Tatsuma Meguro
University of Tsukuba
Sensitivity Study on Top Quark Pair
Resonance Search
motivation
● The large mass of top quark could be sensitive to the physics beyond the standard model.● Many theories predict new particles preferentially coupled to the 3rd family, in particular top quark
● Model independent search is desirable.
Motivation 2•Current interest is in the high mass tail region (signal model can be anything)
• There are few background in the interested region (700~1000GeV) .• We can’t tell whether there is interference effect or not because of small data size.• Analysis is a little bit simpler without interference case.
•increase the signal acceptance• include pretag events
[~100%: assuming tag/jet=30% at high mass region]
• include >=5jet events [~30%]
(jet assignment become difficult)• include more lepton category [~20%]
(don’t need not-good quality lepton with very high pt)
• Separate events into different sensitivity categories• By tagging information
Event Selection (standard selection)
ttbar invariant mass reconstruction
Background
Process Pretag Tagged
QCD 169.93+-62.26 13.62+-11.68
ww 30.85+-2.07 2.53+-0.27
wz 5.87+-0.40 0.94+-0.08
zz 0.50+-0.04 0.09+-0.01
top 409.87+-55.05 48.23+-15.57
s-channel single top 3.33+-0.31 2.00+-0.20
t-channel single top 3.41+-0.27 1.94+-0.17
Z->l l 35.31+-3.17 2.39+-0.28
HF(Wbb,Wcc,Wc) 137.19+-31.32
LF(W+np) 425.74+-79.21 16.65+-3.64
Total Background 1222.00+-0.00 117.80+-19.38
Wbb(16.53+-6.66)
Wcc,Wc(12.88+-5.23)
<The background estimation>
Ex) Pretag LF background
from Method 2 tableUse Official MC sample of each process
0 parton + W(->e nu)
1 parton + W(->e nu)
2 parton + W(->e nu)
3 parton + W(->e nu)
4 parton + W(->e nu)
0 parton + W(->mu nu)
1 parton + W(->mu nu)
2 parton + W(->mu nu)
3 parton + W(->mu nu)
4 parton + W(->mu nu)
1,Apply MDL for each MC sample2,Calculate Acc.*Xs*Br and merge samples in this ratio3,Normalize according to method 2 table4,we can get background shape (ex: pretag LF background(below) )
Signal template
sqrt(s)
Neve black:Hepgred:Reconstructed
400GeV 600GeV
1000GeV
1200GeV
800GeV
Signal : Z' -> ttbar (width=1 GeV)
Old sampleodd tail in low mass region.I guess this comes from p.d.f distribution.
ttbar invariant mass distributiontaddeg sample pretag sample
mass(GeV)mass(GeV)
rough estimation
<Significance(pretag)/Significance(tag)>
Z' mass (GeV)
How to estimate:1. Count events in each Z mass window.2. Calculate significance = S/sqrt(B) pretag , tagged events separately. 3. Take ratio to see how much sensitivity improve by adding pretag events.
Low mass region ~ 30%High mass region ~ 40% !!
Likelihood Fit
Signal : qqbar -> Z' -> ttbarBackground : others(10~11 shapes)
mu : Number of expectation in i th bin (= Nbkg + Nsig(Xs x Br x Lumi. x Acc.) )ni : Number of pseudo data in i th binG(Nbkg,) for background constrainG(Nsig,) for signal systematics
We can retrieve 95% C.L. upper limit from L(x) plot
G:Gaussian
Cross section
Lik
elih
ood 95%CL
hMtt_ttop75 hMtt_zpr500
95%CL distribution
Cross section
Lik
elih
ood
normalized BG(left) , signal(right) distributionGeV
1, Make distribution from Pseudo experiment (w/o Z' event , Ndata = 500 )
2,Calculate the likelihood for Xs of Z' and set 95%C.L upper limit
Templete
95%CL
we can get one point from 1-pseudo experiment
How To Set Limit
Expected 95% C.L upper limit
cross section
Mz' = 600GeV
cross section
Mz' = 700GeV
cross section
Mz' = 400GeV
cross section
Mz' = 500GeV
cross section
Mz' = 800GeV
cross section
Mz' = 900GeV Mz' (
Z’)
BR
(Z’
ttba
r)
95% upper limit
Estimation of systematic uncertainty(JES)●JES●top mass●PDF●LO/NLO●hadronization(generator)●ISR/FSR●b-tag Efficiency
Suppose JES is dominant, I used only the uncertainty from JES in this analysis.
black:defaultred:JES + sigma_JESblue:JES - sigma_JES
Mttbar
Mttbar
Apply to G(Nbkg,sigma_bkg)
summary●made Signal MC●reconstruct ttbar inbariant mass (signal and background)●estimate 95% upper limit Z' cross section for each mass region , pretag ,tag event separately.
●Add pretag events to see how much improve the sensitivity... XX% Improvement in low mass region (500GeV) XX% Improvement in high mass region (1000GeV)●Other models will indicate same trend for sensitivity search
backup
rough estimate
Total pretag events : 851 Total tagged events : 371 (from method 2 table)
Sensitivity = Signal/sqrt(Background)
pretag : Nsig / sqrt(851) ~ N*0.6 / 29.17tagged : Nsig' / sqrt(371) ~ N*0.4 / 19.26
-> about 40% improvement in rough estimate!!
signal template350(GeV) 400 450 500 550
600 650 700 750 800
850 900 950 1000 1050
1100 1150 1200
sqrt(s)
Neve red:Ntag=0blue:Ntag>1
Identical Matched Jet (Old sample)
(Hepg-Rec)/HepgReconstructed
These plots are normalized
28762074
33742440
31802427
31722336
30292205
29332061
29932201
30102307
30462346
31692363
31262385
33142493
24921941
26061964
25151964
26352059
25591984
29002065
Red Identical JetBlue Not Identical
Problem of Pythia
sqrt(shat) GeV
num
ber
of e
vent
influence from p.d.f ?
So, I am planning to make new Z' template (Z' width = 1 GeV)
default width width = 1 Gev