CMS-SMP-22-016

CMS-SMP-22-016 ; CERN-EP-2024-107
Search for the Z boson decay to $ \tau\tau\mu\mu $ in proton-proton 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 proton-proton collisions at a center-of-mass 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 flavor-conserving four-lepton effective-field-theory operators involving two muons and two tau leptons, for the first time testing all such operators.
Links: e-print arXiv:2404.18298 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Leading-order Feynman diagram of the Z boson decay to four charged leptons.
png pdf	Figure 2: Distribution of $ m_{4\mu} $ after the maximum likelihood fit of the background-only model (stacked histograms) to the data (black points). The nuisance parameters are set to their post-fit 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.
png pdf	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 3-a: 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 3-b: 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

png pdf

Table 1:
Intervals allowed at the 95% CL for $ \mathcal{C}/\Lambda^2 $ for all 6 dimension-six 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 proton-proton 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 flavor-conserving four-lepton Wilson coefficients involving two muons and two tau leptons.

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