semileptonic ttbar + jet events Kenneth Wraight semileptonic ttbar + jet events pt spectra of extra jets comparison of matching methods preliminary reconstruction results
Motivations Top physics is an opportunity to test multiscale QCD calculations by comparing tt+j and tt+jj channels, as well as cross checking generator (e.g. Alpgen) predictions with W+jet events. Glasgow’s interest lies in associated Higgs production. Understanding the extra jet pt is a major step to controlling the background to ttH, Hbb and SUSY channels (10s of ttbar events per second for σ=833pb at L=1034cm-2s-1). 1
matching is used as a measure of merit In physics: hard processes hadronise and interact with the detector to produce jets which are the input of analysis algorithms In simulation: generated samples reflect this tripartite structure: Generator Level (GL) -> Shower jets -> Simulation Level (SL) to measure theoretical success (i.e. efficiency) of an analysis a connection between GL partons from the hard process and SL jets is therefore required aim is to unambiguously match a parton to jet through the simulation process by a generator independent procedure Hence kinematic (e.g. η, pt, Et) properties are used rather than individual mapping if an analysis product is composed of the matched jets it is well-matched What if there is more than one matching procedure? What if they do not give the same results? 2
Nikhef method (ATL-COM-PHYS-2008-046 M. Gosselink et al.) top daughter partons = 2 b quarks & 2 light quarks Two protocols Nikhef method (ATL-COM-PHYS-2008-046 M. Gosselink et al.) Compare reconstructed top vector to generated top vector. If ΔR<0.4, then declare top well-matched Alternative methods A: Match generated top daughter partons to shower jets (ΔR<0.4) then on to simulated jets (ΔR<0.4). The event is then matched. If top is reconstructed from jets matched to generator level partons, then declare top well-matched B: Match generated top daughter bs and hadronic W to shower jets (ΔR<0.4) then on to simulated jets (ΔR<0.4). The event is then matched. If top is reconstructed from jets matched to generator level partons, then declare top well-matched C: Match generated top daughter bs and hadronic W to simulation jets (ΔR<0.4). The event is then matched. If top is reconstructed from jets matched to generator level parton, then declare top well-matched Which is ‘The best’ method of matching? (I will not answer this) 3
defining efficiencies to compare matching ξsel = #events passing trig.&sel. cuts / #events in sample depends on triggering and selection criteria ξmatch = #events matched / #events passing trig.& sel. cuts depends on matching criteria matching dependent ξwell = #events with ‘well-matched’ top chosen / #events matched combinatorics (, some selection) matching dependent ξpotential = #events with ‘well-matched’ present / #events matched 4
relevance of efficiencies Product gives a global efficiency: ξglobal = ξsel * ξmatch * ξwell = #events with ‘well-matched’ top chosen / #events in sample In comparing matching methods ξmatch and ξwell are used. ξmatch to assess how many sample events can be used to check analysis ξwell to assess the success of the analysis according to the matching In addition, ξwell / ξpotential provides a measure of success of reconstruction algorithm. 5
standard commissioning * electron or muon with Ptlep>20GeV within |η|<2.5 fulfilling isolation requirement Et,ΔR=0.2<6. GeV. Also ignore electrons in crack region 1.37<|η|<1.52. ** jets with Ptlep>20GeV within |η|<2.5. Also remove jets with good electron nearby i.e. ΔRj-el<0.2. standard commissioning trigger isolated electron Ptel>25GeV or isolated muon Ptmu>20GeV or analysis cuts exactly one good* lepton remove dilepton events Etmiss>20GeV neutrino >3 good** jets with Ptjet>20GeV four quarks from tops >2 good** jets with Ptjet>40GeV hadronic top products (n b-tagged jets (n=1 or 2), not used here) reconstruction simple: take highest Pt tri-jet combination Wmass constraint: take highest Pt tri-jet combination provided it has di-jet combination with Mjj-MW<10.GeV... used here centrality: take highest Pt tri-jet combination with jets in |η|<1 both: take highest Pt tri-jet with both Wmass and centrality constraints 6
efficiencies from standard commissioning events from 5200 sample (MC@NLO, fullsim) normalised to 100pb-1 ξsel= 19.5% (es:16.6%, μs:21.9%) efficiencies from standard commissioning method matched ξmatch(%) well-recon. ξwell(%) potential ξpot.(%) ξglobal(%) Nikhef 4707 100. 1063 22.6 1523 32.4 4.3 A 2682 60.8 346 12.1 698 24.4 1.4 B 2936 62.4 341 11.6 678 23.1 C 3434 73.0 397 832 24.2 1.6 ξmatch Nikhef matching criterion fulfilled by default other methods more complex criteria non-maximal matching especially A&B where two matching steps are required events passing other methods are subset of Nikhef passed events (ΔRsimtop<0.4) Not all (~74%) of Nikhef tops are within 10GeV of other matched top (in case C). ξwell & ξpotential ... comparing recon. algorithm Loss of events from ξmatch to ξpotential is due to kinematic cuts (pt, η, jetcut=3) By Nikhef method, 1/3 of events incorrectly reconstructed (combinatorial) By, other methods, ~1/2 of events are incorrectly reconstructed (combinatorial) ξglobal Nikhef method defines ~3 times as many selected and well-matched events 7
* quoted values are not exclusive Non-unique matches exist in the case of each protocol... multiple matching Nikhef: 26% A*: 55% 1st step 44% 2nd step B*: 74% 1st step 35% 2nd step C: 31% 8
more top combinations making reconstruction more difficult Concentrate on Nikhef and C methods... 5212 sample 5212 sample (MC@NLO&Herwig, fullsim) has at least two extra jets in each event selection efficiencies: ξsel= 29.1% (es:25.1%, μs:32.6%) method ξmatch(%) ξwell(%) ξpot.(%) ξglobal(%) Nikhef 100. 20.0 31.7 5.8 C 72.0 10.1 25.3 2.1 multiple matches are more common in this higher multiplicity sample: Nikhef: 35% and method C: 36% more jets are present to pass the jet selection and fake lepton selection. more top combinations making reconstruction more difficult & more jets combinations to pass matching criteria. 9
properties of matched jets using method C Hadronic W daughter in green & red, hadronic b in blue properties of matched jets using method C b-weight: ΔEjet->parton: 5200 sample 5212 sample 10
reconstructed top mass reconstructed top (black) with non-well-matched proportion (red) reconstructed top mass Nikhef method: method C: 5200 sample 5212 sample 11
Nikhef method in black, method C in red area normalised Nikhef method in black, method C in red extra jet pt spectra X≥1 X≥2 5200: 5212: 12
Calculating cross-section Split sample in two sample A sample B use half to determine efficiencies and define acceptance: Purity= #well-matched / #reconstructed matching dependent Efficiency: #well-matched / #sample events Acceptance = Efficiency / Purity = #reconstructed / #sample events matching independent apply acceptance to other half to get cross-section plots Ngen = (Ndet / Lum.) * (1/ Acc.) and compare to generated cross-section
cross-sections (unscaled) Generated mass & pt plots from shower level cross-sections (unscaled) tri-jet mass: tri-jet pt: calculated generated 5200 sample 5212 sample 13
Matching independent method used to calculate cross-sections Summary Comparison of two different matching procedures shows disparities between results. Different matching procedures give different degrees of confidence in analysis method. This ambiguity makes testing multiscale QCD calculations and generator predictions more difficult. These difficulties are particularly important for extra-jet studies where the identity of the simulated jets is crucial. Matching independent method used to calculate cross-sections Next step is to investigate fully leptonic channel where it is easier to identify extra jets and not as reliant on jet matching. 14
method ξmatch(%) ξwell(%) ξpot.(%) ξglobal(%) Nikhef 100. 22.6 33.4 Three parton (hadronic side) efficiencies BACKUP 5200 sample: selection efficiencies: (as before) ξsel= 29.1% (es:25.5%, μs:32.6%) method ξmatch(%) ξwell(%) ξpot.(%) ξglobal(%) Nikhef 100. 22.6 33.4 4.3 C_3 76.9 11.3 23.7 1.7 multiple matches are more common in this higher multiplicity sample: Nikhef: 35% and method C_3: 23% with only three partons to match: fewer opportunities to double cross match partons (so fewer events rejected) fewer opportunities to match parton to multiple jets 9