Unki Yang

1
Top Mass Measurementin the leptonjets channel
at CDF
Un-ki Yang University of Chicago For the CDF
collaboration
Wine-Cheese Seminar April 12, 2005
Healthy food to make the Tevatron Physics
program strong!
2
New CDF Result on Top Mass in LeptonJets channel
with 318pb-1

• Better than RunI world average

Analysis Information http//www-cdf.fnal.gov/phys
ics/new/top/top.htmlMASS
3
Outline

• Physics with top mass?
• CDF experiment
• Mtop measurement using the template method
• Mtop measurement with improved jet energy scale
  using in tt events
• Conclusions

4
Why is Top Mass so special?

• Top mass is a fundamental SM parameter
• important in radiative corrections
• Yukawa coupling 1

• Together with Mw and other electroweak precision
• measurements, it constrains MHiggs
• A key to understand electroweak symmetry
  breaking
• Constraint on SUSY models

5
Production of the Top quark at the Tevatron
Top quarks are primarily produced in pairs
(s7pb), via qq (85) and gg(15), - LHC via
gg (90) ttop 4 x 10 -25 s (due to large
mass) Top decays as free quark (L-1 (200MeV)
-1 10 -23 s)

• Dilepton (5, small background)
• 2 high-PT leptons(e/m), 2 b jets,
  large missing ET
• LeptonJet (30, manageable bgrnd)
• 1 high-PT lepton(e/m), 4 jets (2 b jets),
  large missing ET(30)
• All-hadronic (44, large background)
• 6 jets (44)

6
Top mass measurements (by 2004)

• Consistency among different channels? Are we
  looking for same top? Non-SM decays make
  different (like t-gtbH)
• All of previous RunII measurements havent
  reached to RunI precision level yet even with
  more data, bur our RunII goal dMtop is about 2
  GeV
• What are the challenges
• against our goal in RunII?

Matrix Element Method (DLM) WCheese, June
11, 2004 Todays talk Template method in
LeptonJets
7
Challenges of Mtop Measurement
LeptonJets Channel

• Leading 4 jets combinations
• 12 possible jet-parton assignments
• 6 with 1 b-tag (b-tag helps)
• 2 with 2 b-tags
• Poor jet energy scale and resolution
• Hard to find the correct combination

Observed Final state Complicated final state
to reconstruct Mtop
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Challenges of Mtop Measurement
LeptonJets Channel

• Leading 4 jets combinations
• 12 possible jet-parton assignments
• 6 with 1 b-tag (b-tag helps)
• 2 with 2 b-tags
• Poor jet energy scale and resolution
• Hard to find the correct combination
• Extra jets from initial/final state gluons
• Only 50 of the time, leading 4 jets correspond
  to 4 partons (qqbb)

Observed Final state Complicated final state
to reconstruct Mtop
Good b-tagging and jet energy scale and
resolution and good algorithm to reconstruct Mtop
9
CDF at Tevatron
Multi-purpose detector precision meas. search
for new physics

• Silicon detector (SVX)
• top event b-tag 55
• COT drift chamber
• Coverage hlt1
• sPt / Pt 0.15 PT
• Calorimeters
• Central, wall, plug
• Coverage hlt3.6
• EM sE / E 14 /ÖE
• HAD sE / E 80 /ÖE
• Muon scintillatorchamber
• muon ID up-to h1.5

SVX
EM cal
Muon
COTtracking
Had cal
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Top Mass Analysis using the template method

• c2 mass fitter
• Finds top mass that fits event best
• One number per event
• Additional selection cut on resulting c2

Data
Wbb MC
Massfitter
tt MC
Signals/background templates
Datasets
Data
Likelihoodfit
Result
Likelihood fit Best
signal bkgd templates to fit datawith
constraint on background normalization
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Event Selection

• Final State (LeptonJet channel) lepton,
  neutrino plus 4jets
• Datasets (Mar, 2002 Aug. 2004)
• 318pb-1 for High-pt central electron/muon
  triggers
• High pt electron or muon with Pt gt 20 GeV
• Isolated
• Electron EM cluster in calorimeter with
  matched track
• Muon track matched to hits in muon
  chambers,
• MIP ionizing energy in
  calorimeter
• Large missing Et gt20 GeV
• Leading 4 jets
• Reconstructed with cone algorithm (0.4) using
  calorimeter towers
• hlt2.0

12
B-tagging and Sample Division

• Signal/Background improvement
• (only 1-2 of Wjets contains heavy flavor)
• Big help to remove wrong choice in jet-parton
  assignment for Mtop
• Samples are divided to get the best sensitivity
  on top mass

Top Event Tag Efficiency 55 False Tag Rate (per
jet) 0.5
13
Jet Energy Correction
Determine true parton E from measured jet E in
a cone 0.4
Non-uniform response
Correction to central region using dijet balance
to make response uniform in ?
Correction to particle jets using dijet MC tuned
for single particle E/P, material, and
fragmentations due to non-linear and
non-compensating cal.
Diff. resp. of p0/pi- Non-linearity
Out-of-Cone correction to parton top-specific
correction to light quark jets and b-jets
separately
Shower, frag.
14
Top specific corrections

• Separate top-specific corrections are applied to
  b-quark jets and
• light-quark jets(from W), because of
  different response of
• b-parton and light parton.

15
Jet Energy Systematics
A lot of work has been done to reduce the syst.
from jet-energy scale (a factor of two
improvement compared to last year). The new Run
II systematic uncertainties are at the same level
or better than Run I.
16
Test of the Jet Energy Corrections
0.1
0.1
-0.1
-0.1
PT
80
40
40
80
PT
Photonjets, di-jet, Zjets are used to
cross-check the jet energy corrections. Observed
differences between data, Pythia, and Herwig are
contained by the jet systematic uncertainties in
different ? regions.
17
Mass Fitter (event by event)

• Try all jet-parton assignments with kinematic
  constraints, but assign b-tagged jets
  to b-partons
• Select the rec. mass Mt from the choice of lowest
  c2
• Badly reconstructed Mt (c2 gt 9 ) is removed

Top mass isfree parameter
All jets are allowed to be float according to
their resolutions to satisfy that
M(W)M(W-)80.4 GeV, M(t)M(t)
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Reconstructed Top Mass Dist. at Mtop 178 GeV
More correct combination with b-tag
Mt(GeV/c2)
Mt(GeV/c2)
Mt(GeV/c2)
Mt(GeV/c2)
Bkgd is large in the 0-tag
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Signal templates

• Samples Herwig with
• Mtop 130 to 230 GeV
• Get analytical functions
• (2 Gaussian gamma)
• of reconstructed mass, Mt
• as a function of true mass, Mtop
• Fit parameters linear depend.
• on Mtop

Smooth PDFs (Mt true Mtop)
Mt(GeV/c2)
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Backgrounds
gt1-btag

• Wheavy flavor jets(bb,cc,c)
• Heavy flavor fraction from MC
• Normalized to data
• Wjets(mistag)
• Use measured mistag rate, applied to the data
• Multijetfake-W (jet-gte, track-gtm)
• Estimated from data
• Single top, dibson (WW,WZ)
• Estimated from MC

control
signal
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Background templates
Mt(GeV/)
Mt(GeV)

• 0tag Wjets
• Tagged WHF, Mistag, fake-W, Single-top
• Shape mostly by ALPGEN MC, cross-check with data

22
Comparison of c2 distributions
2tag
1tagT
1tagL
0tag
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Combined Likelihood

• Bkgd constraints on tagged samples,
• but no constraint on 0-tag sample

• Data was blinded until top mass fitter was fully
  tested
• And major syst. evaluation were done

24
Sanity Checks
Mtop pull mean

• Pseudo-exps are performed to check any bias in
  the mean and error of Mtop
• No bias appear. Pull width 1.03 is due to
  non-Gaussian tail of the Likelihood

Min vs Mout
Mtop pull width
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Open the box!!! Result on Mtop
Comb. Log Likelihood
Expected error
The best single measurement
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-Log Likelihoods
Sensitivity mainly comes from 2tag and 1tagT
samples
27
Systematic Errors
Can we improve jet systematic?
28
Systematic Errors
Though we find that 70 of JES uncertainty comes
from b-jet, B-jet uncertainty are mainly due to
generic jet corrections.
29
Jet Energy Scale (JES)

• Unique way to obtain JES in tt events.
• Do a cross-check for our standard JES calibration
  obtained in dijet, photonjet environment
• With combined information on JES from W --gt 2
  jets and standard calibration, we improve top
  mass resolution

Mjj(W)
Use W -gt2 jets to calibrate JES(fully in situ
) This scale is applied to b-jets and light-quark
jets
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JES and Mtop measurements- 2D Template Analysis -

• Simultaneous fit to JES and Mtop
• Templates Mt ( true Mtop, JES), Mjj ( true
  Mtop, JES)

Mjj templates
Mt templates

• Use same c2 fit
• Build templates for various Mtop, JES
• - Mtop same mass range
• - JES -3s to 3s 1s defined by the
  standard JES uncert. (3 in JES uncert., 3 GeV
  in Mtop)
• Obtain PDFs (Mt Mtop, JES)

• All pairs of non b-tagged jets from 4 leading
  jets are used. No c2 cut
• Templates are built in the same way Mt templates
  are made.
• Obtain PDFs (Mjj Mtop,JES)

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Mjj templates (true Mtop, JES)
PDFs( Mjj JES, Mtop 180 ) 1tagL
PDFs ( Mjj Mtop, JES0)2tag
Mtop
Mjj
Mjj

• Mjj strongly depends on JES,
• but independent of Mtop

Mjj
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Mt templates (true Mtop, JES)
PDFs ( Mt JES, Mtop180)1tagT
PDFs ( Mt Mtop, Mtop180)1tagT
Mtop
Rec Mt
Rec Mt

• Mt strongly depends on JES and Mtop

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JES and Mtop Fit

• Mtop and JES are simultaneously determined in
  likelihood fit using shape comparisons of Mt and
  , Mjj distributions, taking correlations between
  them
• Mjj sensitive to JES, but mostly independent of
  Mtop
• Mt sensitive to both JES and Mtop
• For optimal performance use combined information
  on JES from W-gtjj and standard calibration
• A Gaussian constraint on JES from the standard
  calibration is included in likelihood as a priori
  
• All other things are same as the 1-D template
  analysis

34
Sanity Checks
JES0
Mtop 180 GeV
Small bias in Mtop mean will be taken as a a
part of method syst.
35
Sanity Checks
JES0
Mtop 180 GeV
Uncertainties on Mtop JES will be scaled by
1.027 1.013
36
Results on the Mtop from the 2-D Fit
Comb. Log Likelihood
Expected error
37
Results on the JES from the 2D Fit
Comb. Log Likelihood
Expected error
38
Summary on the 2-D Fits

• Combined Mtop and JES Fit
• 20 improvement in JES
• Compared with 1-D Template Fit

Log Likelihood vs Mtop, JES
JES(s)
Mtop (GeV)
39
Various Cross-Checks
All measurements are consistent each other
40
Summary on syst. errors
41
How to control ISR?

• In Run I, switch ISR on/off
• using PYTHIA, dMtop 1.3GeV
• In Run II systematic approach
• ISR/FSR effects are governed
• by DGALP evolution eq.
• ltPtgt of the DY(ll) as a function of Q2

qq -gt tt vs mm-
(2Mt)2
log(M2)
42
Conclusions
Conclusions Outlook
2-D Template
1-D Template

• The worlds best measurement has been made in the
  leptonjets channel at CDF
• Our syst. due to JES and bkgd shape are expected
  to be improved soon

43
Backup Slides
44
Tevatron Performance
45
Signal templates

• Gamma 2 Gaussian
• each param linear in Mttotal 18 parameters
• Top Mass templates using HERWIG MC samples (31
  diff. top masses from 130 to 230 GeV )

46
Bkgd templates (Wbb/cc/c)
47
Multi-jets vs Mistag
48
Backgrounds with c2 lt9
49
Jet Energy Systematics
Syst. uncert.() on relative corr. vs h
Frac. Syst. uncert. vs Pt
2
RunII
RunII
RunI
-2
RunII 2004
RunI
Central region
Central region
A lot of work has been done to reduce the syst.
from jet-energy scale (a factor of two
improvement compared to last year). The new Run
II systematic uncertainties are at the same level
or better than Run I.
50
Jet Energy Systematics
51
1D Fit results
52
Cross-check on the JES using only Mjj (Mtop178
GeV)
Comb. Log Likelihood
Expected error

• Fitted JES shows a good agreement with the
  standard JES

53
Other Systematics
54
Other Systematics
55
Prospects

• More top mass results will come within two months
• LeptonJets using Matrix Element
• Dilepton using Matrix Element, neutrino weighting
  
• Dedicated studies on b-jet energy scale using
  Z-gtbb, photon-bjets are underway.
• Z(bb) peaks in the right place withing 1
• Another WC seminar
• on MtopXXX 2 GeV
• not far away!!!

Di-bjet mass spectrum