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tW 共振态 b* 在 LHC/CMS 上的寻找 孟祥伟
Guoming Chen, Xiangwei Meng, Sarmad Shaheen, Huaqiao Zhang and Shihai Zhu IHEP,CAS
粒子天体中心 . 高能物理研究所中国科学院
中国物理学会高能物理分会 第 9 届全国会员代表大会暨学术年会 武汉 . 湖北 4 月 18-22 日 .2014 年
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Contents Introduction
Data Strategy and methods
preSelection Background estimation Selection optimization Systematic uncertainties Limit setting
Search results
Summary
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Introduction Single top quark signature sensitive to many models of new physics
b* framework Excite quark/lepton could reveal the substructure of SM fundamental particles Composite 3rd generation quark model allows FCNC/SM coupling
Previous searches mostly exploit the coupling between excited and u/d
Recent search by ATLAS using full 7 TeV data, set lower limit to ~1TeV with 4.7/fbPhys. Lett. B 721 (2013) 171-189
Aim at full 8TeV 2012 data at CMS Sharing the same selection and background modeling as other tW analysis
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Single b* theory b* predicted by composite 3’rd generation model J. Nutter, R. Schwienhorst, D. Walker, J.-H. Yu, Phys. Rev. D 86 (2012) 094006, arXiv:1207.5179 [hep-ph] generated coupling to a gloun and a b quark decay could be bg, bZ, bH or tW purely left-handed, purely right-handed, or vector like with equal couplings to both left and
right-handed parts
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Leading-order Feynman diagram for single-b∗-quark production and decay to Wt
Data and MC samples 2012 data: 8TeV 19.8/fb Muon/Electron data Stream
MC samples b* samples MadGraph5+pythia Other backgrounds: official production
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process generator Cross section [pb]
tW Powheg+pythia 22.2 (NNLL)Single top t-channel Powheg+pythia 87.1(NNLL)Single top s-channel Powheg+pythia 5.55(NNLL)ttbar MadGraph+Pythia 245.8(NNLL)W+jets MadGraph+Pythia 36257.2(LO)Z+jets MadGraph+Pythia 3503.71(NNLO
)WW Pythia 57.110(LO)WZ Pythia 32.316(LO)ZZ Pythia 8.255(LO)QCD(EM enriched, 20<pT<30 GeV) Pythia 2914860(LO)QCD(EM enriched, 30<pT<80 GeV) Pythia 3866200(LO)QCD(EM enriched, 80<pT<170 GeV) Pythia 183294.9(LO)QCD(EM enriched, 170<pT<250 GeV) Pythia 4586.5 (LO)QCD(Mu enriched, Pt>20GeV) Pythia 134680(LO)
Dedicated b*tWlepton+jet, left-/right-handed b*full simulation, central produced
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Object definition/Event Selection Electron
MVA Id, Pt>30GeV, |eta| < 2.4 && not in 1.4442<|eta|<1.5660, leptonsRhoCorrectedRelIso<0.1
Muon PF/global Muon, Pt>26GeV,|eta|<2.1 leptonDeltaCorrectedRelIso<0.1Delta R(muon,jet)>0.3
Jet: PF AK5 jets, Pt>40 GeV, B-tagged Jet: CSVT jets MET: unclustered MET
CMS Single Top Group definition
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Preselection high Pt trigger Chain
Muon: HLT_IsoMu24_eta2p1_v13/ HLT_Ele27_WP80_v* Primary vertex, Good Run, data cleaning etc. Exactly one electron or muon: Veto additional loose leptons Select only 3 Jets, one b-tagged
Optimized with central jets of |eta|<2.4 Preselection + lepton Pt>130 GeV
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QCD enriched control region
QCD from these plots are demonstrate onlyfrom MC simulation predictions, limited by MC statistics
Anti-isolation control region: QCD purity ~99%Electron: RhoCorrectedRelIso > 0.3Muon: DeltaCorrectedRelIso > 0.3
Electron Channel Muon Channel
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QCD background Fitting
QCD Data Driven WeightMuon+3J1T : (2227 ± 240) / 33731 Electron+3J1T: (3143 ± 298) / 46291
Muon ChannelQCD = 2227
Electron ChannelQCD = 3143
Fit methods: p0*MC + p1*QCD, MC: MC simulation of single top, ttbar, W, Z, dibosonQCD: data QCD enriched control region
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region contaminated for uncertainties Muon+3J1T : ± 130 / 33731 Electron+3J1T: ± 53 / 46291
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Data Driven W+Jets estimation
W+Jets Data Driven WeightMuon+3J1T : (8724 ± 545) / 4598 Electron+3J1T : (9211 ± 478) / 3956
Fit methods: p0*MC + p1*Wjets + QCDQCD: from DataW+Jets: MC sampleMC: MC samples of single top, ttbar, Z, diboson
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Muon ChannelW+jets = 8724
Electron ChannelW+jets = 9211
MC to model the shape of W+jets, other detector systematics like JES/JER/Btag/mis-Tag/leptonSF etc also considered as the uncertainties that impact W+jets shape modeling
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Data driven Estimation CheckFor QCD estimation, we use W+jets from MC predictionNow fixed this W+jets to data driven value and redo QCD
estimation as cross check
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Electron ChannelQCD = 2299
Muon ChannelQCD = 3304
Changes on QCD estimation within the fitting uncertaintiesScale factors on other MCs are very close to 1
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Selection Optimization Methodology
Optimize the cuts on jet pT, leading jet pT, MET, transverse W mass and MET
1. Change cut on jet pT, redo the whole analysis to find the best EXPECTED limit on the 1300 GeV b*
2. Fix jet pT to the value that find above, redo step1 with other variables and fix to the optimized cut
3. After optimized on all variables, loop again to the first optimized variable and so, until the optimization does not change the cut
This lead to the final selection Preselection (jet |eta|<2.4) + lepton Pt > 130 GeV
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Events Yield
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Statistical uncertainties only
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Data/MC at final Selection
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MET
Electron Channel
Muon Channel
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Systematic Uncertainties I Theory modeling (both Acceptance and
Shape) Q2: Dedicated samples with Factorization
and Renormalization scale vitiate a factor 4 or ¼6-8% impact on the acceptance
Top Mass: Dedicated sample with top mass variant +-1 GeV2-4% impact on the acceptance
Matching: Dedicated sample with MLE matching threshold changed13% on ttbar
PDF: Use CT10 Error PDF to evaluate both acceptance and shape unc. 4-12%, on the backgrounds, 17-58% on
the signal
Bkg Normalization Acceptance only
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sample Q2 Top Mass
Matching PDF
ttbar Y Y Y YtW N Y N YT-channel Y Y N YS-channel N N N YW+jets N N N YZ+jets N N N YWW/WZ/ZZ N N N Y
QCD N N N Yb* N N N Y
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Systematic Uncertainties II Detector effects
Lepton Scale factors ID/Trigger/Isolation
B-tagging Mis-tagging JES/JER Pile-Up
Luminosity: 2.6%, Acceptance only
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Theory modeling Theory: >~10%
Bkg Normalization ~10%-30%
Detector effects JES: 1-5% B-tag: 2-4% Others: <1%
Luminosity: 2.6%
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following recipes from Top group recommendation, including both acceptance and shape uncertainties
Limit Setting
Tool: Theta Method: Asymptotic CLs limits
Cross-checked by Bayesian limits within Theta Likelihood function
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Fitting variable: b* mass
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Electron channel
Muon channel
b* mass
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Limit results
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With Linear assumption for the theory cross-section
Exp:Obv:
Left-handed
right-handed
Vector Like
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Systematics Impact on limit Remove one systematic each time to see the change of
expected limits Biggest impact comes from PDF (11GeV) , all background
normalization(7GeV), JES(4GeV)
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Unc. removed Exp. Limit Unc. removed Exp. Limit--- 951 Mis-tag 951JES 955 Pile-Up 952JER 951 Matching 952lepSFID 951 PDF 962lepSFIso 951 Q2 952lepSFTrig 949 Top Mass 949B-tag 951 Normalization 958Lumi 951
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Summary and Outlook
b* searched on full 8 TeV data CMS with tW Lepton+Jets channel Data driven QCD and W+Jets estimation Systematic uncertainties
No tW resonance bump found, Limit setting for left-,right-handed and vector like b* b* below 950GeV excluded at 95% CL (purely left-handed
couplings) the systematics impact checked
Will combine results from dilepton channel and full hadronic channel
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THANKS!
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