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| CMS-PAS-BPH-21-005 | ||
| Search for lepton flavor violating $ \tau \!\to\! $ 3$\mu $ decays in proton-proton collisions at $ \sqrt{s} $ = 13 TeV | ||
| CMS Collaboration | ||
| 26 May 2023 | ||
| Abstract: A search for lepton flavor violating $ \tau\to $ 3$\mu $ decays is performed using 97.7 fb$ ^{-1} $ of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment at the LHC in 2017 and 2018. Tau leptons produced in heavy-flavor hadron decays and produced in W boson decays are exploited in the analysis. The results of this search are combined with the previous result based on data collected in 2016, to obtain a total integrated luminosity of 131 fb$ ^{-1} $. The observed (expected) upper limit at 90% confidence level on the branching fraction $ \mathcal{B}(\tau\to3\mu) $ is 2.9 $ \times$ 10$^{-8} $ (2.4 $ \times $ 10$^{-8} $). The observed (expected) upper limit at 95% confidence level on the branching fraction $ \mathcal{B}(\tau\to3\mu) $ is 3.6 $ \times $ 10$^{-8} $ (3.0 $ \times $ 10$^{-8} $). | ||
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Links:
CDS record (PDF) ;
Physics Briefing ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, Accepted by PLB. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
The $ \mu\mu\pi $ invariant mass distribution with the fits to the $ \mathrm{D^+} $ and $ \mathrm{D}_{s}^{+} $ peaks and the background in 2017 data. |
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Figure 2:
Signal and background distributions for the four observables with the highest discrimination power used for the heavy-flavor analysis BDT training: $ \alpha_{3D} $ as defined in the text (top left), the normalized $ \chi^2 $ of the trimuon vertex fit (top right), the smallest distance of closest approach to the trimuon vertex of all the other tracks in the event with $ p_{\mathrm{T}} > $ 1 GeV (bottom left), and the muon reconstruction quality BDT score of the lowest $ p_{\mathrm{T}} $ muon of the triplet (bottom right). The background distributions are from 2018 data in the mass sidebands. |
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Figure 2-a:
Signal and background distributions for one of the variables used for the heavy-flavor analysis BDT training: $ \alpha_{3D} $ as defined in the text. The background distributions are from 2018 data in the mass sidebands. |
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Figure 2-b:
Signal and background distributions for one of the variables used for the heavy-flavor analysis BDT training: the normalized $ \chi^2 $ of the trimuon vertex fit. The background distributions are from 2018 data in the mass sidebands. |
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Figure 2-c:
Signal and background distributions for one of the variables used for the heavy-flavor analysis BDT training: the smallest distance of closest approach to the trimuon vertex of all the other tracks in the event with $ p_{\mathrm{T}} > $ 1 GeV. The background distributions are from 2018 data in the mass sidebands. |
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Figure 2-d:
Signal and background distributions for one of the variables used for the heavy-flavor analysis BDT training: the muon reconstruction quality BDT score of the lowest $ p_{\mathrm{T}} $ muon of the triplet. The background distributions are from 2018 data in the mass sidebands. |
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Figure 3:
Trimuon mass distributions in the highest BDT score subcategory of each of the three mass resolution categories of the heavy-flavor analysis: A1, B1, and C1, in the 2018 data events with three global muons. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) = $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 3-a:
Trimuon mass distribution in the A1 category of the heavy-flavor analysis, in the 2018 data events with three global muons. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) = $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 3-b:
Trimuon mass distribution in the B1 category of the heavy-flavor analysis, in the 2018 data events with three global muons. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) = $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 3-c:
Trimuon mass distribution in the C1 category of the heavy-flavor analysis, in the 2018 data events with three global muons. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) = $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 4:
Trimuon mass distributions in the highest BDT score subcategories of each of the three mass resolution categories of the heavy-flavor analysis: A1, B1, and C1, in the 2018 data events with two global muons and one tracker muon. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) < $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 4-a:
Trimuon mass distribution in the A1 category of the heavy-flavor analysis, in the 2018 data events with two global muons and one tracker muon. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) < $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 4-b:
Trimuon mass distribution in the B1 category of the heavy-flavor analysis, in the 2018 data events with two global muons and one tracker muon. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) < $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 4-c:
Trimuon mass distribution in the C1 category of the heavy-flavor analysis, in the 2018 data events with two global muons and one tracker muon. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) < $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 5:
Signal and background distributions for the four observables with the highest discrimination power used for the W analysis BDT training: cos($ \alpha_{2D} $) (top left), isolation observable of the $ \tau $ candidate (top right), trimuon vertex fit p-value (bottom left), $ \tau $ candidate $ p_{\mathrm{T}} $ (bottom right). |
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Figure 5-a:
Signal and background distributions for one of the observables used for the W analysis BDT training: cos($ \alpha_{2D} $). |
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Figure 5-b:
Signal and background distributions for one of the observables used for the W analysis BDT training: isolation observable of the $ \tau $ candidate. |
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Figure 5-c:
Signal and background distributions for one of the observables used for the W analysis BDT training: trimuon vertex fit p-value. |
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Figure 5-d:
Signal and background distributions for one of the observables used for the W analysis BDT training: $ \tau $ candidate $ p_{\mathrm{T}} $. |
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Figure 6:
Trimuon mass distributions of the 2018 data events in the three mass resolution categories of the W analysis. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) = $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 6-a:
Trimuon mass distribution of the 2018 data events in category A of the W analysis. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) = $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 6-b:
Trimuon mass distribution of the 2018 data events in category B of the W analysis. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) = $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 6-c:
Trimuon mass distribution of the 2018 data events in category C of the W analysis. Data are shown with black markers. The background-only fit and the expected signal for $ \mathcal{B}(\tau \!\to\! 3\mu) = $ 10$^{-7} $ are shown with blue and red lines, respectively. |
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Figure 7:
Observed and expected upper limits on $ \mathcal{B}(\tau \!\to\! 3\mu) $ at 90% CL, from the heavy-flavor analysis, the W analysis, the combination of the two analyses, as well as their combination with the previously published result using 2016 data. |
| Summary |
| A search for the lepton flavor violating decay $ \tau \!\to\! 3\mu $, using pp collisions with a center-of-mass energy of 13 TeV recorded by the CMS experiment at the CERN LHC in the years 2017--2018 has been presented. Tau leptons produced from heavy-flavor hadron decays and W boson decays are exploited in the analysis. The results are combined with a previous result using 2016 data, to obtain a total integrated luminosity of 131 fb$ ^{-1} $, yielding an observed upper limit on the branching fraction $ \mathcal{B}(\tau \!\to\! 3\mu) $ of 2.9 $ \times 10^{-8} $ at 90% confidence level, with an expected upper limit of 2.4 $ \times 10^{-8} $. |
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