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CMS RESULTS ON THE PHYSICS OF TOP QUARKS
James Keaveney Vrije Universiteit Brussel


Today's talk
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Introduction and context Measurements of Top cross-sections
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For Single Top measurements, see talk from Natalia Tsirova.

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Measurements of Top mass. Measurements of Top properties. Conclusions

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Introduction and context
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Top physics measurements at 7 and 8 TeV are a central part of the CMS physics program.
Mass: Top mass Mt , is crucial parameter of Standard Model, SM. Precise measurements provide electroweak constraints on new physics.

Cross sections:
provide precise test of perturbative QCD, pQCD, and constrain backgrounds in searc hes for new physics.

Properties: W polarisation tests V-A structure of Wtb vertex, new physics
contributions can modify helicity fractions. Searc hes for FCNCs in top decays sensitive to new physics.


Inclusive cross sections: top pairs (8TeV) (CMS-PAS-TOP-12-007, CMS-PAS-TOP-006)
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e/+ jets: Template fit to distribution of lepton + b-jet invariant mass. Systematics: b-tagging, JES.
Events
3500 3000
CMS preliminary, 2410 pb-1 eµ channel s=8TeV

Obs/Exp Ratio

ee//e cut based, high purity Systematics: Lepton ID, JES.

data +Z/ * l l (l=e,µ , ) VV tW Non W/Z leptons t t signal

2500 2000 1500 1000 500 0
1.6 1.4 1.2 1 0.8 0.6
0 1 2 3

b-tagged jet multiplicity

Close agreement with predictions.


Differential cross sections: top pairs (8TeV) CMS-PAS-TOP-12-028, CMS-PAS-TOP-12-041
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Differential cross section ­ top pair production as a function of the kinematics of leptons, jets, top quark and ttbar system. Compares well with approx. NNLO pQCD predictions. Measurement of jet multiplicity in top pair eventsdifferential cross-section as a function of jet multiplicity. Data compared with MADGRAPH +Pythia, MC@NLO+Herwig and POWHEG+Pythia. Data prefers MadGraph, POWHEG to MC@NLO
Top Pt

nJets


Cross sections: top pair + bottom pair (7TeV) CMS PAS TOP-12-024
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First measurement of the cross section ratio: ttbb/ttjj · The ttbb process is an irreducible background to the ttH (H->bb) process. · ttH measurements potentially allow direct measurement of Yukawa coupling to H. · Fit to b-jet multiplicity distribution in dilepton c hannel to extract ttbb/

ttbb

ttjj

/ttjj = 3.6 ± 1.1(stat) ± 0.9(syst)% (MADGRAPH 1.2% POWHEG 1.3%)


Cross sections: top pair + vector boson (7 TeV) Phys. Rev. Lett. 110 (2013) 172002
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Tri-lepton analysis: searc hes for process -

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Same-sign dilepton analysis: searc hes for processes -

Final result:


Top mass (7TeV): JHEP 12(2012) 105 [arXiv:1209.2319]
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Event Selection: 1 lepton, 4 jets, 2 b-tagged jets. Kinematic fit to jet combinations to extract top mass. Fit solutions weighted by fit probability to suppress effect of wrong combinations. 2D Max. Likelihood fit to extract Mt and JES

mt = 173.49 ± 0.43(stat. + JES) ± 0.98 (sys.) GeV


Study of the dependence of the top-quark mass measurement on event kinematics (7TeV): CMS-PAS-TOP-12-027
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· First data study of possible kinematic biases in the measurement of Mt · Mt is determined in bins of numerous kinematic obser vables. · Effects on Mt well reproduced by MADGRAPH, POWHEG and MC@NLO.


Top mass difference (8TeV) CMS-PAS-TOP-12-031
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Tests CPT invariance -> involved in extensions of the Standard Model. Kinematic fit in lepton (anti-lepton) + jets events to extract anti-top (top) masses and Mt = Mt ­ Manti-t

Final result:
Significantly more precise than previous measurements. Consistent with CPT invariance.


Top properties: Searc h for FCNC in top decay (8 TeV)

CMS PAS TOP-12-037
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Searc h for decay c hain tt Wb+Zq l b + l l q - Highly suppressed in SM, but enhanced in new physics models. Event selection: eee, ee e or , 2 jets, 1 b-jet and missing transverse energy > 30 GeV Reconstruct top candidates from Wb and Zj systems Br.(t Zq) greater than 0.07 % excluded @ 95 % C.L.


Top properties: W polarisation (7TeV) CMS-PAS-TOP-12-015
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Test V-A structure of Wtb vertex W helicity fractions (FL, F0, FR) sensitive to new physics. In limit of massless leptons, FR=0. Extract fractions from fit to * distribution. * = angle between the c harged lepton in the W rest frame and the W boson in the top rest frame
FL = 0.288 ± 0.035(stat) ± 0.040(sys) F0 = 0.698 ± 0.057(stat) ±0.063(sys) FR = 0.014 ± 0.027(stat) ± 0.042(sys) Consistent with SM.

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Conclusions
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Large and varied Top quark physics program ongoing at CMS. Measurements of cross sections (tt, ttbb, ttV) provide tests of pQCD and contraints on background to new physics searc hes. Measurements of top mass (mt, mt-tbar ) provide tests of SM consistency and CPT invariance and constrain new physics models. Measurements of top properties (W polarisation, FCNCs) are sensitive to the presence of new physics.
Top sector tested to higher precision than ever before...

No sign of deviations from SM observed.