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.

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.