CMSPASEXO21016  
Search for heavy neutral resonances decaying to $ \tau $ lepton pairs in pp collisions at $ \sqrt{s}= $ 13 TeV  
CMS Collaboration  
4 June 2024  
Abstract: A search for heavy neutral gauge bosons (Z') decaying to two tau leptons is performed in protonproton collisions at $ \sqrt{s}= $ 13 TeV at the CERN LHC. The data were collected with the CMS detector and correspond to an integrated luminosity of 138 fb$ ^{1} $. The yields of candidate events are found to be consistent with the predictions from standard model processes. Limits on the production cross section times branching fraction of the Z' as a function of the Z' mass are set at the 95% confidence level. For a narrow resonance in the sequential standard model scenario, a Z' with mass below 3.5 TeV is excluded. This is the most stringent limit to date from this type of search.  
Links: CDS record (PDF) ; CADI line (restricted) ; 
Figures  
png pdf 
Figure 1:
Prefit reconstructed mass distributions for the (upper row) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, (middle row and lower row left) $ \tau_{\mu}\tau_\mathrm{h} $, and (lower row right) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. In the upper and middle rows, the W$+$jets ( $ \mathrm{t} \overline{\mathrm{t}} $) CR is shown in the left (right) column. The lower row shows the DY CR for the $ \tau_{\mu}\tau_\mathrm{h} $ ($ \tau_\mathrm{h}\tau_\mathrm{h} $) channel in the left (right) column. The background contributions appear as stacked solidfilled histograms, while the observed data are shown as black solid markers with error bars. Overflow counts are included in the last bin. The grey shadow band represents the statistical uncertainty in the background prediction based on the number of simulated events and scale factor uncertainties. The lower panel of each plot shows the ratio of data to predicted background yields. 
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Figure 1a:
Prefit reconstructed mass distributions for the (upper row) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, (middle row and lower row left) $ \tau_{\mu}\tau_\mathrm{h} $, and (lower row right) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. In the upper and middle rows, the W$+$jets ( $ \mathrm{t} \overline{\mathrm{t}} $) CR is shown in the left (right) column. The lower row shows the DY CR for the $ \tau_{\mu}\tau_\mathrm{h} $ ($ \tau_\mathrm{h}\tau_\mathrm{h} $) channel in the left (right) column. The background contributions appear as stacked solidfilled histograms, while the observed data are shown as black solid markers with error bars. Overflow counts are included in the last bin. The grey shadow band represents the statistical uncertainty in the background prediction based on the number of simulated events and scale factor uncertainties. The lower panel of each plot shows the ratio of data to predicted background yields. 
png pdf 
Figure 1b:
Prefit reconstructed mass distributions for the (upper row) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, (middle row and lower row left) $ \tau_{\mu}\tau_\mathrm{h} $, and (lower row right) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. In the upper and middle rows, the W$+$jets ( $ \mathrm{t} \overline{\mathrm{t}} $) CR is shown in the left (right) column. The lower row shows the DY CR for the $ \tau_{\mu}\tau_\mathrm{h} $ ($ \tau_\mathrm{h}\tau_\mathrm{h} $) channel in the left (right) column. The background contributions appear as stacked solidfilled histograms, while the observed data are shown as black solid markers with error bars. Overflow counts are included in the last bin. The grey shadow band represents the statistical uncertainty in the background prediction based on the number of simulated events and scale factor uncertainties. The lower panel of each plot shows the ratio of data to predicted background yields. 
png pdf 
Figure 1c:
Prefit reconstructed mass distributions for the (upper row) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, (middle row and lower row left) $ \tau_{\mu}\tau_\mathrm{h} $, and (lower row right) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. In the upper and middle rows, the W$+$jets ( $ \mathrm{t} \overline{\mathrm{t}} $) CR is shown in the left (right) column. The lower row shows the DY CR for the $ \tau_{\mu}\tau_\mathrm{h} $ ($ \tau_\mathrm{h}\tau_\mathrm{h} $) channel in the left (right) column. The background contributions appear as stacked solidfilled histograms, while the observed data are shown as black solid markers with error bars. Overflow counts are included in the last bin. The grey shadow band represents the statistical uncertainty in the background prediction based on the number of simulated events and scale factor uncertainties. The lower panel of each plot shows the ratio of data to predicted background yields. 
png pdf 
Figure 1d:
Prefit reconstructed mass distributions for the (upper row) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, (middle row and lower row left) $ \tau_{\mu}\tau_\mathrm{h} $, and (lower row right) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. In the upper and middle rows, the W$+$jets ( $ \mathrm{t} \overline{\mathrm{t}} $) CR is shown in the left (right) column. The lower row shows the DY CR for the $ \tau_{\mu}\tau_\mathrm{h} $ ($ \tau_\mathrm{h}\tau_\mathrm{h} $) channel in the left (right) column. The background contributions appear as stacked solidfilled histograms, while the observed data are shown as black solid markers with error bars. Overflow counts are included in the last bin. The grey shadow band represents the statistical uncertainty in the background prediction based on the number of simulated events and scale factor uncertainties. The lower panel of each plot shows the ratio of data to predicted background yields. 
png pdf 
Figure 1e:
Prefit reconstructed mass distributions for the (upper row) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, (middle row and lower row left) $ \tau_{\mu}\tau_\mathrm{h} $, and (lower row right) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. In the upper and middle rows, the W$+$jets ( $ \mathrm{t} \overline{\mathrm{t}} $) CR is shown in the left (right) column. The lower row shows the DY CR for the $ \tau_{\mu}\tau_\mathrm{h} $ ($ \tau_\mathrm{h}\tau_\mathrm{h} $) channel in the left (right) column. The background contributions appear as stacked solidfilled histograms, while the observed data are shown as black solid markers with error bars. Overflow counts are included in the last bin. The grey shadow band represents the statistical uncertainty in the background prediction based on the number of simulated events and scale factor uncertainties. The lower panel of each plot shows the ratio of data to predicted background yields. 
png pdf 
Figure 1f:
Prefit reconstructed mass distributions for the (upper row) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, (middle row and lower row left) $ \tau_{\mu}\tau_\mathrm{h} $, and (lower row right) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. In the upper and middle rows, the W$+$jets ( $ \mathrm{t} \overline{\mathrm{t}} $) CR is shown in the left (right) column. The lower row shows the DY CR for the $ \tau_{\mu}\tau_\mathrm{h} $ ($ \tau_\mathrm{h}\tau_\mathrm{h} $) channel in the left (right) column. The background contributions appear as stacked solidfilled histograms, while the observed data are shown as black solid markers with error bars. Overflow counts are included in the last bin. The grey shadow band represents the statistical uncertainty in the background prediction based on the number of simulated events and scale factor uncertainties. The lower panel of each plot shows the ratio of data to predicted background yields. 
png pdf 
Figure 2:
Prefit reconstructed mass distributions for the (upper) $ \tau_{\mu}\tau_\mathrm{h} $, (middle) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and (lower) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. The background contributions appear as stacked solidfilled histograms, while the observed data are shown as black solid markers with error bars. Overflow counts are included in the last bin. The grey shadow band represents the statistical uncertainty in the background prediction based on the number of simulated events and scale factor uncertainties. The dashed lines correspond to the signal expectation, for Z' masses of 250 GeV (red) and 2500 GeV (magenta), normalized to 1846 pb and 0.1313 pb respectively. 
png pdf 
Figure 2a:
Prefit reconstructed mass distributions for the (upper) $ \tau_{\mu}\tau_\mathrm{h} $, (middle) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and (lower) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. The background contributions appear as stacked solidfilled histograms, while the observed data are shown as black solid markers with error bars. Overflow counts are included in the last bin. The grey shadow band represents the statistical uncertainty in the background prediction based on the number of simulated events and scale factor uncertainties. The dashed lines correspond to the signal expectation, for Z' masses of 250 GeV (red) and 2500 GeV (magenta), normalized to 1846 pb and 0.1313 pb respectively. 
png pdf 
Figure 2b:
Prefit reconstructed mass distributions for the (upper) $ \tau_{\mu}\tau_\mathrm{h} $, (middle) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and (lower) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. The background contributions appear as stacked solidfilled histograms, while the observed data are shown as black solid markers with error bars. Overflow counts are included in the last bin. The grey shadow band represents the statistical uncertainty in the background prediction based on the number of simulated events and scale factor uncertainties. The dashed lines correspond to the signal expectation, for Z' masses of 250 GeV (red) and 2500 GeV (magenta), normalized to 1846 pb and 0.1313 pb respectively. 
png pdf 
Figure 2c:
Prefit reconstructed mass distributions for the (upper) $ \tau_{\mu}\tau_\mathrm{h} $, (middle) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and (lower) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. The background contributions appear as stacked solidfilled histograms, while the observed data are shown as black solid markers with error bars. Overflow counts are included in the last bin. The grey shadow band represents the statistical uncertainty in the background prediction based on the number of simulated events and scale factor uncertainties. The dashed lines correspond to the signal expectation, for Z' masses of 250 GeV (red) and 2500 GeV (magenta), normalized to 1846 pb and 0.1313 pb respectively. 
png pdf 
Figure 3:
Upper limits on $ \sigma\mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ for the (upper) $ \tau_{\mu}\tau_\mathrm{h} $, (middle) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and (lower) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. In each plot, the solid black line represents the observed limit, and the black dashed line with green and yellow bands gives with the expected limit with its one and twostandard deviation uncertainties, respectively. The red, pink, and blue dashed lines represent the theoretical predictions for $ \mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ = 1, 3.37, and 10%, respectively. 3.37% is the same value as in the SM. The assumed $ \mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ are fixed for all Z' masses. 
png pdf 
Figure 3a:
Upper limits on $ \sigma\mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ for the (upper) $ \tau_{\mu}\tau_\mathrm{h} $, (middle) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and (lower) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. In each plot, the solid black line represents the observed limit, and the black dashed line with green and yellow bands gives with the expected limit with its one and twostandard deviation uncertainties, respectively. The red, pink, and blue dashed lines represent the theoretical predictions for $ \mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ = 1, 3.37, and 10%, respectively. 3.37% is the same value as in the SM. The assumed $ \mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ are fixed for all Z' masses. 
png pdf 
Figure 3b:
Upper limits on $ \sigma\mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ for the (upper) $ \tau_{\mu}\tau_\mathrm{h} $, (middle) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and (lower) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. In each plot, the solid black line represents the observed limit, and the black dashed line with green and yellow bands gives with the expected limit with its one and twostandard deviation uncertainties, respectively. The red, pink, and blue dashed lines represent the theoretical predictions for $ \mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ = 1, 3.37, and 10%, respectively. 3.37% is the same value as in the SM. The assumed $ \mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ are fixed for all Z' masses. 
png pdf 
Figure 3c:
Upper limits on $ \sigma\mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ for the (upper) $ \tau_{\mu}\tau_\mathrm{h} $, (middle) $ \tau_{\mathrm{e}}\tau_\mathrm{h} $, and (lower) $ \tau_\mathrm{h}\tau_\mathrm{h} $ channel. In each plot, the solid black line represents the observed limit, and the black dashed line with green and yellow bands gives with the expected limit with its one and twostandard deviation uncertainties, respectively. The red, pink, and blue dashed lines represent the theoretical predictions for $ \mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ = 1, 3.37, and 10%, respectively. 3.37% is the same value as in the SM. The assumed $ \mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ are fixed for all Z' masses. 
png pdf 
Figure 4:
Upper limit on the production cross sections times $ \mathrm{Z}^{'} \rightarrow \tau^+ \tau^ $ branching fraction, for the combination of the three channels. The solid black line represents the observed limit, and the black dashed line with green and yellow bands gives with the expected limit with its one and twostandard deviation uncertainties, respectively. The red, pink, and blue dashed lines represent the theoretical predictions for values of $ \mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ = 1, 3.37, and 10%, respectively. 3.37% is the same value as in the SM. The assumed $ \mathcal{B}(\mathrm{Z}^{'}\to\tau^+\tau^) $ are fixed for all Z' masses. 
Tables  
png pdf 
Table 1:
Prefit estimated background and observed event yields for the three channels. For each SM process the yield in events is shown, along with the fractional contribution to the total in percent. 
Summary 
A search has been performed in protonproton collisions at $ \sqrt{s} = $ 13 TeV for the production of a Z' boson decaying to $ \tau^{+}\tau^{} $. The data were recorded at the LHC during 20162018, and correspond to an integrated luminosity of 138 fb$ ^{1} $. As $ \tau $ leptons can decay hadronically ($ \tau_\mathrm{h} $) or leptonically ($ \tau_{\ell} $), with $ \ell $ representing a muon or electron, the analysis includes the three decay channels with the highest branching fraction products: $ \mathrm{Z}^{'}\to\tau_{\mu}^{\pm}\tau_\mathrm{h}^{\mp} $, $ \mathrm{Z}^{'}\to\tau_{\mathrm{e}}^{\pm}\tau_\mathrm{h}^{\mp} $, and $ \mathrm{Z}^{'} \to \tau_\mathrm{h}^{+} \tau_\mathrm{h}^{} $. A discriminating variable $ m_{\textrm{rec}}(\mathrm{Z}^{'}) $ is constructed to approximate the Z' mass in the presence of undetected neutrino daughters from the $ \tau $ lepton decays. The principal background processes are estimated using MC simulation with correction factors derived from data. The observed yields are found to be consistent with the background prediction. A shapebased analysis is performed using the $ m_{\textrm{rec}}(\mathrm{Z}^{'}) $ distribution as the fit discriminant to determine the likelihood of observing signal in the presence of the predicted background rate, given the observed yield in data. An upper limit on the production cross section times Z' decay branching fraction is set at 95% confidence level as a function of the Z' mass. The data exclude a Z' with mass less than 3.0, 3.5, and 4.1 TeV for a branching fraction of 1.0, 3.37, and 10%, respectively. These exclusion limits are the most stringent to date for a Z' decaying to $ \tau^{+}\tau^{} $. 
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