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Register a new userSearch for lepton-number- and baryon-number-violating tau decays at Belle
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We search for lepton-number- and baryon-number-violating decays $tau^{-}tooverline{p}e^{+}e^{-}$, $pe^{-}e^{-}$, $overline{p}e^{+}mu^{-}$, $overline{p}e^{-}mu^{+}$, $overline{p}mu^{+}mu^{-}$, and $pmu^{-}mu^{-}$ using 921 fb$^{-1}$ of data, equivalent to $(841pm12)times 10^6$ $tau^{+}tau^{-}$ events, recorded with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. In the absence of a signal, $90%$ confidence-level upper limits are set on the branching fractions of these decays in the range $(1.8$-$4.0)times 10^{-8}$. We set the worlds first limits on the first four channels and improve the existing limits by an order of magnitude for the last two channels.
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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.
Searches for the lepton flavour violating decay $tau^-rightarrow mu^-mu^+mu^-$ and the lepton flavour and baryon number violating decays $tau^-rightarrow bar{p}mu^+mu^-$ and $tau^-rightarrow pmu^-mu^-$ have been carried out using proton-proton collision data, corresponding to an integrated luminosity of 1.0 fb$^{-1}$, taken by the LHCb experiment at $sqrt{s} = 7$ TeV. No evidence has been found for any signal, and limits have been set at 90% confidence level on the branching fractions: $cal B(tau^-rightarrow mu^-mu^+mu^-) < 8.0 times 10^{-8}$, $cal B(tau^-rightarrow bar{p}mu^+mu^-) < 3.3 times 10^{-7}$ and $cal B(tau^-rightarrow pmu^-mu^-) < 4.4 times 10^{-7}$. The results for the $tau^-rightarrow bar{p}mu^+mu^-$ and $tau^-rightarrow pmu^-mu^-$ decay modes represent the first direct experimental limits on these channels.
We present a search for ten baryon-number violating decay modes of $Lambda$ hyperons using the CLAS detector at Jefferson Laboratory. Nine of these decay modes result in a single meson and single lepton in the final state ($Lambda rightarrow m ell$) and conserve either the sum or the difference of baryon and lepton number ($B pm L$). The tenth decay mode ($Lambda rightarrow bar{p}pi^+$) represents a difference in baryon number of two units and no difference in lepton number. We observe no significant signal and set upper limits on the branching fractions of these reactions in the range $(4-200)times 10^{-7}$ at the $90%$ confidence level.
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.
Recently there has been much interest in the use of single-jet mass and jet substructure to identify boosted particles decaying hadronically at the LHC. We develop these ideas to address the challenging case of a neutralino decaying to three quarks in models with baryonic violation of R-parity. These decays have previously been found to be swamped by QCD backgrounds. We demonstrate for the first time that such a decay might be observed directly at the LHC with high significance, by exploiting characteristics of the scales at which its composite jet breaks up into subjets.
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