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| CMS-PAS-BPH-17-004 | ||
| Search for $\tau \to 3\mu$ decays using $\tau$ leptons produced in D and B meson decays | ||
| CMS Collaboration | ||
| March 2019 | ||
| Abstract: A search for charged lepton flavor violating decays $\tau\to3\mu$ has been performed using proton-proton collisions with a center-of-mass energy of 13 TeV. The analysis uses the data set collected by the CMS detector in 2016, corresponding to an integrated luminosity of 33 fb$^{-1}$, and exploits $\tau$ leptons produced in D and B meson decays. No signal is observed, and an upper limit of $ 8.8 \times 10^{-8}$ is set at the 90% confidence level on the branching fraction of the $\tau$ lepton to three muons. | ||
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Links:
CDS record (PDF) ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, JHEP 01 (2021) 163. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Distributions of trimuon mass resolutions for simulated $ {\tau}\to 3 {{\mu}}$ events (solid line) and $3 {{\mu}}$ sideband data events (dots), passing all selection criteria. The vertical lines at $\delta m/m=$ 0.007 and 0.01 separate three event categories (A, B, and C) used in the analysis. |
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Figure 2:
Trimuon mass distributions for simulated $ {\tau}\to 3 {{\mu}}$ events passing all selection criteria and falling into category A (left), category B (center) or category C (right). The solid line is the result of the fit with Crystal-Ball plus Gaussian function. |
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Figure 2-a:
Trimuon mass distributions for simulated $ {\tau}\to 3 {{\mu}}$ events passing all selection criteria and falling into category A (left), category B (center) or category C (right). The solid line is the result of the fit with Crystal-Ball plus Gaussian function. |
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Figure 2-b:
Trimuon mass distributions for simulated $ {\tau}\to 3 {{\mu}}$ events passing all selection criteria and falling into category A (left), category B (center) or category C (right). The solid line is the result of the fit with Crystal-Ball plus Gaussian function. |
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Figure 2-c:
Trimuon mass distributions for simulated $ {\tau}\to 3 {{\mu}}$ events passing all selection criteria and falling into category A (left), category B (center) or category C (right). The solid line is the result of the fit with Crystal-Ball plus Gaussian function. |
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Figure 3:
Signal and background distributions for the four observables with the best discriminating power used for the BDT training: (top-left) normalized $\chi ^2$ of the trimuon vertex fit; (top-right) pointing angle $\alpha $; (bottom-left) significance of the trimuon vertex 3D displacement; (bottom-right) track kink parameter. All distributions are normalized to unity. |
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Figure 3-a:
Signal and background distributions for the four observables with the best discriminating power used for the BDT training: (top-left) normalized $\chi ^2$ of the trimuon vertex fit; (top-right) pointing angle $\alpha $; (bottom-left) significance of the trimuon vertex 3D displacement; (bottom-right) track kink parameter. All distributions are normalized to unity. |
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Figure 3-b:
Signal and background distributions for the four observables with the best discriminating power used for the BDT training: (top-left) normalized $\chi ^2$ of the trimuon vertex fit; (top-right) pointing angle $\alpha $; (bottom-left) significance of the trimuon vertex 3D displacement; (bottom-right) track kink parameter. All distributions are normalized to unity. |
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Figure 3-c:
Signal and background distributions for the four observables with the best discriminating power used for the BDT training: (top-left) normalized $\chi ^2$ of the trimuon vertex fit; (top-right) pointing angle $\alpha $; (bottom-left) significance of the trimuon vertex 3D displacement; (bottom-right) track kink parameter. All distributions are normalized to unity. |
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Figure 3-d:
Signal and background distributions for the four observables with the best discriminating power used for the BDT training: (top-left) normalized $\chi ^2$ of the trimuon vertex fit; (top-right) pointing angle $\alpha $; (bottom-left) significance of the trimuon vertex 3D displacement; (bottom-right) track kink parameter. All distributions are normalized to unity. |
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Figure 4:
BDT score distributions for signal and background events passing all selection criteria and falling into category A (left) or category C (right). All distributions are normalized to unity. The vertical lines separate subcategories with different signal-to-background ratios. The two subcategories on the right are used in the signal search. |
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Figure 4-a:
BDT score distributions for signal and background events passing all selection criteria and falling into category A (left) or category C (right). All distributions are normalized to unity. The vertical lines separate subcategories with different signal-to-background ratios. The two subcategories on the right are used in the signal search. |
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Figure 4-b:
BDT score distributions for signal and background events passing all selection criteria and falling into category A (left) or category C (right). All distributions are normalized to unity. The vertical lines separate subcategories with different signal-to-background ratios. The two subcategories on the right are used in the signal search. |
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Figure 5:
Trimuon mass distributions in the six independent event categories used in the analysis: A1, A2; B1, B2; C1, C2. The six event categories are defined in the text. Data are shown with points. The background-only fit and the expected signal for $\mathcal {B}({\tau}\to 3 {{\mu}})=10^{-7}$ are shown with lines. |
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Figure 5-a:
Trimuon mass distributions in the six independent event categories used in the analysis: A1, A2; B1, B2; C1, C2. The six event categories are defined in the text. Data are shown with points. The background-only fit and the expected signal for $\mathcal {B}({\tau}\to 3 {{\mu}})=10^{-7}$ are shown with lines. |
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Figure 5-b:
Trimuon mass distributions in the six independent event categories used in the analysis: A1, A2; B1, B2; C1, C2. The six event categories are defined in the text. Data are shown with points. The background-only fit and the expected signal for $\mathcal {B}({\tau}\to 3 {{\mu}})=10^{-7}$ are shown with lines. |
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Figure 5-c:
Trimuon mass distributions in the six independent event categories used in the analysis: A1, A2; B1, B2; C1, C2. The six event categories are defined in the text. Data are shown with points. The background-only fit and the expected signal for $\mathcal {B}({\tau}\to 3 {{\mu}})=10^{-7}$ are shown with lines. |
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Figure 5-d:
Trimuon mass distributions in the six independent event categories used in the analysis: A1, A2; B1, B2; C1, C2. The six event categories are defined in the text. Data are shown with points. The background-only fit and the expected signal for $\mathcal {B}({\tau}\to 3 {{\mu}})=10^{-7}$ are shown with lines. |
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Figure 5-e:
Trimuon mass distributions in the six independent event categories used in the analysis: A1, A2; B1, B2; C1, C2. The six event categories are defined in the text. Data are shown with points. The background-only fit and the expected signal for $\mathcal {B}({\tau}\to 3 {{\mu}})=10^{-7}$ are shown with lines. |
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Figure 5-f:
Trimuon mass distributions in the six independent event categories used in the analysis: A1, A2; B1, B2; C1, C2. The six event categories are defined in the text. Data are shown with points. The background-only fit and the expected signal for $\mathcal {B}({\tau}\to 3 {{\mu}})=10^{-7}$ are shown with lines. |
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Figure 6:
The invariant mass distribution for two muons and a pion after applying signal-like kinematic cuts on two muons and a pion, and after requiring that the two muons have opposite signs and their invariant mass is consistent the $\phi $ meson mass. The two peaks are associated with $ {\mathrm {D}}_\text {s}$ (1.97 GeV) and $ {\mathrm {D^+}}$ (1.87 GeV) decays, and modelled with Crystal-Ball functions, while the background is fitted with an exponential function. |
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Figure 7:
Fit prompt and non-prompt $ {\mathrm {D}}_\text {s}$ contributions to data. The histograms are from $ {\mathrm {D}}_\text {s} \to \phi ({{\mu}} {{\mu}}) {\pi}$ MC. The filled histogram is the $ {{\mathrm {B}}}\to {\mathrm {D}}_\text {s}$ component, while the opened histogram stacked on the other is the prompt $ {\mathrm {D}}_\text {s}$ component. They fit to data (sideband subtracted) using the proper decay length ($L M/p$). |
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Figure 8:
S/(S+B)-weighted trimuon mass distribution including events from all the categories used in the analysis. Data are shown with points. The background-only fit and the expected signal for $\mathcal {B}({\tau}\to 3 {{\mu}})=10^{-7}$ are shown with lines. |
| Tables | |
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Table 1:
The expected inclusive number of $ {\tau}$ leptons produced in D and B meson decays at LHC (13 TeV) for an integrated luminosity of 33 fb$^{-1}$. Numbers are from PYTHIA (without EVTGEN). Charge conjugated states are implied. For comparison, the number of $ {\tau}$ leptons produced in W and Z boson decays is $8 \times 10^{8}$. |
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Table 2:
D and B meson decay branching fractions (and their uncertainties) used in this analysis. |
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Table 3:
Number of expected signal events, assuming $\mathcal {B}({\tau}\to 3 {{\mu}})=10^{-7}$, and the number of observed events in data at each step of the event selection. Events are counted in the trimuon mass range 1.62-2.00 GeV. |
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Table 4:
Sources of systematic uncertainties affecting the signal modeling, and their impacts on the expected signal event yield and trimuon mass distribution shape. |
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Table 5:
Signal and data yields for the six event categories in the mass range 1.62-2.00 GeV. The signal yields are shown for $\mathcal {B}({\tau}\to 3 {{\mu}})=10^{-7}$. The data yields inside parentheses are in the mass ranges of 1.78 GeV $\pm$ 2$ \sigma $, where $\sigma $ is the mass resolution (12 MeV, 19 MeV, and 25 MeV for the category A, B, and C respectively). |
| Summary |
| The first search for the charged lepton flavor violating decay $\tau\to 3\mu$ using the CMS detector has been presented. The analysis uses 33 fb$^{-1}$ of proton-proton collisions collected at a center-of-mass energy of 13 TeV by the CMS detector in 2016. It exploits $\tau$ leptons produced in D and B meson decays, which is the main source of $\tau$ leptons at the LHC. No excess above the expected background is observed. An upper limit of $8.8\times 10^{-8}$ is set on the branching fraction $\mathcal{B}(\tau \rightarrow 3\mu)$ at 90% confidence level. The corresponding upper limit at 95% confidence level is $1.1 \times 10^{-7}$. |
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