of events. Also, bottom quarks from gluon splitting can initiate an additionalb-jet as can be seen in Figure6.1.
Figure 6.1: t-channel event topology [39].
We base our t-channel signal selection on the above listed characteristic features. The selec-tion requirements are described in the following secselec-tions.
would lead to a reduction of signal. This can be seen in Figure6.2, which shows the electron pT distribution for Monte-Carlo signal and background after the trigger+matching+leading electron selection with data superimposed.
[GeV]
Electron pT
Events
1 10 102
103
104
105
106
[GeV]
Electron pT
Events
1 10 102
103
104
105
106
20 40 60 80 100 120
e+jets electron selection
L = 35 pb-1
∫
processest-channelWt di-bosons
t t W+Jets Z+Jets Data
Figure 6.2: ElectronpT distribution after the trigger, the electron to trigger matching and the highpT
electron requirements are applied. The last bin contains overflow. QCD multi-jets contribution absents in the plot due to the lack of its simulated sample.
Further, we define two regions according to the pseudo-rapidity of the electron: 1) central region, |ηe|<1.37; 2) combined sides, 1.52 < |ηe|<2.47. We use these two regions in order to account for the electron reconstruction+identification dependence on ηe or the expected signal and background fractions, which depend onηeas well (see Table6.2).
An event is rejected if it has more than one electron or extra muon(s) withpT > 20GeV.
This helps to reduce the di-lepton background from the production of t¯t, Wt-channel and di-bosons, while keeping the t-channel event yield unchanged (see Figure6.8).
After selecting events with proper lepton configuration, we require two different configur-ations of jets, which havepT jet > 25GeV. The first selection is based on the expectation for t-channel, which predicts one central and one forward jet. Here, a jet is defined to be central if|ηjet|< 2.5. If2.5 <|ηjet|< 4.5, then the jet is forward, where the upper pseudo-rapidity threshold is determined by the availability of a calibration of the jet energy scale. The central jet pseudo-rapidity threshold is determined by the inner detector coverage andb-jets can be identified in this region only. Previous analysis [171] to our study was making use of two jet requirement without looking at the centrality of the untagged jet. We find that rejecting events with both jets in the central region significantly reduces the expected event yields of t¯tand other background. Comparison of the final event yields with and without the forward jet requirement can be seen in Table6.1. The expected signal yield is also reduced but the advantage is that the measurement precision is less sensitive to the background (except of W+jets) event rate uncertainties.
The second configuration of jets is a selection of a control region and is aimed to an auxiliary measurement of QCD multi-jets and W+jets contribution in the signal selection as was already mentioned above. The requirement is to have exactly one central jet,|ηjet|<2.5, in an event.
It is required to be ab-jet. Then the selected events are dominated by QCD multi-jets and the heavy flavored jets component of W+jets. This can be seen in Figure6.3, which presents aE/T distribution in events with only oneb-jet.
[GeV]
ET
Events
0 50 100 150
[GeV]
ET
Events
0 50 100 150
0 20 40 60 80 100 120
e+jets tag selection
Jets total/central = 1/1
|<1.37 ηe
| L = 35 pb-1
∫
processes t-channel Wt di-bosons
t t W+Jets Z+Jets Data
(a)|ηe|<1.37
[GeV]
ET
Events
0 20 40 60 80
[GeV]
ET
Events
0 20 40 60 80
0 20 40 60 80 100 120
e+jets tag selection
Jets total/central = 1/1
|<2.47 ηe 1.52<|
L = 35 pb-1
∫
processes t-channel Wt di-bosons
t t W+Jets Z+Jets Data
(b)1.52<|ηe|<2.47
Figure 6.3:E/T distributions in the selected events with oneb-tagged jet. The left plot corresponds to the events, where the electron is in the central region. The right plot corresponds to the events with the electron is in one of the side regions. QCD multi-jets contribution absents in the plots due to the lack of its simulated sample.
We find that theb-jet requirement significantly reduces Z+jets and light flavored jets com-ponent of the W+jets backgrounds. Figure6.4presents a comparison between theE/T distri-butions before (left plots) and after (right plots) requiring a central jet to beb-tagged in the events with one central and one forward jet. These plots show that the Z+jets contribution is significantly suppressed and the W+jets contribution is also reduced.
The last requirement of the selection is a kinematic cut on a variable introduced for QCD multi-jets suppression. The ’triangular variable’ is a sum of E/T and the reconstructed W boson transverse mass and we require it to be higher than60GeV. TheW boson transverse mass depends onE/T and is defined as,
mT(W) = [2ET(e)E/T(1−cos∆φ)]1/2 (6.1) Where, ET(e) ' |pT(e)| is the module of the electron transverse momentum and ∆φ = min[φE/
T −φpT(e),2π−(φE/
T −φpT(e))]. φE/
T andφpT(e) are the azimuthal angles of E/T and pT(e)vectors.
In Chapter7dedicated to the data driven measurements of background, we show that the triangular variable gives a powerful separation of QCD multi-jets background from the rest and the region below 60GeV can be used as a control region. Figure 6.5 shows the two-dimensional distributions of E/T versus mT(W) in data (left plot) in the simulated W+jets sample (right plot). These distributions are made in the events after the trigger (with electron matching) and the highpT electron requirements are applied. We learn from these distribu-tions thatW like events and other events, which do not haveW boson can be well separated with the triangular cut. The cutE/T +mT(W) > 60GeV is represented on the plots as red lines.
After the selection requirements are applied, good purification of the signal region is reached.
All background processes except W+jets and QCD multi-jets are well suppressed. Figure6.6
[GeV]
ET
Events
0 50 100 150 200 250
0 20 40 60 80 100 120
e+jets pretag selection
Jets total/central = 2/1
|<1.37 ηe
| L = 35 pb-1
∫
processes t-channel Wt di-bosons
t t W+Jets Z+Jets Data
(a) w/ob-tagging,|ηe|<1.37
[GeV]
ET
Events
0 5 10 15
[GeV]
ET
Events
0 5 10 15
0 20 40 60 80 100 120
e+jets tag selection
Jets total/central = 2/1
|<1.37 ηe
| L = 35 pb-1
∫
processes t-channel Wt di-bosons
t t W+Jets Z+Jets Data
(b) withb-tagging,|ηe|<1.37
[GeV]
ET
Events
0 50 100 150 200 250
0 20 40 60 80 100 120
e+jets pretag selection
Jets total/central = 2/1
|<2.47 ηe 1.52<|
L = 35 pb-1
∫
processes t-channel Wt di-bosons
t t W+Jets Z+Jets Data
(c) w/ob-tagging,1.52<|ηe|<2.47
[GeV]
ET
Events
0 2 4 6 8
[GeV]
ET
Events
0 2 4 6 8
0 20 40 60 80 100 120
e+jets tag selection
Jets total/central = 2/1
|<2.47 ηe 1.52<|
L = 35 pb-1
∫
processes t-channel Wt di-bosons
t t W+Jets Z+Jets Data
(d) withb-tagging,1.52<|ηe|<2.47
Figure 6.4:E/T distributions in the selected events with2 jets (one central and one forward) before (left plots) and after (right plots) applying theb-jet requirement. The top row plots correspond to the events, where the electron is in the detector central region. The bottom row plots correspond to the events with the electron in one of the side regions. QCD multi-jets contribution absents in the plots due to the lack of its simulated sample.
showsE/T distributions in the events remaining after all selection requirements are applied.
The left plot corresponds to the selected events with a central electron and the right plot is for the events with a side electron.
0 50 100 150 200 250 300 350 400 450
[GeV]
ET
0 20 40 60 80 100 120
(W) [GeV]Tm
0 50 100 150 200
e+jets electron selection Data
(a) Data
0 50 100 150 200 250 300 350
[GeV]
ET
0 20 40 60 80 100 120
(W) [GeV]Tm
0 50 100 150 200
e+jets electron selection W+Jets, MC
(b) W+jets, Monte-Carlo simulation
Figure 6.5:E/T versus theW transverse mass distributions in data (left plot) and in the Monte-Carlo sample of W+jets. Trigger, trigger matching and highpT electron requirements are applied to events.
The red lines represent the triangular cut,E/T+mT(W)>60GeV.
[GeV]
ET
Events
0 2 4 6 8
[GeV]
ET
Events
0 2 4 6 8
0 20 40 60 80 100 120
e+jets final selection
Jets total/central = 2/1
|<1.37 ηe
| L = 35 pb-1
∫
processes t-channel Wt di-bosons
t t W+Jets Z+Jets Data
(a)|ηe|<1.37
[GeV]
ET
Events
0 1 2 3 4
[GeV]
ET
Events
0 1 2 3 4
0 20 40 60 80 100 120
e+jets final selection
Jets total/central = 2/1
|<2.47 ηe 1.52<|
L = 35 pb-1
∫
processes t-channel Wt di-bosons
t t W+Jets Z+Jets Data
(b)1.52<|ηe|<2.47
Figure 6.6:E/T distributions for selected events. All selection requirements are applied. The left plot corresponds to the events, where the electron is in the central region. The right plot corresponds to the events with the electron in one of the side regions. QCD multi-jets contribution absents in the plots due to the lack of its simulated sample.
We introduce a notation for the event selection steps, which will be used throughout the document as shortcuts,
• Electron selection: trigger and electron trigger matching, highpT electron requirements
• Pretag selection: electron selection, veto lepton and jet multiplicity requirements
• Tag selection: pretag selection, b-jet requirement
• Final selection: tag selection, cut on the triangular variable
Further, we introduce the following notation for the events with different jet configuration and the electron pseudo-rapidity region:
• 1j/1cj: events with only a central jet
– 1j/1cj/central: electron is in the central region – 1j/1cj/sides: electron is in one of the side regions
• 2j/1cj: events with two jets one of which is central and the other is forward – 2j/1cj/central: electron is in the central region
– 2j/1cj/sides: electron is in one of the side regions
We also define other notations for selected events such as2j/2cj/central, 2j/2cj/sides, 3j/2cj/central, 3j/2cj/sides, 3j/3cj/central and 3j/3cj/sides. Meaning of each of these definitions is obvious.
Figure6.7shows a graphical representation of the allowed configurations of jets in events.
The 1j/1cj and2j/1cj events (central jet isb-tagged) are presented on the left and the right sub-figures respectively.
(a)1j/1cj, central jet isb-tagged (b)2j/1cj, central jet isb-tagged
Figure 6.7: A graphical illustration of the selected event configuration. The left figure corresponds to the1j/1cj events. The right figure corresponds to the2j/1cj events. In addition, the central jet is required to beb-tagged.