We present a search for high-mass resonances decaying into two leptons of different flavor: $eμ$, $eτ$ and $μτ$. These resonances are predicted by several models for Physics Beyond the Standard Model, such as R-parity-violating MSSM. The search is based on 1 ${\rm fb^{-1}}$ of Tevatron Run II data collected with the CDF detector at $\sqrt{s}$ = 1.96 TeV in proton anti-proton collisions.
A search is performed for heavy resonances decaying to two long-lived massive neutral particles, each decaying to leptons. The experimental signature is a distinctive topology consisting of a pair of oppositely charged leptons originating at a separated secondary vertex. Events were collected by the CMS detector at the LHC during pp collisions at sqrt(s) = 7 TeV, and selected from data samples corresponding to 4.1 (5.1) inverse femtobarns of integrated luminosity in the electron (muon) channel. No significant excess is observed above standard model expectations, and an upper limit is set with …
I present here the most recent experimental results in the search for Lepton Flavor and Lepton Number Violating decays of charged leptons, and the perspectives in this field for the next future.
The high-mass spectrum of lepton and photon pairs is sensitive to a broad range of new physics. Examples are extra dimensions and new gauge bosons such as the W' and Z'. Additionally, electron compositeness would result in excited electrons that decay into an electron and a photon. We report the latest results of searches for high-mass dilepton, diphoton, and electron-photon resonances by the CDF and D0 experiments at the Tevatron.
A search for a standard model Higgs boson decaying into a pair of tau leptons is performed using events recorded by the CMS experiment at the LHC in 2011 and 2012. The dataset corresponds to an integrated luminosity of 4.9 inverse femtobarns at a centre-of-mass energy of 7 TeV and 19.7 inverse femtobarns at 8 TeV. Each tau lepton decays hadronically or leptonically to an electron or a muon, leading to six different final states for the tau-lepton pair, all considered in this analysis. An excess of events is observed over the expected background contributions, with a local significance larger t…
The search for the Standard Model Higgs boson in the four lepton (electron and muon) final state with the ATLAS detector at the LHC is presented. The analysis strategy and the efficiency for selecting the signal and rejecting the background are discussed, focusing on the performance of the lepton identification that can be achieved with the first data of LHC expected in 2008.
We present the analysis technique and preliminary results of two ongoing analyses at CLEO which put lepton universality and lepton flavor conservation to the test in Upsilon decays.
This Letter presents a search for pair production of heavy down-type quarks decaying via b'->Wt in the lepton + jets channel, as b'b'->WtWt->bbWWWW->lvbbqqqqqq. In addition to requiring exactly one lepton, large missing transverse momentum and at least six jets, the invariant mass of nearby jet pairs is used to identify high transverse momentum W bosons. In data corresponding to an integrated luminosity of 1.04 fb^-1 from pp collisions at sqrt(s) = 7 TeV recorded with the ATLAS detector, a heavy down-type quark with mass less than 480 GeV is excluded at the 95% confidence level.
A search is presented for a singly produced third-generation scalar leptoquark decaying to a $τ$ lepton and a bottom quark. Associated production of a leptoquark and a $τ$ lepton is considered, leading to a final state with a bottom quark and two $τ$ leptons. The search uses proton-proton collision data at a center-of-mass energy of 13 TeV recorded with the CMS detector, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Upper limits are set at 95% confidence level on the production cross section of the third-generation scalar leptoquarks as a function of their mass. From a compariso…
Hadronically decaying tau leptons are of prime importance in numerous physics analyses in ATLAS. The spectrum of the possible applications of hadronically decaying tau leptons reaches from Standard Model measurements, including Higgs searches, to searches for physics beyond the Standard Model. The basic principles behind the sophisticated tau reconstruction and identification techniques, which are specifically designed to identify hadronically decaying taus and reject various background processes, are delineated here along with current data-driven estimates of their respective performance.
A search for heavy leptons decaying to a $Z$ boson and an electron or a muon is presented. The search is based on $pp$ collision data taken at $\sqrt{s}=8$ TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb$^{-1}$. Three high-transverse-momentum electrons or muons are selected, with two of them required to be consistent with originating from a $Z$ boson decay. No significant excess above Standard Model background predictions is observed, and 95% confidence level limits on the production cross section of high-mass trilepton resona…
The presence of tau leptons in the final state is an important signature in many Higgs boson and SUSY searches. With the ATLAS detector, hadronically decaying tau leptons can be reconstructed in a wide range of transverse energies. The reconstruction algorithm for hadronically decaying tau leptons is explained and the performance of tau lepton identification is shown. Particularly important is the discrimination of hadronically decaying tau leptons against overwhelming backgrounds from QCD jets.
A search for high-mass resonances decaying into tau-lepton pairs is performed using a data sample of pp collisions at sqrt(s) = 7 TeV. The data were collected with the CMS detector at the LHC and correspond to an integrated luminosity of 4.9 inverse femtobarns. The number of observed events is in agreement with the standard model prediction. An upper limit on the product of the resonance cross section and branching fraction into tau-lepton pairs is calculated as a function of the resonance mass. Using the sequential standard model resonance Z'(SSM) and the superstring-inspired E(6) model with …
The BaBar experiment has recently obtained some important results in the search for new physics in leptonic and lepton flavor violating decays, exploiting the complete datasets collected at the $Υ(4S)$, $Υ(3S)$ and $Υ(2S)$ energies. In particular, new limits on the ratio $Γ(Υ(1S)\toτ^+τ^-)/Γ(Υ(1S)\toμ^+μ^-)$, on lepton flavor violating decays of the $Υ(3S)$ and $Υ(2S)$, and on $τ$ decays to three charged leptons or $τ\to e/μγ$ are presented.
A search for the standard model Higgs boson decaying to W+W- in pp collisions at sqrt(s) = 7 TeV is reported. The data are collected at the LHC with the CMS detector, and correspond to an integrated luminosity of 4.6 inverse femtobarns. The W+W- candidates are selected in events with two charged leptons and large missing transverse energy. No significant excess of events above the standard model background expectations is observed, and upper limits on the Higgs boson production relative to the standard model Higgs expectation are derived. The standard model Higgs boson is excluded in the mass …
A search for pair production of third-generation scalar leptoquarks decaying into a top quark and a $τ$-lepton is presented. The search is based on a dataset of $pp$ collisions at $\sqrt{s}=13$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb$^{-1}$. Events are selected if they have one light lepton (electron or muon) and at least one hadronically decaying $τ$-lepton, or at least two light leptons. In addition, two or more jets, at least one of which must be identified as containing $b$-hadrons, are required. Si…
A search for supersymmetric partners of quarks is performed in the topology of multijet events accompanied by at least one tau lepton decaying hadronically and large missing transverse energy. Approximately 1 fb-1 of ppbar collision data from the Fermilab Tevatron Collider at a center of mass energy of 1.96 TeV recorded by the D0 detector is analyzed. Results are combined with the previously published D0 inclusive search for squarks and gluinos. No evidence of physics beyond the standard model is found and lower limits on the squark mass up to 410 GeV are derived in the framework of minimal su…
We present a search for lepton-flavor-violating tau decays into three leptons (electrons or muons) using 782 fb^-1 of data collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. No evidence for these decays is observed and we set 90% confidence level upper limits on the branching fractions between 1.5 x 10^-8 and 2.7 x 10^-8.
We search for lepton-flavor-violating tau decays into three leptons (electron or muon) using 535 fb-1 of data collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. No evidence for these decays is observed, and we set 90% confidence level upper limits on the branching fractions of (2.0-4.1)x10^-8. These results improve upon our previously published upper limits by factors of 4.9 to 10.
A search for the neutrinoless, lepton-flavor violating decay of the tau lepton into three charged leptons has been performed using an integrated luminosity of 468 fb^-1 collected with the BABAR detector at the PEP-II collider. In all six decay modes considered, the numbers of events found in data are compatible with the background expectations. Upper limits on the branching fractions are set in the range (1.8-3.3) X 10^-8 at 90% confidence level.
The ATLAS detector at the LHC is used to search for high-mass states, such as heavy charged gauge bosons (W'), decaying to a charged lepton (electron or muon) and a neutrino. Results are presented based on the analysis of pp collisions at a centre-of-mass energy of 7 TeV corresponding to an integrated luminosity of 4.7 fb-1. No excess beyond Standard Model expectations is observed. A W' with Sequential Standard Model couplings is excluded at the 95% credibility level for masses up to 2.55 TeV. Excited chiral bosons (W*) with equivalent coupling strength are excluded for masses up to 2.42 TeV.
The ATLAS detector at the LHC is used to search for high-mass states, such as heavy charged gauge bosons (W'), decaying to a charged lepton (electron or muon) and a neutrino. Results are presented based on the analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 1.04 fb^-1. No excess beyond Standard Model expectations is observed. A W' with Sequential Standard Model couplings is excluded at the 95% confidence level for masses up to 2.15 TeV.
We search for lepton-flavor-violating tau decays into three leptons (electron or muon) using 535 fb^{-1} of data collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. No evidence for these decays is observed and we set 90% confidence level upper limits on the branching fractions between 2.0x10^{-8} and 4.1x10^{-8}. These results improve the best previously published upper limits by factors from 4.9 to 7.0.
A search for heavy narrow resonances decaying into four-lepton final states from cascade decays of a Z' boson has been performed using proton-proton collision data at sqrt(s) = 8 TeV collected by the CMS experiment, corresponding to an integrated luminosity of 19.7 inverse femtobarns. No excess of events over the standard model background expectation is observed. Upper limits for a benchmark model on the product of cross section and branching fraction for the production of these heavy narrow resonances are presented. The limit excludes leptophobic Z' bosons with masses below 2.5 TeV within the…
A search is performed in proton-proton collisions at sqrt(s) = 8 TeV for exotic particles decaying via WZ to fully leptonic final states with electrons, muons, and neutrinos. The data set corresponds to an integrated luminosity of 19.5 inverse femtobarns. No significant excess is observed above the expected standard model background. Upper bounds at 95% confidence level are set on the production cross section of a W' boson as predicted by an extended gauge model, and on the W'WZ coupling. The expected and observed mass limits for a W' boson, as predicted by this model, are 1.55 and 1.47 TeV, r…
A review of the experimental status in searches for charged Lepton Flavor Violation (LFV) is presented. Searches for LFV in decays of leptons, heavy mesons, and bosons are explored, with an emphasis on the experiments providing the best constraints in each sector. In addition, prospects for upper limits by current and upcoming experiments in the next decade are discussed.
Lepton flavor violations in charged lepton give good signatures for the new physics. We review recent searches for lepton flavor violation in tau decays at B-factories. In these searches, optimization for background reduction is important to obtain high sensitivity. No evidence for these decays is observed and 90% confidence level upper limits have been set on the branching fractions at the O(10^{-8}) level.
Searches for lepton flavor violating tau -> l gamma, tau -> lll and tau -> lhh decays at the BaBar experiment are presented. Upper limits on the branching ratios are obtained at the level of 10^{-7} at 90% confidence level.
We present the most recent searches for lepton-flavor-violating tau decays in BaBar. We find no evidence of tau decaying to three charged leptons or to a charged lepton and a neutral meson (KS, rho, phi, K*0, anti-K*0), and set upper limits on the corresponding branching fractions between 1.8 and 19 times 10-8 at 90% confidence level.
A search for lepton-flavor and lepton-number violation in the decay of the tau lepton into one charged lepton and two charged hadrons is performed using 221.4 $\mathrm{fb}^{-1}$ of data collected at an $e^+e^-$ center-of-mass energy of 10.58 GeV with the BaBar detector at the PEP-II storage ring. In all 14 decay modes considered, the observed data are compatible with background expectations, and upper limits are set in the range ${\cal B}(τ\to\ell hh')<(0.7-4.8)\times 10^{-7}$ at 90% confidence level.
We search for lepton flavor and lepton number violating tau decays into a lepton (ell = electron or muon) and two charged mesons (h,h' = pi^\pm or K^\pm), tau- -> ell-h+h'- and tau- -> ell+h-h'-, using 671 fb^{-1} of data collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. We obtain 90% C.L. upper limits on the branching fractions in the range (4.4-8.8)x10^{-8} for tau -> ehh', and (3.3-16)x10^{-8} for tau -> muhh' processes. These results improve upon previously published upper limits by factors between 1.6 to 8.8.
Using almost 10 million B Bbar decays produced at CESR, the CLEO collaboration has set the most stringent limits to date on the lepton flavor violating decays B0 to mu(e) tau.
A search is performed for a new resonance decaying into a lighter resonance and a Z boson. Two channels are studied, targeting the decay of the lighter resonance into either a pair of oppositely charged tau leptons or a b b-bar pair. The Z boson is identified via its decays to electrons or muons. The search exploits data collected by the CMS experiment at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.8 inverse femtobarns. No significant deviations are observed from the standard model expectation and limits are set on production cross sections and parameters …
A search for neutral Higgs bosons in the minimal supersymmetric extension of the standard model (MSSM) decaying to tau-lepton pairs in pp collisions is performed, using events recorded by the CMS experiment at the LHC. The dataset corresponds to an integrated luminosity of 24.6 inverse femtobarns, with 4.9 inverse femtobarns at 7 TeV and 19.7 inverse femtobarns at 8 TeV. To enhance the sensitivity to neutral MSSM Higgs bosons, the search includes the case where the Higgs boson is produced in association with a b-quark jet. No excess is observed in the tau-lepton-pair invariant mass spectrum. E…
Searches for the direct electroweak production of supersymmetric charginos, neutralinos, and sleptons in a variety of signatures with leptons and W, Z, and Higgs bosons are presented. Results are based on a sample of proton-proton collision data collected at center-of-mass energy sqrt(s) = 8 TeV with the CMS detector in 2012, corresponding to an integrated luminosity of 19.5 inverse femtobarns. The observed event rates are in agreement with expectations from the standard model. These results probe charginos and neutralinos with masses up to 720 GeV, and sleptons up to 260 GeV, depending on the…
A search for heavy resonances that decay to tau lepton pairs is performed using proton-proton collisions at sqrt(s) = 13 TeV. The data were collected with the CMS detector at the CERN LHC and correspond to an integrated luminosity of 2.2 inverse femtobarns. The observations are in agreement with standard model predictions. An upper limit at 95% confidence level on the product of the production cross section and branching fraction into tau lepton pairs is calculated as a function of the resonance mass. For the sequential standard model, the presence of Z' bosons decaying into tau lepton pairs i…
A search has been made for lepton-flavor-violating interactions, e+ p -> mu/tau X, at a center-of-mass energy of 300 GeV with an integrated luminosity of 47.7 pb^-1 using the ZEUS detector at HERA. No evidence was found for lepton-flavor violation and constraints were derived on leptoquarks that could mediate such interactions. Some of the constraints are also applicable to lepton-flavor-violating processes mediated by squarks in R-parity-violating supersymmetric models. In some cases involving heavy quarks and especially for the tau channel, the ZEUS constraints are the most stringent publish…
We search for lepton-flavor-violating tau-> ell V^0 decays, where ell is an electron or muon and V^0 is one of the vector mesons rho^0, phi, omega, K*0 and K*0-bar. We use 854 fb^{-1} of data collected with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. No evidence for a signal is found in any decay mode, and we obtain 90% confidence level upper limits on the individual branching fractions in the range (1.2-8.4)*10^{-8}.
Several models of physics beyond the Standard Model predict neutral particles that decay into final states consisting of collimated jets of light leptons and hadrons (so-called "lepton jets"). These particles can also be long-lived with decay length comparable to, or even larger than, the LHC detectors' linear dimensions. This paper presents the results of a search for lepton jets in proton--proton collisions at the centre-of-mass energy of $\sqrt{s}$ = 8 TeV in a sample of 20.3 fb$^{-1}$ collected during 2012 with the ATLAS detector at the LHC. Limits on models predicting Higgs boson decays t…
A search for a new high-mass resonance decaying to a tau lepton and a neutrino is reported. The analysis uses proton-proton collision data collected by the CMS experiment at the LHC at $\sqrt{s} =$ 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The search utilizes hadronically decaying tau leptons. No excess in the event yield is observed at high transverse masses of the $τ$ and missing transverse momentum. An interpretation of results within the sequential standard model excludes W' boson masses below 4.0 TeV at 95% confidence level. Existing limits are also improved on …
We search for lepton-flavor-violating tau decays into a lepton (electron or muon) and an f0(980) meson using 671 fb-1 of data collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. No events are observed and we set the following 90% C.L. upper limits on the branching fraction products: B(tau- -> e-f0(980))*B(f0(980)->pi+pi-)<3.2x10^-8 and B(tau- -> mu-f0(980))*B(f0(980) -> pi+pi-)<3.4x10^-8. This is the first search performed for these modes.
A measurement of the cross section of top quark pair production in proton-proton collisions recorded with the ATLAS detector at the LHC at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb^-1. Events with an isolated electron or muon and a tau lepton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, sigma_ttbar = 186+/- 13 (stat) +/- 20 (syst) +/- 7 (…
Searching for vector-like leptons decaying into an electron and missing transverse energy in e$^{+}$e$^{-}$ collisions with $\sqrt{s} = 240$ GeV at the FCC-ee
Centre for Theoretical Physics, The British University in Egypt（埃及英国大学理论物理中心）
This paper describes the trigger and offline reconstruction, identification and energy calibration algorithms for hadronic decays of tau leptons employed for the data collected from $pp$ collisions in 2012 with the ATLAS detector at the LHC center-of-mass energy $\sqrt{\mathrm{s}} = 8$ TeV. The performance of these algorithms is measured in most cases with $Z$ decays to tau leptons using the full 2012 dataset, corresponding to an integrated luminosity of 20.3 fb$^{-1}$. An uncertainty on the offline reconstructed tau energy scale of $2-4$%, depending on transverse energy and pseudorapidity, is…
A search for production of supersymmetric particles in final states containing jets, missing transverse momentum, and at least one hadronically decaying tau lepton is presented. The data were recorded by the ATLAS experiment in sqrt(s) = 7 TeV proton-proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 2.05 fb-1 of data. The results are interpreted in the context of gauge mediated supersymmetry breaking models with Mmess = 250 TeV, N5 = 3, mu > 0, and Cgrav = 1. The production of supersymmetric particles is excluded at 95% C.…
We present a measurement of the mass difference Delta m_d for the B0 meson and the statistical power of the b flavor tagging methods used. The measurement uses 90 (1/pb) of data from p p-bar collisions at sqrt[s] = 1.8 TeV collected with the CDF detector. An inclusive lepton trigger is used to collect a large sample of B hadron semileptonic decays. The mass difference Delta m_d is determined from the proper time dependence of the fraction of B hadrons that undergo flavor oscillations. The flavor at decay is inferred from the charge of the lepton from semileptonic B decay. The initial flavor is…
A search for pair production of heavy scalar leptoquarks (LQs), each decaying into a top quark and a $τ$ lepton, is presented. The search considers final states with an electron or a muon, one or two $τ$ leptons that decayed to hadrons, and additional jets. The data were collected in 2016 in proton-proton collisions at $\sqrt{s}=$ 13 TeV with the CMS detector at the LHC, and correspond to an integrated luminosity of 35.9 fb$^{-1}$. No evidence for pair production of LQs is found. Assuming a branching fraction of unity for the decay LQ$ \to $t$τ$, upper limits on the production cross section ar…
Both the B-factories BaBar and Belle have ended data-taking and have mostly completed the data analysis aimed at searching Lepton Flavor Violation in Tau decays. No evidence of LFV in tau decays has been found yet. We review in the following the experimental upper limits that have been set.
We have searched a sample of 9.6 million BBbar events for the lepton-flavor-violating leptonic B decays, B^0 -> μ^\pm τ^\mp and B^0 -> e^\pm τ^\mp. The τ-lepton was detected through the decay modes τ-> \ellν\barν, where \ell = e, μ. There is no indication of a signal, and we obtain the 90% confidence level upper limits [Br(B^0 -> μ^\pm τ^\mp)] < 3.8 X 10^-5 and [Br(B^0 -> e^\pm τ^\mp)] < 1.1 X 10^-4.
The invariant mass of tau lepton pairs turns out to be smaller than the resonant mass of their mother particle and the invariant mass distribution is stretched wider than the width of the resonant mass as significant fraction of tau lepton momenta are carried away by neutrinos escaping undetected at collider experiments. This paper describes a new approach to reconstruct resonant masses of heavy particles decaying to tau leptons at such experiments. A typical example is a Z or Higgs boson decaying to a tau pair. Although the new technique can be used for each tau lepton separately, I combine t…
The Higgs and some of the Standard Model superpartners may have been copiously produced at LEP and the Tevatron without being detected. We study a novel scenario of this type in which the Higgs decays predominantly into a light hidden sector either directly or through light SUSY states. Subsequent cascades increase the multiplicity of hidden sector particles which, after decaying back into the Standard Model, appear in the detector as clusters of collimated leptons known as lepton jets. We identify the relevant collider observables that characterize this scenario, and study a wide range of LEP…
The observation of a charged-lepton flavor violating process would be a definite sign for physics beyond the Standard Model, but would actually only prove that one particular linear combination of lepton numbers is violated. We categorize lepton-flavor-violating processes by their quantum numbers and show how their discovery can be interpreted model independently, studying in particular which processes are required to establish that the entire flavor group is broken. We also comment on total lepton number, seeing as lepton number violation practically implies lepton flavor violation as well.
We propose a novel possibility to detect a very distinctive signal with more than four muons originating from pair-produced vector-like leptons decaying to a muon-philic $Z^\prime$ boson. These new particles are good candidates to explain the anomalies in the muon anomalous magnetic moment and the $b\to s\ell\ell$ processes. The doublet (singlet) vector-like leptons lighter than 1.3 (1.0) TeV are excluded by the latest data at the LHC if $\mathrm{BR}(E\to Z^\prime μ) = 1$. We also show that the excess in the signal region with more than five leptons can be explained by this scenario if the vec…
We report on the recent results of searches for lepton-flavor-violating tau decays as well as lepton-number-violating B decays with the world-highest data samples accumulated at the Belle.
Implications of a simple model for the mass generation of leptons are studied, in particular for the upcoming long-baseline neutrino experiments. The flavor mixing angles are large (nearly maximal). The probability for the long-baseline ν_μ\leftrightarrow ν_e oscillation is predicted to be about 1%.
We present an algorithm for the reconstruction of the Higgs mass in events with Higgs bosons decaying into a pair of tau leptons. The algorithm is based on matrix element (ME) techniques and achieves a relative resolution on the Higgs boson mass of typically 15-20%. A previous version of the algorithm has been used in analyses of Higgs boson production performed by the CMS collaboration during LHC Run 1. The algorithm is described in detail and its performance on simulated events is assessed. The development of techniques to handle tau decays in the ME formalism represents an important result …
Flavor violating processes in the lepton sector have highly suppressed branching ratios in the standard model. Thus, observation of lepton flavor violation (LFV) constitutes a clear indication of physics beyond the standard model (BSM). We review new physics searches in the processes that violate the conservation of lepton (muon) flavor by two units with muonia and muonium--antimuonium oscillations.
We present a model for calculating charged-lepton mixing matrices. These matrices are an essential ingredient for predicting lepton flavor-violating rates in the lepton number nonuniversal models recently proposed to explain anomalies in B-meson decays. The model is based on work on "constrained flavor breaking" by Appelquist, Bai and Piai relating the charged-lepton mass matrix, M_l, to those for the up and down-type quarks, M_{u,d}. We use our recent model of lepton nonuniversality to illustrate the magnitudes of flavor-violating B-decay rates that might be expected. Decays with mu tau final…
