CMSB2G17003 ; CERNEP2017224  
Search for pair production of vectorlike quarks in the ${\mathrm{b}\mathrm{W}\mathrm{\bar{b}}\mathrm{W}} $ channel from protonproton collisions at $\sqrt{s} = $ 13 TeV  
CMS Collaboration  
2 October 2017  
Phys. Lett. B 779 (2018) 82  
Abstract: A search is presented for the production of vectorlike quark pairs, ${\mathrm{T}\overline{\mathrm{T}}} $ or ${\mathrm{Y}\overline{\mathrm{Y}}} $, with electric charge of $ 2 / 3 $ (T) or $  4 / 3 $ (Y), in protonproton collisions at $\sqrt{s} = $ 13 TeV. The data were collected by the CMS experiment at the LHC in 2016 and correspond to an integrated luminosity of 35.8 fb$^{1}$. The T and Y quarks are assumed to decay exclusively to a W boson and a b quark. The search is based on events with a single isolated electron or muon, large missing transverse momentum, and at least four jets with large transverse momenta. In the search, a kinematic reconstruction of the final state observables is performed, which would permit a signal to be detected as a narrow mass peak ($\approx$ 7% resolution). The observed number of events is consistent with the standard model prediction. Assuming strong pair production of the vectorlike quarks and a 100% branching fraction to bW, a lower limit of 1295 GeV at 95% confidence level is set on the T and Y quark masses.  
Links: eprint arXiv:1710.01539 [hepex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; 
Figures & Tables  Summary  Additional Figures  References  CMS Publications 

Figures  
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Figure 1:
The T quark reconstructed mass spectra for the $\mu $+jets (upper plot) and e+jets (lower plot) channels from data and from MC simulations of signal and background processes. The MC prediction for pair production of a T quark with a mass of 1300 GeV is shown by a dotted line, enhanced by a factor of 20. The lower panels show the ratio of the data to the background prediction. The uncertainties represented by the vertical bars on the points are statistical only. The shaded regions show the total statistical uncertainties in the background. 
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Figure 1a:
The T quark reconstructed mass spectrum for the $\mu $+jets channel from data and from MC simulations of signal and background processes. The MC prediction for pair production of a T quark with a mass of 1300 GeV is shown by a dotted line, enhanced by a factor of 20. The lower panel shows the ratio of the data to the background prediction. The uncertainties represented by the vertical bars on the points are statistical only. The shaded region shows the total statistical uncertainty in the background. 
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Figure 1b:
The T quark reconstructed mass spectrum for the e+jets channel from data and from MC simulations of signal and background processes. The MC prediction for pair production of a T quark with a mass of 1300 GeV is shown by a dotted line, enhanced by a factor of 20. The lower panel shows the ratio of the data to the background prediction. The uncertainties represented by the vertical bars on the points are statistical only. The shaded region shows the total statistical uncertainty in the background. 
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Figure 2:
The T quark reconstructed mass spectra for the sum of $\mu $+jets and e+jets channels from data and from MC simulations of signal and background processes. The lower plot has a logarithmic xaxis scale and the bin size corresponds to 7% of the mass value in the middle of the bin. The MC prediction for pair production of a T quark with a mass of 1300 GeV is shown by a dotted line, enhanced by a factor of 20. The lower panels show the ratio of the data to the background prediction. The uncertainties represented by the vertical bars on the points are statistical only. The shaded regions show the total statistical uncertainties in the background. 
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Figure 2a:
The T quark reconstructed mass spectrum for the sum of $\mu $+jets channel from data and from MC simulations of signal and background processes. The MC prediction for pair production of a T quark with a mass of 1300 GeV is shown by a dotted line, enhanced by a factor of 20. The lower panel shows the ratio of the data to the background prediction. The uncertainties represented by the vertical bars on the points are statistical only. The shaded region shows the total statistical uncertainty in the background. 
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Figure 2b:
The T quark reconstructed mass spectrum for the sum of e+jets channel from data and from MC simulations of signal and background processes. The plot has a logarithmic xaxis scale and the bin size corresponds to 7% of the mass value in the middle of the bin. The MC prediction for pair production of a T quark with a mass of 1300 GeV is shown by a dotted line, enhanced by a factor of 20. The lower panel shows the ratio of the data to the background prediction. The uncertainties represented by the vertical bars on the points are statistical only. The shaded region shows the total statistical uncertainty in the background. 
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Figure 3:
The T quark reconstructed mass spectra for the sum of the $\mu $+jets and e+jets channels for the "no W tag'' category of events (upper plot), and for the "Wtagged'' category (lower plot) from data and from MC simulations of the background processes. The MC prediction for $ {\mathrm {T}\overline {\mathrm {T}}} $ production of a T quark with a mass of 1300 GeV is shown by a dotted line, enhanced by a factor of 100 (upper) and 10 (lower). The lower panels show the ratio of the data to the background prediction. The uncertainties represented by the vertical bars on the data points are statistical only. 
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Figure 3a:
The T quark reconstructed mass spectra for the sum of the $\mu $+jets and e+jets channels for the "no W tag'' category of events from data and from MC simulations of the background processes. The MC prediction for $ {\mathrm {T}\overline {\mathrm {T}}} $ production of a T quark with a mass of 1300 GeV is shown by a dotted line, enhanced by a factor of 100. The lower panel shows the ratio of the data to the background prediction. The uncertainties represented by the vertical bars on the data points are statistical only. 
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Figure 3b:
The T quark reconstructed mass spectra for the sum of the $\mu $+jets and e+jets channels for the "Wtagged'' category from data and from MC simulations of the background processes. The MC prediction for $ {\mathrm {T}\overline {\mathrm {T}}} $ production of a T quark with a mass of 1300 GeV is shown by a dotted line, enhanced by a factor of 10. The lower panel shows the ratio of the data to the background prediction. The uncertainties represented by the vertical bars on the data points are statistical only. 
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Figure 4:
Upper plot: Observed and expected Bayesian upper limits at 95% CL on the product of the $ {\mathrm {T}\overline {\mathrm {T}}} $ or $ {\mathrm {Y}\overline {\mathrm {Y}}} $ production cross section and the branching fraction to bW using only the Wtagged events. The inner and outer bands show the 1 and 2 standard deviation uncertainty ranges in the expected limits, respectively. The dasheddotted line shows the prediction of the theory. Lower plot: The postfit distribution of the reconstructed T quark mass, $ {m_{\text {reco}}}$. Horizontal bars on the data points show bin size. The shaded band on the histogram and on the ratio plot shows the quadrature sum of the statistical and systematic uncertainties. The MC prediction for heavy quark production with a mass of 1300 GeV is shown by a dashed line, enhanced by a factor of 10. 
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Figure 4a:
Observed and expected Bayesian upper limits at 95% CL on the product of the $ {\mathrm {T}\overline {\mathrm {T}}} $ or $ {\mathrm {Y}\overline {\mathrm {Y}}} $ production cross section and the branching fraction to bW using only the Wtagged events. The inner and outer bands show the 1 and 2 standard deviation uncertainty ranges in the expected limits, respectively. The dasheddotted line shows the prediction of the theory. 
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Figure 4b:
The postfit distribution of the reconstructed T quark mass, $ {m_{\text {reco}}}$. Horizontal bars on the data points show bin size. The shaded band on the histogram and on the ratio plot shows the quadrature sum of the statistical and systematic uncertainties. The MC prediction for heavy quark production with a mass of 1300 GeV is shown by a dashed line, enhanced by a factor of 10. 
Tables  
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Table 1:
The numbers of expected background events for each process in the $\mu $+jets and e+jets channels, normalized to the integrated luminosity of the data, the number of observed events, and the ratio of the observed to predicted events. The uncertainties in the predicted numbers of background events are statistical only. 
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Table 2:
Selection efficiencies from MC simulation for the $ {\mathrm {T}\overline {\mathrm {T}}} $ signal as a function of the T quark mass assuming $\mathcal {B}({\mathrm {T}} \to {\mathrm{b} \mathrm{W}}) = 100%$, and the numbers of expected signal events after the final selection for the integrated luminosity of the data. The uncertainties are statistical only. 
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Table 3:
Variations in percent on the yield of the selected MC events due to shape systematic uncertainties for a signal with a T quark mass of 1200 GeV and background. 
Summary 
The results of a search for vectorlike quarks, either T or Y, with electric charge of $ 2/3 $ and $ 4/3 $, respectively, that are pair produced in pp interactions at $\sqrt{s} = $ 13 TeV and decay exclusively via the bW channel have been presented. Events are selected requiring that one W boson decays to a lepton and neutrino, and the other to a quarkantiquark pair. The selection requires a muon or electron, significant missing transverse momentum, and at least four jets. A kinematic fit assuming ${\mathrm{T}\overline{\mathrm{T}}} $ or ${\mathrm{Y}\overline{\mathrm{Y}}} $ production is performed and for every event a candidate T/Y quark mass ${m_{\text{reco}}}$ is reconstructed. The analysis provides a highresolution (7%) mass scan of the bW spectrum in the range from the top quark mass up to $\approx$2 TeV, in which the signal from pair production of equal mass objects decaying to bW would show up in a modelindependent way as a narrow peak. A binned maximumlikelihood fit to the ${m_{\text{reco}}}$ distribution is made and no significant deviations from the standard model expectations are found. Upper limits are set on the ${\mathrm{T}\overline{\mathrm{T}}} $ and ${\mathrm{Y}\overline{\mathrm{Y}}} $ pair production cross sections as a function of their mass. By comparing these limits with the predicted theoretical cross section of the pair production, the production of T and Y quarks is excluded at 95% confidence level for masses below 1295 GeV (1275 GeV expected). More generally, the results set upper limits on the product of the production cross section and branching fraction to bW for any new heavy quark decaying to this channel. 
Additional Figures  
png pdf 
Additional Figure 1:
The $ {\mathrm {T}} $ quark reconstructed mass spectra for the sum of $\mu $+jets and e+jets channels from data and from MC simulations of signal and background processes. The upper plot has a logarithmic xaxis scale and the bin size corresponds to 7% of the mass value in the middle of the bin. The MC prediction for the pair production of a $ {\mathrm {T}} $ quark with a mass of 800 GeV is shown by a dotted line, without an enhancement factor. The MC prediction for pair production of a $ {\mathrm {T}} $ quark with a mass of 1300 GeV is shown by a dashed line, enhanced by a factor of 20. The lower panels show the ratio of the data to the background prediction. The uncertainties represented by the vertical bars on the points are statistical only. The shaded regions show the total statistical uncertainties in the background. 
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Additional Figure 2:
The $ {\mathrm {T}\overline {\mathrm {T}}} $ mass distributions for the $e$+jets (upper) and $\mu $+jets (middle) channels and their sum (the lowest). The lower panels on each plot show the ratio of the data to the background prediction. The dashed lines shown the expected signal for a T quark mass of 1300 GeV, multiplied by a factor of 20. The uncertainties shown by the vertical bars are statistical only. 
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Additional Figure 2a:
The $ {\mathrm {T}\overline {\mathrm {T}}} $ mass distributions for the $e$+jets channel. The lower panel shows the ratio of the data to the background prediction. The dashed lines shown the expected signal for a T quark mass of 1300 GeV, multiplied by a factor of 20. The uncertainties shown by the vertical bars are statistical only. 
png pdf 
Additional Figure 2b:
The $ {\mathrm {T}\overline {\mathrm {T}}} $ mass distributions for the $\mu$+jets channel. The lower panel shows the ratio of the data to the background prediction. The dashed lines shown the expected signal for a T quark mass of 1300 GeV, multiplied by a factor of 20. The uncertainties shown by the vertical bars are statistical only. 
png pdf 
Additional Figure 2c:
The $ {\mathrm {T}\overline {\mathrm {T}}} $ mass distributions for the sum of $e$+jets and $\mu$+jets channels. The lower panel shows the ratio of the data to the background prediction. The dashed lines shown the expected signal for a T quark mass of 1300 GeV, multiplied by a factor of 20. The uncertainties shown by the vertical bars are statistical only. 
png pdf 
Additional Figure 3:
Display of the data event with the highest reconstructed TTbar mass of 2775 GeV for the hypothesis $ {\mathrm {T}\overline {\mathrm {T}}} \rightarrow \mathrm {bWbW} \rightarrow \mathrm {e}\nu + 4\mathrm {jets} $ 
png 
Additional Figure 3a:
Display of the data event with the highest reconstructed TTbar mass of 2775 GeV for the hypothesis $ {\mathrm {T}\overline {\mathrm {T}}} \rightarrow \mathrm {bWbW} \rightarrow \mathrm {e}\nu + 4\mathrm {jets} $ 
png 
Additional Figure 3b:
Display of the data event with the highest reconstructed TTbar mass of 2775 GeV for the hypothesis $ {\mathrm {T}\overline {\mathrm {T}}} \rightarrow \mathrm {bWbW} \rightarrow \mathrm {e}\nu + 4\mathrm {jets} $ 
png 
Additional Figure 3c:
Display of the data event with the highest reconstructed TTbar mass of 2775 GeV for the hypothesis $ {\mathrm {T}\overline {\mathrm {T}}} \rightarrow \mathrm {bWbW} \rightarrow \mathrm {e}\nu + 4\mathrm {jets} $ 
Signal reconstruction of a 800 GeV T quark Additional Fig. 1 displays the reconstructed mass distribution of a vectorlike T quark with a mass of 800 GeV. This is near the mass of the previously indicated "ghost state" at $\approx$750 GeV decaying into two photons, though the quantum numbers of the two states are different. This figure is similar to Fig. 2b in the paper, except it also shows the expected signal (without an enhancement factor) for a T quark with a mass of 800 GeV. The resulting narrow peak with $\approx$300 events shows the high sensitivity of the analysis for this mass region. $\mathrm{ T \overline{T} }$ mass distributions If we assume the existence of a heavy resonance decaying to a pair of VLQs (like, for example $\mathrm{Z}' \rightarrow \mathrm{ T \overline{T} }$), we can consider the mass spectrum $ M(\mathrm{ T \overline{T} } )$ as another signal distribution that permits the search for heavy resonances. This distribution is shown in Additional Fig. 2 for selected events. We do not observe any significant excess of data over the expected background. Our measured lower limit on the T quark mass of 1295 GeV implies that $ M(\mathrm{ T \overline{T} }) > $ 2590 GeV at the 95%CL. Properties of the data event with highest reconstructed mass Additional Fig. 3 displays the event in data that passed all the selection criteria and had the highest reconstructed TTbar mass. Some of the properties of this particular event include:

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