TOP-12-037
Search for flavor changing neutral currents in top quark decays in proton-proton collisions at √s= 8 TeV

Abstract

The results of a search for flavor changing neutral currents in top quark decays t → Zq in events with a topology compatible with the decay chain tt → Wb+Zq → l ν b + l l q are presented. The search is performed with a data sample corresponding to an integrated luminosity of 19.5 fb^−1 of proton-proton collisions at a center-of-mass energy of 8 TeV, collected with the CMS detector at the LHC. The observed number of events are compatible with the standard model prediction and noevidence for flavor changing neutral currents in top quark decays is found. A t → Zq branching fraction greater than 0.07 % is excluded at the 95 % confidence level.

Signal Efficiencies

Channel efficiencies [%]
eee 0.485 ± 0.012
eeμ 0.531 ± 0.013
μμe 0.554 ± 0.013
μμμ 0.549 ± 0.013
All 2.118 ± 0.026

Signal selection efficiency for each three-lepton channels in percent. The efficiency is calculated as the fraction of events with leptonically (e, μ, τ) decaying W and Z bosons passing the selection. The branching fractions of W and Z leptonic decay channels are not included. Only statistical uncertainties are shown.

Figures

Basic kinematic distributions

Figure Caption
hist_met.png The missing transverse energy distribution in events which pass the basic selection assuming BF(t → Zq) = 0.1 %.
hist_wen.png The reconstructed eν transverse mass of the W boson candidate, in events which pass the basic selection assuming BF(t → Zq) = 0.1 %.
hist_wmn.png The reconstructed μν transverse mass of the W boson candidate, in events which pass the basic selection assuming BF(t → Zq) = 0.1 %.

Reconstructed top quark Mass Distributions in b-tagging selection

Figure Caption
hist_mass_tcz.png Reconstructed mass of a top candidate from a non-b-jet and Z for an integrated luminosity of 19.5 fb^−1.
hist_mass_tcz.png Reconstructed mass of a top candidate from a b-jet and W for an integrated luminosity of 19.5 fb^−1.
hist_mass_tcz.png 2D scatter distributions of the two top candidates for an integrated luminosity of 19.5 fb^−1.

Results

Selection Data-driven estimation SM MC prediction
t → Zq (B=0.1%) --- 6.36±0.08±1.27
WZ   0.87±0.10±0.62
ZZ 1.54±0.12±0.74 0.07±0.01±0.05
Drell-Yan   0.00±0.03±0.02
tt   0.74±0.70±0.52
Ztt   1.09±0.13±0.77
Wtt 1.60±4.96±0.44 0.09±0.05±0.06
tbZ   0.33±0.02±0.23
Total background 3.14±4.97±1.17 3.19±0.72±2.26
Data Observed 1 ---
Expected limit at the 95% CL BF( t → Zq) < 0.10% ---
Observed limit at the 95% CL BF( t → Zq) < 0.07% ---

-- YuanChao - 15-May-2013

Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf Mass_Correlation.pdf r1 manage 14.4 K 2013-05-15 - 18:27 YuanChao  
PNGpng Mass_Correlation.png r1 manage 17.9 K 2013-05-15 - 17:28 YuanChao 2D scatter distributions of the two top candidates
PDFpdf hist_mass_tbw.pdf r1 manage 16.3 K 2013-05-15 - 18:27 YuanChao  
PNGpng hist_mass_tbw.png r1 manage 15.8 K 2013-05-15 - 17:23 YuanChao Reconstructed mass of a top candidate from a b-jet and W
PDFpdf hist_mass_tcz.pdf r1 manage 16.4 K 2013-05-15 - 18:27 YuanChao  
PNGpng hist_mass_tcz.png r1 manage 15.7 K 2013-05-15 - 17:22 YuanChao Reconstructed mass of a top candidate from a non-b-jet and Z
PDFpdf hist_met.pdf r1 manage 18.4 K 2013-05-15 - 18:25 YuanChao  
PNGpng hist_met.png r1 manage 18.0 K 2013-05-15 - 17:19 YuanChao The missing transverse energy distribution
PDFpdf hist_wen.pdf r1 manage 17.2 K 2013-05-15 - 18:25 YuanChao  
PNGpng hist_wen.png r1 manage 18.8 K 2013-05-15 - 17:20 YuanChao The reconstructed eν transverse mass of the W
PDFpdf hist_wmn.pdf r1 manage 17.1 K 2013-05-15 - 18:26 YuanChao  
PNGpng hist_wmn.png r1 manage 18.9 K 2013-05-15 - 17:21 YuanChao The reconstructed μν transverse mass of the W
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Topic revision: r3 - 2013-05-15 - RobertoTenchini
 
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