1. 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
PIC Michael Strauss The University of Oklahoma

2. PIC 2001 Michael Strauss The University of Oklahoma
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
PIC Michael Strauss The University of Oklahoma

3. 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
d2s
dET dh ET
Central h region
pdf ?
Compositeness ?
PIC Michael Strauss The University of Oklahoma

4. Developments in Jet Physics
(with proton initial states)
Inclusion of error estimates in the PDFs
covariance
matrices
More rigorous treatment of experimental errors
calculation of virtual corrections
Progress toward NNLO predictions
jet algorithms
workshop
More consistent ET calculations between experiments at the Tevatron
PIC Michael Strauss The University of Oklahoma

5. Cone Definition of Jets
Centroid found with 4-vector addition
Cone Definition R=0.7 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)
R=2 + 2
h = -ln[tan(q/2)] 2R
1.3R
D0 uses Ellis and Soper’s definition
PIC Michael Strauss The University of Oklahoma

6. PIC 2001 Michael Strauss The University of Oklahoma
KT Definition of Jets
KT Definition
cells/clusters are combined if their relative kT2 is “small” (D=1.0 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
PIC Michael Strauss The University of Oklahoma

7. PIC 2001 Michael Strauss The University of Oklahoma
KT and Cone Algorithm
Use CTEQ4M and Herwig
Match KT jets with cone jets
DO Preliminary
DO Preliminary
99.9% of Jets have DR< pT of KT algorithm is slightly higher
PIC Michael Strauss The University of Oklahoma

8. KT Algorithm and Subjets
For subjets, define “large” KT
(ycut = 10-3)
Theoretically it is infrared safe. Cone algorithm is not because of soft radiation in NNLO calculations.
The same algorithm can be applied to partons from fixed-order or resummed calculations, and to MC particles and to data. In the cone algorithm, R_sep is an ad-hoc parameter used to accommodate the difference between jet definitions at the parton and detector levels.
An arbitrary procedure must be used to split and merge jets when using the cone algorithm
Increasing ycut
PIC Michael Strauss The University of Oklahoma

9. Jet Selection Criteria
Central Tracking
EM Calorimeter
Hadronic
Calorimeter
Inner Muon
Magnet
Outer Muon e g
jet m
noise
Typical selections on EM fraction, hot cells, missing ET, vertex position, etc.
> 97% efficient
> 99% pure
PIC Michael Strauss The University of Oklahoma

10. Jet Energy Corrections
no distinction between jets of different kinds
Response functions
Noise and underlying event
“Showering”
Resolutions: Uncertainty on ET Estimated with dijet balancing or simulation
d2s
dET dh ET
d2s
dET dh
Response function restores energy loss in calorimeter
Noise and underlying event added
Showering correction removes effect where energy from particle inside the cone is lost outside of the cone
Resolution causes energy. I.e. in cross section plot. ET
Observed
Cross Section
Important for cross section measurement
PIC Michael Strauss The University of Oklahoma

11. Jet and Event Quantities
Low ET Multijet Studies
Subjet Multiplicity
PIC Michael Strauss The University of Oklahoma

12. 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 jet’s ET>20 GeV.
Theory normalized to 2-jet data >40 GeV.
Looks at low x gluon radiation
The normalization corresponds to a K-factor in Pythia of 1.31
Looking also at Jetrad and Herwig
PIC Michael Strauss The University of Oklahoma

13. 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
PIC Michael Strauss The University of Oklahoma

14. 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:
<M> = fg Mg + (1-fg) MQ
DO Preliminary
Mean Jet Multiplicity
Gluon Jet Fraction
Quark Jet Fraction
PIC Michael Strauss The University of Oklahoma

15. D Subjet Multiplicity Using KT Algorithm
Assume Mg, MQ independent of √s
Measure M at two √s energies and extract the g and Q components
DO Preliminary
PIC Michael Strauss The University of Oklahoma

16. 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
PIC Michael Strauss The University of Oklahoma

17. D Subjet Multiplicity Using KT Algorithm
HERWIG prediction =1.91±0.16(stat)
Largest uncertainty comes from the gluon fractions in the PDFs
Coming soon as a PRD
PIC Michael Strauss The University of Oklahoma

18. ZEUS Subjet Multiplicity
Comparison at hadron level
Unfolded using Ariadne MC
NLO QCD describes data Sensitive to as
PIC Michael Strauss The University of Oklahoma

19. PIC 2001 Michael Strauss The University of Oklahoma
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
PIC Michael Strauss The University of Oklahoma

20. PIC 2001 Michael Strauss The University of Oklahoma
Cross Sections
Inclusive cross sections
Rapidity dependence
KT central inclusive
R32
630/1800 ratio of jet cross sections
Di-Jets
as Conclusions
PIC Michael Strauss The University of Oklahoma

21. PIC 2001 Michael Strauss The University of Oklahoma
Jet Cross Sections
How well are pdf’s known?
Are quarks composite particles?
What are appropriate scales?
What is the value of as?
Is NLO (as3) sufficient?
PIC Michael Strauss The University of Oklahoma

22. 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
PIC Michael Strauss The University of Oklahoma

23. Experimental Differential Cross Section
Theoretical cross section
Physics variables are q and x
Detector measures ET and h
Counting experiment with detector inefficiencies
PIC Michael Strauss The University of Oklahoma

24. CDF Inclusive Jet Cross Section
0.1 < |h| < 0.7
Complete c2 calculation
PRD 64, (2001)
PIC Michael Strauss The University of Oklahoma

25. x-Q2 Measured Parameter Space
Q2 (GeV2 )
From D Inclusive Cross Section Measurement
PIC Michael Strauss The University of Oklahoma

26. 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)
PIC Michael Strauss The University of Oklahoma

27. D Inclusive Jet Cross Section
MRSTU includes no intrinsic parton transverse momentum and therefore has effectively increased gluon distributions at all x relative to MRST.
CTEQ4HJ
CTEQ4M
MRSTg
MRST
PIC Michael Strauss The University of Oklahoma

28. PIC 2001 Michael Strauss The University of Oklahoma
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
PIC Michael Strauss The University of Oklahoma

29. Inclusive Cross Section Using KT Algorithm
D = 1.0
D Preliminary
Predictions IR and UV safe
Merging behavior well-defined for both experiment and theory
PIC Michael Strauss The University of Oklahoma

30. 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
MRSTg
MRSTg
CTEQ3M
CTEQ4M
CTEQ4HJ
D Preliminary
PIC Michael Strauss The University of Oklahoma
Strauss The University of Oklahoma

31. 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)
PIC Michael Strauss The University of Oklahoma
Michael Strauss The University of Oklahoma

32. 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
PIC Michael Strauss The University of Oklahoma
Michael Strauss The University of Oklahoma

33. CDF as from Inclusive Cross Section
m scale is the major source of theoretical uncertainty
ET/2 < m < 2ET
PDF affects as
CTEQ4M minimizes c2
Theoretical uncertainties each ~ 5%
PIC Michael Strauss The University of Oklahoma

34. ZEUS Inclusive Jet Production
PIC Michael Strauss The University of Oklahoma

35. ZEUS Inclusive Jet Production
Measured cross section slightly above NLP pQCD in forward section
PIC Michael Strauss The University of Oklahoma

36. ZEUS Inclusive Jet Production
as Results:
Uses various fits of ds/dQ2 and ds/dET
Full phase-space
High-Q2 region (Q2>500 GeV)
High-ET region (>14GeVT)
Uses mu_R=ET, and mu_R=Q, uses MRST99 and CTEQ4
PIC Michael Strauss The University of Oklahoma

37. 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 > 20, 30, 40 GeV for <3 and ET > 20 GeV for <2
PIC Michael Strauss The University of Oklahoma

38. PIC 2001 Michael Strauss The University of Oklahoma
Inclusive R32
Features:
Rapid rise HT<200GeV
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
PIC Michael Strauss The University of Oklahoma

39. R32 Sensitivity to Renormalization Scale
ET>20 GeV, h<2 show greatest sensitivity to scale
PIC Michael Strauss The University of Oklahoma

40. PIC 2001 Michael Strauss The University of Oklahoma
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
R=0.6ETmax, for 20 GeV thresholds
R= HT, .3 for all criteria
Introduction of additional scales unnecessary.
ET>20 GeV, h<2
PRL 86, 1955 (2001)
PIC Michael Strauss The University of Oklahoma

41. 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
0.0 xT
0.4
Naive Parton model
PIC Michael Strauss The University of Oklahoma

42. D Inclusive Cross Section
s = 1800 GeV
s = 630 GeV
PIC Michael Strauss The University of Oklahoma

43. PIC 2001 Michael Strauss The University of Oklahoma
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)
PIC Michael Strauss The University of Oklahoma

44. PIC 2001 Michael Strauss The University of Oklahoma
CDF DiJet
Provides precise information about initial state partons
Cone of R=0.7
Both Jets: ET>10 GeV
Jet 1: 0.1<|h|<0.7
Jet 2: Four h regions
0.1<|h|<0.7
0.7<|h|<1.4
1.4<|h|<2.1
2.1<|h|<3.0
PIC Michael Strauss The University of Oklahoma

45. CDF DiJet Cross Section
PDF c2/dof
MRST 2.68
MRST 3.63
MRST 4.49
CTEQ4M 2.88
CTEQ4HJ 2.43
Mrstd has average kt of 0.64 GeV
All < 1% Probability
PIC Michael Strauss The University of Oklahoma

46. PIC 2001 Michael Strauss The University of Oklahoma
ZEUS DiJet
kT algorithm used
ET > 8 GeV (leading)
ET > 5 GeV (other)
-1<h<2 (leading)
470<Q2<20000 GeV2
Phys Lett B507, 70 (2001)
PIC Michael Strauss The University of Oklahoma

47. PIC 2001 Michael Strauss The University of Oklahoma
ZEUS DiJet
R2+1 parameterized as:
R2+1 (MZ) = A1as(MZ) + A1as2(MZ)
PIC Michael Strauss The University of Oklahoma

48. PIC 2001 Michael Strauss The University of Oklahoma
ZEUS as Summary
Dijets has lowest total error of all Zeus measurements.
All measurements consistent with PDG value of ±20
Cdf was / / plus theory
PIC Michael Strauss The University of Oklahoma

49. Great reach at high x and Q2, A great place to look for new physics!
Tevatron Run II
Run II: Ecm = 1.96 TeV, L  2fb-1
expect: ~100 events ET > 490 GeV
and ~1K events ET > 400 GeV
Run I: Ecm = 1.8 TeV, L  0.1fb-1
yielded 16 Events ET> 410 GeV
Great reach at high x and Q2,
A great place to look for new physics!
PIC Michael Strauss The University of Oklahoma

50. 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 PDF’s
Cross section measurements will continue to refine PDF’s
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
PIC Michael Strauss The University of Oklahoma