Title: Recent Results on Jet Physics and as
1Recent Results on Jet Physics and as
- XXI Physics in Collision Conference
- Seoul, Korea
- June 28, 2001
- Presented by
- Michael Strauss
- The University of Oklahoma
2Outline
- 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
3Motivation 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 ?
4Developments in Jet Physics
(with proton initial states)
Inclusion of error estimates in the PDFs
5Cone 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)
6kT 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
7kT 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
8kT Algorithm and Subjets
For subjets, define large kT
(ycut 10-3)
Increasing ycut
9Jet Selection Criteria
Typical selections on EM fraction, hot cells,
missing ET, vertex position, etc. gt 97
efficient gt 99 pure
10Jet 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
11Jet and Event Quantities
- Low ET Multijet Studies
- Subjet Multiplicity
12D? 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
13D? 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
14D? 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
15D? 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
16D? 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
17D? 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
18ZEUS Subjet Multiplicity
- Comparison at hadron level
- Unfolded using Ariadne MC
NLO QCD describes data Sensitive to as
19as 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
20Cross Sections
- Inclusive cross sections
- Rapidity dependence
- KT central inclusive
- R32
- 630/1800 ratio of jet cross sections
- Di-Jets
- as Conclusions
21Jet 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?
22CTEQ 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
23Experimental Differential Cross Section
Detector measures ET and h
24CDF Inclusive Jet Cross Section
- 0.1 lt h lt 0.7
- Complete c2 calculation
PRD 64, 032001 (2001)
25x-Q2 Measured Parameter Space
From D? Inclusive Cross Section Measurement
26D? 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)
27D? Inclusive Jet Cross Section
CTEQ4HJ CTEQ4M
MRSTg? MRST
28Gluon PDF Conclusions
- c2 determined from complete covariance matrix
- Best constraint on gluon PDF at high x
- Currently being incorporated in new global PDF
fits
29Inclusive 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
30Comparison with Theory
- Normalization differs by 20 or more
- No statistically 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
31Further kT Developments (since PIC 2001)
Detected Particles
Hadronization
Jet
Single parton
With hadronization correction PDF c/dof
Prob MRST 0.86 71 CTEQ4M 1.06 44
Parton
Correction for hadronization explains low ET
behavior
32CDF 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
33CDF 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
34CDF 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
35ZEUS Inclusive Jet Production
36ZEUS Inclusive Jet Production
Measured cross section slightly above NLP pQCD in
forward section
37ZEUS Inclusive Jet Production
as Results Uses various fits of ds/dQ2 and ds/dET
Full phase-space High-Q2 region (Q2gt500
GeV2) High-ET region (gt14 GeV)
38R32 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
39Inclusive 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
40R32 Sensitivity to Renormalization Scale
ETgt20 GeV, hlt2 show greatest sensitivity to scale
41R32 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)
42D? 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
43D? Inclusive Cross Section
?s 1800 GeV
?s 630 GeV
44Cross 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)
45CDF 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
46CDF 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
47ZEUS DiJet
kT algorithm used
- ET gt 8 GeV (leading)
- ET gt 5 GeV (other)
- -1lthlt2 (leading)
- 470ltQ2lt20000 GeV2
Phys Lett B507, 70 (2001)
48ZEUS DiJet
R21 parameterized as R21 (MZ) A1as(MZ)
A1as2(MZ)
49ZEUS as Summary
- Dijets has lowest total error of all Zeus
measurements. - All measurements consistent with PDG value of
0.118520
50Tevatron 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!
51Conclusions 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