CMS-BPH-17-004

CMS-BPH-17-004 ; CERN-EP-2020-113
Search for the lepton flavor violating decay $ {\tau \!\to\! 3\mu} $ in proton-proton collisions at $ \sqrt{s} = $ 13 TeV
CMS Collaboration
11 July 2020
JHEP 01 (2021) 163
Abstract: Results are reported from a search for the lepton flavor violating decay $ {\tau \!\to\! 3\mu}$ in proton-proton collisions at $\sqrt{s} = $ 13 TeV. The data sample corresponds to an integrated luminosity of 33.2 fb$^{-1}$ recorded by the CMS experiment at the LHC in 2016. The search exploits $\tau$ leptons produced in both W boson and heavy-flavor hadron decays. No significant excess above the expected background is observed. An upper limit on the branching fraction $\mathcal{B}({\tau \!\to\! 3\mu} )$ of 8.0 $\times$ 10$^{-8}$ at 90% confidence level is obtained, with an expected upper limit of 6.9 $\times$ 10$^{-8}$.
Links: e-print arXiv:2007.05658 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Trimuon invariant mass distributions for barrel (left) and endcap (right) categories of the W boson analysis. The data are shown with filled circles and vertical bars representing the statistical uncertainty. The background-only fit and the expected signal for $\mathcal {B}({\tau \,\to \, 3\mu}) = $ 10$^{-7}$ are shown with solid and dashed lines, respectively.
png pdf	Figure 1-a: Trimuon invariant mass distributions for the barrel category of the W boson analysis. The data are shown with filled circles and vertical bars representing the statistical uncertainty. The background-only fit and the expected signal for $\mathcal {B}({\tau \,\to \, 3\mu}) = $ 10$^{-7}$ are shown with solid and dashed lines, respectively.
png pdf	Figure 1-b: Trimuon invariant mass distributions for the endcap category of the W boson analysis. The data are shown with filled circles and vertical bars representing the statistical uncertainty. The background-only fit and the expected signal for $\mathcal {B}({\tau \,\to \, 3\mu}) = $ 10$^{-7}$ are shown with solid and dashed lines, respectively.
png pdf	Figure 2: Trimuon invariant mass distribution in the six independent event categories used in the heavy-flavor analysis and defined in the text: A1, A2, B1, B2, C1, C2. The data are shown with filled circles and vertical bars representing the statistical uncertainty. The background-only fit and the expected signal for $\mathcal {B}({\tau \,\to \, 3\mu}) = $ 10$^{-7}$ are shown with solid and dashed lines, respectively.
png pdf	Figure 2-a: Trimuon invariant mass distribution in the A1 event category. The data are shown with filled circles and vertical bars representing the statistical uncertainty. The background-only fit and the expected signal for $\mathcal {B}({\tau \,\to \, 3\mu}) = $ 10$^{-7}$ are shown with solid and dashed lines, respectively.
png pdf	Figure 2-b: Trimuon invariant mass distribution in the A2 event category. The data are shown with filled circles and vertical bars representing the statistical uncertainty. The background-only fit and the expected signal for $\mathcal {B}({\tau \,\to \, 3\mu}) = $ 10$^{-7}$ are shown with solid and dashed lines, respectively.
png pdf	Figure 2-c: Trimuon invariant mass distribution in the B1 event category. The data are shown with filled circles and vertical bars representing the statistical uncertainty. The background-only fit and the expected signal for $\mathcal {B}({\tau \,\to \, 3\mu}) = $ 10$^{-7}$ are shown with solid and dashed lines, respectively.
png pdf	Figure 2-d: Trimuon invariant mass distribution in the B2 event category. The data are shown with filled circles and vertical bars representing the statistical uncertainty. The background-only fit and the expected signal for $\mathcal {B}({\tau \,\to \, 3\mu}) = $ 10$^{-7}$ are shown with solid and dashed lines, respectively.
png pdf	Figure 2-e: Trimuon invariant mass distribution in the C1 event category. The data are shown with filled circles and vertical bars representing the statistical uncertainty. The background-only fit and the expected signal for $\mathcal {B}({\tau \,\to \, 3\mu}) = $ 10$^{-7}$ are shown with solid and dashed lines, respectively.
png pdf	Figure 2-f: Trimuon invariant mass distribution in the C2 event category. The data are shown with filled circles and vertical bars representing the statistical uncertainty. The background-only fit and the expected signal for $\mathcal {B}({\tau \,\to \, 3\mu}) = $ 10$^{-7}$ are shown with solid and dashed lines, respectively.
png pdf	Figure 3: Left: the $\mu \mu \pi $ invariant mass distribution with fits to the ${\mathrm{D^+}}$ and ${\mathrm{D^+}_{s}}$ peaks and the background. Right: background-subtracted proper decay length distribution for ${{\mathrm{D^+}_{s}} \,\to \, \phi \pi^{+} \,\to \, \mu^{+} \mu^{-} \pi^{+}}$ events (points) and the fitted contributions from ${\mathrm{D^+}_{s}}$ mesons produced directly (open histogram) and from B meson decays (filled histogram). The highest bin also contains the overflow events. The vertical bars in both plots represent the statistical uncertainties in the data.
png pdf	Figure 3-a: The $\mu \mu \pi $ invariant mass distribution with fits to the ${\mathrm{D^+}}$ and ${\mathrm{D^+}_{s}}$ peaks and the background. The vertical bars represent the statistical uncertainties in the data.
png pdf	Figure 3-b: Background-subtracted proper decay length distribution for ${{\mathrm{D^+}_{s}} \,\to \, \phi \pi^{+} \,\to \, \mu^{+} \mu^{-} \pi^{+}}$ events (points) and the fitted contributions from ${\mathrm{D^+}_{s}}$ mesons produced directly (open histogram) and from B meson decays (filled histogram). The highest bin also contains the overflow events. The vertical bars represent the statistical uncertainties in the data.

Tables
png pdf	Table 1: Sources of systematic uncertainties in the W boson analysis and their effect on the signal efficiency and normalization for the barrel and endcap categories.
png pdf	Table 2: The sources of systematic uncertainties in the heavy-flavor analysis affecting signal modeling and their impact on the expected signal event yield. The columns labeled Uncertainty and Yield give the uncertainty associated with the source, and the resulting effect on the yield, respectively.

Summary

The results of a search for the lepton flavor violating decay ${\tau \!\to\! 3\mu} $, using proton-proton collisions with a center-of-mass energy of 13 TeV at the LHC, are presented. The search uses data collected by CMS in 2016, corresponding to an integrated luminosity of 33.2 fb$^{-1}$, and, for the first time, combines the result of two analyses: one targeting $\tau$ leptons produced in W boson decays and the other using $\tau$ leptons from heavy-flavor hadron decays. No signal is observed, and the branching fraction $\mathcal{B}({\tau \!\to\! 3\mu} )$ is determined to be less than 8.0 $\times$ 10$^{-8}$ at 90% confidence level, with an expected upper limit of 6.9 $\times$ 10$^{-8}$. While the limit obtained in this measurement is still a factor of four away from the current most restrictive one from the Belle experiment [8], we have achieved similar sensitivity to that by BaBar [7] and LHCb [9].

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