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Measurement of the $tau$-lepton lifetime at Belle

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 Added by Mikhail Shapkin
 Publication date 2013
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and research's language is English




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The lifetime of the $tau$-lepton is measured using the process $e^+e^-rightarrowtau^+tau^-$, where both $tau$-leptons decay to $3pi u_tau$. The result for the mean lifetime, based on $711,mathrm{fb}^{-1}$ of data collected with the Belle detector at the $Upsilon(4S)$ resonance and $60,mathrm{MeV}$ below, is $tau = (290.17 pm 0.53(mathrm{stat.}) pm 0.33(mathrm{syst.}))cdot10^{-15},mathrm{s}$. The first measurement of the lifetime difference between $tau^+$ and $tau^-$ is performed. The upper limit on the relative lifetime difference between positive and negative $tau$-leptons is $|Deltatau| / tau < 7.0 times 10^{-3}$ at 90% CL.



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The reconstruction of tau-pair production, $e^{+}e^{-} to tau^{+}tau^{-}$, from the subsequent 3-prong ($tau^{+} rightarrow pi^{+} pi^{-} pi^{+} bar{ u}_{tau}$) and 1-prong ($tau^{-} to ell^{-} bar{ u}_{ell} u_{tau}$, $tau^{-} to h^{-} u_{tau}$ or $tau^{-} to pi^{-} pi^0 u_{tau}$) decays, is presented using 8.8 fb$^{-1}$ of $e^{+}e^{-}$ collision data of Belle II at the center-of-mass energy $sqrt{s} = m_{Upsilon(4S)}$. The pseudomass technique developed by the ARGUS experiment is used to measure the $tau$-lepton mass $m_{tau}$ in the 3-prong $tau^{+} to pi^{+} pi^{-} pi^{+} bar{ u}_{tau} $ decay, resulting in $m_{tau} = 1777.28 pm 0.75~{rm (stat.)} pm 0.33 ~{rm (sys.)}~{rm{MeV}/rm{c}^2}$.
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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.
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.
75 - S. Hirose , T. Iijima , I. Adachi 2017
With the full data sample of $772 times 10^6$ $B{bar B}$ pairs recorded by the Belle detector at the KEKB electron-positron collider, the decay $bar{B} rightarrow D^* tau^- bar{ u}_tau$ is studied with the hadronic $tau$ decays $tau^- rightarrow pi^- u_tau$ and $tau^- rightarrow rho^- u_tau$. The $tau$ polarization $P_tau(D^*)$ in two-body hadronic $tau$ decays is measured, as well as the ratio of the branching fractions $R(D^{*}) = mathcal{B}(bar {B} rightarrow D^* tau^- bar{ u}_tau) / mathcal{B}(bar{B} rightarrow D^* ell^- bar{ u}_ell)$, where $ell^-$ denotes an electron or a muon. Our results, $P_tau(D^*) = -0.38 pm 0.51 {rm (stat)} ^{+0.21}_{-0.16} {rm (syst)}$ and $R(D^*) = 0.270 pm 0.035{rm (stat)} ^{+0.028}_{-0.025}{rm (syst)}$, are consistent with the theoretical predictions of the Standard Model. The polarization values of $P_tau(D^*) > +0.5$ are excluded at the 90% confidence level.
70 - Markus Tobias Prim 2019
The Belle Collaboration presented three recent Belle analyses: The search for $Brightarrow l ugamma$ ($l = e, mu$) with improved hadronic tagging, the search for $Brightarrow mu u_mu$ with inclusive tagging and the test of lepton universality in $Brightarrow K^*ll$ ($l = e, mu$) decays.
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