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Register a new userPrecision Measurement of the Mass of the $tau$ Lepton
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M. Ablikim
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An energy scan near the $tau$ pair production threshold has been performed using the BESIII detector. About $24$ pb$^{-1}$ of data, distributed over four scan points, was collected. This analysis is based on $tau$ pair decays to $ee$, $emu$, $eh$, $mumu$, $mu h$, $hh$, $erho$, $murho$ and $pirho$ final states, where $h$ denotes a charged $pi$ or $K$. The mass of the $tau$ lepton is measured from a maximum likelihood fit to the $tau$ pair production cross section data to be $m_{tau} = (1776.91pm0.12 ^{+0.10}_{-0.13}$) MeV/$c^2$, which is currently the most precise value in a single measurement.
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The mass of the $tau$ lepton has been measured in the decay mode $tau to 3pi u_tau$ using a pseudomass technique. The result obtained from $414 mathrm{fb}^{-1}$ of data collected with the Belle detector is $M_tau = (1776.61pm 0.13 {(stat.)} pm 0.35 {(sys.)})$ MeV/$c^2$. The upper limit on the relative mass difference between positive and negative $tau$ leptons is $|M_{tau^+}-M_{tau^-}|/M_{tau} < 2.8 times 10^{-4}$ at 90% confidence level.
The mass of the $tau$-lepton has been measured in the decay modes $tau to 3pi u_tau$ and $tau to 3pi pi^0 u_tau$ using a pseudomass technique. The preliminary result is $1776.71pm 0.25 {(stat)} pm 0.62 {(syst)}$ MeV. The preliminary value of an upper limit on the relative mass difference between positive and negative $tau$ leptons is $|(M_{tau^+}-M_{tau^-})|/M_{mathrm{average}}$ is $5.0 times 10^{-4}$ at 90% CL.
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 $m_{tau} = 1777.28 pm 0.75~{rm (stat.)} pm 0.33 ~{rm (sys.)}~{rm{MeV}/rm{c}^2}$.
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.
We report the first measurement of the $tau$ lepton polarization in the decay ${bar B} rightarrow D^* tau^- {bar u_{tau}}$ as well as a new measurement of 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, with the decays $tau^- rightarrow pi^- u_{tau}$ and $tau^- rightarrow rho^- u_{tau}$. We use the full data sample of $772 times 10^6$ $B{bar B}$ pairs accumulated with the Belle detector at the KEKB electron-positron collider. Our preliminary results, $R(D^*) = 0.276 pm 0.034{rm (stat.)} ^{+0.029} _{-0.026}{rm (syst.)}$ and $P_{tau} = -0.44 pm 0.47 {rm (stat.)} ^{+0.20} _{-0.17} {rm (syst.)}$, are consistent with the theoretical predictions of the Standard Model within $0.6$ standard deviation.
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