measuring properties of top quark; is it really top of sm?
DESCRIPTION
Measuring properties of top quark; Is it really top of SM?. Yen-Chu Chen Institute of Physics Academia Sinica. Content. Introduction Production of top quark Properties of top quark: Understanding the top quark Or is it really the top of the Standard Model?. The Tevatron. Chicago. CDF. - PowerPoint PPT PresentationTRANSCRIPT
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Measuring properties of top quark;Is it really top of SM?
Yen-Chu Chen
Institute of Physics
Academia Sinica
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Content
• Introduction
• Production of top quark
• Properties of top quark:– Understanding the top quark– Or is it really the top of the Standard Model?
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Tevatron Ring( ~4 miles)!
Main injector
Chicago
CDF
D0
• pp collisions at √s = 1.96 TeV!• Already 4 fb-1 recorded!• Expect to collect
6 fb-1 by the end of 2009! ( 8 fb-1 by the end of 2010)
The Tevatron
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Muon stations
Calorimeter
Silicon detector
COT
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Top quark physics
P Pt
tX
b
b
W
W
l
q
l
q
q'_
q'
__
Production Cross section
Top spin
Rare decays, charged Higgs, etc.
W helicity
Top charge
Top mass
Charge asymmetry
Life time, decay width
Production mechanism
FCNC
Br(t→Wb)/Br(t→Wq)
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Sample separation
• Di-lepton (DIL) channel:– both W decay to leptons,– in practice uses only e,μ.
• Lepton+Jets (LJ) channel: – one W decays to leptons,– the other decays to quark
s.
• Hadronic channel:– both W decay to quarks.
t
t
W +
W ̅
b
b
e+, μ+, u, c
e –, μ–, u, c
_
_ _
e, μ , d, s _ _
e, μ , d, s _ _
_
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Production of top quark
• Production mechanism
• Production cross section
• Forward backward asymmetry
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Production of top quark• In the SM: 6.7 pb (mt = 175 GeV/c2)
• Single top production:
t-channelt-channel1.98 ± 0.21 pb1.98 ± 0.21 pb
VVtbtbb
q q
t
s-channels-channel0.88 ± 0.07 pb0.88 ± 0.07 pb
VVtbtb
tq
bq
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Production mechanism
• Di-lepton events; using data of 2 fb-1
=azimuthal correlation of the two leptons
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• Fit result (DIL, 2 fb-1): Fgg = 0.53 +0.36 -0.38 (+0.35 -0.37(stat) +0.07 -0.08(sys))
• Previous result (L+J, 1 fb-1):
Fgg = 0.07± 0.14(stat)± 0.07(syst)
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Production cross section• DIL, 2.8 fb-1
= 6.67 ± 0.77(stat) ± 0.43(sys) ±0.39(lum) pb = 7.81 ± 0.92(stat) ± 0.68(sys) ± 0.45(lum) pb
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Single top production
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Forward backward asymmetry
• Afb = 0.17 ± (0.07)stat ± (0.04)syst• To be compared with theory: Afb = 0.04 ± 0.01
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Properties of top quark• Lift time
– Expected: ~10-25 sec
• Decay width• Decay mode• Mass and its implication
– Expected: none, a free parameter in the SM
• W helicity in top decay– Expected: 70% LH, 30% Lo, ~0 RH
• Charge– Expected: +2/3
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Life time
ct < 52.5 m @ 95% CL
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Decay width
top < 12.7 GeV @ 95% CL
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Decay mode
• Br(t→Wb)/Br(t->Wq) = 1.12 +0.21 –0.19(stat) +0.17 -0.13 (sys) (160 pb-1)
• Br(t→Zq) < 3.7% (1.9 fb-1)
• Br(t→Zc) < 13% (1.9 fb-1)
• Br(t→gc) < 12% (1.9 fb-1)
• Br(t→c) < 18% (1.9 fb-1)
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Mass of the top quark
Tevatron Top mass, 2007 Mar.
• Reaching below 1% uncertainty!
• CDF and D0 are working together on the common systematic issues to reduce uncertainty.
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MTM method (CDF, 2.7 fb-1)
• log Lsig(mt, JES) = Σi[log Li(mt, JES)] - nbg log Lavg(mt, JES | background)
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• mt = 172.2 ± 1.0 (stat.) ± 0.9 (JES) GeV/c2
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W helicity in top decay
• (1/Γ) (dΓ/dcosθ*) = f- (3/8) (1- cosθ*)2
+ f0 (3/4) ( 1 - cosθ* 2 )
+ f+ (3/8) ( 1 + cosθ* )2 ,
• θ* is the decay angle of lepton in the W rest frame with respect to the W direction in the top rest frame
• f0 = 0.703, f-=0.297, f+=3.410-4
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• f0 = 0.65 ± 0.19 ± 0.03
• f+ = -0.03 ± 0.07 ± 0.03
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Combination in CDFfor W helicity studies
• f0 = 0.62 ± 0.11 assuming f_+ = 0.0 (the SM value)
f+ = -0.04 ± 0.05 assuming f_0 = 0.7 (the SM value)
• f0 = 0.66 ± 0.16 and f+ = -0.03 ± 0.07,
correlation coeff = -0.82 with no assumption
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Charge of the top quark
• The particle we have been called “top” for more than ten years could be a top like quark from an exotic model. – D. Chang, and E. Ma, Phys. Rev. D58, 097301 (1998).
– D. Chang, W.-F. Chang, and E. Ma, Phys. Rev. D59, 091503 (1999);
• The only way to tell is the charge of the top quark, all the other properties are the same:– SM : +2/3, XM : -4/3
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To get result of top chargein DIL samples
• Study top charge based on – jet charge calculation and
– two kinds of pairing methods: MlbsMax and KIN
• Using single b tagged samples to reduce the background.
• Taking the part of not b tagged samples as control region. (Higher contamination, lower purity)
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Using jet charge to identify b flavor- common in both LJ and DIL -
Pjet
pi Qi
Additional tracks from the particles interacting with the detectors could cause bias, since they are mostly protons and !
=> pt > 1.5 GeV/c, |d0| < 0.15 cm
, α = 0.5
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b quark charge from jet
• Assume jet charge symmetry:
• If > 0 → ; < 0 →
• Purity of identifying jet charge correctly:
• PYTHIA, Top mass 175 GeV/c2 (ttop75) : – = 0.601 ± 0.007; = 0.614 ± 0.007 (DIL)
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Jet charge from di-jet ( trigger) data• Calculate of the away jet:
– Nos : number of opposite charge sign jets with respect to the lepton
– Nss : number of same sign jets
• Found b fraction in jets.– There are also c decay, b mixing and backgrou
nd.
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• Purity in di-jet data analysis is consistent with jet charge from Top MC.
• Take purity from MC study and apply the scale factor (SF) to take care of the difference between data and MC.
• SF = 1.01 ± 0.01 ± 0.03
• No difference seen between jet charge from different muon charge sign samples!
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Pairing of lepton and b in DIL
• MlbsMax :– Calculate the invariant mass of lepton + jet paired
– For wrong pairing this value tends to go larger than the correct pair.
– Reject the pairing having the largest Mlb2 of the four possibl
e pairs.
– Make cut on the maximum Mlb2 to increase the purity.
• KIN :– Take the top mass as input and resolve the kinematic equati
on set.
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Pairing using MlbsMaxEfficiency
Purity
Mlbs max cut
Mlbs max cut
• correct pairing
• wrong pairing
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Efficiency and purity of MlbsMax
• Pythia MC
• Based on ttop75, top mass 175 GeV/c2
• Mlb2 max cut at 21K
• Efficiency = 0.395 ± 0.004
• Purity = 0.948 ± 0.002
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Pairing using KIN
• Assuming top mass to resolve the equation set of energy momentum conservation.
• Details described in:– CDF note 8638, page 51.
– Presentation at top property meeting, 2008/08/22, Yen-Chu Chen
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Brief review of KIN pairing• For given lepton and jet momenta, try to resolve the equation s
et via Newton’s method.
• Multiple solutions could be found
– Pick the one having smaller mtt2
• Two possible way of pairing
– Pick the one having higher reconstruction probability
• Probability of miss-reconstruction is higher in cases similar to RH W helicity than the cases similar to LH W helicity.
• The nature is mostly LH (30%) and LO (70%).
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Efficiency and purity of KIN
SM
(Pythia)
SM
(Herwig)
LH
(GGwig)
LO
(GGwig)
RH
(GGwig)
t t rec. eff. 0.97 ± 0.02 0.97 ± 0.02 0.98 ± 0.02 0.95 ± 0.01 0.87 ± 0.01
Pairing purity 0.72 ± 0.01 0.72 ± 0.01 0.73 ± 0.01 0.73 ± 0.01 0.67 ± 0.01
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MlbsMax vs. KIN
Efficiency Purity eD2
KIN pairing 0.97 ± 0.02 0.72 ± 0.01 0.19 ± 0.02
Mlb2 pairing with
cut at 21K
0.395 ± 0.004
0.948 ± 0.002
0.317 ± 0.004
Choose MlbsMax as nominal and KIN for cross check!
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Study of top charge, MC only• Basic formula to get final top charge :
• Final form:
Ptq = fs * Ps + fnb * Pnb + fLJ * PLJ + fbk * Pbk
The Four components:– Signal: true (l, b) pair from top decay– None b: the jet used in pair is not b– LJ faking DIL: L+J in HEP but identified as DIL– General DIL background,
• FAKE, DY, WW, WZ, ZZ
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DIL Lumi. Scaled b tgd. eff. b tgd events Not b tgd events.
Total 143.3 ± 13.1 134.6 ± 12.3
t /t 94.9 ± 7.1 88.1 ± 6.7 0.598 ± 0.003
52.8 ± 4.0 35.4 ± 2.7
WW 6.8 ± 1.2 6.4 ± 1.1 0.034 ± 0.007
0.22 ± 0.06 6.18 ± 1.06
WZ 1.6 ± 0.3 1.5 ± 0.2 0.022 ± 0.006
0.03 ± 0.01 1.46 ± 0.24
ZZ 1.1 ± 0.9 1.0 ± 0.8 0.067 ± 0.013
0.07 ± 0.06 0.95 ± 0.74
Wr 0.17 ± 0.18 0.16 ± 0.17 0.0 0.0 0.16 ± 0.17
DY-> 5.3 ± 1.0 4.9 ± 1.0 0.028 ± 0.009
0.14 ± 0.05 4.8 ± 0.93
DY->ee, 12.8 ± 2.2 12.0 ± 2.0
ztopzb 0.028 ± 0.006
0.14 ± 0.04 4.74 ± 0.80
ztopzt 0.039 ± 0.005
0.19 ± 0.04 4.69 ± 0.80
xtoppb 0.057 ± 0.011
0.13 ± 0.03 2.12 ± 0.36
FAKES 21.8 ± 6.3 20.4 ± 5.9 0.34 ± 0.17 6.9 ± 3.9 13.51 ± 5.17
btop1w 0.42 ± 0.06
btop2w 0.35 ± 0.09
LJ 0.37 ± 0.01 1.2 ± 0.1 2.1 ± 0.3
Event/pair estimation
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Efficiencies of jet charge cal.
• Jet charge calculation requires good tracks. The efficiencies of applying jet charge calculation are studied based on MC for top signal and various background samples respectively.
• The jets being b tagged or not do make difference in term of efficiency.
ttop75 b matched Not b matched
b tagged 0.983 ± 0.001 0.968 ± 0.014
Not b tagged 0.706 ± 0.004 0.647 ± 0.009
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Estimation of (l, b) pair of top(b tagged, jet q applied)
b tgd evt. (l,b) pair estimate b tagged or not After jet q applied
top signal 52.7 ± 4.0 b jets 97.1 ± 7.4 b tgd 63.9 ± 4.9 b tgd 62.8 ± 4.8
Not b tgd 33.2 ± 2.5 Not b tgd 23.4 ± 1.8
Non-b jets
8.4 ± 0.7 b tgd 0.41 ± 0.05 b tgd 0.40 ± 0.05
Not b tgd 8.03 ±0.63 Not b tgd 5.2 ± 0.4
BKG
(not LJ)
6.6 ± 3.9 Non-b jets
13.2 ± 7.8 b tgd 6.59 ± 4.03 b tgd 6.6 ± 4.0
Not b tgd 6.59 ± 4.03 Not b tgd 5.1 ± 3.1
LJ faking DIL
1.2 ± 0.1 b jets 1.5 ± 0.1 b tgd 1.27 ± 0.12 b tgd 1.2 ± 0.1
Not b tgd 0.24 ± 0.03 Not b tgd 0.2 ± 0.03
Non-b jets
1.0 ± 0.1 b tgd 0.08 ± 0.02 b tgd 0.1 ± 0.02
Not b tgd 0.89 ± 0.09 Not b tgd 0.7 ± 0.1
Tot. evt/pair 60.6 ± 5.6 121.1 ± 10.8 105.6 ± 7.2
Data 61 122 105
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MlbsMax + jet chargenot b tagged samples
Signal Non-b bkg LJ
Fraction 0.395 ± 0.047 0.075 ± 0.010 0.517 ± 0.057 0.031 ± 0.005
Plb 0.920 ± 0.003 N/A N/A 0.74 ± 0.03
Pb 0.589 ± 0.005 N/A N/A 0.61 ± 0.04
Ptq (est) 0.575 ± 0.001 0.50 0.50 0.55 ± 0.02
Ptq (MC) 0.576 ± 0.006 0.52 ± 0.02 0.50 ± 0.1 0.49 ± 0.02
Ps (sig+LJ) = 0.570 ± 0.006
Pbkg (non-b + bkg) = 0.502 ± 0.012
These samples are dominated by backgrounds. •Using jets that are b matched.
•None b fraction = 0.61
•Assumed values
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MlbsMax + jet chargeb tagged samples
Signal Non-b bkg LJ
Fraction 0.78 ± 0.05 0.062 ± 0.006 0.14 ± 0.05 0.018 ± 0.002
Plb 0.932 ± 0.002 N/A N/A 0.67 ± 0.03
Pb 0.608 ± 0.005 N/A N/A 0.61 ± 0.04
Ptq (est) 0.593 ± 0.004 0.50 0.50 0.54 ± 0.01
Ptq (MC) 0.588 ± 0.004 0.51 ± 0.02 0.57 ± 0.08 0.52 ± 0.03
Ps (sig+LJ) = 0.586 ± 0.008
Pbkg (non-b + bkg) = 0.55 ± 0.07
•Using jets that are b matched.
•None b fraction = 0.38
•Assumed values
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Systematic uncertainties of MlbsMax
on b tagged samplesmore less
Uncertainty of jet charge calibration ± 0.007
PDF (± 0.34% of pairing purity) 0.0007 -0.0007
ISR/FSR -0.006 -0.002
JES ( ± 1 sigma) 0.001 -0.002
Top mass (175 on samples of 170) 0.003 ---
MC generator (Herwig – Pythia) -0.002
W hel. -0.001 (LH/Lo)
Ptq (Comb) = 0.584 ± 0.004 (stat) +0.008 -0.010 (sys)
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KIN + jet chargenot b tagged samples
Signal Non-b bkg LJ
Fraction 0.388 ± 0.047 0.073 ± 0.010 0.526 ± 0.056 0.033 ± 0.005
Plb 0.720 ± 0.003 N/A N/A 0.67 ± 0.02
Pb 0.593 ± 0.003 N/A N/A 0.60 ± 0.02
Ptq (est) 0.541 ± 0.001 0.50 0.50 0.54 ± 0.01
Ptq (MC) 0.543 ± 0.004 0.51 ± 0.01 0.50 0.53 ± 0.02
Ps (sig+LJ) = 0.546 ± 0.004
Pbkg (non-b + bkg) = 0.501 ± 0.009
•Using jets that are b matched.
•None b fraction = 0.59
•Assumed values
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KIN + jet chargeb tagged samples
Signal Non-b bkg LJ
Fraction 0.81 ± 0.04 0.064 ± 0.006 0.11 ± 0.04 0.021 ± 0.002
Plb 0.706 ± 0.002 N/A N/A 0.65 ± 0.02
Pb 0.614 ± 0.003 N/A N/A 0.61 ± 0.02
Ptq (est) 0.547 ± 0.001 0.50 0.50 0.53 ± 0.01
Ptq (MC) 0.544 ± 0.003 0.513 ± 0. 01 0.56 ± 0.06 0.58 ± 0.02
Ps (sig+LJ) = 0.545 ± 0.003
Pbkg (non-b + bkg) = 0.543 ± 0.066
•Using jets that are b matched.
•None b fraction = 0.37
•Assumed values
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Systematic uncertainties of KINb tagged samples
more less
Uncertainty of jet charge calibration ± 0.013
PDF (± 1.0% of pairing purity) 0.002 -0.002
ISR/FSR -0.010 -0.003
JES ( ± 1 sigma) 0.003 0.001
Top mass (175/170 on samples of 170/175) 0.002 0.001
MC generator (Herwig – Pythia) 0.002
W hel. 0.002 (Lo) -0.001(LH)
Ptq (Comb) = 0.545 ± 0.003 (stat) +0.014 -0.020 (sys)
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Result from 1.5 fb-1
• This combines both DIL and LJ.• Result shows good agreement with SM!• Exclude the XM at 87% CL.• But with 2 fb-1, which we have investigated for more than a year, the
result from DIL will come soon; final blessing next week!
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Is it really the truth/top of SM?
• Up to date most of the results are consistent with SM expectation.
• However unexpected result(s) might arise in near future!
• Experience learned at Tevatron is very important to LHC.
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Back up slides
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The issue in LJ faking DIL
• There is correlation between pairing and jet charge calculation!• The formula of top charge fails!
Pairing correct Pairing wrong
Jet q correct
Jet q wrong
Jet q purity
Pairing correct 0.73 ± 0.04
Pairing wrong 0.48 ± 0.05
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Checking top signal
Jet q purity
Pairing correct 0.611 ± 0.006
Pairing wrong 0.602 ± 0.023
• Such correlation is not seen in top signal MC!
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Top QMlbsMax, not b tagged samples
Ptq (sig+bkg) = 0.527 ± 0.004 +0.008 -0.010 ; Ptq (data) = 0.50 ± 0.08
-4/3+4/3-2/3 +2/3
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Top QKIN, not b tagged samples
Ptq (sig+bkg) = 0.519 ± 0.002 ± 0.015 ; Ptq (data) = 0.52 ± 0.05
-4/3 +4/3-2/3 +2/3