CMSPASEXO17016  
Search for heavy neutrinos and thirdgeneration leptoquarks in final states with two hadronically decaying $\tau$ leptons and two jets in protonproton collisions at $\sqrt{s} = $ 13 TeV  
CMS Collaboration  
July 2018  
Abstract: A search for new particles has been conducted using events with two high transverse momentum ($p_{\textrm{T}}$) $\tau$ leptons that decay hadronically, at least two high$p_{\textrm{T}}$ jets, and missing transverse momentum from the $\tau$ lepton decays. The analysis is performed using data from protonproton collisions, collected by the CMS experiment at the LHC in 2016 at $\sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{1}$. The observed data are consistent with the standard model expectation. The results are interpreted with two physics models. The first model involves righthanded charged bosons, $\textrm{W}_\mathrm{R}$, that decay to heavy righthanded neutrinos, $N_{\ell}$ ($\ell = \textrm{e}$, $\mu$, $\tau$), arising in a leftright symmetric extension of the standard model. The model considers that $\textrm{N}_{\textrm{e}}$ and $\textrm{N}_{\mu}$ are too heavy to be detected at the LHC. Assuming that the $\textrm{N}_{\tau}$ mass is half of the $\textrm{W}_\mathrm{R}$ mass, masses of the $\textrm{W}_\mathrm{R}$ boson below 3.5 TeV are excluded at a 95% confidence level (expected exclusion of 3.5 TeV). Exclusion bounds are also presented considering different scenarios for the mass ratio between $\textrm{N}_{\tau}$ and $\textrm{W}_\mathrm{R}$ as function of $\textrm{W}_\mathrm{R}$ mass. In the second model, pair production of thirdgeneration scalar leptoquarks that decay into $\tau\tau\textrm{b}\textrm{b}$ is considered, resulting in an observed (expected) exclusion region with leptoquark masses below 1.02 (1.0) TeV, assuming a 100% branching fraction for the leptoquark decay to a $\tau$ lepton and a bottom quark. These results represent the most stringent limits to date.  
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These preliminary results are superseded in this paper, JHEP 03 (2019) 170. The superseded preliminary plots can be found here. 
Figures  
png pdf 
Figure 1:
Distributions in (left) $S^{MET}_{\textrm {T}}$, for the $\mathrm{ t \bar{t} } (\mu \mu \textrm {j j})$ control sample, and (right) $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2},\textrm {j}_{1}, \textrm {j}_{2},p_{\textrm {T}}^{\textrm {miss}})$ (right) for the $\textrm {Z} (\tau \tau)$ control sample with relaxed $\tau _{\textrm {h}}$ $ p_{\textrm {T}} $ thresholds and $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2}) < $ 100 GeV. 
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Figure 1a:
Distributions in (left) $S^{MET}_{\textrm {T}}$, for the $\mathrm{ t \bar{t} } (\mu \mu \textrm {j j})$ control sample, and (right) $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2},\textrm {j}_{1}, \textrm {j}_{2},p_{\textrm {T}}^{\textrm {miss}})$ (right) for the $\textrm {Z} (\tau \tau)$ control sample with relaxed $\tau _{\textrm {h}}$ $ p_{\textrm {T}} $ thresholds and $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2}) < $ 100 GeV. 
png pdf 
Figure 1b:
Distributions in (left) $S^{MET}_{\textrm {T}}$, for the $\mathrm{ t \bar{t} } (\mu \mu \textrm {j j})$ control sample, and (right) $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2},\textrm {j}_{1}, \textrm {j}_{2},p_{\textrm {T}}^{\textrm {miss}})$ (right) for the $\textrm {Z} (\tau \tau)$ control sample with relaxed $\tau _{\textrm {h}}$ $ p_{\textrm {T}} $ thresholds and $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2}) < $ 100 GeV. 
png pdf 
Figure 2:
QCD multijet validation test in CR$B$ defined by $\textrm {N}_{\textrm {j}} \ge $ 2, $p_{\textrm {t}^{miss}} < $ 50 GeV, tight $\tau _{\textrm {h}}$ isolation, comparing the observed $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2},\textrm {j}_{1},\textrm {j}_{2},p_{\textrm {T}}^{\textrm {miss}})$ (left) and $S_{\textrm {T}}^{MET}$ (right) shapes against extrapolation from the loose $\tau _{\textrm {h}}$ region, CR$A$. For both samples nonQCD contributions estimated from simulation have been subtracted, as discussed in the text. Note that the normalizations match by construction. The bottom frame shows the ratio between the observed "Data" in CR$B$ and the total background estimation. 
png pdf 
Figure 2a:
QCD multijet validation test in CR$B$ defined by $\textrm {N}_{\textrm {j}} \ge $ 2, $p_{\textrm {t}^{miss}} < $ 50 GeV, tight $\tau _{\textrm {h}}$ isolation, comparing the observed $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2},\textrm {j}_{1},\textrm {j}_{2},p_{\textrm {T}}^{\textrm {miss}})$ (left) and $S_{\textrm {T}}^{MET}$ (right) shapes against extrapolation from the loose $\tau _{\textrm {h}}$ region, CR$A$. For both samples nonQCD contributions estimated from simulation have been subtracted, as discussed in the text. Note that the normalizations match by construction. The bottom frame shows the ratio between the observed "Data" in CR$B$ and the total background estimation. 
png pdf 
Figure 2b:
QCD multijet validation test in CR$B$ defined by $\textrm {N}_{\textrm {j}} \ge $ 2, $p_{\textrm {t}^{miss}} < $ 50 GeV, tight $\tau _{\textrm {h}}$ isolation, comparing the observed $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2},\textrm {j}_{1},\textrm {j}_{2},p_{\textrm {T}}^{\textrm {miss}})$ (left) and $S_{\textrm {T}}^{MET}$ (right) shapes against extrapolation from the loose $\tau _{\textrm {h}}$ region, CR$A$. For both samples nonQCD contributions estimated from simulation have been subtracted, as discussed in the text. Note that the normalizations match by construction. The bottom frame shows the ratio between the observed "Data" in CR$B$ and the total background estimation. 
png pdf 
Figure 3:
Distributions in $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2},\textrm {j}_{1},\textrm {j}_{2},p_{\textrm {T}}^{\textrm {miss}})$ (left) and $S_{T}^{MET}$ (right) for the estimated background in the signal region. The bottom frame shows the ratio between the observed data and the background estimation; the band corresponds to the statistical uncertainty for the background. The $\mathrm{ t \bar{t} }$, QCD multijet and \textrm {Z}+jets contributions are estimated employing MC and data driven techniques, while the other contributions are obtained from the MC prediction. 
png pdf 
Figure 3a:
Distributions in $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2},\textrm {j}_{1},\textrm {j}_{2},p_{\textrm {T}}^{\textrm {miss}})$ (left) and $S_{T}^{MET}$ (right) for the estimated background in the signal region. The bottom frame shows the ratio between the observed data and the background estimation; the band corresponds to the statistical uncertainty for the background. The $\mathrm{ t \bar{t} }$, QCD multijet and \textrm {Z}+jets contributions are estimated employing MC and data driven techniques, while the other contributions are obtained from the MC prediction. 
png pdf 
Figure 3b:
Distributions in $m(\tau _{\textrm {h},1},\tau _{\textrm {h},2},\textrm {j}_{1},\textrm {j}_{2},p_{\textrm {T}}^{\textrm {miss}})$ (left) and $S_{T}^{MET}$ (right) for the estimated background in the signal region. The bottom frame shows the ratio between the observed data and the background estimation; the band corresponds to the statistical uncertainty for the background. The $\mathrm{ t \bar{t} }$, QCD multijet and \textrm {Z}+jets contributions are estimated employing MC and data driven techniques, while the other contributions are obtained from the MC prediction. 
png pdf 
Figure 4:
Upper limits at the 95% confidence level on the cross section times branching fraction for production of (above) $\textrm {W}_{\textrm {R}}$ decaying to $\textrm {N}_{\tau}$ and (below) a pair of leptoquarks each decaying to $\tau \textrm {b}$, as functions of the produced particle mass. The observed limits are shown as solid black lines. Expected limits and their one (two) standard deviation limits are shown by dashed lines with green (yellow) bands. The theoretical cross sections are indicated by the solid blue lines. 
png pdf 
Figure 4a:
Upper limits at the 95% confidence level on the cross section times branching fraction for production of (above) $\textrm {W}_{\textrm {R}}$ decaying to $\textrm {N}_{\tau}$ and (below) a pair of leptoquarks each decaying to $\tau \textrm {b}$, as functions of the produced particle mass. The observed limits are shown as solid black lines. Expected limits and their one (two) standard deviation limits are shown by dashed lines with green (yellow) bands. The theoretical cross sections are indicated by the solid blue lines. 
png pdf 
Figure 4b:
Upper limits at the 95% confidence level on the cross section times branching fraction for production of (above) $\textrm {W}_{\textrm {R}}$ decaying to $\textrm {N}_{\tau}$ and (below) a pair of leptoquarks each decaying to $\tau \textrm {b}$, as functions of the produced particle mass. The observed limits are shown as solid black lines. Expected limits and their one (two) standard deviation limits are shown by dashed lines with green (yellow) bands. The theoretical cross sections are indicated by the solid blue lines. 
png pdf 
Figure 5:
Expected and observed limits at the 95% confidence level as a function of $m(\textrm {W}_{\textrm {R}})$ and $x=m(\textrm {N}_{\tau})/m(\textrm {W}_{\textrm {R}})$. 
Tables  
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Table 1:
Summary of systematic uncertainties. Values are given in percent. "s'' indicates a shape uncertainty. For example, "12,s" means the normalization uncertainty is 12%, but varied mass and $S_{\textrm {T}}^{MET}$ distributions are also considered to account for shape systematics. These varied mass and $S_{\textrm {T}}^{MET}$ distributions are normalized to the predicted background yields outlined in Section 6 and Table xxxxx. 
png pdf 
Table 2:
Estimated prefit background and signal yields in the SR and their statistical uncertainties. The \textrm {Z}+jets and $\mathrm{ t \bar{t} }$ background normalizations are determined by correcting the predictions obtained from the simulated samples with scale factors, $SF^{\textrm {Z}\to \mu \mu}_{dijet}$ and $SF^{\mathrm{ t \bar{t} }}$, determined in dedicated data control samples. The QCD multijet background event rate is determined with a datadriven method utilizing the number of QCD multijet events containing two nonisolated $\tau _{\textrm {h}}$ candidates and scaled by the TL ratio. The expected number of events for the $ {\mathrm {W}}_\mathrm {R}$ signal sample assume $m(\textrm {N}_{\tau}) = m({\mathrm {W}}_\mathrm {R})/2$. 
Summary 
A search is performed for physics beyond the standard model (BSM) in events with two energetic $\tau$ leptons, two energetic jets, and large momentum imbalance, using data corresponding to an integrated luminosity of 35.9 fb$^{1}$ collected in 2016 by the CMS detector in protonproton collisions at $\sqrt{s}= $ 13 TeV. The search focuses on two benchmark scenarios: (1) production of heavy righthanded neutrinos, $\textrm{N}_{\ell}$, and righthanded $\textrm{W}_{\textrm{R}}$ bosons, which arise in the leftright symmetric extensions of the SM and where the $\textrm{W}_{\textrm{R}}$ and $\textrm{N}_{\ell}$ decay chains result in a pair of hightransverse momentum $\tau$ leptons; (2) pair production of thirdgeneration scalar leptoquarks that decay to $\tau\tau$\textrm{bb}. The observed $m(\tau_{\textrm{h},1},\tau_{\textrm{h},2},\textrm{j}_{1},\textrm{j}_{2},p_{\textrm{T}}^{\textrm{miss}})$ and $S^{MET}_{\textrm{T}}$ distributions do not reveal any evidence for physics beyond the SM. Assuming that only the $\textrm{N}_{\tau}$ flavor contributes significantly to the $\mathrm{W}_\mathrm{R}$ decay width, $\mathrm{W}_\mathrm{R}$ masses below 3.45 (2.75) TeV are excluded at the 95% confidence level, assuming the $\textrm{N}_{\tau}$ mass is 0.8 (0.2) times the mass of $\mathrm{W}_\mathrm{R}$ boson. In the second BSM scenario, leptoquarks with a mass less than 1.02 TeV are excluded at the 95% confidence level, to be compared with an expected mass limit of 1.00 TeV. Both of these results represent the most stringent limits to date in $\tau \tau \textrm{j} \textrm{j}$ final states, exceeding the previous limits by CMS using 12.9 fb$^{1}$ of data recorded at 13 TeV [48]. 
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