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Register a new userLepton flavor violating decay Z-> l_i l_j in the 331 model
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We study the lepton flavor violating (LFV) decays Z-> l_i l_j (l_{i,j}=e,mu,tau) in the framework of the minimal 331 model. The main contributions arise at the one-loop level via a doubly charged bilepton with general LFV couplings. We obtain an estimate for the corresponding branching ratios by using the bounds on the LFV couplings of the doubly charged bilepton from the current experimental limits on the decays l_i-> l_jgamma and l_i-> l_j l_k l_k. A bound on the bilepton mass is also obtained through the current limit on the anomalous magnetic moment of the muon. It is found that the bilepton contributions to LFV Z decays are not expected to be at the reach of experimental detection. In particular, the branching ratio for the Z-> mu tau decay is below the 10^{-10} level for a bilepton mass of the order of 500 GeV.
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In the simplest little Higgs model the new flavor-changing interactions between heavy neutrinos and the Standard Model leptons can generate contributions to some lepton flavor violating decays of $Z$-boson at one-loop level, such as $Z to tau^{pm}mu^{mp}$, $Zto tau^{pm}e^{mp}$, and $Z to mu^{pm}e^{mp}$. We examine the decay modes, and find that the branching ratios can reach $10^{-7}$ for the three decays, which should be accessible at the Giga$Z$ option of the ILC.
We analyse the dependence of the rates of the LFV charged lepton decays mu to e + gamma, tau to e + gamma, tau to mu + gamma (l_i to l_j + gamma) and their ratios, predicted in the class of SUSY theories with see-saw mechanism of nu-mass generation and soft SUSY breaking with universal boundary conditions at the GUT scale, on the Majorana CP-violation phases in the PMNS neutrino mixing matrix and the ``leptogenesis CP-violating (CPV) parameters. The case of quasi-degenerate in mass heavy Majorana neutrinos is considered. The analysis is performed for normal hierarchical (NH), inverted hierarchical (IH) and quasi-degenerate (QD) light neutrino mass spectra. We show, in particular, that for NH and IH nu-mass spectrum and negligible lightest neutrino mass, all three l_i to l_j + gamma decay branching ratios, BR(l_i to l_j + gamma), depend on one Majorana phase, one leptogenesis CPV parameter and on the 3-neutrino oscillation parameters; if the CHOOZ mixing angle theta_13 is sufficiently large, they depend on the Dirac CPV phase in the PMNS matrix. The ``double ratios R(21/31) sim BR(mu to e + gamma)/BR(tau to e + gamma) and R(21/32) sim BR(mu to e + gamma)/BR(tau to mu + gamma) are determined by these parameters. The same Majorana phase enters into the NH and IH expressions for the effective Majorana mass in neutrinoless double beta decay, <m>.
The lepton flavor violating $Z^{prime}totaumu$ decay is studied in the context of several extended models that predict the existence of the new gauge boson named $Z^prime$. A calculation of the strength of the lepton flavor violating $Z^primemutau$ coupling is presented by using the most general renormalizable Lagrangian that includes lepton flavor violation. We used the experimental value of the muon magnetic dipole moment to bound this coupling, from which the $mathrm{Re}(Omega_{Lmutau}Omega^ast_{Rmutau})$ parameter is constrained and it is found that $mathrm{Re}(Omega_{Lmutau}Omega^ast_{Rmutau})sim 10^{-2}$ for a $Z^prime$ boson mass of 2 TeV. Alongside, we employed the experimental restrictions over the $tautomugamma$ and $tautomumu^+mu^-$ processes in the context of several models that predict the existence of the $Z^prime$ gauge boson to bound the mentioned coupling. The most restrictive bounds come from the calculation of the three-body decay. For this case, it was found that the most restrictive result is provided by a vector-like coupling, denoted as $|Omega_{mutau}|^2$, for the $Z_chi$ case, finding around $10^{-2}$ for a $Z^prime$ boson mass of 2 TeV. We used this information to estimate the branching ratio for the $Z^primetotaumu$ decay. According to the analyzed models the least optimistic result is provided by the Sequential $Z$ model, which is of the order of $10^{-2}$ for a $Z^prime$ boson mass around 2 TeV.
We calculate lepton flavor violating Z -> l^+ l^- decay in the framework of the general two Higgs Doublet model. In our calculations we used the constraints for the Yukawa couplings bar{xi}^{D}_{N,tau e} and bar{xi}^{D}_{N,taumu} coming from the experimental result of muon electric dipole moment and upper limit of the BR(mu -> egamma). We observe that it is possible to reach the present experimental upper limits for the branching ratios of such Z decays in the model III.
This dissertation reviews the Standard Model formalism as well as the Lepton Flavour Violating (LFV) decay processes which cause its extension, known as the physics beyond the SM. Firstly, using the experimental bounds on three body LFV decays, the corresponding bounds on two body LFV decays are reviewed. The dynamical suppression of three body LFV decays due to momentum dependent couplings is also reviewed. Secondly, the role of the LFV decays to explain the LSND excess is discussed in detail, for which the experimental bounds on three body LFV decays, i.e. mu -> 3e are used to constraint the coupling tilde{g}_{Z_{mu e}}, which is needed to calculate the anomalous muon decay mu -> e u_lbar{ u}_l. Then comparing the effective coupling of anomalous muon decay to r>1.6times 10^{-3} [9809524], it is proved that LFV is not the correct hypothesis to explain the LSND excess. Finally, LFV decays at loop order are studied in Seesaw model of neutrino masses [PRL. 86 2502 (2001)] where the smallness of the Seesaw neutrino mass may be naturally realized with m_N (mass of right-handed singlet neutrinos) of order 1 TeV. It is shown that the Higgs mass of a new scalar doublet with lepton number L=-1 needed in the model has to be larger than 50 TeV to get the branching ratio of mu -> 3e to be consistent with the existing bound on mu -> 3e. This defeats the original motivation of the model, namely that there is no physics beyond the TeV energy scale.
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