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CMS-EXO-16-033 ; CERN-EP-2018-020
Search for high-mass resonances in final states with a lepton and missing transverse momentum at $\sqrt{s} = $ 13 TeV
JHEP 06 (2018) 128
Abstract: A search for new high-mass resonances in proton-proton collisions having final states with an electron or muon and missing transverse momentum is presented. The analysis uses proton-proton collision data collected in 2016 with the CMS detector at the LHC at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The transverse mass distribution of the charged lepton-neutrino system is used as the discriminating variable. No significant deviation from the standard model prediction is found. The best limit, from the combination of electron and muon channels, is 5.2 TeV at 95% confidence level for the mass of a W' boson with the same couplings as those of the standard model W boson. Exclusion limits of 2.9 TeV are set on the inverse radius of the extra dimension in the framework of split universal extra dimensions. In addition, model-independent limits are set on the production cross section and coupling strength of W' bosons decaying into this final state. An interpretation is also made in the context of an R parity violating supersymmetric model with a slepton as a mediator and flavor violating decay.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Production and decay of a new heavy boson, an SSM W' or a $ {{\mathrm {W}}_{\mathrm {KK}}} $ (left). The coupling strength, $g$, may vary. In RPV SUSY, a tau slepton ($ {\tilde{\tau}} $) could also act as a mediator (right) with corresponding $\lambda $ couplings for the decay that are different for the two final states, denoted by $\lambda _{231}$ and $\lambda _{132}$ for the electron and muon final states, respectively.

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Figure 1-a:
Production and decay of a new heavy boson, an SSM W' or a $ {{\mathrm {W}}_{\mathrm {KK}}} $. The coupling strength, $g$, may vary.

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Figure 1-b :
In RPV SUSY, a tau slepton ($ {\tilde{\tau}} $) could also act as a mediator with corresponding $\lambda $ couplings for the decay that are different for the two final states, denoted by $\lambda _{231}$ and $\lambda _{132}$ for the electron and muon final states, respectively.

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Figure 2:
Distributions in $ {p_{\mathrm {T}}} $ (left) and ${{p_{\mathrm {T}}} ^\text {miss}}$ (right), for the electron (upper row) and muon (lower row) for data and for expected SM backgrounds, after applying complete selection criteria. The QCD multijet background in the electron channel is derived from data. The background labelled as "diboson" includes WW, ZZ, and WZ contributions. Also shown are SSM W' signal examples for the two indicated masses. The last bin shows the total overflow. The lower panel shows the ratio of data to prediction and the shaded band includes the systematic uncertainties.

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Figure 2-a:
Distribution in $ {p_{\mathrm {T}}} $, for the electron for data and for expected SM backgrounds, after applying complete selection criteria. The QCD multijet background in the electron channel is derived from data. The background labelled as "diboson" includes WW, ZZ, and WZ contributions. Also shown are SSM W' signal examples for the two indicated masses. The last bin shows the total overflow. The lower panel shows the ratio of data to prediction and the shaded band includes the systematic uncertainties.

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Figure 2-b:
Distribution in ${{p_{\mathrm {T}}} ^\text {miss}}$, for the electron for data and for expected SM backgrounds, after applying complete selection criteria. The QCD multijet background in the electron channel is derived from data. The background labelled as "diboson" includes WW, ZZ, and WZ contributions. Also shown are SSM W' signal examples for the two indicated masses. The last bin shows the total overflow. The lower panel shows the ratio of data to prediction and the shaded band includes the systematic uncertainties.

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Figure 2-c:
Distribution in $ {p_{\mathrm {T}}} $, for the muon for data and for expected SM backgrounds, after applying complete selection criteria. The QCD multijet background in the electron channel is derived from data. The background labelled as "diboson" includes WW, ZZ, and WZ contributions. Also shown are SSM W' signal examples for the two indicated masses. The last bin shows the total overflow. The lower panel shows the ratio of data to prediction and the shaded band includes the systematic uncertainties.

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Figure 2-d:
Distribution in ${{p_{\mathrm {T}}} ^\text {miss}}$, for the muon for data and for expected SM backgrounds, after applying complete selection criteria. The QCD multijet background in the electron channel is derived from data. The background labelled as "diboson" includes WW, ZZ, and WZ contributions. Also shown are SSM W' signal examples for the two indicated masses. The last bin shows the total overflow. The lower panel shows the ratio of data to prediction and the shaded band includes the systematic uncertainties.

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Figure 3:
Observed $ {M_\mathrm {T}} $ distributions for the electron (left) and muon (right) channels after all selections. Signal examples for W' masses of 1.8 and 3.8 TeV, including detector simulation, are shown in both channels. In addition, signal examples for RPV SUSY and split-UED are depicted. The lower panel shows the ratio of data to prediction and the hatched band includes the systematic uncertainties. The last bin shows the total overflow.

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Figure 3-a:
Observed $ {M_\mathrm {T}} $ distribution for the electron channel after all selections. Signal examples for W' masses of 1.8 and 3.8 TeV, including detector simulation, are shown in both channels. In addition, signal examples for RPV SUSY and split-UED are depicted. The lower panel shows the ratio of data to prediction and the hatched band includes the systematic uncertainties. The last bin shows the total overflow.

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Figure 3-b:
Observed $ {M_\mathrm {T}} $ distribution for the muon channel after all selections. Signal examples for W' masses of 1.8 and 3.8 TeV, including detector simulation, are shown in both channels. In addition, signal examples for RPV SUSY and split-UED are depicted. The lower panel shows the ratio of data to prediction and the hatched band includes the systematic uncertainties. The last bin shows the total overflow.

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Figure 4:
Cross section upper limits at 95% CL on the effective cross section $\sigma ({\mathrm {W}'}) \, \mathcal {B}({\mathrm {W}'} \to \ell \nu) \, A \, \epsilon $ above a threshold $ {M_\mathrm {T}^\mathrm {min}} $ for the individual electron (left) and muon (right) channels. Shown are the observed limit (solid line), expected limit (dashed line), and the expected limit $\pm $1 and $\pm $2 standard deviation (s.d.) intervals.

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Figure 4-a:
Cross section upper limits at 95% CL on the effective cross section $\sigma ({\mathrm {W}'}) \, \mathcal {B}({\mathrm {W}'} \to \ell \nu) \, A \, \epsilon $ above a threshold $ {M_\mathrm {T}^\mathrm {min}} $ for the electron channel. Shown are the observed limit (solid line), expected limit (dashed line), and the expected limit $\pm $1 and $\pm $2 standard deviation (s.d.) intervals.

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Figure 4-b:
Cross section upper limits at 95% CL on the effective cross section $\sigma ({\mathrm {W}'}) \, \mathcal {B}({\mathrm {W}'} \to \ell \nu) \, A \, \epsilon $ above a threshold $ {M_\mathrm {T}^\mathrm {min}} $ for the muon channel. Shown are the observed limit (solid line), expected limit (dashed line), and the expected limit $\pm $1 and $\pm $2 standard deviation (s.d.) intervals.

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Figure 5:
Expected (dashed line) and observed (solid line) 95% CL limits in the SSM interpretation for the electron (left) and muon (right) channels. The shaded bands represent the one and two standard deviation (s.d.) uncertainty bands. Also shown are theoretical cross sections for the SSM benchmark model (black with a grey band for the PDF uncertainties) and split-UED (red and blue solid lines) interpretations.

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Figure 5-a:
Expected (dashed line) and observed (solid line) 95% CL limits in the SSM interpretation for the electron muon channel. The shaded bands represent the one and two standard deviation (s.d.) uncertainty bands. Also shown are theoretical cross sections for the SSM benchmark model (black with a grey band for the PDF uncertainties) and split-UED (red and blue solid lines) interpretations.

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Figure 5-b:
Expected (dashed line) and observed (solid line) 95% CL limits in the SSM interpretation for the electron muon channel. The shaded bands represent the one and two standard deviation (s.d.) uncertainty bands. Also shown are theoretical cross sections for the SSM benchmark model (black with a grey band for the PDF uncertainties) and split-UED (red and blue solid lines) interpretations.

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Figure 6:
Expected (dashed line) and observed (solid line) 95% CL limits on the product of the cross section and branching fraction, in the SSM W' model (left) and the model-independent interpretation (right). Shown are the combination of the electron and muon channels, with the shaded bands corresponding to one and two standard deviations (s.d.). For comparison, the results from Ref. [9] for the regions investigated in 2015 are shown as dotted lines.

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Figure 6-a:
Expected (dashed line) and observed (solid line) 95% CL limits on the product of the cross section and branching fraction, in the SSM W' model. Shown are the combination of the electron and muon channels, with the shaded bands corresponding to one and two standard deviations (s.d.). For comparison, the results from Ref. [9] for the regions investigated in 2015 are shown as dotted lines.

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Figure 6-b:
Expected (dashed line) and observed (solid line) 95% CL limits on the product of the cross section and branching fraction, in the model-independent interpretation. Shown are the combination of the electron and muon channels, with the shaded bands corresponding to one and two standard deviations (s.d.). For comparison, the results from Ref. [9] for the regions investigated in 2015 are shown as dotted lines.

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Figure 7:
Expected (dashed line) and observed (solid line) 95% CL limits on the coupling strength $g_{{\mathrm {W}'}}/g_{{\mathrm {W}}}$ as a function of the W' mass, for the electron (left) and muon (right) channels. The area above the limit line is excluded. The shaded bands represent the one and two standard deviation (s.d.) uncertainty bands. The SSM W' couplings are shown as a dotted line.

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Figure 7-a:
Expected (dashed line) and observed (solid line) 95% CL limits on the coupling strength $g_{{\mathrm {W}'}}/g_{{\mathrm {W}}}$ as a function of the W' mass, for the electron channel. The area above the limit line is excluded. The shaded bands represent the one and two standard deviation (s.d.) uncertainty bands. The SSM W' couplings are shown as a dotted line.

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Figure 7-b:
Expected (dashed line) and observed (solid line) 95% CL limits on the coupling strength $g_{{\mathrm {W}'}}/g_{{\mathrm {W}}}$ as a function of the W' mass, for the muon channel. The area above the limit line is excluded. The shaded bands represent the one and two standard deviation (s.d.) uncertainty bands. The SSM W' couplings are shown as a dotted line.

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Figure 8:
Exclusion limit in the plane ($\mu $, 1/R) for the split-UED interpretation for the $n=2$ case in the electron (left) and muon (middle) channels, respectively. On the right, the result for the combination of both channels is shown. The expected limit is depicted as a dashed line and the experimentally excluded region as a solid black line (filled as a yellow area). For comparison, the 8 TeV result from Ref. [8] is given as a red, dotted line.

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Figure 8-a:
Exclusion limit in the plane ($\mu $, 1/R) for the split-UED interpretation for the $n=2$ case in the electron channel. The expected limit is depicted as a dashed line and the experimentally excluded region as a solid black line (filled as a yellow area). For comparison, the 8 TeV result from Ref. [8] is given as a red, dotted line.

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Figure 8-b:
Exclusion limit in the plane ($\mu $, 1/R) for the split-UED interpretation for the $n=2$ case in the muon channel. The expected limit is depicted as a dashed line and the experimentally excluded region as a solid black line (filled as a yellow area). For comparison, the 8 TeV result from Ref. [8] is given as a red, dotted line.

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Figure 8-c:
Exclusion limit in the plane ($\mu $, 1/R) for the split-UED interpretation for the $n=2$ case in the combination of both electron and muon channels. The expected limit is depicted as a dashed line and the experimentally excluded region as a solid black line (filled as a yellow area). For comparison, the 8 TeV result from Ref. [8] is given as a red, dotted line.

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Figure 9:
Observed (solid line) and expected (dashed line) exclusion limits for different couplings in the model with a $\tau $ slepton as a mediator. The couplings $\lambda ^{'}_{3ij}$, $\lambda _{231}$, and $\lambda _{132}$ are defined in Fig. 1. Results are shown for the final states consisting of $ {\mathrm {e}}+\nu _{\mu}$ on the left and $\mu +\nu _{{\mathrm {e}}}$ on the right.

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Figure 9-a:
Observed (solid line) and expected (dashed line) exclusion limits for different couplings in the model with a $\tau $ slepton as a mediator. The couplings $\lambda ^{'}_{3ij}$, $\lambda _{231}$, and $\lambda _{132}$ are defined in Fig. 1. Results are shown for the final states consisting of $ {\mathrm {e}}+\nu _{\mu}$.

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Figure 9-b:
Observed (solid line) and expected (dashed line) exclusion limits for different couplings in the model with a $\tau $ slepton as a mediator. The couplings $\lambda ^{'}_{3ij}$, $\lambda _{231}$, and $\lambda _{132}$ are defined in Fig. 1. Results are shown for the final states consisting of $\mu +\nu _{{\mathrm {e}}}$.
Tables

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Table 1:
Expected and observed numbers of signal and background events, for a selected set of $ {M_\mathrm {T}} $ thresholds. Also shown are the total systematic uncertainties in the estimate of the event numbers. The signal yields are based on NNLO cross sections.
Summary
A search for high-mass resonances in the lepton plus the missing transverse momentum final state in proton-proton collisions at $\sqrt{s} = $ 13 TeV has been performed, using a data sample corresponding to an integrated luminosity of 35.9 fb$^{-1}$. No evidence for new physics is observed when examining the transverse mass distributions in the electron and muon channels. These observations are interpreted as 95% confidence limits on the parameters of several models.

The exclusion limits on a sequential standard model-like W' are calculated to be 4.9 (4.9) TeV in the individual electron (muon) channels. A combination of both channels increases the limit to 5.2 TeV, assuming standard model couplings. Additionally, variations in the coupling strength are studied and couplings above 2 $\times 10^{-2}$ are excluded for low W' masses. These results are also applied to the split universal extra dimension model, and the inverse radius of the extra dimension $1/R$ is constrained by this analysis to be above 2.9 TeV.

These results are presented in a model-independent form, making possible their interpretation in a number of other models. An example of this application is given using a supersymmetric model with R-parity violation, and a tau slepton mediator with flavor-violating decays into either $\mathrm{e}+\nu_{\mu}$ or $\mu+\nu_{\mathrm{e}}$. Limits on the coupling strengths at the decay vertex have been derived as a function of the mediator mass, for various values of the coupling at the production vertex $\lambda^{'}_{3ij}$.
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Compact Muon Solenoid
LHC, CERN