a game. Poor MC poor corrections Larger systematic uncertainties. Our bias…: Our bias… The ’interesting’ hard scale physics accounts for only a small part of the cross-section.. The Underlying Event: The Underlying Event Everything but the hard sub-process Can include Initial and final state radition multiple interactions… Underlying Event at the Tevatron: Underlying Event at the Tevatron Tuning models for the underlying event: Tuning models for the underlying event Charged Particle Multiplicities at the LHC: Charged Particle Multiplicities at the LHC dNchg/dη at η=0 √s (GeV) PYTHIA models favour ln2(s); PHOJET suggests a ln(s) dependence. LHC Fragmentation Functions: 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. e+e- & ep : Breit Frame: e+e- & ep : Breit Frame DIS event Lab Frame Breit Frame Breit 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-. PT PL Slide15: Scaling violations in fragmentation functions in e+e- and DIS Universality of fragmentation confirmed. Single particles provide precision probe of QCD Extract as from fragmentation functions: Extract as from fragmentation functions Convolute NLO with data-derived quark and gluon fragmentation functions to extract as p+- K+- p,p Testing the MLLA: Testing the MLLA Assuming gluon coherence MLLA/LPHD describes The evolution of the moments of ln(1/xp) distribution Higher moments of ln(1/x): Higher moments of ln(1/x) Challenge to pQCD/MLLA Is Hadronisation Universal ?: Is Hadronisation Universal ? How can we tell ? Different environments. Role of the spectator ? Flavour and baryonic composition.. Slide20: Precision measurements of identified particles e+e- Slide21: Identified Particles at LEP hh and ep have some catching up to do. Flavour Composition of the Hadronic Final State: Flavour Composition of the Hadronic Final State P(s)/P(u,d) in hadronisation process. DIS e+e- Require consistent Definitions/procedures to draw any conlusions G. Bocquet et al., Phys.Lett.B366:447-450,1996 Baryon Suppression: Baryon Suppression gp e+e- expectation (theory assumption) Slide24: Results consistent with LO and NLO _ bb production Fragmentation of heavy quarks: Fragmentation of heavy quarks Slide26: Charm fragmentation ratios Neutral/Charged D-mesons Strangeness suppression factor Fraction of charged D-mesons in vector state Slide27: DIS e+e- KNO Scaling Koba-Nielsen-Olesen + Polyakov Asympotically high energies Slide28: Differences between quark and gluon jets. Quark and Gluon Jet multiplicities: Quark and Gluon Jet multiplicities Search after Colour factors CA/ CF Multiplicities in quark and gluon enriched jets Multiplicities in squark and gluino jets: 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. QCD Instantons: 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. Slide32: QCD Instantons Discriminate between instantons and ’QCD’ Require Summary…: 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.