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Final State QCD
Studies @ LEP, part 2
Nuno Anjos
LIP, Lisbon – DELPHI and L3 Collaborations
EPS HEP05 Lisboa, Portugal July 22nd, 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)
Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22nd 2/12
Photon-Photon Collisions
• Main source of hadron production in LEP2• Probe the QCD structure of the photon
• Hadrons and jets at large pT
Anti-tagged mode:electron and positronescape through beam pipe
• Quasi-real photons: photon virtualities Q2~0• Sensitive to quark and gluon components of photon• Along with quasi-real photon DIS studies
measure parton density function of photon
Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22nd 3/12
Selection Criteria
• Hadronic Events
Nch ≥ 5
• Low Angle Anti-tag Luminosity Monitor E < 25 or 30 GeV
• Background suppression
Wvis < 35 GeV/c2
(check papers for more criteria)
• Acceptance | < 1 or 1.5
Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22nd 4/12
Single Particle Production
• 617 pb-1, 161 to 209 GeV 910K selected events
Background sub. + MC correction
PYTHIA
Excal., KoralZ, BDKRS
Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22nd 5/12
Di-jet Production
Direct (33%) Single Resolved (23%)
Double Resolved Double Resolved VDM
Sub. Back. e+e-→ qq pairs WW, ZZ Multiple Parton Int.
Non 2-to-2
Jet Events
1605±16 500±5 43±3 38±4 131±7 893±13
(44%)
Jet reconstruction
•Durham algorithm•|jets| <1• pT > 3 GeV/c•‹pT› > 4 GeV/c•Softer remnant jets allowed outside domain
550 pb-1189 to 209 GeV5147 Sel. Ev.
Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22nd 6/12
MC total
Direct
Single Resolved
Double Resolved
Analysis of the Di-jet ProcessesDirect
phase space domain
Double Resolvedphase 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: Eout jets, |∑piT|, Wtot)
•Background subtractedCharged
Multiplicity(all domains)
Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22nd 7/12
Di-jet Cross-sectionLeading Order Next-to-Leading Order
Measured LO NLO
17.1±0.3 20.2±0.1 17.8±0.1
Cross Section (pb)
Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22nd 8/12
qq Hadronic Event Structure
e+e- → qq @ LEP: ideal environment for QCD studiesWell defined initial sate; QCD affect only final state; no beam remnants
Very wide energy rangeZ0 peak: large statistics collected at LEP1Low 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
Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22nd 9/12
Jet Rates
Event Shape
Variables
Perturbative effects
Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22nd 10/12
s from event shapes
s from combined measurementsEvent Shapes Variables
first moments
αs(mZ) = 0.1227 ± 0.0012 ± 0.0058
Non-perturbative effects
Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22nd 11/12
Charged Multiplicity ‹Nch›(206 GeV)=28.09±0.23±0.24
distribution
=-ln(p/Ebeam)
Hadronization
Nuno Anjos EPS HEP 2005 Lisboa, Portugal, July 22nd 12/12
Conclusions
DELPHI studies of collisions:• Inclusive single-particle pT spectrum was measured and found to agree with
PYTHIA MC model.• Differential inclusive d/dpT 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:αs(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.