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Korean Physical Society 201226/10/2012
Study of top mass at CMS using Bi-Event
Subtraction TechniqueKorean Physical Society 2012
Junghwan Goh, Minsuk Kim, Intae Yu (SKKU),Tai Sakuma, Teruki Kamon (Texas A&M)
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Outline Motivation of Bi-Event Subtraction Technique (BEST) BEST on tt̄ semi-leptonic channel Datasets and event selection Closure test with MC Results Summary and prespects
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Motivation of BEST
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How to find X→jj ?
‣ Choose a best dijet candidate among cascade decays of interest
‣ Or, allow combinations and estimate effect of random combinations
Challenge for background modeling
‣ Should be model independent & no kinematic preferences
‣ Data driven method
‣ More sensitivity for new physics search with multi jets
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Korean Physical Society 2012
Combinatorial background with BEST Mixture of jet information from different events will give
random & uncorrelated pair of dijets Mass distributions from bi-event to subtract from the one with
all jet combinations - resulting dijet distribution with most of the combinatorial background removed
Published in the paper : Kamon et.al, PLB Vol 703, p475-478
4Mixture of two events
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Top quark physics at CMS
5 [GeV]topm160 165 170 175 180 185-0.5
6.8
CMS combined result 0.9± 0.4 ±173.4 up to L= 5.0/fb syst.)± stat. ±(val.
CMS 2011 all-jets 1.3± 0.7 ±173.5 PAS-TOP-11-017 (L=3.54/fb) syst.)± stat. ±(val.
CMS 2011 lepton+jets 1.2± 0.3 ±173.5 CMS PAPER TOP-11-015 (L=5.0/fb) syst.)± stat. ±(val.
CMS 2011 dilepton 1.4± 0.4 ±172.5 CMS PAPER TOP-11-016 (L=5.0/fb) syst.)± stat. ±(val.
CMS 2010 lepton+jets 2.7± 2.1 ±173.1 )-1PAS-TOP-10-009 (L=36 pb syst.)± stat. ±(val.
CMS 2010 dilepton 4.6± 4.6 ±175.5 )-1JHEP 07 (2011) (L=36 pb syst.)± stat. ±(val.
CMS Preliminary
CMS combined result
CMS Preliminary
Available Top reconstruction methods :
Kinematic fitX2 fit to assumed mass constraints
MT2Applicable to dileptons
M3Invariant mass of 3 jet & dijet with highest pT
And so on...
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Top quark study with BEST
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Event topology with many jets
Jet combination problem can be solved by BEST
Strategy Start from semi-leptonic channel Extend to full-hadronic channel
Lepton+JetsDilepton Hadronic
Missing neutrino Jet combination
Clean event Full kinematics
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Dataset and event selection
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CM energy : 8TeVMC : tt̄ MadGraph + Pythia & tauolaReal data : 2012 summer (~5fb-1)
Event selectionMuon+3Jet triggered dataset1 isolated muon |η| < 2.1≥2 jets with pT > 45GeV≥4 jets with pT > 35GeV≥2 b tagged jets
High purity after event selection
Jet reconstruction and selectionParticle Flow, Anti-kT (R=0.5) with JESpT > 35GeV, |η| < 2.4b Tag algorithm based on SecVtx
W and top reconstructionNo overlap between jets (∆R > 0.4)2 untagged jet for W, +1 bjet for topNo lepton/MET for mass reconstruction
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Counting combinations
8Same eventSame event Bi eventBi event
Pairs Counts Pairs Counts
W signal
biW
J+K
K+K
HadB+J(same top)
LepB+J
B+XB+X
J+J 1
J+J’ 4
J+K 2n J+K’, K+J’ 2(n+m)
K+K n(n-1)/2 K+K’ nm
J+b 2
J+B 2 J+B’, B+J’ 8
b+B 1 b+B’, B+B’ 4
b+K, B+K 2n b+K’, B+K’,... 2(n+m)
Objects in shorthand notationJ : Jets from Wb/HadB : b quark in hadronic topB/LepB : b quark in leptonic topk : jets not from top decay
Reducible by b tagging
W signalArtificial background
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MC closure test
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Same event8TeV
Bi event8TeV
Work in progress
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Three-jet mass distributions
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Same event8TeV
Bi event8TeV
Work in progress
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First look on real data
11Work in progress
Same event8TeV
Same event8TeV
Bi event8TeV
Bi event8TeV
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Bi event subtraction with fitting
12Work in progress
CMS work in progress, 5fb-1 at √s=8TeV
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Korean Physical Society 2012
Possible applications Jet energy correction
A calibration tool with W jet resonance
Measurement of top quark properties Top quark mass and production cross section
without kinematic assumptions
Dijet and multi-jet resonance search Model independent, data driven background estimation Beyond Standard Model searches with multijets
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Korean Physical Society 2012
Summary and prospects Combinatorial background with
Bi-Event Subtraction Technique Successfully applied BEST to tt̄ system
W and top mass peak extraction with CMS 2012 data Shown to be consistent with W and top quark mass Validation of technique with real data
Method is rapidly developing Aiming for independent measurement of top mass in 2012 Extension to BSM physics with Multi-jet
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