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Register a new userLepton Flavor Violating Decays - Review & Outlook
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Authors
Toshinori Mori
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Here I review the status and prospects of experimental investigations into lepton flavor violation (LFV) in charged leptons. Rare LFV processes are naturally expected to occur through loops of TeV scale particles predicted by supersymmetric theories or other models beyond the Standard Model. In contrast to physics of quark flavors that is dominated by the Cabibbo-Kobayashi-Maskawa matrix, LFV in charged leptons is a definitive signal of new physics. Currently active researches are rare tau decay searches at the B factories. The MEG experiment will soon start a sensitive search for the LFV muon decay, mu to e gamma. Prospects for searches at the LHC, a possibility of a fixed target LFV experiment with high energy muons, and a sensitivity of leptonic kaon decays to LFV are also briefly discussed.
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Exotic Higgs decays are promising channels to discover new physics in the near future. We present a simple model with a new light scalar that couples to the Standard Model through a charged lepton-flavor violating interaction. This can yield exciting new signatures, such as $h to e^+ e^+ mu^-mu^-$, that currently have no dedicated searches at the Large Hadron Collider. We discuss this model in detail, assess sensitivity from flavor constraints, explore current constraints from existing multi-lepton searches, and construct a new search strategy to optimally target these exotic, lepton-flavor violating Higgs decays.
Charged lepton flavor violation is forbidden in the Standard Model but possible in several new physics scenarios. In many of these models, the radiative decays $tau^{pm}rightarrowell^{pm}gamma$ ($ell=e,mu$) are predicted to have a sizeable probability, making them particularly interesting channels to search at various experiments. An updated search via $tau^{pm}rightarrowell^{pm}gamma$ using full data of the Belle experiment, corresponding to an integrated luminosity of 988 fb$^{-1}$, is reported for charged lepton flavor violation. No significant excess over background predictions from the Standard Model is observed, and the upper limits on the branching fractions, $mathcal{B}(tau^{pm}rightarrow mu^{pm}gamma)$ $leq$ $4.2times10^{-8}$ and $mathcal{B}(tau^{pm}rightarrow e^{pm}gamma)$ $leq$ $5.6times10^{-8}$, are set 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 of (2.0-4.1)x10^-8. These results improve upon our previously published upper limits by factors of 4.9 to 10.
We have searched for neutrinoless $tau$ lepton decays into $ell$ and $V^0$, where $ell$ stands for an electron or muon, and $V^0$ for a vector meson ($phi$, $omega$, $K^{*0}$, $bar{K}^{*0}$ or $rho^0$), using 543 fb$^{-1}$ of data collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. No excess of signal events over the expected background has been observed, and we set upper limits on the branching fractions in the range $(5.9-18) times 10^{-8}$ at the 90% confidence level. These upper limits include the first results for the $ell omega$ mode as well as new limits that are significantly more restrictive than our previous results for the $ell phi$, $ell K^{*0}$, $ell bar{K}^{*0}$ and $ell rho^0$ modes.
We have searched for the lepton-flavor-violating decays tau- -> ell-K0s and ell-K0sK0s (ell = e or mu), using a data sample of 671 fb^-1 collected with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. No evidence for a signal was found in any of the decay modes, and we set the following upper limits for the branching fractions: B(tau^- -> e^-K0s) < 2.6 x 10^-8, B(tau^- -> mu^-K0s) < 2.3 x 10^-8, B(tau^- -> e^-K0sK0s) < 7.1 x 10^-8 and B(tau^- -> mu^-K0sK0s) < 8.0 x 10^-8 at the 90% confidence level.
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