Single Top Event Selection

Current Single Top Preselection

The followin event selection is based upon that proposed by Arnaut Lucotte and recently discussed by the Single-Top group. It is the most general event selection applied to single top samples and is refined further when selecting for specific channels (s, t or Wt). The second Jet Pt cut is at 25 GeV for low luminosity studies, and at 30 GeV for others (see section on jet pt2 study). Note that for the current sample, there is a selection at the reconstruction level to keep only muons and electrons with Pt > 10 GeV, so the secondary lepton cut effectively removes all secondary leptons.

A clearer wording of the jet cuts is: at least two jets with Pt > 30 GeV and no more than four jets with Pt > 15 GeV.

An isolated muon or electron is defined as having Etcone20 > 6 with respect to all jets in the event.

Lepton Selection

  • At least 1 high-pT lepton (e or isolated μ)
    • Isolated Electron pT ≥ 25 GeV
    • Isolated Muon pT ≥ 20 GeV
    • Lepton (isolated e or isolated μ) |eta| ≤ 2.5
    • Electron outside of 1.35 < |eta| < 1.65
  • No secondary lepton. A secondary lepton is any e or isolated μ, except the high-pT lepton, with pT ≥ 10 GeV.

Jet Selection

  • 2 ≤ NJet ≤ 4
  • Jet pT
    • Jet 1 pT ≥ 30 GeV
    • Jet 2 pT ≥ 25 GeV
    • Jet 3 pT ≥ 15 GeV
    • Jet 4 pT ≥ 15 GeV
  • Jet |eta| ≤ 5.0 for Jets 1, 2, 3, and 4
  • NB-Jet ≥ 1; where the 1 B-jet has pT ≥ 30 GeV and |eta| ≤ 2.5

Other Selection Criteria

  • ETMiss > 20 GeV

Trigger Selection

  • Electron Triggers
    • L1: EM25i OR EM60
    • L2: e25i OR e60
    • EF: e25i OR e60
  • Muon Triggers
    • L1: MU20i
    • L2: mu20i
    • EF: mu20i

Initial Selection Cuts for the Current Analysis

There are four different cut selections, which vary based on the tagged jet number cut and the lepton cuts. This selection contains all events above, but excludes secondary lepton events at this time, as well as events with more than two tagged jets. Both leptons are isolated, even if not stated. The important cut in this four selection sample is in the b-tagged jet number, since we only look at events with 1 or 2 b-tags.

Cuts for the first sample:

  • Nelectron = 1
  • Nmuon = 0
  • NB-Jet = 1

Cuts for the second sample:

  • Nelectron = 1
  • Nmuon = 0
  • NB-Jet = 2

Cuts for the third sample:

  • Nmuon = 1
  • Nelectron = 0
  • NB-Jet = 1

Cuts for the fourth sample:

  • Nmuon = 1
  • Nelectron = 0
  • NB-Jet = 2

Investigation of the Jet 2 pT cut

Signal Samples

In the original event preselection, the pT cut on both jets was 30 GeV. However, requiring that the Jet 2 pT was greater than 30 GeV removed a disproportionatly large number of events when compared to the other selection criteria. Different values of this cut on signal and background samples were studied to see if more signal events could be obtained without drastically increasing the background.

Distributions of Jet 2 pT for s, t, and Wt channels are shown below. The plots on the left are for pTJet 2 > 30 GeV, the center plots are pTJet 2 > 25 GeV, and the plots on the right are for pTJet 2 > 20 GeV. For pTJet 2 > 30 GeV, it can be seen in all channels that a large number of events, as represented by the light blue are, are removed. For pTJet 2 > 25 GeV, a moderate number of events are removed, and for pTJet 2 > 20 GeV, amost no events are removed.

evt_sel_jet_005501_pt2_one_pt30.png" evt_sel_jet_005501_pt2_one_pt25.png" evt_sel_jet_005501_pt2_one_pt20.png"

evt_sel_jet_005502_pt2_one_pt30.png" evt_sel_jet_005502_pt2_one_pt25.png" evt_sel_jet_005502_pt2_one_pt20.png"

evt_sel_jet_005500_pt2_one_pt30.png" evt_sel_jet_005500_pt2_one_pt25.png" evt_sel_jet_005500_pt2_one_pt20.png"

Background Samples

The effects of different Jet 2 pT cuts on a Wbb background sample was also studied since a major role of this cut is to limit contamination from soft jets in the background. However, in the t-channel, the Wbb is probably not a significant background, and in the s-channel, the requirement that NJets ≥ 2 removes Wbb background, so lowering the pT cut to obtain more signal events would probably be a good idea regardless of its direct effect on the background.

Plots of the Wbb FastSim sample are shown below for pTJet 2 > 30 GeV, pTJet 2 > 25 GeV, and pTJet 2 > 20 GeV. It can been seen that as the cut is lowered, more events remain in the sample.

evt_sel_jet_009901_pt2_one_pt30.png" evt_sel_jet_009901_pt2_one_pt25.png" evt_sel_jet_009901_pt2_one_pt20.png"

Table Sumarizing Results

Effect of Jet 2 pT Cut

The table belows the immediate effect of the Jet 2 pT cut. It shows the number of events before the cut, the number of events after the cut, and the percentage decrease in the number of events as a result of the cut.
Channel Jet 2 pT Cut (GeV) Events Before Cut Events After Cut % Decrease
s 20 20602 20570 0.2
s 25 20602 18860 9.2
s 30 20602 16806 22.6
         
t 20 2627 2626 0.04
t 25 2627 2513 4.5
t 30 2627 2361 11.3
         
Wt 20 26139 26121 0.07
Wt 25 26139 25105 4.1
Wt 30 26139 23593 10.8
         
Wbb 20 158474 158461 0.01
Wbb 25 158474 132094 20.0
Wbb 30 158474 106307 49.1

Effect of Jet 2 pT Cut on Final Sample

The table belows the effect of the Jet 2 pT cut on the final sample. It shows the number of events in the final sample for different Jet 2 pT cut in addition to the number of events in the final sample for a Jet 2 pT cut of 30 GeV. The percent increase in the number of events in the final sample for a certain Jet 2 pT cut over the number of events in the sample with a Jet 2 pT cut of 30 GeV is also shown.
Channel Jet 2 pT Cut (GeV) Final Events Final (30 GeV) % Increase
s 20 16177 13604 18.9
s 25 15038 13604 10.5
s 30 13604 13604 0.0
         
t 20 1845 1708 8.0
t 25 1792 1708 4.9
t 30 1708 1708 0.0
         
Wt 20 16858 15709 7.3
Wt 25 16430 15709 4.6
Wt 30 15709 15709 0.0
         
Wbb 20 132829 93821 41.6
Wbb 25 113996 93821 21.5
Wbb 30 93821 93821 0.0

Summary of Jet 2 pT Study

Moving the Jet 2 pT cut causes a 5-10% increase of signal events depending on the channel while also causing about a 20% increase in Wbb background. For early data studies it is important to include as many signal events as possible and therefore the increase in signal events outweights the subsequent increase in the background.

However, the Wbb sample is very sensitive in the region of the cut (29% for 5 GeV change) and it may not be safe to lower the value, especially since the normalization of the Wbb background is not known apriori (20% errors taken apriori). Also, the jet re-calibration for low-pt jets may introduce uncertainties in the JES, (corrections are very important for lower pT jets) which will reflect in the systematics.

The single top group has decided that the cut of 25 GeV will be used for early data studies while the cut of 30 GeV will be used for high luminosity studies.

-- PatRyan - 22 Feb 2007, -- JennyHolzbauer - 10 Sep 2007-- JennyHolzbauer - 30 Jan 2008
Topic attachments
I Attachment Action Size Date Who Comment
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evt_sel_jet_005500_pt2_one_pt25.pngpng evt_sel_jet_005500_pt2_one_pt25.png manage 23.2 K 06 Apr 2007 - 12:49 UnknownUser  
evt_sel_jet_005500_pt2_one_pt30.pngpng evt_sel_jet_005500_pt2_one_pt30.png manage 23.2 K 06 Apr 2007 - 12:49 UnknownUser  
evt_sel_jet_005501_pt2_one_pt20.pngpng evt_sel_jet_005501_pt2_one_pt20.png manage 21.7 K 06 Apr 2007 - 12:49 UnknownUser  
evt_sel_jet_005501_pt2_one_pt25.pngpng evt_sel_jet_005501_pt2_one_pt25.png manage 21.7 K 06 Apr 2007 - 12:49 UnknownUser  
evt_sel_jet_005501_pt2_one_pt30.pngpng evt_sel_jet_005501_pt2_one_pt30.png manage 21.7 K 06 Apr 2007 - 12:50 UnknownUser  
evt_sel_jet_005502_pt2_one_pt20.pngpng evt_sel_jet_005502_pt2_one_pt20.png manage 21.8 K 06 Apr 2007 - 12:50 UnknownUser  
evt_sel_jet_005502_pt2_one_pt25.pngpng evt_sel_jet_005502_pt2_one_pt25.png manage 21.8 K 06 Apr 2007 - 12:51 UnknownUser  
evt_sel_jet_005502_pt2_one_pt30.pngpng evt_sel_jet_005502_pt2_one_pt30.png manage 21.9 K 06 Apr 2007 - 12:51 UnknownUser  
evt_sel_jet_009901_pt2_one_pt20.pngpng evt_sel_jet_009901_pt2_one_pt20.png manage 26.5 K 06 Apr 2007 - 12:52 UnknownUser  
evt_sel_jet_009901_pt2_one_pt25.pngpng evt_sel_jet_009901_pt2_one_pt25.png manage 26.5 K 06 Apr 2007 - 12:52 UnknownUser  
evt_sel_jet_009901_pt2_one_pt30.pngpng evt_sel_jet_009901_pt2_one_pt30.png manage 26.4 K 06 Apr 2007 - 12:53 UnknownUser  
Topic revision: r46 - 16 Oct 2009, TomRockwell
 

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