Lepton Flavor Violation Processes Research Articles

Search for lepton flavor violation process J/ψ→eμ

Using a sample of 5.8×107J/ψ events, the Beijing Spectrometer experiment has searched for the decay J/ψ→eμ. Four candidates, consistent with the estimated background, are observed, and an upper limit on the branching fraction of J/ψ→eμ of 1.1×10−6 at the 90% C.L. is obtained.

Prospects of muonium to antimuonium conversion and μ −–μ + conversion at PRISM

High-quality muon beam produced at PRISM facility is very useful to pursue experimental researches for Lepton Flavor Violation (LFV) process with |Δ L i |=2, such as muonium to antimuonium conversion and μ −–μ + conversion.

Lepton flavor-violating process in a supersymmetric grand unified theory with right-handed neutrinos

Motivated by the recent results of neutrino oscillation experiments, we investigated lepton flavor-violating (LFV) processes in a SU(5) supersymmetric grand unified theory with right-handed neutrinos. The current experimental upper bound for the μ→eγ process already gives some constraint on the model. The correlation between μ→eγ and the SUSY contribution to the muon anomalous magnetic moment is also discussed. Future LFV experiments will have considerable impacts on this type of SUSY GUT model equipped with seesaw neutrino mass generation.

Neutrino masses and lepton-flavor violation in a supersymmetric model with lopsided Froggatt–Nielsen charges

We analyze in detail lepton-flavor violation (LFV) in the charged-lepton sector, such as μ→eγ and τ→μγ within the framework of supersymmetric models with lopsided Froggatt–Nielsen charges. We show that the present experimental limits on the LFV processes already exclude some of the models. The proposed future search for LFV, especially in muon processes, can provide a significant probe to this framework.

Lepton flavor-changing scalar interactions and g-2 of the muon

A systematic investigation on muon anomalous magnetic moment and related lepton flavor-violating process such as $\m\to e\g$, $\t\to e\g$ and $\t\to \m\g$ is made at two loop level in the models with flavor-changing scalar interactions. The two loop diagrams with double scalar exchanges are studied and their contributions are found to be compatible with the ones from Barr-Zee diagram. By comparing with the latest data, the allowed ranges for the relevant Yukawa couplings $Y_{ij}$ in lepton sector are obtained. The results show a hierarchical structure of $Y_{\m e, \t e} \ll Y_{\m \t} \simeq Y_{\m\m}$ in the physical basis if $\Delta a_{\mu}$ is found to be $>50\times 10^{-11}$. It deviates from the widely used ansatz in which the off diagonal elements are proportional to the square root of the products of related fermion masses. An alternative Yukawa coupling matrix in the lepton sector is suggested to understand the current data. With such a reasonable Yukawa coupling ansatz, the decay rate of $\t\to \m\g$ is found to be near the current experiment upper bound.

Lepton–Flavor Violating Processes li → liγ in Topcolor Assisted Technicolor Models**The project supported by National Natural Science Foundation of China (19905004), the Excellent Youth Foundation of Henan Scientific Committee (9911)

We consider the lepton-flavor violating (LFV) processes li → liγ in the framework of topcolor-assisted technicolor (TC2) models. We find that the new gauge boson Z′ predicted by TC2 models can give significantly contributions to these processes via the flavor-changing couplings Z′lilj. The present experimental bound on the LFV process μ → eγ gives severe constraints on the TC2 models. Using other experimental constraints on the Z′ mass MZ, we obtain constraints on the lepton mixing factors Kτμ and Kτe. The future LFV experiments will probe into TC2 models.

Lepton flavor violation in tau decays

We study lepton flavor violation (LFV) in tau decays induced by heavy Majorana neutrinos within two models: (I) the Standard Model with additional right–handed heavy Majorana neutrinos, i.e., a typical seesaw–type model; (II) the Standard Model with left–handed and right–handed neutral singlets, which are inspired by certain scenarios of SO(10) models and heterotic superstring models with E6 symmetry. We calculate various LFV branching ratios and a T–odd asymmetry. The seesaw Model I predicts very small branching ratios for LFV processes in most of the parameter space, although in a very restricted parameter region it can reach maximal branching ratios Br(τ → �γ )∼10 −9 and Br(τ → 3� )∼10 −10 . In contrast, Model II may show branching ratios Br(τ → eγ)∼10 −8 and Br(τ → 3e) < ∼ 10 −9 , over a sizable region of the parameter space, large enough to be tested by experiments in the near future.

Lepton flavor violating Z→ lilj in flavor-universal topcolor-assisted technicolor

In the context of flavor-universal topcolor-assisted technicolor (TC2) models, we calculate the lepton flavor violating (LFV) Z→ l i l j decays. We find that the extra U(1) gauge boson Z′ can give significant contributions to these LFV processes. With reasonable values of the parameters, the branching ratios of the processes Z→ τμ and Z→ τe can approach the experimental upper limits. The indirect bound on the process Z→ μe can give a severe constraint on the flavor-universal TC2 models.

Lepton-flavor violation with non-universal soft terms

We study the lepton-flavor violation processes tau ->mu gamma and mu->e gamma in two different examples of models with non-universal soft breaking terms derived from strings. We show that the predictions are quite different from those of universal scenarios. Non-universal A-terms provide an interesting framework to enhance the supersymmetric contributions to CP violation effects. We observe that in the case of the lepton-flavor violation we study, the non-universality of the scalar masses enhances the branching ratios more significantly than the non-universality of the A-terms. We find that the current experimental bounds on these processes restrict both the parameter space of the models and the texture of the Yukawa couplings which predicts the lepton masses, providing at the same time an interesting experimental test for physics beyond the Standard Model.

Neutrino masses and lepton-flavor violation in supersymmetric models with lopsided Froggatt-Nielsen charges

We analyze in detail lepton-flavor violation (LFV) in the charged-lepton sector such as $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}e\ensuremath{\gamma}, \stackrel{\ensuremath{\rightarrow}}{\ensuremath{\tau}}\ensuremath{\mu}\ensuremath{\gamma}, \stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}eee,$ and $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}e$ conversion in nuclei, within the framework of supersymmetric models with lopsided Froggatt-Nielsen charges, in which the large mixing in the neutrino sector as well as small mixing in the quark sector can be naturally accommodated. We show that the present experimental limits on the LFV processes already exclude some of the models. The future proposed search for LFV, especially in muon processes, can provide a significant probe to this framework. We also stress the importance of the measurement of ${U}_{e3}^{\mathrm{MNS}}$ in neutrino experiments, and the fact that the KamLAND experiment could play a significant role to test a certain class of models.

Lepton flavor violation in supersymmetric SO(10) grand unified models

The study for lepton flavor violation combined with neutrino oscillations may provide more information about the lepton flavor structure of the grand unified theory. In this paper, we study two lepton flavor violation processes $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\tau}}\ensuremath{\mu}\ensuremath{\gamma}$ and $\stackrel{\ensuremath{\rightarrow}}{Z}\ensuremath{\tau}\ensuremath{\mu}$ in the context of supersymmetric SO(10) grand unified models. We find the first process is detectable in a quite large supersymmetric parameter space if the experimental sensitivity for this process can reach up to $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}.$ The latter process may be important in some supersymmetric parameter space where the former is suppressed. It is especially important in the region with small $\mathrm{tan}\ensuremath{\beta}$ and large scalar lepton masses. However, it is detectable at Giga-$Z$ option of the Tesla project only when the value of $\mathrm{tan}\ensuremath{\beta}$ is close to the limit set by the present Higgs boson search experiments.

Lepton-flavor violation in supersymmetric models with trilinearR-parity violation

Supersymmetry with R-parity violation (RPV) provides an interesting framework for naturally accommodating small neutrino masses. Within this framework, we discuss the lepton-flavour violating (LFV) processes mu -> e gamma, mu -> eee, and mu -> e conversion in nuclei. We make a detailed study of the observables related to LFV in different RPV models, and compare them to the expectations of R-conserving supersymmetry with heavy right-handed neutrinos. We show that the predictions are vastly different and uniquely characterise each model, thus providing a powerful framework for experimentally distinguishing between different theories of LFV. Besides the obvious possibility of amplified tree-level generation of mu -> eee and mu -> e conversion in nuclei, we find that even in the case where these processes arise at the one-loop level, their rates are comparable to that of mu -> e gamma, in clear contrast to the predictions of R-conserving models. We conclude that in order to distinguish between the different models, such a combined study of all the LFV processes is necessary, and that measuring P-odd asymmetries in polarised mu -> eee can play a decisive role. We also comment on the intriguing possibility of RPV models yielding a large T-odd asymmetry in the decay of polarised mu -> eee.

A constraint on Yukawa-coupling unification from lepton-flavor violating processes

We present a new constraint on a lepton mixing matrix V from lepton-flavor violating (LFV) processes in supersymmetric standard models with massive neutrinos. Here we assume Yukawa-coupling unification f ν3 ≃ f top, in which τ-neutrino Yukawa coupling f ν3 is unified into top-quark Yukawa coupling f top at the unification scale M ∗≃3×10 16 GeV. We show that the present experimental bound on μ→ eγ decay already gives a stringent limit on the lepton mixing (typically V 13<0.02 for V 23=1/ 2 ). Therefore, many existing neutrino-mass models are strongly constrained. Future improvement of bounds on LFV processes will provide a more significant impact on the models with the Yukawa-coupling unification. We also stress that a precise measurement of a neutrino mixing ( V MNS) e3 in future neutrino experiments would be very important, since the observation of non-zero ( V MNS) e3 , together with negative experimental results for the LFV processes, have a robust potential to exclude a large class of SUSY standard models with the Yukawa-coupling unification.

Lepton flavor violation in the standard model extended by heavy singlet Dirac neutrinos

Low energy neutrinoless lepton flavor violating (LFV) processes are studied in an extension of the Standard Model (SM) by heavy SU(2)xU(1) singlet Dirac neutrinos. An upper bound procedure is elaborated for the evaluation of amplitudes. A comment on the extraction of heavy neutrino mixings from astrophysical observations is given. For processes not treated in the applied model the formalism not treated in the applied model the formalism for evaluating the branching ratios (BRs) is presented. The processes previously studied in the model are carefully examined. Some of the previous results are improved. Special attention is paid to the structure of the amplitudes and BRs as well as to the relations between BRs of different LFV processes. Numerical analysis of the BRs is done. The decoupling of heavy neutrinos is discussed and it is explicitely shown that the very heavy neutrinos decouple when the upper bound procedure is applied. The upper limits of the BRs are compared with the current experimental upper bounds and the processes interesting for the search for LFV are proposed. The LFV decays are shown to be unsuitable for finding upper bounds on "diagonal" LFV parameters. The B-meson LFV processes are suggested for the search of LFV in future B-factories.

Slepton oscillation ateγcolliders

We study the possible signature for lepton-flavor violation processes in the scalar-lepton sector of the minimal supersymmetric model at future electron-photon colliders. We find that an $\egm$ collider can provide a good opportunity to probe the mass differences and flavor mixing angles between the sleptons of the first generation and the other two generations for the right-handed as well as the left-handed sector. The sensitivity reach for lepton-flavor violation is comparable to that obtained at an $e^+e^-$ and $e^-e^-$ collider, and is significantly better than the current bound from low-energy rare processes such as $\mu \to e \gamma$.

Intermediate scale as a source of lepton flavor violation in SUSY SO(10).

In supersymmetric SO(10) grand unified models, we examine the lepton flavor violation process $\mu \rightarrow e\gamma$ from having the SU(2)$_R\times $U(1)$_{B-L}$ gauge symmetry broken at an intermediate scale $M_I$ below the SO(10) grand unification scale $M_G$. Even in the case that supersymmetry is broken by universal soft terms introduced at the scale $M_G$, we find significant rates for $\mu\rightarrow e\gamma$ with $M_I \sim 10^{12}$ GeV or less. These rates are further enhanced if the universal soft terms appear at a scale greater than $M_G$.

Lepton-flavor violation via right-handed neutrino Yukawa couplings in the supersymmetric standard model.

Various lepton-flavor violating (LFV) processes in the supersymmetric standard model with right-handed neutrino supermultiplets are investigated in detail. It is shown that large LFV rates are obtained when $\tan \beta $ is large. In the case where the mixing matrix in the lepton sector has a similar structure as the Kobayashi-Maskawa matrix and the third-generation Yukawa coupling is as large as that of the top quark, the branching ratios can be as large as $Br(\mu\rightarrow e\gamma)\simeq 10^{-11}$ and $Br(\tau\rightarrow\mu\gamma)\simeq 10^{-7}$, which are within the reach of future experiments. If we assume a large mixing angle solution to the atmospheric neutrino problem, rate for the process $\tau\rightarrow\mu\gamma$ becomes larger. We also discuss the difference between our case and the case of the minimal $SU(5)$ grand unified theory.