1. Philippe Doublet, LAL Roman Pöschl & François Richard, LAL
Btagging results and LeptonFinder performances with semileptonic top events
Philippe Doublet, LAL
Roman Pöschl & François Richard, LAL

2. Ongoing study : semileptonic top events
We study tt(bW)(bW)(bqq)(blv) events with l = e , µ
Topology : 4 jets + 1 lepton
2 important steps :
Find the lepton
Find one B jet
Btagging discussed 1 month ago : remarks added today
Lepton selection improved : results

3. Btagging in semileptonic top environment
4 jets + 1 lepton to identify
The lepton with highest energy was removed
Jet clustering forced to 4 jets and B tagging were reapplied on the signal events
The 2nd highest B tagged jet has a rather small Btag value. Why ?
Look at inputs of the NeuralNet
Smaller decay length (+ larger significance)
pT corrected mass smaller
Lead to suggestion that leptonic decays of B mesons are the culpurit
We are in touch with the LCFI Vertex Group

4. Investigating leptonic B decays
We have a look at B  lvX decays
Histograms shows Btag value for real Bjets : all jets and leptonic B jets (re-scalled)
 No difference is seen
Confirmed by LCFI Group : leptonic decays of Bs should not be a problem
Black = All Bjets
Red = leptonic B jets
Distribution of Btag value for real B jets (all and leptonic only)

5. A problem of decay length
We then investigate the true decay length of B mesons w.r.t. the Btag of the jet
We find that an important fraction is lost there
Nothing can be done against this
True B decay length (from MC table) vs Btag of the corresponding jet

6. NN input variables for B, C and light jets
Major differencies

7. NN input variables for B, C and light jets with Btag > 0.8
Small fraction of non Bjets with Btag > 0.8
Purity for the jet with highest Btag = 97%

8. NN input variables for B, C and light jets with Btag < 0.8
For small Btag, Bjets have a tendency to mimic C jets (thus leading to small Btag)
Maybe due to bad jet clustering

9. Partial conclusion on Btag
Efficiency to find one event with highest Btag > 0.8 = 82%
Purity = 97% (highest Btagged jet)
Btagging performances get worse
If decay length is small

10. Finding the lepton of the semileptonic top decays
Marlin Processor developped : LeptonFinder
Ask for a lepton : electron or muon
Looks for candidates in PandoraPFO collection
Best candidate put apart in a new collection
Other PFOs separated
Number of candidates associated to the lepton collection

11. Identification
We apply identifiaction criteria on each PFOs with Ptrack > 5 GeV :
If Ecluster/Ptrack < 0.5, then muon
If Ecluster/Ptrack > 0.8 and Eecal/Ecluster > 0.9, then electron
We identify 99.3% of the good muons, 98.1% of the good electrons
Remark : Ptrack > 5 GeV suppresses 0.4% of muon events

12. Isolation (1/2)
We require isolation criteria inside the jet to define a lepton as a good candidate
Rejects most of leptonic B decays
Rejects most of particles inside the jet
Real leptons (from MC table)
Non isolated candidates  suppressed
Remark : if condition looser, larger contamination (not desired)

13. Isolation (2/2)
Events left after identification and isolation = 93.9% for muons, 93.6% for electrons
Maximum efficiency reachable ≈ 94%
Remark : tracking assumed perfect, i.e. we only look at events where the MC lepton’s track is reconstructed  real efficiency will be smaller than 94%

14. Number of candidates (all events)
We draw the distribution of the number of candidates
We define x = rate of good lepton identified (the one from tbW decay), y = rate of misidentification (pions, …)
Histogram permits to calculate x and y
x(1-y)
+ y(1-x)
Contamination term
Negligible
xy²
Population in « bin 2 » should decay in « bin 1 » when a criteria for the best lepton is defined
xy (+y²)
(1-x)(1-y)

15. Results
Type x y
contamination
Muon
92.5%
8.2%
0.6%
Electron
87.7%
10.5%
1.3%
Contamination for muons = 0.6%, for electrons = 1.3% (i.e. if only one lepton found, rate of wrong lepton)
We deduce the tracking inefficiencies, assumed to be xmax – x (with xmax≈94%)

16. Tracking inefficiencies
Type x
xmax
difference
Muon
92.5%
93.9%
1.4%
Electron
87.7%
93.6%
5.9%
If difference consists only of tracking inefficiencies, it is
1.4% for muons
5.9% for electrons
(TPC central disk + |cos θ|>0.97)

17. What if Ncandidates > 1 ?
What is the best method to select a lepton among several ?  isolation criteria (i.e. highest jet energy fraction z = Elepton / Ejet)
93% (91%) of good muon (electron) candidates selected if N > 1
In the end,
contamination = 1% for muons and 2% for electrons
93% of events with muons found and 89% of events with electrons found

18. Conclusion on the LeptonFinder
Efficiency (≈ 90%) and purity (≈99%) of the sample are satisfying
Now, LeptonFinder is applied on all DST files, then :
Jet clustering forced to 4 jets
B tagging
Cut based analysis
Preliminary results on semileptonic tt cross-section and ALR available for ECFA meeting