Search for Singletop production

1
Search for Singletop production
Univ. of TsukubaS Kim, K Nakamura

• Introduction
• Double tag
• Kinematics
• TODO

2
Motivation of Double tag
Large amount of Background for this
channel. About half of the background comes from
Mistags and Wc(c).
Double tag is the powerful method to reject these
background for s-channel !
At least 1tag
CDF NOTE 8112
But, statistics of the signal is limited by low
efficiency of double tag.
We plan to apply tight loose b-tagging SJ
SecVtx
JetProb
3
Event Selection

• Lepton ID

Standard tight lepton selection of leptonjet
group.
Muon
Electron
Ptgt20 GeV/c Isolated Cosmic Veto
Etgt20 GeV Isolated Conversion Veto

• Jet Energy Correction

Level4 Jet Energy Correction not to apply abs
correction
Etgt15 GeV, Eta lt 2.8

• MET

Et is corrected with lepton and Level4 jets
Etgt20 GeV
But event selection of this talk is
unexacting We will move into gen6 soon!
4
Double tag
5
Double tag performance
We can use two type of b-tagging methods.
SECVTX Lxy based alg. JETPROB d0(each track)
based alg.
We can use below two categories of double tag.

A) SECVTX SECVTX B) SECVTX JETPROB (not
include category A)
Performance for the 2-jet-bin
6
Event Yield _at_L955 pb-1
7
s-channel specific kinimatics study
8
S-channel single top event _at_ CM Frame
ambiguity
Selection of bjet from top quark
t-channel tagged jet -gt 95
s-channel tagged jet -gt 53
top mass resolution 15 GeV
Selection of neutrino Pz solution
30 no solution
CDFNOTE
Mlnb for 4 cases
This Top Mass constraint gave some bias to Mlnb
distribution??
9
Likelihood Function for neutrino Pz
2 solution case
0 or 1 solution case
Move three parameters METx METy PnZ
10
PnZ _at_GenLev
PnZ Qlep _at_Sim
---- W ---- W-
---- correct ---- wrong
correct
wrong
PnZ
PnZ
PnZQlep
11
Good agreement!
recPnZ
recPnZ
genPnZ
genPnZ
2 solutions no solution 71.6
28.4 Probability to choose correct solution
75.2 (of 2 solutions case)
12
Likelihood Function for bbbar ambiguity
Using L5 jet
No b jet correction
Sim Lev
Sim Lev
-- from top -- from W
-- from top -- from W
bjet Pt
Mlb
13
cosfb x Qlep
_at_ Wstar Rest Frame
n
n
l-
l
t
t
W-
W
f
b
b
P
P
f
Z
b
b
tb production process has the V-A coupling, so
it is easy for the b-jet to be produced in the
direction of quark in proton, and bbar-jet to
be produced in the direction of anti-quark in
anti-proton _at_ the Wstar rest frame.
cosf angle shown in upper figure times lepton
charge is the powerful variable to select b-jet
which is directory produced from Wstar.
14
cosfb distribution
bjet from top
bjet from W
Gen Lev
---- W ---- W-
bjet from W
bjet from top
Sim Lev
15
cosfb x Qlep distribution
Sim Lev
Gen Lev
-- from top -- from W
-- from top -- from W
Use these distribution as the Probability
Density Function
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bbbar ambiguity
Correct choice 81 !
Mlnb distribution
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Likelihood for Event Selection
signal
Wbb
MbbGeV
MbbGeV
PtbbGeV
PtbbGeV
18
Mlnb distribution at the best significance
L955fb-1
Before event selection
After event selection
S/N6.0/61.7 S/sqN0.75
S/N5.4/39.3 S/sqN0.86
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Summery ToDo

• Double tag including sj tagging has a potential
  to improve the s/n ratio and significance.
• This shows an expected performance
• We examine some kinematics variables for
  eliminating ambiguity and background rejection.
• We find a lot of effective variables.
• To Do in a couple of weeks
• Signal selection will be reconfirmed with gen6.
• To Do in 3 months
• We will improve JetProb tagger.
• Estimation of Data based background
• second lepton veto study?
• make results for 1.2 or 2 fb-1 data

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Backup
21

• ttbar pair production

• SingleTop production

sttbar7.30.50.60.4 pb-1
ssingle2.860.28 pb-1
stat syst lumi
g
W
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Motivation of SingleTop Search

• Produced electroweakly coupling
• Direct access to W-t-b vertex
• ssingletopG(t?bW)Vtb2
• Top-polarization study and V-A structure of EW
  top interaction
• Background study for SM Higgs search
• Exactly the same final state as WH-gtWbb
• Background of all channels to use
• Wnjet background
• Non-SM phenomena
• Heavy W boson
• Anomalous W-t-b coupling

t-channel
s-channel
23
W mass
W has the mass as small as possible to keep
larger than Mt??
24
cosfb x Qlep
_at_ Wstar Rest Frame
n
n
l-
l
t
t
W-
W
f
b
b
P
P
f
Z
b
b
cosfb x Qlep
Sim Lev
cosfb
---- W ---- W-
-- top -- other
top
other
Sim Lev
25
MW _at_Sim
Using L5 jet
No b jet correction
26
Likelihood correlation check
MW
PnZ Qlep
27
MET
Double Gaussian
28
Using L5 jet
B-jet Pt distribution
No b jet correction
Gen Lev
Sim Lev
-- top -- other
-- top -- other
Use these distribution as the Probability
Density Function
29
Using L5 jet
Mlb distribution
No b jet correction
Gen Lev
Sim Lev
-- top -- other
-- top -- other
Use these distribution as the Probability
Density Function
30
Parameter correlation
These figure shows some correlations
between the Likelihood parameters But
Likelihood is only used as the choosing bbbar
ambiguity, so we use this likelihood !
31
Top Specific Correlation Factor
gen P Rec P
Rec P
p02.12 p123.3 p20.057
p01.83 p124.8 p20.032
p01.31 p125.0 p20.081
From takeuchi-san
From takeuchi-san
From takeuchi-san
distribution
Xaxis value of each bin is weighted mean
32
W boson direction at the W rest frame
Dominant channel of Wbb background is t-channel
like upper right feynman diagram. -gt W boson
direction is close to p or pbar direction
33
Signal
W
W
f
b
f
b
t
t
d
u
d
P
P
u
W-
W
Z
_at_ Wstar Rest Frame
b
b
Wbb
b
b
b
b
u
d
u
d
f
d
u
d
u
P
P
f
-
-
Z
W
W
_at_ Wbb Rest Frame
34
cosfw distribution
generator level w/ acceptance cut
simulated
35
cosfwxQlep distribution
We can use this variable for event selection !
36
Mtop(lnb) distribution
At least 1 tag
Double tag