CMSSMP22016 ; CERNEP2024107  
Search for the Z boson decay to $ \tau\tau\mu\mu $ in protonproton collisions at $ \sqrt{s}= $ 13 TeV  
CMS Collaboration  
29 April 2024  
Phys. Rev. Lett. 133 (2024) 161805  
Abstract: The first search for the $\mathrm{Z}$ boson decay to $ \tau\tau\mu\mu $ at the CERN LHC is presented, based on data collected by the CMS experiment at the LHC in protonproton collisions at a centerofmass energy of 13 TeV and corresponding to an integrated luminosity of 138 fb$ ^{1} $. The data are compatible with the predicted background. For the first time, an upper limit at the 95% confidence level of 6.9 times the standard model expectation is placed on the ratio of the $ \mathrm{Z}\to\tau\tau\mu\mu $ to $ \mathrm{Z}\to4\mu $ branching fractions. Limits are also placed on the six flavorconserving fourlepton effectivefieldtheory operators involving two muons and two tau leptons, for the first time testing all such operators.  
Links: eprint arXiv:2404.18298 [hepex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ; 
Figures  
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Figure 1:
Leadingorder Feynman diagram of the Z boson decay to four charged leptons. 
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Figure 2:
Distribution of $ m_{4\mu} $ after the maximum likelihood fit of the backgroundonly model (stacked histograms) to the data (black points). The nuisance parameters are set to their postfit values and the signal (black dotted line) is overlaid, scaled to the upper limit on its cross section of 6.9 times the SM expectation. The gray shaded areas in both panels correspond to the total uncertainty in the background prediction. The black vertical bars indicate the statistical uncertainty in the data. 
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Figure 3:
Observed limits at the 95% CL on different combinations of two Wilson coefficients (colors) showing the full ranges (left) and an enlarged view of the region around values of 0 (right). In the left (right) figure, the limit on $ \mathcal{C}_{\text{LR}}^{2233} $ vs. $ \mathcal{C}_{\text{LL}}^{2233} $ ($ \mathcal{C}_{\text{LL}}^{2332} $) is scaled by a factor of 5 (100) for visibility. The SM expectation is indicated by a black dot. The hatches indicate the excluded side of each limit. 
png pdf 
Figure 3a:
Observed limits at the 95% CL on different combinations of two Wilson coefficients (colors) showing the full ranges (left) and an enlarged view of the region around values of 0 (right). In the left (right) figure, the limit on $ \mathcal{C}_{\text{LR}}^{2233} $ vs. $ \mathcal{C}_{\text{LL}}^{2233} $ ($ \mathcal{C}_{\text{LL}}^{2332} $) is scaled by a factor of 5 (100) for visibility. The SM expectation is indicated by a black dot. The hatches indicate the excluded side of each limit. 
png pdf 
Figure 3b:
Observed limits at the 95% CL on different combinations of two Wilson coefficients (colors) showing the full ranges (left) and an enlarged view of the region around values of 0 (right). In the left (right) figure, the limit on $ \mathcal{C}_{\text{LR}}^{2233} $ vs. $ \mathcal{C}_{\text{LL}}^{2233} $ ($ \mathcal{C}_{\text{LL}}^{2332} $) is scaled by a factor of 5 (100) for visibility. The SM expectation is indicated by a black dot. The hatches indicate the excluded side of each limit. 
Tables  
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Table 1:
Intervals allowed at the 95% CL for $ \mathcal{C}/\Lambda^2 $ for all 6 dimensionsix Wilson coefficients conserving lepton flavor and involving two muons and two tau leptons. The second column shows the allowed intervals considering only terms linear in $ \mathcal{C} $, in which case the lower and upper bounds originate from unphysical behavior and the limit on $ \mathcal{R}_{\tau\tau\mu\mu} $, respectively. For the third column, quadratic terms are considered as well and all bounds are determined by the limit on $ \mathcal{R}_{\tau\tau\mu\mu} $. 
Summary 
In summary, the first search for the Z boson decay to $ \tau\tau\mu\mu $ at the LHC has been presented. The sample of protonproton collision data analyzed was collected at $ \sqrt{s}= $ 13 TeV by the CMS experiment and corresponds to an integrated luminosity of 138 fb$ ^{1} $. No excess over the standard model background prediction was observed. For the first time, an upper limit at 95% confidence level of 6.2 was placed on the ratio of the $ \mathrm{Z}\to\tau\tau\mu\mu $ to $ \mathrm{Z}\to4\mu $ branching fractions in the phase space considered, equivalent to 6.9 times the standard model expectation of 0.90. Also for the first time, constraints were placed on all flavorconserving fourlepton Wilson coefficients involving two muons and two tau leptons. 
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Compact Muon Solenoid LHC, CERN 