Jet Measurements at D0 using a KT Algorithm

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Jet Measurements at D0 using a KT Algorithm
V.Daniel Elvira daniel_at_fnal.gov
HEP International Conference in Quantum
Chromodynamics 2-9th July 2002
Montpellier (France)
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Outline

• Introduction to the KT jet algorithm, the D0
  definition, general systematics
• Physics Results
• - Subjet Multiplicities
• Phys. Rev. D65 052008, 2002
  (hep-ex/0108054)
• - Thrust Cross Sections
  Preliminary
• - The Inclusive Jet Cross Section
  Phys. Lett. B525 211, 2002
  (hep-ex/0109041)

D0 Run I Data at 1.8 TeV
630 GeV Run IB (94-95) Run 1C
(95-96)
90 pb-1 600 nb-1
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The D0 Calorimeters
p
y
p
q
j
x
Z
h 0.7
h 1.5

• Liquid argon sampling and uranium absorber

• Transverse segmentation (towers)

• Hermetic with full coverage

h - ln tan (q / 2) h lt 4.2 l intgt
7.2 (total)
Electrons sE / E 15 /ÖE 0.3 Pions
sE / E 45 /ÖE 4
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Jet Definition

• Hard Scattering ( , qg, ,gg)
• Parton shower (higher orders)
• Fragment./Hadroniz. (colorless particles)

calorimeter jet

• Parton jet q and g (before hadroniz.)
• Particle jet final state (after hadroniz.)
• Calorimeter jet measured object (after
  calorimeter shower)

• Jet Algorithm
• Fixed cone ( D0 Run I results published
  before 2002, all CDF measurements)
• Clustering (New 2002 D0 Run I KT results)
  Less split-merge ambiguities, infrared safe to
  all orders in perturbation theory.

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Run I KT Algorithm at D0
Ellis, Soper Phys. Rev. D48 3160, 1993
Catani, Dokshitzer, Seymour, Webber Nucl. Phys
B406 187, 1993
Successively associate pairs of Jets
Soft Collinear
(if DRltlt1 )
Resolution parameter
For each pair of particles and for each particle
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Run I KT Algorithm at D0
Find the minimum of If
merge particles If make
it a jet
Iterate over the list of particles until I have
only jets
Subjets
Merge criteria adjusted to study jet structure.
Cone jet
KT jet
Re-run KT algorithm on all particles already
assigned to a jet
resolution parameter
Jet ET
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Subjet Multiplicity in Quark Gluon Jets
R. Snihurs thesis

• Motivation
• Test of QCD ( Q G jets are different)
• Separate Q jets from G jets (top, Higgs,
  WJets events)
• Measure the subjet multiplicity in quark and
  gluon jets

• Method
• Select quark enriched gluon enriched jet
  sample (KT with D0.5)
• Compare jets at same (ET , h) produced at
  different and assume relative q/g
  content is known

s
630GeV
qq
Contributions of different initial states to the
cross section for fixed Jet ET vary with
gg
qg
100 200 300
Jet ET
Jet ET
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Subjet Multiplicity in Quark Gluon Jets
Phys. Rev. D65 052008, 2002 (hep-ex/0108054)

• Subjet Multiplicity
• M fgMg (1-fg )Mq
• (g jet fractions fg obtained from HERWIG
  prediction)
• f 630 33 f 1800 59 (55ltETlt100 GeV)
• Assuming that the subjet multiplicity is
  independent of

Quark Jets
Gluon Jets
0.5 0.4 0.3 0.2 0.1
Preliminary
Mq
1 2 3 4
(1-f 630)M1800 - (1- f 1800)M630
Mg
f 1800- f 630
HEWIG prediction 1.91 Forshaw Seymour fully
resummed calculation 2.12

• Use kT algorithm unravel jets until all
  subjets are separated by ycut .001
• Measure number of subjets for events with
  jets with 55ltETlt100 GeV

Dominant uncertainties come from g jet fraction
and energy calibration
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Event Shapes Dijet Transverse Thrust
V. Sorins thesis
Test QCD predictions (new NLO-three jet
calculations), study significance of resummation
calculations

• Sum done over jets (KT with D1)
• Only the two leading jets in thrust
• Bin thrust in HT3 (3 lead jets)
• Use transverse pT

Reduce noise
(Lorenz invariance)
T Pencil-likeness of the event
Jet production rate
as2 is LO
as3 is NLO Event shape (thrust)
as3 is LO
as4 is NLO
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Dijet Transverse Thrust cross section
CTEQ4HJ,
JETRAD
Giele, Glover, Kosower, Nucl.Phys.B403 633,1993
DØ preliminary
Stat. Errors Only
Syst. Err 14-22
Syst. Err 14-25
Stat. Errors Only
DØ preliminary
DØ preliminary
Stat. Errors Only
DØ preliminary
Stat. Errors Only
Syst. Err 85-15
Syst. Err 22-14

• In the range , the
  LO calculation is O(?s4) ? excellent
  opportunity to test new NLO-three jet predictions

• In the limit (1-T) 1 ? Need resummation

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Inclusive Jet Cross Section
Phys. Lett. B525 211, 2002
Motivation test pQCD predictions, study proton
structure, search for quark
compositeness
Stat Errors only
dominant
KT algorithm (D1)
Tot. Err 14 (27) at 60 (450) GeV
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Comparison with Theory Cone Result
( See PRD 64, 032003, 2001 for cone measurements )
Using full covariance matrix
Each result is compared to itsown prediction
(JETRAD)
PDF c2 /ndf Prob () CTEQ4HJ 1.13
29 CTEQ4HJ 0.75 77
Phys. Lett. B525 211, 2002 (KT)
Leaving out the first four bins

• KT data/theory agreement is reasonable marginal
  at low PT
• NLO predictions s(KT, D1) s(cone,R0.7)
  within 1 data nominal results diverge
  at low pT

Remember - the data is corrected back to
particle level
- Error correlations are large point-to-point in
pT, but largely
uncorrelated between the two measurements.
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• Ratio between HERWIG particle parton cross
  sections
• Particle cone KT cross sections must be
  different
• KT absorbs energy
• Cone gives up energy

Energy difference between matched KT and cone
jets consistent with HERWIG (only within 2s at
low PT)
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Summary

• D0 has successfully implemented and calibrated
  a KT jet algorithm in a hadron
  collider
• Articles on subjet multiplicities inclusive
  jet cross sections were published or are in
  preparation (Thrust Distributions)
• q g jets have a different structure consistent
  with HERWIG prediction
• Thrust distributions offer an excellent
  opportunity to test the recently developed NLO
  3-jet generators
• The marginal agreement of the particle level
  inclusive KT jet cross section with NLO theory
  opened a discussion on matters such us
  hadronization, underlying event, and algorithm
  definition