The Mu2e experiment will search for the neutrino-less conversion of a muon into an electron in the field of an aluminum nucleus. An observation would be the first signal of charged lepton flavor violation and de facto evidence for new physics beyond the Standard Model. The clean signature of the conversion process offers an opportunity for a powerful search: Mu2e will probe four orders of magnitude beyond current limits, with real discovery potential over a wide range of well motivated new physics models. This goal requires an integrated system of solenoids that will create the most intense muon beam in the world, and suppression of all possible background sources. The Mu2e components are currently being constructed, with the experiment planned to begin operations in the Fermilab Muon Campus within the next few years.
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.
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.
The experimental status of charged lepton flavor violation searches is briefly reviewed, with particular emphasis on the three classical searches involving muon transisions: $μ\to e γ$, $μ\to e$ conversion and $μ\to 3e$.
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.
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.
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.
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.
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.
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.
Since 1999, the B-factories collaborations BABAR and Belle have accumulated and studied large samples of tau lepton pairs. The experimental results on Lepton Universality checks and Lepton Flavour Violation searches are reported.
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.
In the Standard Model of particle physics, lepton flavour and lepton number are conserved quantities although no fundamental symmetry demands their conservation. I present recent results of searches for lepton flavour and lepton number violating hadron decays measured at the B factories and LHCb. In addition, the LHCb collaboration has recently performed a search for the lepton flavour violating decay τ^- \to μ^-μ^-μ^+. The obtained upper exclusion limit, that has been presented in this talk for the first time, is of the same order of magnitude as those observed at the B factories. This is the…
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.
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%.
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}.
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.
Searches for lepton flavour and lepton number violation in kaon decays by the NA48/2 and NA62 experiments at CERN are presented. A new measurement of the helicity suppressed ratio of charged kaon leptonic decay rates $RK=BR(Ke2)/BR(Kmu2) to sub-percent relative precision is discussed. An improved upper limit on the lepton number violating K+- --> pi-+mu+-mu+- decay rate is also presented.
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…
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.
Recent hints for lepton-universality violation in $b\to c\ellν$ and $b\to s\ell\ell$ transitions could imply the existence of lepton-flavour violating $b$ decays. The LHCb experiment is well suited for the search for these decays due to its large acceptance and trigger efficiency, as well as its excellent invariant mass resolution and particle identification capabilities. Recent results on searches for lepton-flavour violating decays from the LHCb experiment are presented.
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.
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…
In the standard model (SM), lepton flavor violating (LFV) Higgs decay is absent at renormalizable level and thus it is a good probe to new physics. In this article we study a type of new physics that could lead to large LFV Higgs decay, i.e., a lepton-flavored dark matter (DM) model which is specified by a Majorana DM and scalar lepton mediators. Different from other similar models with similar setup, we introduce both left-handed and right-handed scalar leptons. They allow large LFV Higgs decay and thus may explain the tentative Br$(h\raτμ)\sim1\%$ experimental results from the LHC. In partic…
Recent Kaon decay studies seeking lepton-flavor violating (LFV) new-physics effects are briefly discussed. The main focus is set on precise measurements of rare or not-so-rare decays aiming at finding evidence of deviations from the SM prediction rather than on the results from direct searches of LFV transitions forbidden or ultra-rare in the SM.
We study the lepton flavor violating process, $e+ T \to τ+T'$, at a few GeV. This process can be studied by experiments directing GeV scale electron or positron beams on internal or fixed targets. We study the effects of some low energy lepton flavor violating interactions on this process. We study the sensitivities of this process on these low energy lepton flavor violating interactions and compare them to the sensitivities of lepton flavor violating $τ$ decay processes. Comparing with $τ$ decay processes, this process provides another way to study the lepton flavor violating effects with $e-…
Neutrino masses and mixings have important implications for models of fermion masses, and, most directly, for the charged lepton sector. We consider supersymmetric Abelian flavor models, where neutrino mass parameters are related to those of charged leptons and sleptons. We show that processes such as τto μγ, μto eγand μ-e conversion provide interesting probes. In particular, some existing models are excluded by current bounds, while many others predict rates within reach of proposed near future experiments. We also construct models in which the predicted rates for charged lepton flavor violat…
The simplest realization of the inverse seesaw mechanism in a $SU(3)_C \otimes SU(3)_L \otimes U(1)_X$ gauge theory offers striking flavor correlations between rare charged lepton flavor violating decays and the measured neutrino oscillations parameters. The predictions follow from the gauge structure itself without the need for any flavor symmetry. Such tight complementarity between charged lepton flavor violation and oscillations renders the scenario strictly testable.
Lepton Flavor Violating (LFV) processes are clear signals of physics beyond the Standard Model. We investigate the possibility of measuring this kind of processes at present and foreseeable future muon-electron colliders, taking into account present day bounds from existing experiments. As a model of new physics we consider a Z' boson with a $U'(1)$ gauge symmetry and generic couplings. Processes that violate lepton flavor by two units seem to be particularly promising.
We investigate collider signatures of standard model extensions featuring vector-like leptons and a flavorful scalar sector. Such a framework arises naturally within asymptotically safe model building, which tames the UV behavior of the standard model towards the Planck scale and beyond. We focus on values of Yukawa couplings and masses which allow to explain the present data on the muon and electron anomalous magnetic moments. Using a CMS search based on $77.4 \, \rm{fb}^{-1}$ at the $\sqrt{s}=13$ TeV LHC we find that flavorful vector-like leptons are excluded for masses below around $300$ Ge…
In the coming years, experiments underway will increase the sensitivity to charged lepton flavor violation by four orders of magnitude. These experiments will stringently probe weak scale physics. I review the status of global symmetries in the standard model and present several well-motivated models that predict observable lepton flavor violation. Finally, I describe what we might learn from future experimental results, whether positive or null.
Most extensions of the Standard Model lepton sector predict large lepton flavor violating rates. Given the promising experimental perspectives for lepton flavor violation in the next few years, this generic expectation might offer a powerful indirect probe to look for new physics. In this review we will cover several aspects of lepton flavor violation in supersymmetric models beyond the Minimal Supersymmetric Standard Model. In particular, we will concentrate on three different scenarios: high-scale and low-scale seesaw models as well as models with R-parity violation. We will see that in some…
A recent proposal for explaining discrepancies in angular observables in the rare decay B --> K*mu+mu- with a gauged L_mu - L_tau current carried with it the prediction of lepton flavor universality violation in related B-meson decays. This prediction gained empirical support with a subsequent hint for lepton flavor universality violation in the B --> K l+l- decay by LHCb. In this short paper we fully quantify the prediction including the associated uncertainties. We also provide new predictions for a variety of additional observables sensitive to lepton flavor universality violation in B-meso…
Non-Abelian discrete symmetries provide an interesting opportunity to address the flavor puzzle in the lepton sector. However, the number of currently viable models based on such symmetries is rather large. High-precision measurements of the leptonic mixing parameters by future neutrino experiments, including ESSnuSB, T2HK, DUNE, and JUNO, will be crucial to test such models. We show that the complementarity among these experiments offers a powerful tool for narrowing down this broad class of lepton flavor models.
New physics scenarios beyond the Standard Model (SM) for neutrino mass mechanism often necessitate the existence of a neutral scalar $H$ and/or doubly-charged scalar $H^{\pm\pm}$, which couple to the SM charged leptons in a flavor violating way, while evading all existing constraints. Such scalars could be effectively produced at future lepton colliders like CEPC, ILC, FCC-ee and CLIC, either on-shell or off-shell, and induce striking charged lepton flavor violating (LFV) signals. We find that a large parameter space of the scalar masses and the LFV couplings can be probed at lepton colliders,…
We introduce the program SuSeFLAV for computing supersymmetric mass spectra with flavor violation in various supersymmetric breaking scenarios with/without seesaw mechanism. A short user guide summarizing the compilation, executables and the input files is provided.
Many new physics scenarios beyond the Standard Model often necessitate the existence of a (light) neutral scalar $H$, which might couple to the charged leptons in a flavor violating way, while evading all existing constraints. We show that such scalars could be effectively produced at future lepton colliders, either on-shell or off-shell depending on their mass, and induce lepton flavor violating (LFV) signals, i.e. $e^+ e^- \to \ell_α^\pm \ell_β^\mp (+H)$ with $α\neq β$. We find that a large parameter space of the scalar mass and the LFV couplings can be probed, well beyond the current low-en…
Dark, chiral fermions carrying lepton flavor quantum numbers are natural candidates for freeze-in. Small couplings with the Standard Model fermions of the order of lepton Yukawas are `automatic' in the limit of Minimal Flavor Violation. In the absence of total lepton number violating interactions, particles with certain representations under the flavor group remain absolutely stable. For masses in the GeV-TeV range, the simplest model with three flavors, leads to signals at future direct detection experiments like DARWIN. Interestingly, freeze-in with a smaller flavor group such as $SU(2)$ is …
We present here two concrete examples of models where a sub-TeV scale breaking of their respective $\mathcal{T}_{13}$ and $A_5$ flavor symmetries is able to account for the recently observed discrepancy in the muon anomalous magnetic moment, $(g-2)_μ$. Similarities in the flavor structures of the charged-lepton Yukawa matrix and dipole matrix yielding $(g-2)_μ$ give rise to strong constraints on low-scale flavor models when bounds from lepton flavor violation (LFV) are imposed. These constraints place stringent limits on the off-diagonal Yukawa structure, suggesting a mostly (quasi-)diagonal t…
In this work productions of charged and doubly charged scalars and their lepton flavor violating collider signals in the framework of littlest Higgs model at LHC are studied. In the allowed parameter region of the littlest Higgs model, the production rates of the scalars of littlest Higgs model via $pp \to ϕ^{++}ϕ^{-}$, $pp \to ϕ^+ϕ^-$ and $pp \to ϕ^{++}ϕ^{--}$ processes are calculated. We obtained that charged and doubly charged scalar pairs can be produced at LHC . Considering the possible lepton flavor violating decays of charged scalars found in literature, final state analysis is done. Th…
Studies of lepton-flavor violation in strangeness-changing ($|ΔS|=1$) transitions have a~long tradition in the kaon sector where they provide some of the strongest limits on physics beyond the standard model. Recent hints of violation of lepton-flavor universality in $B$-meson decays have revived interest in lepton-flavor violation as the two phenomena appear simultaneously in many extensions of the standard model. At the same time, the LHCb experiment has produced new results for the hyperon process $Σ^+\to pμ^+μ^-$ and may be in a position to study other rare hyperon decay modes. With this i…
We explore scalar dark matter that is part of a lepton flavor triplet satisfying symmetry requirements under the hypothesis of minimal flavor violation. Beyond the standard model, the theory contains in addition three right-handed neutrinos that participate in the seesaw mechanism for light neutrino mass generation. The dark-matter candidate couples to standard-model particles via Higgs-portal renormalizable interactions as well as to leptons through dimension-six operators, all of which have minimal flavor violation built-in. We consider restrictions on the new scalars from the Higgs boson me…
