2008/12/10national tsing hus univ.1 measuring properties of top quark; is it really top of sm?...

2008/12/10 National Tsing Hus Univ. 1

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?


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


Muon stations

Calorimeter

Silicon detector

COT


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)


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 _ _

_


Production of top quark

• Production mechanism

• Production cross section

• Forward backward asymmetry


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


Production mechanism

• Di-lepton events; using data of 2 fb-1

=azimuthal correlation of the two leptons


• 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)


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


Single top production


Forward backward asymmetry

• Afb = 0.17 ± (0.07)stat ± (0.04)syst• To be compared with theory: Afb = 0.04 ± 0.01


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


Life time

ct < 52.5 m @ 95% CL


Decay width

top < 12.7 GeV @ 95% CL


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)


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.


MTM method (CDF, 2.7 fb-1)

• log Lsig(mt, JES) = Σi[log Li(mt, JES)] - nbg log Lavg(mt, JES | background)


• mt = 172.2 ± 1.0 (stat.) ± 0.9 (JES) GeV/c2


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


• f0 = 0.65 ± 0.19 ± 0.03

• f+ = -0.03 ± 0.07 ± 0.03


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


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


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)


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


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)


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.


• 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!


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.


Pairing using MlbsMaxEfficiency

Purity

Mlbs max cut

Mlbs max cut

• correct pairing

• wrong pairing


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


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


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%).


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


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!


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


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


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


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


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


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.


•Assumed values


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)


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



•Assumed values


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



•Assumed values


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)


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!


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.


Back up slides


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


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!


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


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

2008/12/10national tsing hus univ.1 measuring properties of top quark; is it really top of sm?...

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