Документ взят из кэша поисковой машины. Адрес оригинального документа : http://zebu.uoregon.edu/~uochep/talks/talks02/mas01-02.pdf Дата изменения: Fri Mar 14 00:30:51 2003 Дата индексирования: Tue Oct 2 10:50:57 2012 Кодировка: Поисковые слова: jet

Study of top-quark production and decay vertices at NLC

Jan 8, 2002 M. Iwasaki University of Oregon


Top-quark physics Top-quark is already discovered ... no longer `search' physics It also has potential to search new physics mtop = 175 GeV ... Uniquely heavy t = 1.4 GeV ... Decays very fast NLC is excellent and much cleaner place to do it ...than LHC!!


Top-quark anomalous coupling analysis Top-quark decays before forming a hadron ! Top spin information is transferred to its daughters ! can probe the couplings with their angular distributions At NLC, we can probe both top-production + decay couplings
t

production

/Z0 t W t b

" can't do at LHC!

decay


b t /Z0 t Form Factors at /Z0!t t vertex: F1V + F1A 5]+iµq/2mt[ F2V + F2A 5]} iM = {µ[ 0 0 SM(Z): (1/4-sinW2)/sinWcosW (-1/4)/sinWcosW (): 2/3 0 0 0 F2V: Electroweak Magnetic Dipole moment Electric Dipole moment .. Non-zero ! CP F2A: Form Factors at t!b W vertex: iM = ig/2{µ[F1LW PL + F1RW PR] + iµq/2mt[F2LW PL + F2RW PR]} SM: 1 0 0 0 W
+

: Form Factors


b t t W+ Wb

Signal of t t production ... 2b's and 2W's

Depending on the W 1) 2 leptons + 2 jets 2) 1 lepton + 4jets 3) 6 jets

decay, there are 3 final states: (both leptonic) (1 leptonic 1 hadronic) (both hadronic)


Here we define the angles
Top decay angle W decay angle

t et e+ t b t

W

l or d-type quark W or u-type quark

We want to measure these angles Which one is t or t ? ... use charge of the lepton 4-momentum of the leptonic decayed top-quark ... from the opposite top-quark


In this analysis, we use tt4 jets + 1 lepton (µ or e)
Generate tt events with Pandora-Pythia mt = 175 GeV, ECM = 500 GeV including QCD effect(parton shower), ISR+beamstrahlung Beam polarization... 1) P(e-) = -0.8 P(e+) 2) No polarization 3) P(e-) = +0.8 P(e+) 4) P(e-) = -0.8 P(e+) ! Peff =0 =0 = +0.5 = 0.93 tt = 742 565 398 1078 fb fb fb fb

· Generate 100 fb-1 each ... Through LCD Fast Detector simulator · Reconstruct 4 jets by Jet-clustering (JADE) ... use charged tracks + Neutral clusters (charged/neutral cluster separation .. Perfect) · Reconstruct W by 2 jets · Reconstruct Top by W + b-jet (Apply 0.85 < E3jets/Ebeam < 1.05)


Flavor-tagging To tag b-quark, we use mass-tag method
t W b

1. Reconstruct Secondary Vertex 2. Form 'PT-corrected mass' of SV M
corr.

= (Mvtx2+|PTvtx| 2) + |P
SV eIP

T

vtx

|

PTvtx Pvtx

e

+

3. Identify heavy-quark signals with Mcorr


Flavor tagging heavy-quark
22000 1200 20000 18000 1000 16000 14000 800 12000 600 10000 8000 400 6000 4000 2000 0 0 1 2 3 4 5 6 7 0 0

uds-quark
b c uds

200

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PT corrected mass(GeV)

# of significant tracks in jet

b

efficiency = 67% purity = 95% efficiency = 67% purity = 97%

: PT corrected mass > 1.8 GeV

uds: Nsig = 0
efficiency = 87% purity = 79%

c+b: PT corrected mass > 0.5 GeV


Reconstructed W with 2 jets
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With flavor-tag

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(GeV)

Reconstructed top with 3 jets
With flavor-tag
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Selection efficiency:: Event selection = Top selection Flavor-tag Total

60% 50% 67% 20%

Top-reconstruction performance:: purity 88% (80% without flavor-tag) correct b-assign 86% (53%) Mass resolution 7.6 GeV (10.0 GeV)


Reconstructed Angular Distributions
900 350 300 250 200 150 300 100 200 50 0 -1 100 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 800 700 600 500 400

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180 160 140 120 100 80 60 40 20 0 -1 -0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8 1 160 140 120 100 80 60 40 20 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2


Preliminary sensitivities to the couplings (/z t t vertex) 1) F 2) F
1,2A 1,2V Z, Z,

! Maximum Likelihood method ! L/R asymmetry
Z,

1) Sensitivities to F1,2A

for 100 fb-1 (68% CL) F1A F1AZ F2A F2AZ P(e-) = -0.8 0.011 0.013 0.016 0.049 P(e-) = -0.8 P(e+) = +0.5 0.009 0.011 0.021 0.033 No polarization 0.011 0.014 0.013 0.059 P(e-) = +0.8 0.011 0.015 0.014 0.052 ! positron polarization is useful (~20% effect) 2) Sensitivities to F1,2V, using L/R asymmetry (200 fb-1) ALR = (0.335 ± 0.017 )/0.8 (for |Pe| = 0.8) F1V : 0.012 ! F1V : 0.047 F2V : 0.038 F2V : 0.009


P(e1) 2) 3)

) = -0.8 Without ISR, Beamstlahlung, parton-shower With .. With .. + Energy Flow (detector LD)

1) 2) 3) Top efficiency 24% 20% 14% Top purity 90% 87% 86% 5.8 GeV 7.6 GeV 9.4 GeV mtop 14 mrad 23 mrad 39 mrad 0.011 0.012 F1A sensitivity 0.010 F1A 0.012 0.013 0.015 F2A 0.012 0.016 0.018 F2A 0.039 0.049 0.058


Summary We estimate very preliminary sensitivities (real parts) 1) From Maximum Likelihood analysis (100 fb-1) F1A ~0.01 F1A ~0.01 F2A ~0.02 F2A ~0.05 2) From L/R asymmetry (100+100 fb-1) F1V ~0.05 F1V ~0.01 F2V ~0.04 F2V ~0.01 Positron polarization is useful (P=0.5 ~20% effect) ISR, beamstrahlung, parton-showering ... inflate error by 10-30% Perfect Energy-Flow ! Realistic analysis ... inflate error by 10-20% Future plan 1) tt! 6 jets analysis top-quark sign.. determined by vertex charge of b-jet 2) Kinematical constraint fit


How to tag t or t-bar?? Using Vertex charge of b-jet, we know b or b ! t or t !
b-quark
4000 3500 3000 2500 2000 1500 1000 500 0 -5
Qvtx_B Nent = 8028 Mean = -0.3332 RMS = 0.7901

(Vertex Charge ... Charge sum of the reconstructed Secondary Vertex)

b-bar-quark
4000 3500 3000 2500 2000 1500 1000 500 0 -5

Qvtx_Bbar Nent = 8020 Mean = 0.3271 RMS = 0.7789

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Require at least 1 b-jet has Vertex Charge 0 83% ! We can determine t or t with purity efficiency 57%