qcd3

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.