qcd3

Information about qcd3

Published on September 27, 2007

Author: Mertice

Source: authorstream.com

Content

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.

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