Search for Lepton-Flavor Violation in Z-Boson Decays with tau Leptons with the ATLAS Detector

Search for Lepton-Flavor Violation in Z-Boson Decays with τLeptons with the ATLAS Detector G. Aad et al.* (ATLAS Collaboration) (Received 1 June 2021; accepted 5 October 2021; published 28 December 2021) A search for lepton-flavor-violating Z→eτand Z→μτ decays with pp collision data recorded by the ATLAS detector at the LHC is presented. This analysis uses 139 fb−1of Run 2 pp collisions at ffiffiffi s p¼ 13 TeV and is combined with the results of a similar ATLAS search in the final state in which the τlepton decays hadronically, using the same data set as well as Run 1 data. The addition of leptonically decaying τ leptons significantly improves the sensitivity reach for Z→lτdecays. The Z→lτbranching fractions are constrained in this analysis to BðZ→eτÞ<7.0×10−6and BðZ→μτÞ<7.2×10−6at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: BðZ→eτÞ<5.0×10−6and BðZ→μτÞ<6.5×10−6at 95% confidence level. DOI: 10.1103/PhysRevLett.127.271801 Three lepton families (flavors) exist in the standard model (SM) of particle physics [1–4], and the number of leptons of each family is conserved in their interactions. Nevertheless, this conservation is not postulated by any fundamental principle of the theory, and neutrino oscillations [5,6] indicate that processes violating this conservation do occur in nature. According to current knowledge, lepton-flavor-violating (LFV) processes in charged-lepton interactions can occur via neutrino mixing but are too rare to be detected by current experiments [7]. An observation of these would be an unambiguous sign of physics beyond the SM. LFV processes occur, for example, in models predicting the existence of heavy neutrinos [8], which may also explain the observed tiny masses and large mixing of the SM neutrinos. In such models, up to one in 105Z bosons would undergo an LFV decay involving τleptons. In an earlier analysis, the ATLAS experiment at the LHC set the strongest constraints on the branching fractions (B) of the LFV decays of the Zboson involving a τlepton by searching for such decays in which the τlepton decays hadronically [9]. This result was achieved by analyzing proton-proton (pp) collision data corresponding to an integrated luminosity of 139 fb−1at a center-of-mass energy ffiffiffi s p¼13 TeV and 20.3fb−1at ffiffiffi s p¼8TeV. In that search, ATLAS measured the branching fractions to be BðZ→eτÞ<8.1×10−6and BðZ→μτÞ<9.5×10−6at 95% confidence level (C.L.), superseding former limits set by the LEP experiments of BðZ→eτÞ<9.8×10−6[10] and BðZ→μτÞ<1.2×10−5[11] at 95% C.L. This Letter presents a complementary search for Z→lτ decays (l¼light charged lepton, i.e., eor μ) in which the τ leptons decay into electrons or muons (lτl0channel) using 139 fb−1of pp collision data at ffiffiffi s p¼13 TeV collected by the ATLAS experiment [12–14]. The search is performed here for the first time at the LHC and is combined with the similar ATLAS search using hadronic τ-lepton decays (lτhad channel) [9]. The two searches follow similar analysis strategies. Neural network classifiers are used for optimal discrimination of signal from backgrounds and their distributions are employed in a binned maximum-likelihood fit to achieve better sensitivity. ATLAS is a multipurpose particle detector with a forward-backward symmetric cylindrical geometry and a near 4πcoverage in solid angle [12,15,16]. It consists of an inner tracking detector surrounded by a superconducting solenoid, electromagnetic and hadronic calorimeters, and a muon spectrometer based on superconducting air-core toroidal magnets. This search analyzes pp collision events recorded by the ATLAS experiment using single-electron or single-muon triggers [17–19]. Prompt electrons and muons from the Z-boson decays and those from the τ-lepton decays are reconstructed and selected in the same way. Candidates for electrons [20], muons [21], jets [22–24], and visible decay products of hadronic τ-lepton decays (τhad-vis)[25,26] are reconstructed from energy deposits in the calorimeters and charged-particle tracks measured in the inner detector and the muon spectrometer. These candidates are selected with sets of requirements similar to those used in Ref. [9]. Electron candidates are required to pass the medium likelihood-based identification requirement [20] and have a transverse momentum *Full author list given at the end of the article. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3. PHYSICAL REVIEW LETTERS 127, 271801 (2021) 0031-9007=21=127(27)=271801(20) 271801-1 © 2021 CERN, for the ATLAS Collaboration pT>15 GeV and a pseudorapidity jηj<1.37 or 1.52< jηj<2.47. The latter selection vetoes electron candidates passing through the transition region between the barrel and end-cap electromagnetic calorimeters. Muon candidates are required to pass the medium identification requirement [27] and have a pT>10 GeV and jηj<2.5. Both the electron and muon candidates must satisfy the tight isolation requirement [20,27], which is intended to reject misidentified candidates produced from the hadronization of quarks or gluons based on tracks and clusters reconstructed collinear to the candidates. Events with exactly one electron and one muon candidate are selected with the requirement that the lepton with higher transverse momentum has a pT>27 GeV. This selection lies above the threshold for constant efficiency of both single-lepton trigger selections. Events with same-flavor lepton pairs are rejected, in order to reduce the background from Z→ll decays. Events with a leading-pTelectron are used in the search for Z→ eτdecays (eτμchannel), while those with a leading-pT muon are used in the search for Z→μτ decays (μτe channel), assuming the prompt lepton from the Z-boson decay is the leading one in pT. In the μτechannel, the ratio of the electron’spTreconstructed in the inner tracking detector to the transverse energy reconstructed in the electromagnetic calorimeter, ptrack TðeÞ=ETclusterðeÞ,is required to be smaller than 1.1 in order to reject Z→μμ events. Opposite-charge lepton-pair events are analyzed in the search for signal events, while events with same-charge lepton pairs are used for estimates of background processes. Quarkor gluon-initiated particle showers (jets) are reconstructed using the anti-ktalgorithm [22,23] with a radius parameter R¼0.4. Jets fulfilling pT>20 GeV and jηj<2.5are identified as containing bhadrons if tagged by a dedicated multivariate algorithm [28]. To ensure the samples of selected events do not overlap with those used in the lτhad channel, events with a τhad-vis candidate are vetoed. The τhad-vis candidates reconstructed from jets with apT>10 GeV and with one or three associated tracks are selected in jηj<1.37 or 1.52 <jηj<2.5.Theτhad-vis identification is performed by a recurrent neural network algorithm [25].Aτhad-vis candidate is required to have a pT>25 GeV and pass the tight identification selection. The missing transverse momentum (Emiss T) is calculated as the negative pTsum of all fully reconstructed and calibrated physics objects [29,30]. Additionally, the calculation includes inner detector tracks that originate from the vertex associated with the hard-scattering process but are not associated with any of the reconstructed objects. The Z→lτ→ll0þ2νsignal events are characterized by a final state which has two light charged leptons with different flavor and opposite electric charge, two neutrinos, and an invariant mass of all these particles compatible with the Z-boson mass. In most cases, these two leptons are emitted approximately back-to-back in the plane transverse to the proton beam direction. Since the τlepton is typically boosted due to the large difference between its mass and the mass of its parent Zboson, the two neutrinos from its decay are usually almost collinear with the charged lepton from the τ-lepton decay. The dominant background contribution is from the lepton-flavor-conserving Z→ττ →ll0þ4ν decays, where the two τleptons decay leptonically. Subleading background contributions from other SM processes with final states with two prompt leptons include the decays of a top-antitop-quark pair (t¯ t), two gauge bosons (diboson), or a Higgs boson. Finally, small background contributions come from Z→ll decays, where one of the light charged leptons is misidentified with the wrong flavor, and events with “fake leptons.”The latter type of background events includes mostly Wð→lνÞþjets events with leptons from heavy-flavor quark decays or with lightquark-initiated jets that are misidentified as electrons or muons. The signal and background events are separated by using a set of selection criteria that define a signalenhanced sample, referred to as the signal region (SR). The selection criteria are listed in Table I. Three neural network (NN) binary classifiers similar to those used in TABLE I. Selection criteria for events in the signal region. The invariant transverse mass of land Emiss Tis defined as mTðl;Emiss TÞ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pTðlÞEmiss T½1−cosðϕl−ϕEmiss TÞ q. Selection criterion Purpose Exactly two isolated light leptons ðl0;l1Þwith opposite electric charge and different flavor (eor μ); pTðl0Þ>p Tðl1Þ Select events consistent with signal decays. No τhad-vis candidate Complementarity to the lτhad channel. Transverse mass mTðl1;Emiss TÞ<35 GeV Reject top-quark and diboson events. jΔϕðl0;Emiss TÞj >1rad No b-tagged jets (using the 77% efficiency working point [28]) Invariant mass of the l0-l1pair mðl0;l1Þ>40 GeV Reject events incompatible with Z-boson decays. Neural network (optimized for signal vs Z→ττ) output >0.2Complementarity to the CRZττ region. In μτechannel: ptrack TðeÞ=Ecluster TðeÞ<1.1Reject Z→μμ events. PHYSICAL REVIEW LETTERS 127, 271801 (2021) 271801-2 Ref. [9] are trained on simulated events to distinguish signal events from Z→ττ, top-quark pair, and diboson background events individually. The input to these NNs is a mixture of lowand high-level kinematic variables, following the same strategy as in the lτhad channel [9]. The low-level variables are the momentum components of the reconstructed electron and muon candidates, and the Emiss T. The high-level variables are kinematic properties of the e-μ-Emiss Tsystem, such as the collinear mass mcollðe; μÞ, defined as the invariant mass of the e-μ-2νsystem, where the two neutrinos are assumed to have a vectorial momentum sum that is equal in pTand the azimuthal angle ϕ around the beam axis to the measured Emiss Tand equal in η to the subleading-pTlepton momentum. The outputs of the individual NNs (NNiwith values between zero and one) are combined into a final discriminant as shown in Eq. (1), hereafter referred to as the “combined NN output”: combined NN output ¼1− ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 3X 3 i¼1ð1−NNiÞ2 v u u t:ð1Þ Events classified by the NN trained for Z→ττ as backgroundlike are excluded from the SR and used in a control region to better determine the Z→ττ background in the maximum-likelihood fit (see Table I). The signal acceptance in the SR is 19.5% for the eτμchannel and 11.2% for the μτechannel, as determined from simulated signal samples. The lower acceptance in the μτechannel is due to the higher pTthreshold on the subleading-pTlepton and the additional selection on ptrack T=Ecluster T. Predictions for signal and background contributions are based partly on Monte Carlo (MC) simulations and partly on estimates from data. Signal and background processes were simulated as in Ref. [9]. The signal events were simulated using PYTHIA 8[32] with matrix elements calculated at leading order (LO) in the strong coupling constant. Nominal signal samples were generated with a parityconserving Zlτvertex and unpolarized τleptons. Scenarios where the decays are maximally parity violating were considered by reweighting the simulated events using TAUSPINNER [33], as discussed in Ref. [9]. The Z→ττ background events were simulated with the SHERPA 2.2.1 [34] generator using the NNPDF 3.0 NNLO PDF set [35] and next-to-leading-order (NLO) matrix elements for up to two partons, and LO matrix elements for up to four partons, calculated with the COMIX [36] and OPENLOOPS [37–39] libraries. Background Z→ll events were simulated using the POWHEG - BOX [40] generator with NLO matrix elements. All MC samples include a detailed simulation of the ATLAS detector with GEANT [41,42]. As in Ref. [9], the simulation of Z-boson production is improved through a correction derived from measurements in data. The simulated pTspectra of the Zboson are reweighted to match the unfolded distribution measured by ATLAS in Ref. [43]. The predicted overall yields of signal and Z→ττ events are determined by a binned maximum-likelihood fit to the combined data in the SR and in a control region enhanced in Z→ττ events (CRZττ). This eliminates the theoretical uncertainties in the total Z-boson production cross section (σZ), as well as the experimental uncertainties related to the acceptance of the common ll0final state. The selection criteria for events in the CRZττ are the same as those for events in the SR, except that events are required to be classified as Z→ττ-like, i.e., with an output smaller than 0.2 for the Z→ττ NN and greater than 0.2 for both the topquark and diboson NNs. In the μτechannel, a small contribution to the total background originates from Z→ μμ events in which one muon is misreconstructed as an electron. Such electron candidates may originate from muons that fail the muon selection requirements and whose tracks are associated with a calorimeter energy cluster and reconstructed as electrons. They may also originate from muons undergoing bremsstrahlung. Such events are modeled with simulation and their predicted yield is based on the measured σZ[44]. The modeling is validated in a dedicated region which has the same selection as the μτe SR except for the inverse selection on ptrack TðeÞ=Ecluster TðeÞ. Based on the observed level of agreement between data and simulation, a systematic uncertainty of 15% is assigned to the predicted yield of Z→μμ events in the SR, with no further correction. Events with fake leptons yield a small but still significant background contribution. In most cases, the fake lepton is the subleading one. These events are estimated from data using a “fake-factor method”similar to the one used in Ref. [9]. The fake factor is defined as the ratio Npass-iso fake =Nfail-iso fake , where “fake”indicates events with at least one fake lepton and “pass-iso”or “fail-iso”indicate whether the subleading lepton passes or fails the isolation requirement. The fake factor is measured in events with pairs of same-sign leptons (SS). These events are enhanced in Wð→lνÞþjets, which is the dominant source of events with fake leptons in the SR. Events in the SS region pass the same event selections as those in the SR except for a same-charge requirement. The fake factors are measured as functions of the transverse momentum and pseudorapidity of the leptons, separately for eτμand μτeevents. The kinematic properties of events with fake leptons in the SR or in the CRs are estimated by the distributions of events with the subleading lepton failing the isolation requirement, but otherwise satisfying all other selection criteria for that region, multiplied by the fake factor. The total predicted yields of the events with fake leptons in the SR and CRs are instead determined by a combined maximum-likelihood fit to data, separately for eτμand μτeevents. The remaining background processes are estimated using simulations. These backgrounds include events from the production and decay of top quarks [32,40], pairs of gauge bosons [34,35], and the Higgs boson [32,40]. The yield of the PHYSICAL REVIEW LETTERS 127, 271801 (2021) 271801-3 events with top quarks is determined in the maximumlikelihood fit to data via the inclusion of a top-quark control region (CRTop). The selection requirements for the CRTop are the same as for the SR except that at least one b-tagged jet is required. The expected event yields of the remaining processes are determined based on their production cross section, the integrated luminosity, and the simulated selection efficiency. A statistical analysis of the selected events is performed to assess the presence of signal events, following the same method used in Ref. [9]. A simultaneous binned maximumlikelihood fit to the combined NN output distribution in the SR, the mcollðe; μÞdistribution in the CRZττ, and the event yield in CRTop is used to constrain uncertainties in the predictions and extract evidence of a possible signal. The fit is performed independently for the eτand μτ channels. The fraction of Z→eτevents selected in the μτ channel (and vice versa) is negligible and is therefore neglected. In order to improve the discrimination between signal and the events with fake leptons, the events in the SR are further split into two regions based on the transverse momentum of the subleading-pTlepton l1. The low-pTSR contains events with a pTðl1Þ<20ð25ÞGeV in the eτμ(μτe) channel, while the high-pTSR contains the events above these thresholds. Both SRs in the eτμchannel have comparable sensitivity, while the low-pTSR in the μτe channel is more sensitive than the high-pTSR. Both SRs are fitted simultaneously. There are four unconstrained parameters in the fits: the parameter of interest determines the LFV branching fraction BðZ→lτÞby modifying an arbitrary prefit signal yield, μZdetermines σZtimes the overall acceptance and reconstruction efficiency of the ll0 final state in Z→ττ and signal events, μtop determines the yield of the top-quark events, and μfakes determines the yield of the events with fake leptons. Constrained parameters are also introduced to account for systematic uncertainties in the signal and background predictions, as in Ref. [9]. These include uncertainties in simulated events in the modeling of trigger, reconstruction, identification and isolation efficiencies, as well as energy calibrations and resolutions of reconstructed objects. No systematic uncertainties are assigned to the overall yields of events with Zboson decays, fake leptons, or top quarks as these yields are determined from data. Uncertainties related to events with fake leptons include statistical uncertainties due to the size of the data sample used to measure the fake factors as well as to model their distributions in the SRs and CRs. Systematic uncertainties assigned to events with fake leptons account for: shape differences in the modeling of the combined NN output in the SS events; differences in the composition of the events with fake leptons between SS events and the events in the SRs; and uncertainties affecting the number of events with prompt leptons failing the isolation requirements as estimated by simulation. The dominant uncertainties of the search are statistical in nature. Among the systematic uncertainties, the dominant ones are those in the jet calibration which enter through the calculation of the Emiss T[24]. A summary of the uncertainties and their impact on the LFV branching fraction is given in Table II. The observed and best-fit predicted distributions of the combined NN output in the SRs with the highest sensitivity as well as distributions of the collinear mass in the high-pT SRs are shown in Fig. 1. The best-fit yield of Z→lτsignal corresponds to the branching fractions BðZ→eτÞ¼ ½−2.63.5ðstatÞ2.7ðsystÞ×10−6and BðZ→μτÞ¼ ½−4.43.9ðstatÞ3.4ðsystÞ×10−6. The best-fit yields of Z→ττ, top quarks, and events with fake leptons are close to the prefit predicted values and are determined with a relative precision of 2%–4%, except the events with fake leptons in the μτechannel, which have an uncertainty of 30%. As no significant excess of data over the predicted background is observed, a combined fit of the lτl0and lτhad channels is used to set upper limits on BðZ→lτÞ. The analysis of the lτhad channel with Run 2 data [9] uses a similar scheme of regions and unconstrained parameters. In the statistical combination, the parameters of interest are correlated among the different SRs and CRs. The other unconstrained parameters are uncorrelated as these account either for backgrounds specific to each channel or for different acceptances of the lτl0or lτhad final states. Common systematic uncertainties are correlated, besides those related to the jet energy calibrations, which are uncorrelated. TABLE II. Summary of the contributions to the uncertainty in the measured BðZ→lτl0Þ. The uncertainties related to light charged leptons include those in the trigger, reconstruction, identification, and isolation efficiencies, as well as energy calibrations. The uncertainties related to jets and Emiss Tinclude those in the energy calibration and resolution. The uncertainty in the Z→μμ yield is only applicable in the μτ channel. The total systematic uncertainty can differ from the sum in quadrature of the different contributions due to correlations among uncertainties as a result of the likelihood fit to data. Uncertainty in BðZ→lτÞ[×10−6] Source of uncertainty eτμτ Statistical 3.53.9 Fake leptons (statistical) 0.10.1 Systematic 2.73.4 Light charged leptons 0.40.4 Emiss T0.40.8 Jets 1.92.2 Flavor tagging 0.50.9 Z-boson modeling <0.10.1 Z→μμ yield 0.8 Other backgrounds 0.10.6 Fake leptons (systematic) 0.40.9 Total 4.45.2 PHYSICAL REVIEW LETTERS 127, 271801 (2021) 271801-4 This conservative correlation scheme was chosen because of different best-fit values for the parameters associated with these uncertainties in the two channels. However, the fit with correlated jet energy calibration uncertainties yields compatible combined upper limits. The analysis of the lτhad channel with Run 1 data is combined using the same correlation scheme as in Ref. [9]. The combined best-fit amount of Z→lτsignal corresponds to the branching fractions BðZ→eτÞ¼½−1.42.5ðstatÞ 1.8ðsystÞ×10−6and BðZ→μτÞ¼½1.72.2ðstatÞ 1.6ðsystÞ×10−6. Since no significant deviation from the SM background hypothesisisobserved,exclusionlimitsare setusingtheCLS method [45]. The upper limits are shown in Table III for LFV 0 500 1000 1500 2000 2500 3000 3500 Events / 0.025 Data Fake lepton ττ→Z ll→Z Top quark Diboson Higgs ) -4 10×=3B ( τ e→Z Data Fake lepton ττ→Z ll→Z Top quark Diboson Higgs ) -4 10×=3B ( τ e→Z ATLAS -1 = 13 TeV, 139 fbs μ τ e SR, T pLow0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Combined NN output 0.75 0.875 1 1.125 1.25 Data / pred. (a) 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Events / 0.025 Data Fake lepton ττ→Z ll→Z Top quark Diboson Higgs ) -4 10×=3B ( τμ →Z Data Fake lepton ττ→Z ll→Z Top quark Diboson Higgs ) -4 10×=3B ( τμ →Z ATLAS -1 = 13 TeV, 139 fbs e τμ SR, T pHigh0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Combined NN output 0.75 0.875 1 1.125 1.25 Data / pred. (b) 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 Events / 10 GeV Data Fake lepton ττ→Z ll→Z Top quark Diboson Higgs ) -4 10×=3B ( τ e→Z Data Fake lepton ττ→Z ll→Z Top quark Diboson Higgs ) -4 10×=3B ( τ e→Z ATLAS -1 = 13 TeV, 139 fbs μ τ e SR, T pHigh60 70 80 90 100 110 120 130 140 150 160 ) [GeV]e, μ ( coll m 0.75 0.875 1 1.125 1.25 Data / pred. (c) 0 2000 4000 6000 8000 10000 Events / 10 GeV Data Fake lepton ττ→Z ll→Z Top quark Diboson Higgs ) -4 10×=3B ( τμ →Z Data Fake lepton ττ→Z ll→Z Top quark Diboson Higgs ) -4 10×=3B ( τμ →Z ATLAS -1 = 13 TeV, 139 fbs e τμ SR, T pHigh60 70 80 90 100 110 120 130 140 150 160 ) [GeV] μ ,e( coll m 0.75 0.875 1 1.125 1.25 Data / pred. (d) FIG. 1. Observed and best-fit predicted distributions in the SRs. Distributions of the combined NN output are shown in (a) for the lowpTSR of the eτμchannel, and in (b) for the high-pTSR of the μτechannel. Distributions of the collinear mass in the high-pTSR are shown in (c) and (d) for the eτμand μτechannels, respectively. The expected signal, normalized to an arbitrary BðZ→lτÞ¼3×10−4 for visualization purposes, is shown as a dashed histogram in each plot. In the panel below each plot, the ratios of the observed yield (dots) and the best-fit background-plus-signal yield (solid line) to the best-fit background yield are shown. The hatched uncertainty bands represent one standard deviation of the combined statistical and systematic uncertainties. The first and last bins in each plot include underflow and overflow events, respectively. PHYSICAL REVIEW LETTERS 127, 271801 (2021) 271801-5 decays with different assumptions about the τ-polarization state. The polarization of the τlepton affects the energy of its visible decay products and thus the acceptance for signal events. In the scenario where the τleptons are unpolarized, the observed upper limits at 95% C.L. on BðZ→eτÞand BðZ→μτÞare 5.0×10−6and 6.5×10−6, respectively. In conclusion, this Letter reports the first analysis of the lτl0channel in the search for Z→lτdecays at the LHC. This channel yields a sensitivity similar to the lτhad channel. With the combined results of the two channels, the ATLAS experiment sets the most stringent constraints on LFV Zboson decays involving τleptons to date. The precision of these results is mainly limited by statistical uncertainties. We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWFW and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; ANID, Chile; CAS, MOSTand NSFC, China; Minciencias, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF and Danish Natural Science Research Council, Denmark; IN2P3-CNRS and CEA-DRF/IRFU, France; Shota Rustaveli National Science Foundation of Georgia, Georgia; BMBF, HGF and MPG, Germany; General Secretariat for Research and Innovation, Greece; RGC and Hong Kong SAR, China; ISF and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; NWO, Netherlands; Research Council of Norway, Norway; MNiSW and NCN, Poland; FCT, Portugal; MNE/IFA, Romania; JINR; Ministry of Education and Science of the Russian Federation and NRC KI, Russian Federation; Ministry of Education, Science and Technological Development, Serbia; Ministry of Education, Science, Research and Sport, Slovakia; ARRS and Ministry of Education, Science and Sport, Slovenia; DSI/NRF, South Africa; MICINN, Spain; Swedish Research Council and Wallenberg Foundation, Sweden; Secretariat for Education and Research, Switzerland, SNSF and Cantons of Bern and Geneva, Switzerland; MOST, Taiwan; TAEK, Turkey; STFC, United Kingdom; DOE and NSF, USA In addition, individual groups and members have received support from BCKDF, CANARIE, Compute Canada and CRC, Canada; COST, ERC, ERDF, Horizon 2020 and Marie Skłodowska-Curie Actions, European Union; Investissements d’Avenir Labex, Investissements d’Avenir Idex and ANR, France; DFG and AvH Foundation, Germany; Herakleitos, Thales and Aristeia programmes co-financed by EU-ESF and the Herakleitos, Thales and Aristeia programmes cofinanced by EU-ESF and the Greek NSRF, Greece; BSF-NSF and GIF, Israel; Norwegian Financial Mechanism 2014-2021, Norway; La Caixa Banking Foundation, CERCA Programme Generalitat de Catalunya and PROMETEO and GenT Programmes Generalitat Valenciana, Spain; Göran Gustafssons Stiftelse, Sweden; The Royal Society and Leverhulme Trust, United Kingdom. The crucial computing support from all WLCG partners is acknowledged gratefully, in particular from CERN, the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Sweden), CC-IN2P3 (France), KIT/GridKA (Germany), INFN-CNAF (Italy), NL-T1 (Netherlands), PIC (Spain), ASGC (Taiwan), RAL (UK) and BNL (USA), the Tier-2 facilities worldwide and large non-WLCG resource providers. Major contributors of computing resources are listed in Ref. [46]. [1] S. Glashow, Partial-symmetries of weak interactions, Nucl. Phys. 22, 579 (1961). [2] S. Weinberg, A Model of Leptons, Phys. Rev. Lett. 19, 1264 (1967). TABLE III. Observed and expected (median) upper limits on the signal branching fraction at 95% C.L., in different τ-polarization scenarios. Observed (expected) upper limit on BðZ→lτÞ[×10−6] Final state, polarization assumption eτμτ lτhad Run 1þRun2, unpolarized τ[9] 8.1 (8.1) 9.5 (6.1) lτhad Run 2, left-handed τ[9] 8.2 (8.6) 9.5 (6.7) lτhad Run 2, right-handed τ[9] 7.8 (7.6) 10 (5.8) lτl0Run 2, unpolarized τ7.0 (8.9) 7.2 (10) lτl0Run 2, left-handed τ5.9 (7.5) 5.7 (8.5) lτl0Run 2, right-handed τ8.4 (11) 9.8 (13) Combined lτRun 1þRun 2, unpolarized τ5.0 (6.0) 6.5 (5.3) Combined lτRun 2, left-handed τ4.5 (5.7) 5.6 (5.3) Combined lτRun 2, right-handed τ5.4 (6.2) 7.7 (5.3) PHYSICAL REVIEW LETTERS 127, 271801 (2021) 271801-6 [3] A. Salam, Weak and electromagnetic interactions, in Selected Papers of Abdus Salam (World Scientific, Singapore, 1994), pp. 244–254, 10.1142/9789812795915_0034. [4] G. ’t Hooft and M. Veltman, Regularization and renormalization of gauge fields, Nucl. Phys. B44, 189 (1972). [5] Super-Kamiokande Collaboration, Evidence for Oscillation of Atmospheric Neutrinos, Phys. Rev. Lett. 81, 1562 (1998). 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Diehl,103 J. Dietrich,17 S. Díez Cornell,44 C. Diez Pardos,147 A. Dimitrievska,16 W. Ding,13b J. Dingfelder,22 I-M. Dinu,25b S. J. Dittmeier,59b F. Dittus,34 F. Djama,99 T. Djobava,155b J. I. Djuvsland,15 M. A. B. Do Vale,143a D. Dodsworth,24 C. Doglioni,94 J. Dolejsi,138 Z. Dolezal,138 M. Donadelli,78c B. Dong,58c J. Donini,36 PHYSICAL REVIEW LETTERS 127, 271801 (2021) 271801-9 C. Zhu,13a,13d H. L. Zhu,58a H. Zhu,13a J. Zhu,103 Y. Zhu,58a X. Zhuang,13a K. Zhukov,108 V. Zhulanov,118b,118a D. Zieminska,63 N. I. Zimine,77 S. Zimmermann,50,a M. Ziolkowski,147 L. Živković,14 A. Zoccoli,21b,21a K. Zoch,52 T. G. Zorbas,145 O. Zormpa,42 W. Zou,37 and L. Zwalinski34 (ATLAS Collaboration) 1Department of Physics, University of Adelaide, Adelaide, Australia 2Department of Physics, University of Alberta, Edmonton AB, Canada 3aDepartment of Physics, Ankara University, Ankara, Turkey 3bIstanbul Aydin University, Application and Research Center for Advanced Studies, Istanbul, Turkey 3cDivision of Physics, TOBB University of Economics and Technology, Ankara, Turkey 4LAPP, Univ. Savoie Mont Blanc, CNRS/IN2P3, Annecy, France 5High Energy Physics Division, Argonne National Laboratory, Argonne, Illinois, USA 6Department of Physics, University of Arizona, Tucson, Arizona, USA 7Department of Physics, University of Texas at Arlington, Arlington, Arlington, Texas, USA 8Physics Department, National and Kapodistrian University of Athens, Athens, Greece 9Physics Department, National Technical University of Athens, Zografou, Greece 10Department of Physics, University of Texas at Austin, Austin, Texas, USA 11aBahcesehir University, Faculty of Engineering and Natural Sciences, Istanbul, Turkey 11bIstanbul Bilgi University, Faculty of Engineering and Natural Sciences, Istanbul, Turkey 11cDepartment of Physics, Bogazici University, Istanbul, Turkey 11dDepartment of Physics Engineering, Gaziantep University, Gaziantep, Turkey 12Institut de Física d’Altes Energies (IFAE), Barcelona Institute of Science and Technology, Barcelona, Spain 13aInstitute of High Energy Physics, Chinese Academy of Sciences, Beijing, China 13bPhysics Department, Tsinghua University, Beijing, China 13cDepartment of Physics, Nanjing University, Nanjing, China 13dUniversity of Chinese Academy of Science (UCAS), Beijing, China 14Institute of Physics, University of Belgrade, Belgrade, Serbia 15Department for Physics and Technology, University of Bergen, Bergen, Norway 16Physics Division, Lawrence Berkeley National Laboratory and University of California, Berkeley, California, USA 17Institut für Physik, Humboldt Universität zu Berlin, Berlin, Germany 18Albert Einstein Center for Fundamental Physics and Laboratory for High Energy Physics, University of Bern, Bern, Switzerland 19School of Physics and Astronomy, University of Birmingham, Birmingham, United Kingdom 20aFacultad de Ciencias y Centro de Investigaciónes, Universidad Antonio Nariño, Bogotá, Colombia 20bDepartamento de Física, Universidad Nacional de Colombia, Bogotá, Colombia, Colombia 21aDipartimento di Fisica e Astronomia A. Righi, Universit`a di Bologna, Bologna, Italy 21bINFN Sezione di Bologna, Bologna, Italy 22Physikalisches Institut, Universität Bonn, Bonn, Germany 23Department of Physics, Boston University, Boston, Massachusetts, USA 24Department of Physics, Brandeis University, Waltham, Massachusetts, USA 25aTransilvania University of Brasov, Brasov, Romania 25bHoria Hulubei National Institute of Physics and Nuclear Engineering, Bucharest, Romania 25cDepartment of Physics, Alexandru Ioan Cuza University of Iasi, Iasi, Romania 25dNational Institute for Research and Development of Isotopic and Molecular Technologies, Physics Department, Cluj-Napoca, Romania 25eUniversity Politehnica Bucharest, Bucharest, Romania 25fWest University in Timisoara, Timisoara, Romania 26aFaculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovak Republic 26bDepartment of Subnuclear Physics, Institute of Experimental Physics of the Slovak Academy of Sciences, Kosice, Slovak Republic 27Physics Department, Brookhaven National Laboratory, Upton, New York, USA 28Departamento de Física (FCEN) and IFIBA, Universidad de Buenos Aires and CONICET, Buenos Aires, Argentina 29California State University, Fresno, California, USA 30Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 31aDepartment of Physics, University of Cape Town, Cape Town, South Africa 31biThemba Labs, Western Cape, South Africa 31cDepartment of Mechanical Engineering Science, University of Johannesburg, Johannesburg, South Africa 31dNational Institute of Physics, University of the Philippines Diliman, Philippines 31eUniversity of South Africa, Department of Physics, Pretoria, South Africa PHYSICAL REVIEW LETTERS 127, 271801 (2021) 271801-16 31fSchool of Physics, University of the Witwatersrand, Johannesburg, South Africa 32Department of Physics, Carleton University, Ottawa ON, Canada 33aFacult´e des Sciences Ain Chock, R´eseau Universitaire de Physique des Hautes Energies—Universit´e Hassan II, Casablanca, Morocco 33bFacult´e des Sciences, Universit´e Ibn-Tofail, K´enitra, Morocco 33cFacult´e des Sciences Semlalia, Universit´e Cadi Ayyad, LPHEA-Marrakech, Morocco 33dLPMR, Facult´e des Sciences, Universit´e Mohamed Premier, Oujda, Morocco 33eFacult´e des sciences, Universit´e Mohammed V, Rabat, Morocco 33fMohammed VI Polytechnic University, Ben Guerir, Morocco 34CERN, Geneva, Switzerland 35Enrico Fermi Institute, University of Chicago, Chicago, Illinois, USA 36LPC, Universit´e Clermont Auvergne, CNRS/IN2P3, Clermont-Ferrand, France 37Nevis Laboratory, Columbia University, Irvington, New York, USA 38Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark 39aDipartimento di Fisica, Universit`a della Calabria, Rende, Italy 39bINFN Gruppo Collegato di Cosenza, Laboratori Nazionali di Frascati, Italy 40Physics Department, Southern Methodist University, Dallas, Texas, USA 41Physics Department, University of Texas at Dallas, Richardson, Texas, USA 42National Centre for Scientific Research “Demokritos”, Agia Paraskevi, Greece 43aDepartment of Physics, Stockholm University, Sweden 43bOskar Klein Centre, Stockholm, Stockholm, Sweden 44Deutsches Elektronen-Synchrotron DESY, Hamburg and Zeuthen, Germany 45Lehrstuhl für Experimentelle Physik IV, Technische Universität Dortmund, Dortmund, Germany 46Institut für Kernund Teilchenphysik, Technische Universität Dresden, Dresden, Germany 47Department of Physics, Duke University, Durham, North Carolina, USA 48SUPA—School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 49INFN e Laboratori Nazionali di Frascati, Frascati, Italy 50Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany 51II. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany 52D´epartement de Physique Nucl´eaire et Corpusculaire, Universit´e de Gen`eve, Gen`eve, Switzerland 53aDipartimento di Fisica, Universit`a di Genova, Genova, Italy 53bINFN Sezione di Genova, Genova, Italy 54II. Physikalisches Institut, Justus-Liebig-Universität Giessen, Giessen, Germany 55SUPA—School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 56LPSC, Universit´e Grenoble Alpes, CNRS/IN2P3, Grenoble INP, Grenoble, France 57Laboratory for Particle Physics and Cosmology, Harvard University, Cambridge, Massachusetts, USA 58aDepartment of Modern Physics and State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China, Hefei, China 58bInstitute of Frontier and Interdisciplinary Science and Key Laboratory of Particle Physics and Particle Irradiation (MOE), Shandong University, Qingdao, China 58cSchool of Physics and Astronomy, Shanghai Jiao Tong University, Key Laboratory for Particle Astrophysics and Cosmology (MOE), SKLPPC, Shanghai, China 58dTsung-Dao Lee Institute, Shanghai, China 59aKirchhoff-Institut für Physik, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 59bPhysikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 60aDepartment of Physics, Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China 60bDepartment of Physics, University of Hong Kong, Hong Kong, China 60cDepartment of Physics and Institute for Advanced Study, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China 61Department of Physics, National Tsing Hua University, Hsinchu, Taiwan 62IJCLab, Universit´e Paris-Saclay, CNRS/IN2P3, 91405, Orsay, France 63Department of Physics, Indiana University, Bloomington, Indiana, USA 64aINFN Gruppo Collegato di Udine, Sezione di Trieste, Udine, Italy 64bICTP, Trieste, Italy 64cDipartimento Politecnico di Ingegneria e Architettura, Universit`a di Udine, Udine, Italy 65aINFN Sezione di Lecce, Lecce, Italy 65bDipartimento di Matematica e Fisica, Universit`a del Salento, Lecce, Italy 66aINFN Sezione di Milano, Milano, Italy 66bDipartimento di Fisica, Universit`a di Milano, Milano, Italy 67aINFN Sezione di Napoli,Napoli, Italy PHYSICAL REVIEW LETTERS 127, 271801 (2021) 271801-17 67bDipartimento di Fisica, Universit`a di Napoli, Napoli, Italy 68aINFN Sezione di Pavia, Pavia, Italy 68bDipartimento di Fisica, Universit`a di Pavia, Pavia, Italy 69aINFN Sezione di Pisa, Pisa, Italy 69bDipartimento di Fisica E. Fermi, Universit`a di Pisa, Pisa, Italy 70aINFN Sezione di Roma, Roma, Italy 70bDipartimento di Fisica, Sapienza Universit`a di Roma, Roma, Italy 71aINFN Sezione di Roma Tor Vergata, Roma, Italy 71bDipartimento di Fisica, Universit`a di Roma Tor Vergata, Roma, Italy 72aINFN Sezione di Roma Tre, Roma, Italy 72bDipartimento di Matematica e Fisica, Universit`a Roma Tre, Roma, Italy 73aINFN-TIFPA, Trento, Italy 73bUniversit`a degli Studi di Trento, Trento, Italy 74Institut für Astround Teilchenphysik, Leopold-Franzens-Universität, Innsbruck, Austria 75University of Iowa, Iowa City, Iowa, USA 76Department of Physics and Astronomy, Iowa State University, Ames, Iowa, USA 77Joint Institute for Nuclear Research, Dubna, Russia 78aDepartamento de Engenharia El´etrica, Universidade Federal de Juiz de Fora (UFJF), Juiz de Fora, Brazil 78bUniversidade Federal do Rio De Janeiro COPPE/EE/IF, Rio de Janeiro, Brazil 78cInstituto de Física, Universidade de São Paulo, São Paulo, Brazil 79KEK, High Energy Accelerator Research Organization, Tsukuba, Japan 80Graduate School of Science, Kobe University, Kobe, Japan 81aAGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Krakow, Poland 81bMarian Smoluchowski Institute of Physics, Jagiellonian University, Krakow, Poland 82Institute of Nuclear Physics Polish Academy of Sciences, Krakow, Poland 83Faculty of Science, Kyoto University, Kyoto, Japan 84Kyoto University of Education, Kyoto, Japan 85Research Center for Advanced Particle Physics and Department of Physics, Kyushu University, Fukuoka, Japan 86Instituto de Física La Plata, Universidad Nacional de La Plata and CONICET, La Plata, Argentina 87Physics Department, Lancaster University, Lancaster, United Kingdom 88Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 89Department of Experimental Particle Physics, Jožef Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana, Slovenia 90School of Physics and Astronomy, Queen Mary University of London, London, United Kingdom 91Department of Physics, Royal Holloway University of London, Egham, United Kingdom 92Department of Physics and Astronomy, University College London, London, United Kingdom 93Louisiana Tech University, Ruston, Louisiana, USA 94Fysiska institutionen, Lunds universitet, Lund, Sweden 95Centre de Calcul de l’Institut National de Physique Nucl´eaire et de Physique des Particules (IN2P3), Villeurbanne, France 96Departamento de Física Teorica C-15 and CIAFF, Universidad Autónoma de Madrid, Madrid, Spain 97Institut für Physik, Universität Mainz, Mainz, Germany 98School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom 99CPPM, Aix-Marseille Universit´e, CNRS/IN2P3, Marseille, France 100Department of Physics, University of Massachusetts, Amherst, Massachusetts, USA 101Department of Physics, McGill University, Montreal QC, Canada 102School of Physics, University of Melbourne, Victoria, Australia 103Department of Physics, University of Michigan, Ann Arbor, Michigan, USA 104Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan, USA 105B.I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, Minsk, Belarus 106Research Institute for Nuclear Problems of Byelorussian State University, Minsk, Belarus 107Group of Particle Physics, University of Montreal, Montreal QC, Canada 108P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, Russia 109National Research Nuclear University MEPhI, Moscow, Russia 110D.V. Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, Moscow, Russia 111Fakultät für Physik, Ludwig-Maximilians-Universität München, München, Germany 112Max-Planck-Institut für Physik (Werner-Heisenberg-Institut), München, Germany 113Graduate School of Science and Kobayashi-Maskawa Institute, Nagoya University, Nagoya, Japan 114Department of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico, USA 115Institute for Mathematics, Astrophysics and Particle Physics, Radboud University/Nikhef, Nijmegen, Netherlands 116Nikhef National Institute for Subatomic Physics and University of Amsterdam, Amsterdam, Netherlands PHYSICAL REVIEW LETTERS 127, 271801 (2021) 271801-18 117Department of Physics, Northern Illinois University, DeKalb, Illinois, USA 118aBudker Institute of Nuclear Physics and NSU, SB RAS, Novosibirsk, Russia 118bNovosibirsk State University Novosibirsk, Novosibirsk, Russia 119Institute for High Energy Physics of the National Research Centre Kurchatov Institute, Protvino, Russia 120Institute for Theoretical and Experimental Physics named by A.I. Alikhanov of National Research Centre “Kurchatov Institute”, Moscow, Russia 121Department of Physics, New York University, New York, New York, USA 122Ochanomizu University, Otsuka, Bunkyo-ku, Tokyo, Japan 123Ohio State University, Columbus, Ohio, USA 124Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, Oklahoma, USA 125Department of Physics, Oklahoma State University, Stillwater, Oklahoma, USA 126Palacký University, Joint Laboratory of Optics, Olomouc, Czech Republic 127Institute for Fundamental Science, University of Oregon, Eugene, Oregon, USA 128Graduate School of Science, Osaka University, Osaka, Japan 129Department of Physics, University of Oslo, Oslo, Norway 130Department of Physics, Oxford University, Oxford, United Kingdom 131LPNHE, Sorbonne Universit´e, Universit´e de Paris, CNRS/IN2P3, Paris, France 132Department of Physics, University of Pennsylvania, Philadelphia, Pennsylvania, USA 133Konstantinov Nuclear Physics Institute of National Research Centre “Kurchatov Institute”, PNPI, St. Petersburg, Russia 134Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania, USA 135aLaboratório de Instrumentação e Física Experimental de Partículas—LIP, Lisboa, Portugal 135bDepartamento de Física, Faculdade de Ciências, Universidade de Lisboa, Lisboa, Portugal 135cDepartamento de Física, Universidade de Coimbra, Coimbra, Portugal 135dCentro de Física Nuclear da Universidade de Lisboa, Lisboa, Portugal 135eDepartamento de Física, Universidade do Minho, Braga, Portugal 135fDepartamento de Física Teórica y del Cosmos, Universidad de Granada, Granada (Spain), Spain 135gDep Física and CEFITEC of Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal 135hInstituto Superior T´ecnico, Universidade de Lisboa, Lisboa, Portugal 136Institute of Physics of the Czech Academy of Sciences, Prague, Czech Republic 137Czech Technical University in Prague, Prague, Czech Republic 138Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic 139Particle Physics Department, Rutherford Appleton Laboratory, Didcot, United Kingdom 140IRFU, CEA, Universit´e Paris-Saclay, Gif-sur-Yvette, France 141Santa Cruz Institute for Particle Physics, University of California Santa Cruz, Santa Cruz, California, USA 142aDepartamento de Física, Pontificia Universidad Católica de Chile, Santiago, Chile 142bUniversidad Andres Bello, Department of Physics, Santiago, Chile 142cInstituto de Alta Investigación, Universidad de Tarapacá, Arica, Chile 142dDepartamento de Física, Universidad T´ecnica Federico Santa María, Valparaíso, Chile 143aUniversidade Federal de São João del Rei (UFSJ), São João del Rei, Brazil 143bUniversidad de la Serena, La Serena, Chile 144Department of Physics, University of Washington, Seattle, Washington, USA 145Department of Physics and Astronomy, University of Sheffield, Sheffield, United Kingdom 146Department of Physics, Shinshu University, Nagano, Japan 147Department Physik, Universität Siegen, Siegen, Germany 148Department of Physics, Simon Fraser University, Burnaby BC, Canada 149SLAC National Accelerator Laboratory, Stanford, California, USA 150Department of Physics, Royal Institute of Technology, Stockholm, Sweden 151Departments of Physics and Astronomy, Stony Brook University, Stony Brook, New York, USA 152Department of Physics and Astronomy, University of Sussex, Brighton, United Kingdom 153School of Physics, University of Sydney, Sydney, Australia 154Institute of Physics, Academia Sinica, Taipei, Taiwan 155aE. Andronikashvili Institute of Physics, Iv. Javakhishvili Tbilisi State University, Tbilisi, Georgia 155bHigh Energy Physics Institute, Tbilisi State University, Tbilisi, Georgia 156Department of Physics, Technion, Israel Institute of Technology, Haifa, Israel 157Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel 158Department of Physics, Aristotle University of Thessaloniki, Thessaloniki, Greece 159International Center for Elementary Particle Physics and Department of Physics, University of Tokyo, Tokyo, Japan 160Department of Physics, Tokyo Institute of Technology, Tokyo, Japan 161Tomsk State University, Tomsk, Russia 162Department of Physics, University of Toronto, Toronto ON, Canada PHYSICAL REVIEW LETTERS 127, 271801 (2021) 271801-19 163aTRIUMF, Vancouver BC, Canada 163bDepartment of Physics and Astronomy, York University, Toronto ON, Canada 164Division of Physics and Tomonaga Center for the History of the Universe, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Japan 165Department of Physics and Astronomy, Tufts University, Medford, Massachusetts, USA 166Department of Physics and Astronomy, University of California Irvine, Irvine, California, USA 167Department of Physics and Astronomy, University of Uppsala, Uppsala, Sweden 168Department of Physics, University of Illinois, Urbana, Illinois, USA 169Instituto de Física Corpuscular (IFIC), Centro Mixto Universidad de Valencia—CSIC, Valencia, Spain 170Department of Physics, University of British Columbia, Vancouver BC, Canada 171Department of Physics and Astronomy, University of Victoria, Victoria BC, Canada 172Fakultät für Physik und Astronomie, Julius-Maximilians-Universität Würzburg, Würzburg, Germany 173Department of Physics, University of Warwick, Coventry, United Kingdom 174Waseda University, Tokyo, Japan 175Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot, Israel 176Department of Physics, University of Wisconsin, Madison, Wisconsin, USA 177Fakultät für Mathematik und Naturwissenschaften, Fachgruppe Physik, Bergische Universität Wuppertal, Wuppertal, Germany 178Department of Physics, Yale University, New Haven, Connecticut, USA aDeceased. bAlso at Department of Physics, King’s College London, London, United Kingdom. cAlso at Istanbul University, Dept. of Physics, Istanbul, Turkey. dAlso at Instituto de Fisica Teorica, IFT-UAM/CSIC, Madrid, Spain. eAlso at TRIUMF, Vancouver BC, Canada. fAlso at Department of Physics, University of Fribourg, Fribourg, Switzerland. gAlso at Department of Physics and Astronomy, University of Louisville, Louisville, Kentucky, USA. hAlso at Departament de Fisica de la Universitat Autonoma de Barcelona, Barcelona, Spain. iAlso at Moscow Institute of Physics and Technology State University, Dolgoprudny, Russia. jAlso at Faculty of Physics, Sofia University, ’St. Kliment Ohridski’, Sofia, Bulgaria. kAlso at Department of Physics, Ben Gurion University of the Negev, Beer Sheva, Israel. lAlso at Universita di Napoli Parthenope, Napoli, Italy. mAlso at Institute of Particle Physics (IPP), Victoria, Canada. nAlso at Bruno Kessler Foundation, Trento, Italy. oAlso at Department of Physics, St. Petersburg State Polytechnical University, St. Petersburg, Russia. pAlso at Borough of Manhattan Community College, City University of New York, New York, New York, USA. qAlso at Department of Physics, California State University, Fresno, USA. rAlso at Department of Financial and Management Engineering, University of the Aegean, Chios, Greece. sAlso at Centro Studi e Ricerche Enrico Fermi, Rome, Italy. tAlso at Department of Physics, California State University, East Bay, USA. uAlso at Institucio Catalana de Recerca i Estudis Avancats, ICREA, Barcelona, Spain. vAlso at Graduate School of Science, Osaka University, Osaka, Japan. wAlso at Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany. xAlso at University of Chinese Academy of Sciences (UCAS), Beijing, China. yAlso at Institute of Physics, Azerbaijan Academy of Sciences, Baku, Azerbaijan. zAlso at Yeditepe University, Physics Department, Istanbul, Turkey. aaAlso at CERN, Geneva, Switzerland. bbAlso at Joint Institute for Nuclear Research, Dubna, Russia. ccAlso at Hellenic Open University, Patras, Greece. ddAlso at Center for High Energy Physics, Peking University, China. eeAlso at The City College of New York, New York, New York, USA. ffAlso at Department of Physics, California State University, Sacramento, USA. ggAlso at D´epartement de Physique Nucl´eaire et Corpusculaire, Universit´e de Gen`eve, Gen`eve, Switzerland. hhAlso at Faculty of Physics, M.V. Lomonosov Moscow State University, Moscow, Russia. iiAlso at Institut für Experimentalphysik, Universität Hamburg, Hamburg, Germany. jjAlso at CPPM, Aix-Marseille Universit´e, CNRS/IN2P3, Marseille, France. kkAlso at Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, Budapest, Hungary. llAlso at Giresun University, Faculty of Engineering, Giresun, Turkey. mmAlso at Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan, USA. nnAlso at Institute of Theoretical Physics, Ilia State University, Tbilisi, Georgia. PHYSICAL REVIEW LETTERS 127, 271801 (2021) 271801-20