1. Final State QCD Studies @ LEP, part 2 Nuno Anjos LIP, Lisbon – DELPHI and L3 Collaborations EPS HEP05 Lisboa, Portugal July 22 nd, 2005 1a) Single Particle Inclusive Production in Two-photon Collisions at LEP II with the DELPHI Detector DELPHI 2005-018 CONF-738 1b) Di-jet Production in Collisions at LEP DELPHI 2005-007 CONF-727 2) Studies of Hadronic Event Structure in e + e - Annihilations from 30 GeV to 209 GeV with the L3 Detector CERN-PH-EP/2004-024 (L3 paper)

2. Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22 nd 2/12 Photon-Photon Collisions Main source of hadron production in LEP2 Probe the QCD structure of the photon Hadrons and jets at large p T Anti-tagged mode: electron and positron escape through beam pipe Quasi-real photons: photon virtualities Q 2 ~0 Sensitive to quark and gluon components of photon Along with quasi-real photon DIS studies measure parton density function of photon

3. Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22 nd 3/12 Selection Criteria Hadronic Events N ch 5 Low Angle Anti-tag Luminosity Monitor E < 25 or 30 GeV Background suppression W vis < 35 GeV/c 2 (check papers for more criteria) Acceptance | < 1 or 1.5

4. Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22 nd 4/12 Single Particle Production 617 pb -1, 161 to 209 GeV 910K selected events Background sub. + MC correction PYTHIA Excal., KoralZ, BDKRS

5. Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22 nd 5/12 Di-jet Production Direct (33%)Single Resolved (23%) Double ResolvedDouble Resolved VDM Sub. Back. e + e - qq pairs WW, ZZ Multiple Parton Int. Non 2-to-2 Jet Events 1605±16500±543±338±4131±7893±13 (44%) Jet reconstruction Durham algorithm | jets | <1 p T > 3 GeV/c p T > 4 GeV/c Softer remnant jets allowed outside domain 550 pb-1 189 to 209 GeV 5147 Sel. Ev.

6. Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22 nd 6/12 MC total Direct Single Resolved Double Resolved Analysis of the Di-jet Processes Direct phase space domain Double Resolved phase space domain Phase space divided in Dir, SR, DR domains MC has problems with resolved processes Fix 3 parameter fit performed in each domain (var: E out jets, |p iT |, W tot ) Background subtracted Charged Multiplicity (all domains)

7. Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22 nd 7/12 Di-jet Cross-section Leading OrderNext-to-Leading Order MeasuredLONLO 17.1±0.320.2±0.117.8±0.1 Cross Section (pb)

8. Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22 nd 8/12 qq Hadronic Event Structure e + e - qq @ LEP: ideal environment for QCD studies Well defined initial sate; QCD affect only final state; no beam remnants Very wide energy range Z0 peak: large statistics collected at LEP1 Low energy: LEP1 events with ISR High energy: LEP2 phase –ISR decreases high energy satistics –WW and ZZ backgrounds are significant _ Electroweak Process Perturbative QCD Hadronization Final decays

9. Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22 nd 9/12 Jet Rates Event Shape Variables Perturbative effects

10. Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22 nd 10/12 s from event shapes s from combined measurements Event Shapes Variables first moments α s (m Z ) = 0.1227 ± 0.0012 ± 0.0058 Non-perturbative effects

11. Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22 nd 11/12 Charged Multiplicity N ch (206 GeV)=28.09±0.23±0.24 distribution =-ln(p/E beam ) Hadronization

12. Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22 nd 12/12 Conclusions DELPHI studies of collisions: Inclusive single-particle p T spectrum was measured and found to agree with PYTHIA MC model. Differential inclusive d /dp T cross-section was measured and compared to QCD NLO predictions. Total and differential di-jet cross sections were measured and found to agree with QCD NLO predictions: = 17.1 ± 0.3 pb L3 studies of Hadronic Event Structure in e + e - collisions: Jet rates were measured using diverse jet algorithms, and found to agree with PS models. Energy evolution found to agree with QCD models. Event shape distributions and their moments were measured and found to agree with PS models. Energy evolution of moments was compared with predictions from QCD and power law corrections. Event shape energy evolution was compared with QCD calculations and found in good agreement at all energies. A measurement of s was extracted: α s (mZ) = 0.1227 ± 0.0012 ± 0.0058 Charged particle multiplicity and inclusive charged particle momentum spectrum were measured, and their energy evolution was compared with phenomenological models and QCD calculations.