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Search for a Heavy Top-like Quark in the Dilepton Final State in pp Collisions at sqrt(s)=7 TeV

Abstract

The results of a search for pair production of a heavy top-like $t'$ quark in the decay mode $t' \bar{t'} \to bW^{+} \bar{b}W^{-} \to b \ell^{+} \nu \: \bar{b} \ell^{-} \bar{\nu}$ are presented. The search is performed in a data sample corresponding to a total integrated luminosity of 1.14 fb-1 of pp collisions at a centre-of-mass energy of 7 TeV, collected by the CMS experiment at the LHC. The observed number of events agrees with the standard model prediction, and no evidence of $t' \bar{t'}$ production is found. Upper limits on the production cross section as a function of $t'$ mass are presented. A $t'$ with a mass below 422 GeV/c2 is excluded at the 95% C.L.

Approved Tables:

Table 1

The observed and simulated yields after the preselection described in the text, for an integrated luminosity of 1.14 fb-1. Uncertainties are statistical only.

Sample ee μμ eμ all
$t'\bar{t'}$, $M_{t'} = 350$ GeV/c2 5.63 ± 0.41 5.63 ± 0.38 13.43 ± 0.61 24.69 ± 0.83
$t'\bar{t'}$, $M_{t'} = 400$ GeV/c2 2.51 ± 0.18 2.92 ± 0.19 6.33 ± 0.28 11.76 ± 0.38
$t'\bar{t'}$, $M_{t'} = 450$ GeV/c2 1.45 ± 0.09 1.53 ± 0.09 3.27 ± 0.14 6.25 ± 0.19
$t\bar{t}\rightarrow\ell^+\ell^-$ 167.46 ± 5.85 178.88 ± 5.71 445.45 ± 9.30 791.79 ± 12.38
$t\bar{t}\rightarrow\mathrm{fake}$ 3.35 ± 0.85 0.19 ± 0.19 5.81 ± 1.04 9.35 ± 1.36
$W+\mathrm{jets}$ <2 <2 <2 <2
$DY\rightarrow\ell^+\ell^-$ 2.23 ± 1.39 2.15 ± 1.66 <1 4.38 ± 2.17
Di-boson 0.04 ± 0.01 0.14 ± 0.07 0.14 ± 0.07 0.31 ± 0.10
Single top 2.63 ± 0.28 2.41 ± 0.26 7.03 ± 0.45 12.06 ± 0.59
Total simulated background 175.70 ± 6.08 183.76 ± 5.96 458.43 ± 9.37 817.88 ± 12.66
Data 184 182 512 878

Link to PDF of Table 1

Table 2

The expected yields of $t'\bar{t'}$ events in the signal region for different values of $M_{t'}$, for an integrated luminosity of 1.14 fb-1. Uncertainties are statistical only.

Sample ee μμ eμ all
$t'\bar{t'}$, $M_{t'} = 350$ GeV/c2 1.74 ± 0.23 1.66 ± 0.21 4.61 ± 0.36 8.01 ± 0.47
$t'\bar{t'}$, $M_{t'} = 400$ GeV/c2 1.06 ± 0.12 1.30 ± 0.12 2.56 ± 0.18 4.92 ± 0.24
$t'\bar{t'}$, $M_{t'} = 450$ GeV/c2 0.60 ± 0.06 0.76 ± 0.07 1.55 ± 0.09 2.91 ± 0.13
$t'\bar{t'}$, $M_{t'} = 500$ GeV/c2 0.44 ± 0.05 0.38 ± 0.04 0.96 ± 0.06 1.79 ± 0.09
$t'\bar{t'}$, $M_{t'} = 550$ GeV/c2 0.24 ± 0.02 0.26 ± 0.02 0.53 ± 0.03 1.02 ± 0.05
$t'\bar{t'}$, $M_{t'} = 600$ GeV/c2 0.14 ± 0.02 0.12 ± 0.02 0.33 ± 0.03 0.58 ± 0.03

Link to PDF of Table 2

Table 3

Summary of the observed and predicted yields. Uncertainties include both statistical and systematic errors, apart from for di-boson and single top where uncertainties are statistical only.

Sample Yield Source
$t\bar{t}\rightarrow\ell^+\ell^-$ 1.35 ± 0.67 Data
Fake leptons 0.0+0.4-0.0 Data
$t\bar{t}\rightarrow e^+e^- or \mu^+\mu^-$ 0.07+0.13-0.07 Data
$t\bar{t}\rightarrow\tau^+\tau^-$ 0.11 ± 0.11 Simulation
Di-boson 0.02 ± 0.02 Simulation
Single top 0.07 ± 0.04 Simulation
Total prediction 1.62+0.80-0.70  
Data 1  

Link to PDF of Table 3

Table 4

Efficiency x Acceptance x Branching Ratio in simulated events for different t' masses. Each value has a relative uncertainty of 22%.

Sample Eff x Acc x BR
$t'\bar{t'}$, $M_{t'} = 350$ GeV/c2 0.24%
$t'\bar{t'}$, $M_{t'} = 400$ GeV/c2 0.33%
$t'\bar{t'}$, $M_{t'} = 450$ GeV/c2 0.41%
$t'\bar{t'}$, $M_{t'} = 500$ GeV/c2 0.50%
$t'\bar{t'}$, $M_{t'} = 550$ GeV/c2 0.55%
$t'\bar{t'}$, $M_{t'} = 600$ GeV/c2 0.58%

Link to PDF of Table 4

Table 5

The expected theoretical cross section of $t'\bar{t'}$ production and the 95% C.L. upper limits on the production cross section of $t'\bar{t'}$.

Mt' 350 GeV/c2 400 GeV/c2 450 GeV/c2 500 GeV/c2 550 GeV/c2 600 GeV/c2
Theory (pb) 2.940 1.301 0.617 0.310 0.162 0.088
Expected (pb) 1.517 1.103 0.888 0.728 0.662 0.628
Observed (pb) 1.406 1.022 0.823 0.675 0.613 0.582

Link to PDF of Table 5

Approved Plots from EXO-11-050 ( click on figures to get .pdf )

Figure Caption
mass2d_gen.png Expected distribution of $M_{l1b1}$ and $M_{l2b2}$ as predicted by the simulation for $t\bar{t}$ events (blue points) and for $t'\bar{t'}$ events with $M_{t'}$=350 GeV/c2 (red points): $M_{l1b1}$ vs. $M_{l2b2}$ at generator level, where $M_{l1b1}$ (or $M_{l2b2}$) is the mass of the lepton and b quark from the same t/t'. The signal region is defined by $M_{l1b1}$>170 GeV/c and $M_{l2b2}$>170 GeV/c.
mass2d_reco.png Expected distribution of $M_{l1b1}$ and $M_{l2b2}$ as predicted by the simulation for $t\bar{t}$ events (blue points) and for $t'\bar{t'}$ events with $M_{t'}$=350 GeV/c2 (red points): $M_{l1b1}$ vs. $M_{l2b2}$ at reconstruction level, where we assume the lepton and b-jet from the same t/t' are closest in ΔR. The signal region is defined by $M_{l1b1}$>170 GeV/c and $M_{l2b2}$>170 GeV/c.
ltb.png Comparison of data and the simulated background for $M_{l1b1}$ (when $M_{l2b2}$>170 GeV/c2). The contributions from the minor sources of backgrounds, which are specified in Table 3, are combined. The simulated background yields are scaled in the signal region so that they match the background estimations from the data driven methods, and the yields outside the signal region are from simulated background yields without rescaling. The signal region is defined by $M_{l1b1}$~$>170 GeV/c2. One data event is observed in the signal region. The expected distributions for a $t'\bar{t'}$ signal are also shown for $M_{t'}$=350 GeV/c2.
llb.png Comparison of data and the simulated background for $M_{l2b2}$ (when $M_{l1b1}$>170 GeV/c2). The contributions from the minor sources of backgrounds, which are specified in Table 3, are combined. The contributions from the minor sources of backgrounds, which are specified in Table 3, are combined. The simulated background yields are scaled in the signal region so that they match the background estimations from the data driven methods, and the yields outside the signal region are from simulated background yields without rescaling. The signal region is defined by $M_{l2b2}$>170 GeV/c2. One data event is observed in the signal region. The expected distributions for a $t'\bar{t'}$ signal are also shown for $M_{t'}$=350 GeV/c2.
limit.png The 95% C.L. upper limits on the production cross section of $t'\bar{t'}$ as a function of t' mass are shown. The expected 95% C.L. lower bound on $M_{t'}$ is 415 GeV/c2 and the observed 95\% C.L. lower bound on $M_{t'}$ is 422 GeV/c2.
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PDFpdf limit.pdf r1 manage 16.4 K 2011-08-11 - 17:06 OliverGutsche  
PNGpng limit.png r1 manage 108.6 K 2011-08-11 - 17:06 OliverGutsche  
PDFpdf llb.pdf r2 r1 manage 15.1 K 2011-08-11 - 23:07 OliverGutsche  
PNGpng llb.png r2 r1 manage 65.3 K 2011-08-11 - 23:07 OliverGutsche  
PDFpdf ltb.pdf r2 r1 manage 15.1 K 2011-08-11 - 23:08 OliverGutsche  
PNGpng ltb.png r2 r1 manage 66.9 K 2011-08-11 - 23:08 OliverGutsche  
PDFpdf mass2d_gen.pdf r2 r1 manage 37.3 K 2011-08-12 - 03:22 OliverGutsche  
PNGpng mass2d_gen.png r2 r1 manage 108.3 K 2011-08-12 - 03:22 OliverGutsche  
PDFpdf mass2d_reco.pdf r2 r1 manage 48.6 K 2011-08-12 - 03:22 OliverGutsche  
PNGpng mass2d_reco.png r2 r1 manage 126.6 K 2011-08-12 - 03:22 OliverGutsche  
PDFpdf table1.pdf r1 manage 245.8 K 2011-08-11 - 17:09 OliverGutsche  
PDFpdf table2.pdf r2 r1 manage 277.4 K 2011-08-12 - 20:33 OliverGutsche  
PDFpdf table3.pdf r1 manage 159.3 K 2011-08-11 - 17:09 OliverGutsche  
PDFpdf table4.pdf r1 manage 159.3 K 2011-08-11 - 17:09 OliverGutsche  
PDFpdf table5.pdf r1 manage 159.3 K 2011-08-11 - 17:09 OliverGutsche  
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