ttbar Cross-Section Studies
D. Jana*, M. Saleem*, F. Rizatdinova**, P. Gutierrez*, P. Skubic*
*University of Oklahoma, **Oklahoma State University
1
Motivation (1)
 For all quarks, there are qqbar bound states (mesons),
LHC is the good place to study the existence of ttbar
bound state.
 Existence of ttbar resonance will simply imply
physics beyond SM.
 Several theoretical models (MSSM) predict existence
of ttbar pair resonances.
 ttbar will be the main background to many Higgs and
SUSY searches. Examples: ttH (SM Higgs boson
search), Stop pair production, heavy charged Higgs
boson searches.
 Any new physics related to EWSB should be coupled
to the Top (t), leading to deviations from SM ttbar
production rate.
2
Motivations (2)
 At LHC:
 Production of more than 8 million ttbar pairs(~ 830 pb-1) in
one year at initial luminosity (~1033 cm-2/s)
 LHC is also a Top factory.
 Excellent opportunity to understand top quark at 14 TeV. We
need to understand top quark very well before doing any other
physics (Higgs or SUSY or any other physics with in the SM
or beyond the SM).
 ttbar production cross-section study is also first priority
at 14 TeV (excellent chance for students to graduate)
 Plan is to continue with this study and understand the
ttbar production which will be helpful understanding of
the ttbar background for several analyses and also
proceed towards the ttbar resonance study (later).
3
Decay Process
p p  tt  X
 Multi-jet channel:
 65.5 % of all ttbar events
tt  WWbb  ( jj )( jj )(bb)
Problem : Large QCD background
 Dilepton channel:
 4.9 % of all ttbar events
tt  WWbb  (l )(l )bb
Background process: Drell-Yan process, Z-boson + jets, 2 W-boson jets
and bbar pair production
 Single lepton plus jet channel:
 29.6% of all ttbar events
tt  WWbb  ( jj )(l )bb l= e ,,
4
Topology of single lepton + jets decay channel
40
5
Current Goal
 Our aim is to measure the ttbar x-section using likelihood method.
 No B-tagging is assumed at this moment.
 Need to find topological variables that will help to separate ttbar from
Background (e.g; W+jet).
S ( x1 , x2 , ......)
L 
S ( x1 , x2 , ......)  B ( x1 , x2 , ......)



i
i
Si 
Si

i
Bi

 S
 S /
i
i
i
i


Si  
exp   i  ln

Bi  





Si  
exp   i  ln
  1
Bi  


i


Si 
exp   i  ln


Bi  fitted



i



Si 

exp   i  ln


Bi  fitted 







1
/ Bi
Bi  1
The first line is the best
discrimnator.
Approximated in the 2nd line
neglecting the correlation among
the variables.
Finally, this is transformed to a
expression: where we have log
of the ratio of the topological
variable ‘i’ used to build the
the discriminant.
MC Data
 We are using the following available Samples:
Signal : ttbar “e+jets", Mtop = 170 GeV ; sample: 6201
Total signal events ~ 85000 ;
Background: W + jets
Ntotal
W+2partons : 12000
W+3partons : 11250
W+4partons : 5500
W+5partons : 4950
nreco
15
125
436
709
X-sec(pb)
2032
771
273
91
Luminosity(pb-1)
100
Run on these samples separately and calculated the selection efficiency after
passing all the cuts (next page). The Final BG is the sum of the number of expected
events using:
 x (=nreco/Ntotal) x £ = Nexpect(~ 910 ) for W+n jets.
The expected number for the ttbar signal events:
 x (=nreco/Ntotal) x £ = Nexpect
896.1 x 7.9 x 100
= ( ~ 707919 ) for ttbar
7
Basic Selection Criteria
 Only one lepton (e) with Pt > 20 GeV & |eta|<2
 Missing Et > 20 GeV
 3 jets with Pt > 40 GeV & |eta| <2 (All standard cuts
used)
 4th jet with Pt > 20 GeV and |eta|<2
Electron isolation cut (deltaR, distance from the jet axis) > 0.4
E/P :
8
Jet Momentum Distribustion
1st leading
2nd leading
3rd leading
4th leading
1st Leading Jet
2nd Leading Jet
Jet Momentum (GeV)
In the early data, we will not have
B-tagging performance well
understood and will not be applied.
Assumption: the 2 leading jets are
b-jets.
The next two leading jets (3rd, 4th)
are considered as “light” jets.
1st light jet
2nd light Jet
Jet Momentum (GeV)
9
Invariant Mass Distribustion for W
Transverse W mass MeV
(electron & neutrino)
Invariant jj mass (MeV)
(two light jets)
10
Comparison of variables to discriminate between
signal & background
From the previous experience (with D0):
We are planning to use the following discriminating variables, to differentiate
Our signal form the Background.
-Aplanarity
- Centrality
-Sphericity
- HT
- KT
- 
11
Aplanarity
D0
12
Event Probability
Event Probability
Centrality is defined:
HT
C
H
Centrality
D0
H T = Scalar Sum of the p
T
of the jets
= Scalar Sum of the energy of the jets
13
Event Probability
Something not right
Sphericity
D0
Sphericity(S): Measures summed p2trans w.r.t. event axis.
S=1 (for Isotropic events, ttbar..)
S =0 (for less isotropic, like w+jet, QCD..)
14
Event Probability
HT(MeV)
D0
HT -> Scalar sum of the pT of the four
leading jets
15
Event Probability
KT
KT  R E
min
jj
D0
min
T
W
T
/E
ETW  ETlepton  ET (miss )
16
Event Probability
Delta Phi (electron, Missing ET)
17
Event Probability
Invariant mass of W (combining 2 light jets)
18
Event Probability
Jet Pt (Pt of a leading Jet)
19
Cut flow for W+jets
Cuts
W+2par
(Effeciency)
W+3par
W+4par
W+5par
lepton
cut(Etacone>40,
eta<2
5.59 %
4.97%
37.72%
36.65%
lepton cut
(Elpt>20GeV)
4.63%
3.90%
27.96%
25.05%
1st Jet Cut 40
GeV
4.03%
3.62%
26.65%
24.46%
2nd Jet Cut 40
GeV
2.15%
2.51%
20.89%
21.33%
3rd Jet Cut 40
GeV
0.60%
0.68%
10.22%
15.03%
4th Jet Cut 20
GeV
0.001%
0.01%
7.92%
14.32%
20
Cutflow
Total Signal events : 85966
Cuts
Event Number
lepton
cut(Etacone>40,eta<2
20978
Efficiency
24.4%
lepton cut
(Elpt>20GeV)
16209
18.9%
1st Jet Cut 40 GeV
15224
12892
8124
17.7%
15.0%
9.5%
6802
7.9%
2nd Jet Cut 40 GeV
3rd Jet Cut 40 GeV
4th Jet Cut 20 GeV
21
Future Plan
 Plan is to plot Likelihood discriminant distributions for ttbar & W+jets
once we get enough W+jet events
 After we find Likelihood distributions, we will find top mass
dependence of the ttbar production cross-section.
 We have not yet applied b-tagging, as 2 of our jets will be tagged as
b-jets.
 During the initial running we might not have B-tagging, well
understood. In that case we might have to choose from the
following:
 Do not use any B-tagging initially
 Or only tag one of the jet as B-jet (will be helpful to remove some of the
background).
 background studies (W + jets) already started.
 Will be working on the others: Z-boson + jets, Z-boson pairs, W-boson pairs, Wboson & Z-boson production).
 Will report to this group meeting for suggestions and comments.
22
Backup…..
23
Aplanarity defination
Normalized momentum tensor
M ij 
p
0
i
p 0j
p 0 = Momentum vector of a reconstructed object 0
0
p
0
2
i, j  Cartesian Co-ordinates
0
After standard diagonalization, we can find out 3 eigen values of
M ij
1  2  3 , 1  2  3  1
Aplanarity ( which is a measure of the flatness of the events) is defined by
A
3
3
2
In the sum, we have jets and electron (from W decay) which has best
discriminating power between signal & background
24
Sphericity
S
3
 2  3 
2
0  S 1
25