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Some theoretical issues regarding Method 2. J. Huston Michigan State University. History. Based on a series of talks that I gave at Lepton + Jets meetings May 21, 2004 Nov. 7, 2003 Oct. 10, 2003 April 25, 2003 Plus a paper that I wrote with John Campbell - PowerPoint PPT Presentation
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Top properties workshop 11/11/05
Some theoretical issues regarding Method 2
J. Huston
Michigan State University
Top properties workshop 11/11/05
History
Based on a series of talks that I gave at Lepton + Jets meetings May 21, 2004 Nov. 7, 2003 Oct. 10, 2003 April 25, 2003
Plus a paper that I wrote with John Campbell “Heavy Flavor in W + Jets Production at the
Fermilab Tevatron” hep-ph/0405276 Phys. Rev. D70:094021,2004
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Top properties workshop 11/11/05
Method 2 In Method 2, the calculated
ratio for WbBj(j) to Wjjj(j) is used, along with the measured Wjjj(j) cross sections to estimate the heavy flavor background to top production
The ratio is calculated at LO and then multiplied by a phenomenological factor of 1.5 for use in background subtractions
It would be nicer to put this aspect of the analysis on a firmer basis
NLO is firmer than LO
The state of the art for NLO calculations is WbB and Wjj although recent advances
make the 1 loop calculation of WbBj and Wjjj realistic…but don’t hold your breath
WbB and Wjj are available now in MCFM; may give guidance
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Top properties workshop 11/11/05
Scale dependence
As expected, scale dependence lessened at NLO
More so for exclusive final states than inclusive
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Top properties workshop 11/11/05
K-factors
K-factors depend both on scale choice and pT cuts on jets
Ratio of K-factors (WbB/Wjj) is reasonably stable for scales of MW/2 and above
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Top properties workshop 11/11/05
Wbb/Wjj ratioVery dependent on
kinematics (jet pT) at LOMore stable at NLORatio is higher for
inclusive final states than for exclusive final states
NLO prediction for exclusive ratio is in agreement with ratio assumed in Method 2 analyses (with factor of 1.5 applied) but note that LO and NLO
predictions in MCFM are reasonably close for a jet cut of 15 GeV/c
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Top properties workshop 11/11/05
HT
Look at cross sections for WbB(j) and Wjj(j) as a function of the HT variable
Distributions look similar at LO but not at NLO
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Top properties workshop 11/11/05
Ratios
So it appears that cross sections using very exclusive variables like HT may not have the same shape at LO and NLO so this assumption
should not be made in analyses
But inclusive variables like the lead jet pT are safer
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Top properties workshop 11/11/05
PDF uncertainties
PDF uncertainties easily calculated using weight technique
Note talk given by Craig Group at TeV4LHC meeting in Oct on LHAPDF
Easier to use the weight technique to calculate pdf uncertainties in version 5
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Top properties workshop 11/11/05
Calculation of pdf uncertainties
Note that CDF webpage seems to be recommending option 3
Option 4 is preferred
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Top properties workshop 11/11/05
LO vs NLO pdf’s for parton shower MC’s
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
For NLO calculations, use NLO pdf’s (duh) What about for parton shower Monte
Carlos? somewhat arbitrary assumptions (for
example fixing Drell-Yan normalization) have to be made in LO pdf fits
DIS data in global fits affect LO pdf’s in ways that may not directly transfer to LO hadron collider predictions
LO pdf’s for the most part are outside the NLO pdf error band
LO matrix elements for many of the processes that we want to calculate are not so different from NLO matrix elements
by adding parton showers, we are partway towards NLO anyway
any error is formally of NLO (my recommendation) use NLO pdf’s
pdf’s must be + definite in regions of application (CTEQ is so by def’n)
Note that this has implications for MC tuning, i.e. Tune A uses CTEQ5L
need tunes for NLO pdf’s …but at the end of the day this is still LO physics;There’s no substitute for honest-to-god NLO.
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Top properties workshop 11/11/05
Rick Field at TeV4LHC: CTEQ6.1 Tune
"Transverse" Charged Particle Density: dN/d ηdφ
0.0
0.2
0.4
0.6
0.8
1.0
0 50 100 150 200 250 300 350 400 450 500
( #1) ( / )PT particle jet GeV c
" " Transverse Charged Density1.96 TeV
RDF Preliminary generator level
(|Charged Particles η|<1.0, >0.5 / ) PT GeV c
PY Tune A5CTEQ L
" "Leading Jet
PY Tune Q61CTEQ
PY Tune A61CTEQ
"Transverse" PTsum Density: dPT/d ηdφ
0.0
0.4
0.8
1.2
1.6
0 50 100 150 200 250 300 350 400 450 500
( #1) ( / )PT particle jet GeV c
" " ( / ) Transverse PTsum Density GeV c1.96 TeV (|Charged Particles η|<1.0, >0.5 / ) PT GeV c
RDF Preliminary generator level
" "Leading Jet
PY Tune A5CTEQ L
PY Tune Q61CTEQ
PY Tune A61CTEQ
I used LHAPDF! See the next talk by Craig Group!
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Top properties workshop 11/11/05
W + jets at NLO
Currently are working (B. Cooper, A. Messina, D. Waters, J. Dittmann) to obtain W + jets cross sections that can be compared directly to NLO correct for UE and
hadronization but not out of cone
Hadronization+UE corrections for 0.4 cone different than for 0.7 cone (used for inclusive jet analysis) from Ken Hatakeyama
R=0..4 R=0.7
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Top properties workshop 11/11/05
Tevatron W + jets studies
We can’t help with the VBF Higgs discovery channel at the Tevatron but we can look at the rates for central jet emission in W/Z + jet(s) events
Cross section larger for W + jets so that is primary investigation
Will compare measured cross sections to LO +PS predictions and to fixed order (LO and NLO) predictions from MCFM
In particular, are interested in comparing to CKKW cross sections generated by Steve Mrenna
Predictions will be extrapolated to the LHC
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Top properties workshop 11/11/05
Comparisons
Look at probability for 3rd jet to be emitted as a function of the rapidity separation of the tagging jets
Relatively flat probability, stable with CKKW scale (not shown)
Bracketed by two predictions for MCFM using mW and <pT
jet> as scales MCFM predicts a slight
decrease in the ratio as the tagging jet rapidity separation increases
CKKW and ALGEN+Herwig 3p agrees with the data ALPGEN+Herwig 3p too high
for this kinematic region
MCFM mW
MCFM pTjet
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Top properties workshop 11/11/05
Summary
NLO predictions for Method 2 are more reliable than LO HT is not a good variable to use if you’re expecting
similar behavior at NLO (i.e. real world) as at LO of course, best numbers for heavy flavor fractions
are given by direct measurement; it will be interesting to compare these to the theory predictions and to better understand the factor of 1.5
A lot of theory tools out there (and more on the way); up to use to make use of them
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