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| CMS-SMP-23-003 ; CERN-EP-2025-130 | ||
| Search for charged lepton flavor violating Z and Z' boson decays in proton-proton collisions at $ \sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 10 August 2025 | ||
| Accepted for publication in Phys. Rev. D | ||
| Abstract: A search for flavor violating decays of the Z boson to charged leptons is performed using data from proton-proton collisions at $ \sqrt{s}= $ 13 TeV collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. Each of the decays $ \mathrm{Z}\to\mathrm{e}\mu $, $ \mathrm{Z}\to\mathrm{e}\tau $, and $ \mathrm{Z}\to\mu\tau $ is considered. The data are consistent with the backgrounds expected from standard model processes. For the $ \mathrm{Z}\to\mathrm{e}\mu $ channel the observed (expected) 95% confidence level upper limit on the branching fraction is 1.9 (2.0)$ \times $10$^{-7} $, which is the most stringent direct limit to date on this process; the corresponding limits for the $ \mathrm{Z}\to\mathrm{e}\tau $ and $ \mathrm{Z}\to\mu\tau $ channels are 13.8 (11.4)$ \times $10$^{-6} $ and 12.0 (5.3)$ \times $10$^{-6} $, respectively. Additionally, the $ \mathrm{e}\mu $ final state is used to search for lepton flavor violating decays of Z' resonances in the mass range from 110 to 500 GeV. No significant excess is observed above the predicted background levels. | ||
| Links: e-print arXiv:2508.07512 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Invariant mass of the $ \mathrm{e}\mu $ system for data (points, with bars denoting statistical uncertainty) and simulated background (stacked filled histograms) for events passing the baseline selection. In the legend, ``W, multi-V'' refers to $ \mathrm{W}\!+\!\text{jets} $ events having a jet that is misidentified as a lepton, together with multiple vector boson production. The hatched histogram shows a hypothetical $ \mathrm{Z}\to\mathrm{e}\mu $ signal normalized to a branching fraction of $ 10^{-5} $. The lower panel shows the ratio of the data to simulated background yields, with the statistical (combined systematic and statistical) uncertainty in the simulated yield indicated by the filled (hatched) gray band. |
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Figure 2:
The left plot shows unity-normalized distributions of the $ \mathrm{Z}\to\mathrm{e}\mu $ BDT score for simulated events from the BDT test samples satisfying 70 $ < m_{\mathrm{e}\mu} < $ 110 GeV. The blue and red histograms represent the signal and WW background, respectively. The right plot displays the unity-normalized distribution of $ m_{\mathrm{e}\mu} $ for events in the $ \mathrm{t} \overline{\mathrm{t}} $ data CR used to check for BDT mass-spectrum bias, for several BDT thresholds; the vertical bars show the statistical uncertainties. |
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Figure 2-a:
The left plot shows unity-normalized distributions of the $ \mathrm{Z}\to\mathrm{e}\mu $ BDT score for simulated events from the BDT test samples satisfying 70 $ < m_{\mathrm{e}\mu} < $ 110 GeV. The blue and red histograms represent the signal and WW background, respectively. The right plot displays the unity-normalized distribution of $ m_{\mathrm{e}\mu} $ for events in the $ \mathrm{t} \overline{\mathrm{t}} $ data CR used to check for BDT mass-spectrum bias, for several BDT thresholds; the vertical bars show the statistical uncertainties. |
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Figure 2-b:
The left plot shows unity-normalized distributions of the $ \mathrm{Z}\to\mathrm{e}\mu $ BDT score for simulated events from the BDT test samples satisfying 70 $ < m_{\mathrm{e}\mu} < $ 110 GeV. The blue and red histograms represent the signal and WW background, respectively. The right plot displays the unity-normalized distribution of $ m_{\mathrm{e}\mu} $ for events in the $ \mathrm{t} \overline{\mathrm{t}} $ data CR used to check for BDT mass-spectrum bias, for several BDT thresholds; the vertical bars show the statistical uncertainties. |
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Figure 3:
Fits of the data sidebands with the background functions for the BDT score ranges 0.3-0.7 (upper left), 0.7-0.9 (upper right), and 0.9-1.0 (lower). In the upper panel of each plot, the black (blue) points with bars show the data with statistical uncertainties in the sideband (signal) region, the solid red line shows the average background prediction, and the dashed green curve shows the $ \mathrm{Z}\to\mu\mu^*\to\mu\mu\gamma $ background component. The lower panel in each plot shows the ratio of the difference between the data and the average background prediction to the uncertainty in the data, and the gray band shows the spread of the background estimates from the separate families of parametric functions. |
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Figure 3-a:
Fits of the data sidebands with the background functions for the BDT score ranges 0.3-0.7 (upper left), 0.7-0.9 (upper right), and 0.9-1.0 (lower). In the upper panel of each plot, the black (blue) points with bars show the data with statistical uncertainties in the sideband (signal) region, the solid red line shows the average background prediction, and the dashed green curve shows the $ \mathrm{Z}\to\mu\mu^*\to\mu\mu\gamma $ background component. The lower panel in each plot shows the ratio of the difference between the data and the average background prediction to the uncertainty in the data, and the gray band shows the spread of the background estimates from the separate families of parametric functions. |
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Figure 3-b:
Fits of the data sidebands with the background functions for the BDT score ranges 0.3-0.7 (upper left), 0.7-0.9 (upper right), and 0.9-1.0 (lower). In the upper panel of each plot, the black (blue) points with bars show the data with statistical uncertainties in the sideband (signal) region, the solid red line shows the average background prediction, and the dashed green curve shows the $ \mathrm{Z}\to\mu\mu^*\to\mu\mu\gamma $ background component. The lower panel in each plot shows the ratio of the difference between the data and the average background prediction to the uncertainty in the data, and the gray band shows the spread of the background estimates from the separate families of parametric functions. |
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Figure 3-c:
Fits of the data sidebands with the background functions for the BDT score ranges 0.3-0.7 (upper left), 0.7-0.9 (upper right), and 0.9-1.0 (lower). In the upper panel of each plot, the black (blue) points with bars show the data with statistical uncertainties in the sideband (signal) region, the solid red line shows the average background prediction, and the dashed green curve shows the $ \mathrm{Z}\to\mu\mu^*\to\mu\mu\gamma $ background component. The lower panel in each plot shows the ratio of the difference between the data and the average background prediction to the uncertainty in the data, and the gray band shows the spread of the background estimates from the separate families of parametric functions. |
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Figure 4:
Invariant mass of the $ \mathrm{e}\mu $ system for data (points) and simulated background (stacked filled histograms), for events that pass the baseline selection except for that selection's upper limit on the invariant mass. The lower panel shows the ratio of the data to simulated yields, with the statistical (combined systematic and statistical) uncertainty in the simulated yield indicated by the filled (hatched) gray band. |
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Figure 5:
The $ \mu\tau_\mathrm{h} $ (left) and $ \mu\tau_{\mathrm{e}} $ (right) $ m^{{\text{col}}} $ distributions, for the data (black markers with bars showing the statistical uncertainties) and the simulated backgrounds (filled stacked histograms). The hatched blue histogram shows the shape of the signal, normalized to a branching fraction of 10$^{-3} $, for comparison. The lower panel shows the ratio of the data to simulated yields, with the statistical (combined systematic and statistical) uncertainty in the simulated yield indicated by the filled (hatched) gray band. |
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Figure 5-a:
The $ \mu\tau_\mathrm{h} $ (left) and $ \mu\tau_{\mathrm{e}} $ (right) $ m^{{\text{col}}} $ distributions, for the data (black markers with bars showing the statistical uncertainties) and the simulated backgrounds (filled stacked histograms). The hatched blue histogram shows the shape of the signal, normalized to a branching fraction of 10$^{-3} $, for comparison. The lower panel shows the ratio of the data to simulated yields, with the statistical (combined systematic and statistical) uncertainty in the simulated yield indicated by the filled (hatched) gray band. |
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Figure 5-b:
The $ \mu\tau_\mathrm{h} $ (left) and $ \mu\tau_{\mathrm{e}} $ (right) $ m^{{\text{col}}} $ distributions, for the data (black markers with bars showing the statistical uncertainties) and the simulated backgrounds (filled stacked histograms). The hatched blue histogram shows the shape of the signal, normalized to a branching fraction of 10$^{-3} $, for comparison. The lower panel shows the ratio of the data to simulated yields, with the statistical (combined systematic and statistical) uncertainty in the simulated yield indicated by the filled (hatched) gray band. |
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Figure 6:
Distributions of $ \mu\tau_\mathrm{h} $ signal and estimated backgrounds in $ \alpha_{\mu} $ (left), $ \alpha_{\tau} $ (right), for the data (black markers with bars showing the statistical uncertainties) and the simulated backgrounds (filled stacked histograms). The hatched blue histogram shows the shape of the signal, normalized to a branching fraction of 10$^{-3} $, for comparison. The lower panel shows the ratio of the data to simulated yields, with the statistical (combined systematic and statistical) uncertainty in the simulated yield indicated by the filled (hatched) gray band. |
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Figure 6-a:
Distributions of $ \mu\tau_\mathrm{h} $ signal and estimated backgrounds in $ \alpha_{\mu} $ (left), $ \alpha_{\tau} $ (right), for the data (black markers with bars showing the statistical uncertainties) and the simulated backgrounds (filled stacked histograms). The hatched blue histogram shows the shape of the signal, normalized to a branching fraction of 10$^{-3} $, for comparison. The lower panel shows the ratio of the data to simulated yields, with the statistical (combined systematic and statistical) uncertainty in the simulated yield indicated by the filled (hatched) gray band. |
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Figure 6-b:
Distributions of $ \mu\tau_\mathrm{h} $ signal and estimated backgrounds in $ \alpha_{\mu} $ (left), $ \alpha_{\tau} $ (right), for the data (black markers with bars showing the statistical uncertainties) and the simulated backgrounds (filled stacked histograms). The hatched blue histogram shows the shape of the signal, normalized to a branching fraction of 10$^{-3} $, for comparison. The lower panel shows the ratio of the data to simulated yields, with the statistical (combined systematic and statistical) uncertainty in the simulated yield indicated by the filled (hatched) gray band. |
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Figure 7:
For the $ \mathrm{Z}\to\mathrm{e}\mu $ search, the invariant mass fit results for the BDT score ranges 0.3-0.7 (upper), 0.7-0.9 (middle), and 0.9-1.0 (lower). In each plot, the upper panel shows the data (points with bars showing the statistical uncertainties) together with the fit distribution curve (red) and its separate signal (blue dotted) $ \mathrm{Z}\to\mu\mu $ (yellow dash-dotted) and continuum background (gray dashed) components, and the lower panel shows the deviations of the data from the fit function divided by the data uncertainty. |
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Figure 7-a:
For the $ \mathrm{Z}\to\mathrm{e}\mu $ search, the invariant mass fit results for the BDT score ranges 0.3-0.7 (upper), 0.7-0.9 (middle), and 0.9-1.0 (lower). In each plot, the upper panel shows the data (points with bars showing the statistical uncertainties) together with the fit distribution curve (red) and its separate signal (blue dotted) $ \mathrm{Z}\to\mu\mu $ (yellow dash-dotted) and continuum background (gray dashed) components, and the lower panel shows the deviations of the data from the fit function divided by the data uncertainty. |
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Figure 7-b:
For the $ \mathrm{Z}\to\mathrm{e}\mu $ search, the invariant mass fit results for the BDT score ranges 0.3-0.7 (upper), 0.7-0.9 (middle), and 0.9-1.0 (lower). In each plot, the upper panel shows the data (points with bars showing the statistical uncertainties) together with the fit distribution curve (red) and its separate signal (blue dotted) $ \mathrm{Z}\to\mu\mu $ (yellow dash-dotted) and continuum background (gray dashed) components, and the lower panel shows the deviations of the data from the fit function divided by the data uncertainty. |
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Figure 7-c:
For the $ \mathrm{Z}\to\mathrm{e}\mu $ search, the invariant mass fit results for the BDT score ranges 0.3-0.7 (upper), 0.7-0.9 (middle), and 0.9-1.0 (lower). In each plot, the upper panel shows the data (points with bars showing the statistical uncertainties) together with the fit distribution curve (red) and its separate signal (blue dotted) $ \mathrm{Z}\to\mu\mu $ (yellow dash-dotted) and continuum background (gray dashed) components, and the lower panel shows the deviations of the data from the fit function divided by the data uncertainty. |
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Figure 8:
Upper limits at 95% CL on the branching fraction $ \mathcal{B}(\mathrm{Z}\to\mathrm{e}\mu) $, for each BDT score range and for the final combined fit. The observed limits are denoted by the markers, while the expected limits with their 68 and 95% uncertainties are denoted by the horizontal dashed lines and green and yellow bands, respectively. |
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Figure 9:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mathrm{e}\tau_\mathrm{h} $ channels in the $ m_{\mathrm{e}\tau} $ ranges: (upper left) 40-60 GeV, ``$ \tau\tau $''; (upper right) 60-85 GeV, ``signal-like''; (lower left) 85-100 GeV, ``$ \mathrm{Z}\to\ell\ell $''; (lower right) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
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Figure 9-a:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mathrm{e}\tau_\mathrm{h} $ channels in the $ m_{\mathrm{e}\tau} $ ranges: (upper left) 40-60 GeV, ``$ \tau\tau $''; (upper right) 60-85 GeV, ``signal-like''; (lower left) 85-100 GeV, ``$ \mathrm{Z}\to\ell\ell $''; (lower right) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
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Figure 9-b:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mathrm{e}\tau_\mathrm{h} $ channels in the $ m_{\mathrm{e}\tau} $ ranges: (upper left) 40-60 GeV, ``$ \tau\tau $''; (upper right) 60-85 GeV, ``signal-like''; (lower left) 85-100 GeV, ``$ \mathrm{Z}\to\ell\ell $''; (lower right) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
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Figure 9-c:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mathrm{e}\tau_\mathrm{h} $ channels in the $ m_{\mathrm{e}\tau} $ ranges: (upper left) 40-60 GeV, ``$ \tau\tau $''; (upper right) 60-85 GeV, ``signal-like''; (lower left) 85-100 GeV, ``$ \mathrm{Z}\to\ell\ell $''; (lower right) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
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Figure 9-d:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mathrm{e}\tau_\mathrm{h} $ channels in the $ m_{\mathrm{e}\tau} $ ranges: (upper left) 40-60 GeV, ``$ \tau\tau $''; (upper right) 60-85 GeV, ``signal-like''; (lower left) 85-100 GeV, ``$ \mathrm{Z}\to\ell\ell $''; (lower right) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
png pdf |
Figure 10:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mathrm{e}\tau_{\mu} $ channels in the $ m_{\mathrm{e}\tau} $ ranges: (upper left) 40-50 GeV, ``$ \tau\tau $''; (upper right) 50-100 GeV, ``signal-like''; (lower) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
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Figure 10-a:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mathrm{e}\tau_{\mu} $ channels in the $ m_{\mathrm{e}\tau} $ ranges: (upper left) 40-50 GeV, ``$ \tau\tau $''; (upper right) 50-100 GeV, ``signal-like''; (lower) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
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Figure 10-b:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mathrm{e}\tau_{\mu} $ channels in the $ m_{\mathrm{e}\tau} $ ranges: (upper left) 40-50 GeV, ``$ \tau\tau $''; (upper right) 50-100 GeV, ``signal-like''; (lower) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
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Figure 10-c:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mathrm{e}\tau_{\mu} $ channels in the $ m_{\mathrm{e}\tau} $ ranges: (upper left) 40-50 GeV, ``$ \tau\tau $''; (upper right) 50-100 GeV, ``signal-like''; (lower) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
png pdf |
Figure 11:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mu\tau_\mathrm{h} $ channels in the $ m_{\mu\tau} $ ranges: (upper left) 40-60 GeV, ``$ \tau\tau $''; (upper right) 60-85 GeV, ``signal-like''; (lower left) 85-100 GeV, ``$ \mathrm{Z}\to\ell\ell $''; (lower right) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
png pdf |
Figure 11-a:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mu\tau_\mathrm{h} $ channels in the $ m_{\mu\tau} $ ranges: (upper left) 40-60 GeV, ``$ \tau\tau $''; (upper right) 60-85 GeV, ``signal-like''; (lower left) 85-100 GeV, ``$ \mathrm{Z}\to\ell\ell $''; (lower right) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
png pdf |
Figure 11-b:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mu\tau_\mathrm{h} $ channels in the $ m_{\mu\tau} $ ranges: (upper left) 40-60 GeV, ``$ \tau\tau $''; (upper right) 60-85 GeV, ``signal-like''; (lower left) 85-100 GeV, ``$ \mathrm{Z}\to\ell\ell $''; (lower right) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
png pdf |
Figure 11-c:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mu\tau_\mathrm{h} $ channels in the $ m_{\mu\tau} $ ranges: (upper left) 40-60 GeV, ``$ \tau\tau $''; (upper right) 60-85 GeV, ``signal-like''; (lower left) 85-100 GeV, ``$ \mathrm{Z}\to\ell\ell $''; (lower right) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
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Figure 11-d:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mu\tau_\mathrm{h} $ channels in the $ m_{\mu\tau} $ ranges: (upper left) 40-60 GeV, ``$ \tau\tau $''; (upper right) 60-85 GeV, ``signal-like''; (lower left) 85-100 GeV, ``$ \mathrm{Z}\to\ell\ell $''; (lower right) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
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Figure 12:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mu\tau_{\mathrm{e}} $ channels in the $ m_{\mu\tau} $ ranges: (upper left) 40-50 GeV, ``$ \tau\tau $''; (upper right) 50-100 GeV, ``signal-like''; (lower) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
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Figure 12-a:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mu\tau_{\mathrm{e}} $ channels in the $ m_{\mu\tau} $ ranges: (upper left) 40-50 GeV, ``$ \tau\tau $''; (upper right) 50-100 GeV, ``signal-like''; (lower) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
png pdf |
Figure 12-b:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mu\tau_{\mathrm{e}} $ channels in the $ m_{\mu\tau} $ ranges: (upper left) 40-50 GeV, ``$ \tau\tau $''; (upper right) 50-100 GeV, ``signal-like''; (lower) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
png pdf |
Figure 12-c:
Transformed BDT score distributions for the $ \mathrm{Z}\to\mu\tau_{\mathrm{e}} $ channels in the $ m_{\mu\tau} $ ranges: (upper left) 40-50 GeV, ``$ \tau\tau $''; (upper right) 50-100 GeV, ``signal-like''; (lower) 100-170 GeV, ``misID''. In each plot, the upper panel shows the data (points), the total yield from the signal + background fit (red open histogram), the signal component (blue open histogram), and the background components (stacked filled histograms). The middle panel shows the ratio to the background of the data (points with bars showing the statistical uncertainty in the data) and the combined signal + background (blue dotted histogram). The shaded band shows the systematic uncertainty in the background estimate. The lower panel shows the pull defined in the text (light blue filled histogram). |
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Figure 13:
Observed and expected 95% CL upper limit by category, as well as for the final combined fit, for the $ \mathrm{Z}\to\mathrm{e}\tau $ (left) and $ \mathrm{Z}\to\mu\tau $ (right) searches. The observed limits are denoted by the markers, while the expected limits with their 68 and 95% uncertainties are denoted by the horizontal dashed lines and green and yellow bands, respectively. |
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Figure 13-a:
Observed and expected 95% CL upper limit by category, as well as for the final combined fit, for the $ \mathrm{Z}\to\mathrm{e}\tau $ (left) and $ \mathrm{Z}\to\mu\tau $ (right) searches. The observed limits are denoted by the markers, while the expected limits with their 68 and 95% uncertainties are denoted by the horizontal dashed lines and green and yellow bands, respectively. |
png pdf |
Figure 13-b:
Observed and expected 95% CL upper limit by category, as well as for the final combined fit, for the $ \mathrm{Z}\to\mathrm{e}\tau $ (left) and $ \mathrm{Z}\to\mu\tau $ (right) searches. The observed limits are denoted by the markers, while the expected limits with their 68 and 95% uncertainties are denoted by the horizontal dashed lines and green and yellow bands, respectively. |
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Figure 14:
Distributions in $ m_{\mathrm{e}\mu} $ for the scan points 111 GeV (upper row) and 496 GeV (lower row) from the Z' search. In each row the BDT score range for the left (right) plot is 0.3-0.7 (0.7-1.0). In each plot, the upper panel shows the data (points with bars showing the statistical uncertainty) together with the fit distribution curve (red solid) and its separate signal (blue dotted) and background (red dotted) components, the middle panel shows the background subtracted data with the fit signal distribution, and the lower panel shows the deviations of the data from the fit function divided by the fit uncertainty. |
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Figure 14-a:
Distributions in $ m_{\mathrm{e}\mu} $ for the scan points 111 GeV (upper row) and 496 GeV (lower row) from the Z' search. In each row the BDT score range for the left (right) plot is 0.3-0.7 (0.7-1.0). In each plot, the upper panel shows the data (points with bars showing the statistical uncertainty) together with the fit distribution curve (red solid) and its separate signal (blue dotted) and background (red dotted) components, the middle panel shows the background subtracted data with the fit signal distribution, and the lower panel shows the deviations of the data from the fit function divided by the fit uncertainty. |
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Figure 14-b:
Distributions in $ m_{\mathrm{e}\mu} $ for the scan points 111 GeV (upper row) and 496 GeV (lower row) from the Z' search. In each row the BDT score range for the left (right) plot is 0.3-0.7 (0.7-1.0). In each plot, the upper panel shows the data (points with bars showing the statistical uncertainty) together with the fit distribution curve (red solid) and its separate signal (blue dotted) and background (red dotted) components, the middle panel shows the background subtracted data with the fit signal distribution, and the lower panel shows the deviations of the data from the fit function divided by the fit uncertainty. |
png pdf |
Figure 14-c:
Distributions in $ m_{\mathrm{e}\mu} $ for the scan points 111 GeV (upper row) and 496 GeV (lower row) from the Z' search. In each row the BDT score range for the left (right) plot is 0.3-0.7 (0.7-1.0). In each plot, the upper panel shows the data (points with bars showing the statistical uncertainty) together with the fit distribution curve (red solid) and its separate signal (blue dotted) and background (red dotted) components, the middle panel shows the background subtracted data with the fit signal distribution, and the lower panel shows the deviations of the data from the fit function divided by the fit uncertainty. |
png pdf |
Figure 14-d:
Distributions in $ m_{\mathrm{e}\mu} $ for the scan points 111 GeV (upper row) and 496 GeV (lower row) from the Z' search. In each row the BDT score range for the left (right) plot is 0.3-0.7 (0.7-1.0). In each plot, the upper panel shows the data (points with bars showing the statistical uncertainty) together with the fit distribution curve (red solid) and its separate signal (blue dotted) and background (red dotted) components, the middle panel shows the background subtracted data with the fit signal distribution, and the lower panel shows the deviations of the data from the fit function divided by the fit uncertainty. |
png pdf |
Figure 15:
Expected and observed 95% CL upper limits on $ \sigma(\mathrm{p}\mathrm{p}\to\mathrm{Z}^{'}\!+\!\mathrm{X})\mathcal{B}(\mathrm{Z}^{'}\to\mathrm{e}\mu) $ for Z' masses between 110 and 500 GeV. The solid black line connects filled circles representing the observed upper limits at the scan points, and the dashed line with filled error bands shows the expected limit. |
| Tables | |
png pdf |
Table 1:
Regions in $ m_{\ell\tau} $ for the $ \mathrm{Z}\to\mu\tau $ and $ \mathrm{Z}\to\mathrm{e}\tau $ fits. |
png pdf |
Table 2:
Sources of uncertainty and their impacts on the measured branching fraction, for each of the Z decay channels, and on the product of the production cross section and branching fraction for the Z' resonance scan. The uncertainty ranges for the Z' resonance scan are ordered from the lowest to the highest Z' mass point. Entries to which the specified uncertainty does not apply are denoted with ``$ \text{---} $''. |
png pdf |
Table 3:
The measured branching fraction with its significance (signif.) and observed and expected 95% CL upper limits, for each of the $ \mathrm{Z}\to\mathrm{e}\mu $, $ \mathrm{Z}\to\mathrm{e}\tau $, and $ \mathrm{Z}\to\mu\tau $ decay channels. The prior best published limits are also given for comparison. Included are results for the separate BDT bins for $ \mathrm{Z}\to\mathrm{e}\mu $ and the separate $ \tau $ decay subchannels for $ \mathrm{Z}\to\mathrm{e}\tau $ and $ \mathrm{Z}\to\mu\tau $. |
| Summary |
| A search is presented for flavor violating decays of the Z boson to charged leptons, and for the presence of a heavier vector boson Z' exhibiting such decays. The data from proton-proton collisions at $ \sqrt{s}= $ 13 TeV were collected with the CMS detector at the LHC, and correspond to an integrated luminosity of 138 fb$^{-1}$. The specific decay modes considered are $ \mathrm{Z}^{(\prime)}\to\mathrm{e}\mu $, $ \mathrm{Z}\to\mathrm{e}\tau $, and $ \mathrm{Z}\to\mu\tau $. No significant excess of events over backgrounds from standard model processes is observed. Observed (expected) upper limits of 1.9 $ \times $10$^{-7} $, 1.38 $ \times $10$^{-5} $, and 1.20 $ \times $10$^{-5} $ (2.0 $ \times $10$^{-7} $, 1.14 $ \times $10$^{-5} $, and 0.53 $ \times $10$^{-5} $) at 95% confidence level are set on the branching fractions for $ \mathrm{Z}\to\mathrm{e}\mu $, $ \mathrm{Z}\to\mathrm{e}\tau $, and $ \mathrm{Z}\to\mu\tau $, respectively. The limit for $ \mathrm{Z}\to\mathrm{e}\mu $ is the most restrictive to date, while for $ \mathrm{Z}\to\mu\tau $ the sensitivity in terms of the expected limit is the same as that of the previous best limit. All of these limits are consistent with expectations from the standard model, and with constraints inferred from low-energy experimental limits. For Z' boson masses in the range 110-500 GeV, upper limits are set on the cross section times the branching fraction to $ \mathrm{e}\mu $ that extend from 0.3 to 7 fb, and are the most restrictive to date for this mass range. Future studies can benefit from additional data, since even the systematic uncertainties arise mainly from statistical ones in control samples. |
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