CMS-PAS-EXO-17-016 | ||
Search for heavy neutrinos and third-generation leptoquarks in final states with two hadronically decaying τ leptons and two jets in proton-proton collisions at √s= 13 TeV | ||
CMS Collaboration | ||
July 2018 | ||
Abstract: A search for new particles has been conducted using events with two high transverse momentum (pT) τ leptons that decay hadronically, at least two high-pT jets, and missing transverse momentum from the τ lepton decays. The analysis is performed using data from proton-proton collisions, collected by the CMS experiment at the LHC in 2016 at √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 right-handed charged bosons, WR, that decay to heavy right-handed neutrinos, Nℓ (ℓ=e, μ, τ), arising in a left-right symmetric extension of the standard model. The model considers that Ne and Nμ are too heavy to be detected at the LHC. Assuming that the Nτ mass is half of the WR mass, masses of the WR 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 Nτ and WR as function of WR mass. In the second model, pair production of third-generation scalar leptoquarks that decay into ττbb 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 τ lepton and a bottom quark. These results represent the most stringent limits to date. | ||
Links:
CDS record (PDF) ;
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These preliminary results are superseded in this paper, JHEP 03 (2019) 170. The superseded preliminary plots can be found here.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 | |
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Figure 1:
Distributions in (left) SMETT, for the tˉt(μμj j) control sample, and (right) m(τh,1,τh,2,j1,j2,pmissT) (right) for the Z(ττ) control sample with relaxed τh pT thresholds and m(τh,1,τh,2)< 100 GeV. |
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Figure 1-a:
Distributions in (left) SMETT, for the tˉt(μμj j) control sample, and (right) m(τh,1,τh,2,j1,j2,pmissT) (right) for the Z(ττ) control sample with relaxed τh pT thresholds and m(τh,1,τh,2)< 100 GeV. |
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Figure 1-b:
Distributions in (left) SMETT, for the tˉt(μμj j) control sample, and (right) m(τh,1,τh,2,j1,j2,pmissT) (right) for the Z(ττ) control sample with relaxed τh pT thresholds and m(τh,1,τh,2)< 100 GeV. |
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Figure 2:
QCD multijet validation test in CRB defined by Nj≥ 2, ptmiss< 50 GeV, tight τh isolation, comparing the observed m(τh,1,τh,2,j1,j2,pmissT) (left) and SMETT (right) shapes against extrapolation from the loose τh region, CRA. For both samples non-QCD 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 CRB and the total background estimation. |
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Figure 2-a:
QCD multijet validation test in CRB defined by Nj≥ 2, ptmiss< 50 GeV, tight τh isolation, comparing the observed m(τh,1,τh,2,j1,j2,pmissT) (left) and SMETT (right) shapes against extrapolation from the loose τh region, CRA. For both samples non-QCD 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 CRB and the total background estimation. |
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Figure 2-b:
QCD multijet validation test in CRB defined by Nj≥ 2, ptmiss< 50 GeV, tight τh isolation, comparing the observed m(τh,1,τh,2,j1,j2,pmissT) (left) and SMETT (right) shapes against extrapolation from the loose τh region, CRA. For both samples non-QCD 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 CRB and the total background estimation. |
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Figure 3:
Distributions in m(τh,1,τh,2,j1,j2,pmissT) (left) and SMETT (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 tˉt, 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. |
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Figure 3-a:
Distributions in m(τh,1,τh,2,j1,j2,pmissT) (left) and SMETT (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 tˉt, 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. |
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Figure 3-b:
Distributions in m(τh,1,τh,2,j1,j2,pmissT) (left) and SMETT (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 tˉt, 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. |
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Figure 4:
Upper limits at the 95% confidence level on the cross section times branching fraction for production of (above) WR decaying to Nτ and (below) a pair of leptoquarks each decaying to τb, 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. |
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Figure 4-a:
Upper limits at the 95% confidence level on the cross section times branching fraction for production of (above) WR decaying to Nτ and (below) a pair of leptoquarks each decaying to τb, 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. |
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Figure 4-b:
Upper limits at the 95% confidence level on the cross section times branching fraction for production of (above) WR decaying to Nτ and (below) a pair of leptoquarks each decaying to τb, 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. |
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Figure 5:
Expected and observed limits at the 95% confidence level as a function of m(WR) and x=m(Nτ)/m(WR). |
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 SMETT distributions are also considered to account for shape systematics. These varied mass and SMETT distributions are normalized to the predicted background yields outlined in Section 6 and Table xxxxx. |
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Table 2:
Estimated pre-fit background and signal yields in the SR and their statistical uncertainties. The \textrm {Z}+jets and tˉt background normalizations are determined by correcting the predictions obtained from the simulated samples with scale factors, SFZ→μμdijet and SFtˉt, determined in dedicated data control samples. The QCD multijet background event rate is determined with a data-driven method utilizing the number of QCD multijet events containing two non-isolated τh candidates and scaled by the TL ratio. The expected number of events for the WR signal sample assume m(Nτ)=m(WR)/2. |
Summary |
A search is performed for physics beyond the standard model (BSM) in events with two energetic τ 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 proton-proton collisions at √s= 13 TeV. The search focuses on two benchmark scenarios: (1) production of heavy right-handed neutrinos, Nℓ, and right-handed WR bosons, which arise in the left-right symmetric extensions of the SM and where the WR and Nℓ decay chains result in a pair of high-transverse momentum τ leptons; (2) pair production of third-generation scalar leptoquarks that decay to ττ\textrm{bb}. The observed m(τh,1,τh,2,j1,j2,pmissT) and SMETT distributions do not reveal any evidence for physics beyond the SM. Assuming that only the Nτ flavor contributes significantly to the WR decay width, WR masses below 3.45 (2.75) TeV are excluded at the 95% confidence level, assuming the Nτ mass is 0.8 (0.2) times the mass of WR 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 ττjj final states, exceeding the previous limits by CMS using 12.9 fb−1 of data recorded at 13 TeV [48]. |
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Compact Muon Solenoid LHC, CERN |
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