CMS-PAS-BPH-21-005

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 $^{-8}$ ).
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, Accepted by PLB. The superseded preliminary plots can be found here.

Figures	Summary	References	CMS Publications

Figures
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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).
png pdf	Figure 5-a: Signal and background distributions for one of the observables used for the W analysis BDT training: cos( $\alpha_{2D}$ ).
png pdf	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.
png pdf	Figure 5-c: Signal and background distributions for one of the observables used for the W analysis BDT training: trimuon vertex fit p-value.
png pdf	Figure 5-d: Signal and background distributions for one of the observables used for the W analysis BDT training: $\tau$ candidate $p_{\mathrm{T}}$ .
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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.
png pdf	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

$\times 10^{-8}$ .

References
1	X.-Y. Pham	Lepton flavor changing in neutrinoless $\tau$ decays	EPJC 8 (1999) 513	hep-ph/9810484
2	G. Hernàndez-Tom é , G. López Castro, and P. Roig	Flavor violating leptonic decays of $\tau$ and $\mu$ leptons in the Standard Model with massive neutrinos	EPJC 79 (2019) 84	1807.06050
3	P. Blackstone, M. Fael, and E. Passemar	$\tau \to \mu\mu\mu$ at a rate of one out of 10 $^{14}$ tau decays?	EPJC 80 (2020) 506	1912.09862
4	W. J. Marciano, T. Mori, and J. M. Roney	Charged lepton flavor violation experiments	Ann. Rev. Nucl. Part. Sci. 58 (2008) 315
5	M. Raidal et al.	Flavour physics of leptons and dipole moments	EPJC 57 (2008) 13	0801.1826
6	E. Arganda and M. J. Herrero	Testing supersymmetry with lepton flavor violating $\tau$ and $\mu$ decays	PRD 73 (2006) 055003	hep-ph/0510405
7	Belle Collaboration	Search for lepton flavor violating $\tau$ decays into three leptons with 719 million produced $\tau^+\tau^-$ pairs	PLB 687 (2010) 139	1001.3221
8	BaBar Collaboration	Limits on $\tau$ lepton-flavor violating decays in three charged leptons	PRD 81 (2010) 111101	1002.4550
9	LHCb Collaboration	Search for the lepton flavour violating decay $\tau^- \to \mu^-\mu^+\mu^-$	JHEP 02 (2015) 121	1409.8548
10	ATLAS Collaboration	Probing lepton flavour violation via neutrinoless $\tau \to$ 3 $\mu$ decays with the ATLAS detector	EPJC 76 (2016) 232	1601.03567
11	CMS Collaboration	Search for the lepton flavor violating decay $\tau \to$ 3 $\mu$ in proton-proton collisions at $\sqrt{s} =$ 13 TeV	JHEP 01 (2021) 163	CMS-BPH-17-004 2007.05658
12	CMS Collaboration	The CMS experiment at the CERN LHC	JINST 3 (2008) S08004
13	CMS Collaboration	Performance of the CMS Level-1 trigger in proton-proton collisions at $\sqrt{s} =$ 13 TeV	JINST 15 (2020) P10017	CMS-TRG-17-001 2006.10165
14	CMS Collaboration	The CMS trigger system	JINST 12 (2017) P01020	CMS-TRG-12-001 1609.02366
15	CMS Collaboration	Electron and photon reconstruction and identification with the CMS experiment at the CERN LHC	JINST 16 (2021) P05014	CMS-EGM-17-001 2012.06888
16	CMS Collaboration	Performance of the CMS muon detector and muon reconstruction with proton-proton collisions at $\sqrt{s}=$ 13 TeV	JINST 13 (2018) P06015	CMS-MUO-16-001 1804.04528
17	CMS Collaboration	Description and performance of track and primary-vertex reconstruction with the CMS tracker	JINST 9 (2014) P10009	CMS-TRK-11-001 1405.6569
18	CMS Collaboration	Particle-flow reconstruction and global event description with the CMS detector	JINST 12 (2017) P10003	CMS-PRF-14-001 1706.04965
19	CMS Collaboration	Performance of reconstruction and identification of $\tau$ leptons decaying to hadrons and $\nu_\tau$ in pp collisions at $\sqrt{s}=$ 13 TeV	JINST 13 (2018) P10005	CMS-TAU-16-003 1809.02816
20	CMS Collaboration	Jet energy scale and resolution in the CMS experiment in pp collisions at 8 TeV	JINST 12 (2017) P02014	CMS-JME-13-004 1607.03663
21	CMS Collaboration	Performance of missing transverse momentum reconstruction in proton-proton collisions at $\sqrt{s} =$ 13 TeV using the CMS detector	JINST 14 (2019) P07004	CMS-JME-17-001 1903.06078
22	CMS Collaboration	The CMS trigger system	JINST 12 (2017) P01020	CMS-TRG-12-001 1609.02366
23	T. Sjöstrand et al.	An introduction to PYTHIA 8.2	Comput. Phys. Commun. 191 (2015) 159	1410.3012
24	CMS Collaboration	Event generator tunes obtained from underlying event and multiparton scattering measurements	EPJC 76 (2016) 155	CMS-GEN-14-001 1512.00815
25	D. J. Lange	The EvtGen particle decay simulation package	NIM A 462 (2001) 152
26	GEANT4 Collaboration	GEANT 4---a simulation toolkit	NIM A 506 (2003) 250
27	M. J. Oreglia	A study of the reactions $\psi^\prime \to \gamma \gamma \psi$	PhD thesis, Stanford University, SLAC Report SLAC-R-236, 1980 link
28	T. Chen and C. Guestrin	XGBoost: A scalable tree boosting system	in 22nd ACM SIGKDD International Conference on Knowledge Discovery and Data Mining, KDD '16, New York, 2016 link
29	Particle Data Group, M. Tanabashi et al.	Review of particle physics	PRD 98 (2018) 030001
30	CMS Collaboration	Measurement of the inclusive W and Z production cross sections in pp collisions at $\sqrt{s}=$ 7 TeV	JHEP 10 (2011) 132	CMS-EWK-10-005 1107.4789
31	ATLAS and CMS Collaborations, and LHC Higgs Combination Group	Procedure for the LHC Higgs boson search combination in Summer 2011	Technical Report CMS-NOTE-2011-005, ATL-PHYS-PUB-2011-11, 2011
32	A. L. Read	Presentation of search results: The CL $_{\text{s}}$ technique	JPG 28 (2002) 2693
33	T. Junk	Confidence level computation for combining searches with small statistics	NIM A 434 (1999) 435	hep-ex/9902006
34	R. D. Cousins and V. L. Highland	Incorporating systematic uncertainties into an upper limit	NIMA 320 (1992) 331