Recent Results on Jet Physics and as

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Recent Results on Jet Physics and as

• XXI Physics in Collision Conference
• Seoul, Korea
• June 28, 2001
• Presented by
• Michael Strauss
• The University of Oklahoma

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Outline

• Introduction and Experimental Considerations
• Jet and Event Characteristics
• Low ET Multijet Studies
• Subjet Multiplicities
• Cross Sections
• Three-to-Two Jet Ratio
• Ratio at Different Center-of-Mass Energies
• Inclusive Production
• DiJet Production

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Motivation for Studying Jets

• Investigates pQCD
• Compare with current predictions
• pQCD is a background to new processes
• Investigates parton distribution functions (PDFs)
• Initial state for all proton collisions
• Investigates physics beyond the Standard Model

pdf ? Compositeness ?
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Developments in Jet Physics
(with proton initial states)
Inclusion of error estimates in the PDFs
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Cone Definition of Jets

• Cone DefinitionR0.7 in h-f
• Merging and splitting of jets required if they
  share energy
• Rsep required to compare theoretical predictions
  to data
• (Rsepis the minimum separation of 2 partons
  to be considered distinct jets)

Centroid found with4-vector addition
h -lntan(q/2)
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KT Definition of Jets

• KT Definition
• cells/clusters are combined if their relative kT2
  is small(D1.0 or 0.5 is a scaling parameter)

• Infrared Safe
• Same definition for partons, Monte Carlo and
  data
• Allows subjet definitions

min(dii, dij) dij ? Merge min(dii, dij) dii
? Jet
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KT and Cone Algorithm

• Use CTEQ4M and Herwig
• Match KT jets with cone jets

DO Preliminary
DO Preliminary
99.9 of Jets have DRlt0.5 pT of KT
algorithm is slightly higher
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KT Algorithm and Subjets
For subjets, define large KT
(ycut 10-3)
Increasing ycut
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Jet Selection Criteria
Typical selections on EM fraction, hot cells,
missing ET, vertex position, etc. gt 97
efficient gt 99 pure
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Jet Energy Corrections
no distinction between jetsof different kinds

• Response functions
• Noise and underlying event
• Showering
• Resolutions Uncertainty on ETEstimated with
  dijet balancing or simulation

d2s dET dh
ET
d2s dET dh
ET
Important for cross section measurement
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Jet and Event Quantities

• Low ET Multijet Studies
• Subjet Multiplicity

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D? Low ET Multijet events
ET of Leading Jet
At high-ET, NLO QCD does quite well, but the
number of jets at low- ET does not match as
well. (Comparison with Pythia)
Each jets ETgt20 GeV. Theory normalized to 2-jet
data gt40 GeV.
Looking also at Jetrad and Herwig
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D? Low ET Multijet events
Strong pT ordering in DGLAP shower evolution may
suppress spectator jets in Pythia BFKL has
diffusion in log(pT)
(DATA-THEORY)/THEORY
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D? Subjet Multiplicity Using KT Algorithm
Monte Carlo

• Perturbative and resummed calculations predict
  that gluon jets have higher subjet multiplicity
  than quark jets, on average.
• Linear Combination
• ltMgt fg Mg (1-fg) MQ

DO Preliminary
Mean Jet Multiplicity
Gluon Jet Fraction
Quark Jet Fraction
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D? Subjet Multiplicity Using KT Algorithm

• Assume Mg, MQ independent of vs
• Measure M at two vs energies andextract the g
  and Q components

DO Preliminary
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D? Subjet Multiplicity Using KT Algorithm
Raw Subjet Multiplicities Extracted Quark and
Gluon Mutiplicities
DO Preliminary
DO Preliminary
Higher M ? more gluon jets at 1800 GeV
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D? Subjet Multiplicity Using KT Algorithm
HERWIG prediction 1.910.16(stat)
Largest uncertainty comes fromthe gluon
fractions in the PDFs
Coming soon as a PRD
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ZEUS Subjet Multiplicity

• Comparison at hadron level
• Unfolded using Ariadne MC

NLO QCD describes data Sensitive to as
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as from ZEUS Subjets
?nsub-1? ? ? Proportional to as

• Major Systematic Errors
• Model dependence (2-3)
• Jet energy scale (1-2)
• Major Theoretical Errors
• Variation of renormalization scale

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Cross Sections

• Inclusive cross sections
• Rapidity dependence
• KT central inclusive
• R32
• 630/1800 ratio of jet cross sections
• Di-Jets
• as Conclusions

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Jet Cross Sections

• How well are pdfs known?
• Are quarks composite particles?
• What are appropriate scales?
• What is the value of as?
• Is NLO (as3) sufficient?

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CTEQ Gluon Distribution Studies

• Momentum fraction carried by quarks is very well
  known from DIS data
• Fairly tight constraints on the gluon
  distribution except at high x
• Important for high ET jet production at the
  Tevatron and direct photon production

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Experimental Differential Cross Section
Detector measures ET and h
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CDF Inclusive Jet Cross Section

• 0.1 lt h lt 0.7
• Complete c2 calculation

PRD 64, 032001 (2001)
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x-Q2 Measured Parameter Space
From D? Inclusive Cross Section Measurement
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D? Inclusive Jet Cross Section

• Five rapidity regions
• Largest systematic uncertainty due to jet energy
  scale
• Curves are CTEQ4M

?d2?? dET d?? (fb/GeV)
PRL 86, 1707 (2001)
ET (GeV)
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D? Inclusive Jet Cross Section
CTEQ4HJ CTEQ4M
MRSTg? MRST
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Gluon PDF Conclusions

• c2 determined from complete covariance matrix
• Best constraint on gluon PDF at high x
• Currently being incorporated in new global PDF
  fits

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Inclusive Cross Section Using KT Algorithm
-0.5 lt h lt 0.5 D 1.0
D? Preliminary

• Predictions IR and UV safe
• Merging behavior well-defined for both experiment
  and theory

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Comparison with Theory

• Normalization differs by 20 or more
• No significant deviations of predictions from
  data
• When first 4 data points ignored, probabilities
  are 60-80

PDF c/dof Prob MRST 1.12
31 MRSTg? 1.38 10 MRSTg? 1.17 25 CTEQ3M 1.56
4 CTEQ4M 1.30 15 CTEQ4HJ 1.13 29
D? Preliminary
Strauss
The University of Oklahoma
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CDF as from Inclusive Cross Section

• as2X(0) is LO prediction
• as3X(0)k1 is NLO prediction
• X(0) and k1 determined from JETRAD
• MS scheme used
• Jet cone algorithm used with Rsep 1.3
• as determined in 33 ET bins

ET (GeV)
Michael Strauss The
University of Oklahoma
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CDF as from Inclusive Cross Section

• Experimental systematic uncertainty
• Largest at low ET is underlying event
• Largest at high ET is fragmentation and pion
  response

Michael Strauss The
University of Oklahoma
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CDF as from Inclusive Cross Section
m scale is the major source of theoretical
uncertainty ET/2 lt m lt 2ET
PDF affects as CTEQ4M minimizes c2
Theoretical uncertainties each 5
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ZEUS Inclusive Jet Production
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ZEUS Inclusive Jet Production
Measured cross section slightly above NLP pQCD in
forward section
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ZEUS Inclusive Jet Production
as Results Uses various fits of ds/dQ2 and ds/dET
Full phase-space High-Q2 region (Q2gt500
GeV) High-ET region (gt14GeVT)
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R32 Motivation and Method

• Study the rate of soft jet emission (20-40 GeV)
• QCD multijet production - background to
  interesting processes
• Predict rates at future colliders
• Improve understanding of the limitations of pQCD
• Identify renormalization sensitivity
• Does the introduction of additional scales
  improve agreement with data ?
• Measure the Ratio
• with HT
• for all jets with
• ET gt 20, 30, 40 GeV for ?lt3 and ET gt 20 GeV for
  ?lt2

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Inclusive R32

• Features
• Rapid rise HTlt200GeV
• Levels off at high HT
• Interesting
• 70 of high ET jet events have a third jet above
  20 GeV
• 50 have a third jet above 40 GeV

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R32 Sensitivity to Renormalization Scale
ETgt20 GeV, hlt2 show greatest sensitivity to scale
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R32 Results

• Jet emission best modeled using the same scale
• i.e. the hard scale for all jets
• Best scale is that which minimizes ?2 for all
  criteria
• ?R0.6ETmax, for 20 GeV thresholds
• ?R? HT, ??.3 for all criteria
• Introduction of additional scales unnecessary.

ETgt20 GeV, hlt2
PRL 86, 1955 (2001)
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D? Cross Section Ratio s(630)/s(1800) vs xT

• Ratio of the scale invariant
• cross sections
• at different cm energies
• ( 630 and 1800 GeV)
• Ratio allows substantial reduction
• in uncertainties (in theory and
  experiment). May reveal
• Scaling behavior
• Terms beyond LO ( as2 )

s
ss (ET3/2p) (d2s/dETdh) vs XT ET /
(?s / 2 )
ET
ss
XT
QCD
2
s(630)/s(1800)
1
0.4
0.0
xT
Naive Parton model
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D? Inclusive Cross Section
?s 1800 GeV
?s 630 GeV
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Cross Section Ratio

• ?(630)/?(1800) is 10-15 below NLO QCD
  predictions
• Top plot varying choice of pdf has little effect
• Bottom plot varying ?R scale is more
  significant
• Better agreement where ?R different at 630 and
  1800 (unattractive alternative !)
• Higher order terms will provide more predictive
  power!

Published in PRL 86, 2523 (2001)
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CDF DiJet
Provides precise information about initial state
partons
Cone of R0.7 Both Jets ETgt10 GeV Jet 1
0.1lthlt0.7 Jet 2 Four h regions 0.1lthlt0.7 0
.7lthlt1.4 1.4lthlt2.1 2.1lthlt3.0
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CDF DiJet Cross Section
PDF c2/dof MRST 2.68 MRST? 3.63 MRST?
4.49 CTEQ4M 2.88 CTEQ4HJ 2.43
All lt 1 Probability
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ZEUS DiJet
kT algorithm used

• ET gt 8 GeV (leading)
• ET gt 5 GeV (other)
• -1lthlt2 (leading)
• 470ltQ2lt20000 GeV2

Phys Lett B507, 70 (2001)
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ZEUS DiJet
R21 parameterized as R21 (MZ) A1as(MZ)
A1as2(MZ)
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ZEUS as Summary

• Dijets has lowest total error of all Zeus
  measurements.
• All measurements consistent with PDG value of
  0.118520

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Tevatron Run II
Run II Ecm 1.96 TeV, ?L ? 2fb-1 expect
100 events ET gt 490 GeV and 1K events ET gt
400 GeV Run I Ecm 1.8 TeV, ?L ?
0.1fb-1 yielded 16 Events ETgt 410 GeV
Great reach at high x and Q2, A great place to
look for new physics!
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Conclusions from Jet Physics

• Growing sophistication in jet physics analysis
• Error matrices
• New jet algorithms
• Better corrections
• PDF refinements
• Results generally agree with NLO QCD and PDFs
• Cross section measurements will continue to
  refine PDFs
• as measurements agree with PDG
• Low ET physics still require theoretical
  refinements
• Jet physics should continue to provide fruitful
  developments
• High ET region can reveal compositeness and other
  new physics
• Low ET region reveals soft parton distributions
  in proton
• NNLO and other theoretical refinements needed
• Results needed for discovery measurements