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

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

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

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)

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

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

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%

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

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

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

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

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)

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%

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)

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%)

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)

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

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