David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow 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 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: JetSingle particle ’clean probe’Hadronisation uncertainty Dependence on jet algorithmSimple interpretation Restricted angular coverageAccess wider angles

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

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

David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow 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 String Fragmentation Constant energy between colour Triplet states Density in strong O(1 GeV/fm) Tunneling – exp(  m 2 q /k) s/u – 0.3, c/u – 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

David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow 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.

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

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

David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow Charged Particle Multiplicities at the LHC dN chg /dη at η=0 √s (GeV) PYTHIA models favour ln 2 (s); PHOJET suggests a ln(s) dependence. LHC Tevatron x1.5 x 3 dN chg /dη ~ 30 dN chg /dη ~ 15 Central Region (min-bias dN chg /dη ~ 7) Transverse P t (leading jet in GeV)

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

David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow 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 Extract  s from fragmentation functions Convolute NLO with data-derived quark and gluon fragmentation functions to extract  s     p,p

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

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

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

David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow 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: ,1996

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

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

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

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

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

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

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

David Milstead – Experimental Tests of QCD ITEP06 Winter School, Moscow 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.