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CMS-EXO-19-014 ; CERN-EP-2022-033
Search for heavy resonances and quantum black holes in e$\mu$, e$\tau$, and $\mu\tau$ final states in proton-proton collisions at $\sqrt{s} = $ 13 TeV
JHEP 05 (2023) 227
Abstract: A search is reported for heavy resonances and quantum black holes decaying into e$\mu$, e$\tau$, and $\mu\tau$ final states in proton-proton collision data recorded by the CMS experiment at the CERN LHC during 2016-2018 at $\sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. The e$\mu$, e$\tau$, and $\mu\tau$ invariant mass spectra are reconstructed, and no evidence is found for physics beyond the standard model. Upper limits are set at 95% confidence level on the product of the cross section and branching fraction for lepton flavor violating signals. Three benchmark signals are studied: resonant $\tau$ sneutrino production in $R$ parity violating supersymmetric models, heavy Z' gauge bosons with lepton flavor violating decays, and nonresonant quantum black hole production in models with extra spatial dimensions. Resonant $\tau$ sneutrinos are excluded for masses up to 4.2 TeV in the e$\mu$ channel, 3.7 TeV in the e$\tau$ channel, and 3.6 TeV in the $\mu\tau$ channel. A Z' boson with lepton flavor violating couplings is excluded up to a mass of 5.0 TeV in the e$\mu$ channel, up to 4.3 TeV in the e$\tau$ channel, and up to 4.1 TeV in the $\mu\tau$ channel. Quantum black holes in the benchmark model are excluded up to the threshold mass of 5.6 TeV in the e$\mu$ channel, 5.2 TeV in the e$\tau$ channel, and 5.0 TeV in the $\mu\tau$ channel. In addition, model-independent limits are extracted to allow comparisons with other models for the same final states and similar event selection requirements. The results of these searches provide the most stringent limits available from collider experiments for heavy particles that undergo lepton flavor violating decays.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Leading order Feynman diagrams considered in our search. Left: Resonant production of a $\tau $ sneutrino in an RPV SUSY model that includes the subsequent decay into two leptons of different flavors. The $\nu _\tau $ is produced from the annihilation of two down quarks via the $\lambda '_{311}$ coupling, and then decays via the $\lambda $ couplings. Middle: Resonant production of a Z' boson with subsequent decay into two leptons of different flavors. Right: Production of quantum black holes in a model with extra dimensions that involves subsequent decay into two leptons of different flavors.

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Figure 1-a:
Leading order Feynman diagram considered in our search: Resonant production of a $\tau $ sneutrino in an RPV SUSY model that includes the subsequent decay into two leptons of different flavors. The $\nu _\tau $ is produced from the annihilation of two down quarks via the $\lambda '_{311}$ coupling, and then decays via the $\lambda $ couplings.

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Figure 1-b:
Leading order Feynman diagram considered in our search: Resonant production of a Z' boson with subsequent decay into two leptons of different flavors.

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Figure 1-c:
Leading order Feynman diagram considered in our search: Production of quantum black holes in a model with extra dimensions that involves subsequent decay into two leptons of different flavors.

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Figure 2:
Invariant mass distributions for the e$ \mu $ channel (upper), and collinear mass distributions for the e$ \tau $ (lower left) and $\mu \tau $ (lower right) channels. In addition to the observed data (black points) and the SM prediction (filled histograms), the expected signal distributions for three models are shown: the RPV SUSY model with $\lambda = \lambda ' = $ 0.01 and $\tau $ sneutrino mass of 1.6 TeV, LFV Z' ($\mathcal {B} = $ 0.1) boson with a mass of 1.6 TeV, and the QBH signal expectation for $n=$ 4 and a threshold mass of 1.6 TeV. The bottom panel of each plot shows the ratio of data and SM prediction. The bin width gradually increases with mass.

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Figure 2-a:
Invariant mass distribution for the e$ \mu $ channel. In addition to the observed data (black points) and the SM prediction (filled histograms), the expected signal distributions for three models are shown: the RPV SUSY model with $\lambda = \lambda ' = $ 0.01 and $\tau $ sneutrino mass of 1.6 TeV, LFV Z' ($\mathcal {B} = $ 0.1) boson with a mass of 1.6 TeV, and the QBH signal expectation for $n=$ 4 and a threshold mass of 1.6 TeV. The bottom panel shows the ratio of data and SM prediction. The bin width gradually increases with mass.

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Figure 2-b:
Collinear mass distribution for the e$ \tau $ $\mu \tau $ channel. In addition to the observed data (black points) and the SM prediction (filled histograms), the expected signal distributions for three models are shown: the RPV SUSY model with $\lambda = \lambda ' = $ 0.01 and $\tau $ sneutrino mass of 1.6 TeV, LFV Z' ($\mathcal {B} = $ 0.1) boson with a mass of 1.6 TeV, and the QBH signal expectation for $n=$ 4 and a threshold mass of 1.6 TeV. The bottom panel shows the ratio of data and SM prediction. The bin width gradually increases with mass.

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Figure 2-c:
Invariant mass distribution for the e$ \mu $ channel. Collinear mass distribution for the e$ \tau $ $\mu \tau $ channel. In addition to the observed data (black points) and the SM prediction (filled histograms), the expected signal distributions for three models are shown: the RPV SUSY model with $\lambda = \lambda ' = $ 0.01 and $\tau $ sneutrino mass of 1.6 TeV, LFV Z' ($\mathcal {B} = $ 0.1) boson with a mass of 1.6 TeV, and the QBH signal expectation for $n=$ 4 and a threshold mass of 1.6 TeV. The bottom panel shows the ratio of data and SM prediction. The bin width gradually increases with mass.

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Figure 3:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction as a function of the $\tau $ sneutrino mass in an RPV SUSY model for the e$ \mu $ (upper), e$ \tau $ (lower left), and $\mu \tau $ (lower right) channels. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red and blue solid lines show the predicted product of the cross section and the branching fraction as a function of the tau sneutrino mass for two different values of the couplings.

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Figure 3-a:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction as a function of the $\tau $ sneutrino mass in an RPV SUSY model for the e$ \mu $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red and blue solid lines show the predicted product of the cross section and the branching fraction as a function of the tau sneutrino mass for two different values of the couplings.

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Figure 3-b:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction as a function of the $\tau $ sneutrino mass in an RPV SUSY model for the e$ \tau $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red and blue solid lines show the predicted product of the cross section and the branching fraction as a function of the tau sneutrino mass for two different values of the couplings.

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Figure 3-c:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction as a function of the $\tau $ sneutrino mass in an RPV SUSY model for the $\mu \tau $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red and blue solid lines show the predicted product of the cross section and the branching fraction as a function of the tau sneutrino mass for two different values of the couplings.

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Figure 4:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction for a Z' boson with LFV decays, in the e$ \mu $ (upper), e$ \tau $ (lower left), and $\mu \tau $ (lower right) channels. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red solid lines show the predicted product of the cross section and the branching fraction as a function of the Z' mass assuming $\mathcal {B}=$ 0.1.

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Figure 4-a:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction for a Z' boson with LFV decays, in the e$ \mu $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red solid line show2 the predicted product of the cross section and the branching fraction as a function of the Z' mass assuming $\mathcal {B}=$ 0.1.

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Figure 4-b:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction for a Z' boson with LFV decays, in the e$ \tau $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red solid line show2 the predicted product of the cross section and the branching fraction as a function of the Z' mass assuming $\mathcal {B}=$ 0.1.

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Figure 4-c:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction for a Z' boson with LFV decays, in the $\mu \tau $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red solid line show2 the predicted product of the cross section and the branching fraction as a function of the Z' mass assuming $\mathcal {B}=$ 0.1.

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Figure 5:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction for quantum black hole production in an ADD model with $n=$ 4 extra dimensions, in the e$ \mu $ (upper), e$ \tau $ (lower left), and $\mu \tau $ (lower right) channels. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red solid lines show the predicted product of the cross section and the branching fraction as a function of the QBH threshold mass.

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Figure 5-a:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction for quantum black hole production in an ADD model with $n=$ 4 extra dimensions, in the e$ \mu $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red solid line shows the predicted product of the cross section and the branching fraction as a function of the QBH threshold mass.

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Figure 5-b:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction for quantum black hole production in an ADD model with $n=$ 4 extra dimensions, in the e$ \tau $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red solid line shows the predicted product of the cross section and the branching fraction as a function of the QBH threshold mass.

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Figure 5-c:
Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the product of the cross section and the branching fraction for quantum black hole production in an ADD model with $n=$ 4 extra dimensions, in the $\mu \tau $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits. The red solid line shows the predicted product of the cross section and the branching fraction as a function of the QBH threshold mass.

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Figure 6:
Exclusion limits at 95% CL on the RPV SUSY model in the plane of $\tau $ sneutrino mass and $\lambda '$ coupling, for four values of $\lambda $ couplings. The regions to the left of and above the curves are excluded. The upper plot corresponds to the e$ \mu $ channel, while the lower left and right plots show the e$ \tau $ and $\mu \tau $ channels, respectively.

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Figure 6-a:
Exclusion limits at 95% CL on the RPV SUSY model in the plane of $\tau $ sneutrino mass and $\lambda '$ coupling, for four values of $\lambda $ couplings. The regions to the left of and above the curves are excluded. The plot corresponds to the e$ \mu $ channel.

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Figure 6-b:
Exclusion limits at 95% CL on the RPV SUSY model in the plane of $\tau $ sneutrino mass and $\lambda '$ coupling, for four values of $\lambda $ couplings. The regions to the left of and above the curves are excluded. The plot corresponds to the e$ \tau $ channel.

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Figure 6-c:
Exclusion limits at 95% CL on the RPV SUSY model in the plane of $\tau $ sneutrino mass and $\lambda '$ coupling, for four values of $\lambda $ couplings. The regions to the left of and above the curves are excluded. The plot corresponds to the $\mu \tau $ channel.

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Figure 7:
Model-independent upper limits at 95% CL on the product of the cross section, the branching fraction, acceptance, and efficiency are shown. Observed (expected) limits are shown in black solid (dashed) lines for the e$ \mu $ (upper), e$ \tau $ (lower left), and $\mu \tau $ (lower right) channels. The shaded bands represent 68% and 95% uncertainties in the expected limits.

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Figure 7-a:
Model-independent upper limits at 95% CL on the product of the cross section, the branching fraction, acceptance, and efficiency are shown. Observed (expected) limits are shown in black solid (dashed) lines for the e$ \mu $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits.

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Figure 7-b:
Model-independent upper limits at 95% CL on the product of the cross section, the branching fraction, acceptance, and efficiency are shown. Observed (expected) limits are shown in black solid (dashed) lines for the e$ \tau $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits.

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Figure 7-c:
Model-independent upper limits at 95% CL on the product of the cross section, the branching fraction, acceptance, and efficiency are shown. Observed (expected) limits are shown in black solid (dashed) lines for the $\mu \tau $ channel. The shaded bands represent 68% and 95% uncertainties in the expected limits.
Tables

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Table 1:
The observed and expected (in parentheses) 95% CL lower mass limits on the RPV SUSY, Z', and QBH signals for the e$ \mu $, e$ \tau $, and $\mu \tau $ channels.
Summary
A search has been conducted for heavy particles that undergo lepton flavor violating decays into e$\mu$, e$\tau$, and $\mu\tau$ final states. The search is based on proton-proton collision data at $\sqrt{s} = $ 13 TeV recorded during 2016-2018 in the CMS detector at the CERN LHC, corresponding to an integrated luminosity of 138 fb$^{-1}$. The data are consistent with expectations from the standard model. Lower limits at 95% confidence level are set on the mass of supersymmetric $\tau$ sneutrinos at 4.2 TeV in e$\mu$, 3.7 TeV in e$\tau$, and 3.6 TeV in $\mu\tau$ channels. A Z' vector boson with lepton flavor violating couplings is excluded for masses below 5.0, 4.3, and 4.1 TeV in the e$\mu$, e$\tau$, and $\mu\tau$ channels, respectively, assuming a branching fraction of 10%. In the context of the Arkani-Hamed-Dimopoulos-Dvali model with four extra dimensions, values of the threshold mass for quantum black hole production less than 5.6, 5.2, and 5.0 TeV are excluded in the e$\mu$, e$\tau$, and $\mu\tau$ channels, respectively. In addition, model-independent limits are provided allowing the results to be interpreted in other models with the same final states and similar kinematic distributions. Limits in the e$\tau$ and $\mu\tau$ final states, as well as model-independent limits, are reported for the first time in the context of a high-mass lepton flavor violation search. These are the first results of a high-mass lepton flavor violation search using the full Run 2 data set, and they are currently the most stringent limits from any collider experiment.
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