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Register a new userMeasurement of the branching fraction of leptonic decay $D_s^+totau^+ u_tau$ via $tau^+topi^+pi^0bar u_tau$
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By analyzing $6.32~mathrm{fb}^{-1}$ of $e^+e^-$ annihilation data collected at the center-of-mass energies between 4.178 and 4.226,GeV with the BESIII detector, we determine the branching fraction of the leptonic decay $D_s^+totau^+ u_tau$ with $tau^+topi^+pi^0bar u_tau$, to be $mathcal{B}_{D_s^+totau^+ u_tau}=(5.29pm0.25_{rm stat}pm0.20_{rm syst})%$. We estimate the product of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cs}|$ and the $D_s^+$ decay constant $f_{D^+_s}$ to be $f_{D_s^+}|V_{cs}|=(244.8pm5.8_{rm stat}pm4.8_{rm syst})~mathrm{MeV}$ using the known values of the $tau^+$ and $D_s^+$ masses as well as the $D_s^+$ lifetime, together with our branching fraction measurement. Combining with the value of $|V_{cs}|$ obtained from a global fit in the standard model and $f_{D_s^+}$ from lattice quantum chromodynamics, we obtain $f_{D_s^+}=(251.6pm5.9_{rm stat}pm4.9_{rm syst})$,MeV and $|V_{cs}| = 0.980pm0.023_{rm stat}pm0.019_{rm syst}$.
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We report the first observation of $D^+totau^+ u_tau$ with a significance of $5.1sigma$. We measure ${cal B}(D^+totau^+ u_tau) = (1.20pm0.24_{text{stat.}}pm0.12_{text{syst.}})times10^{-3}$. Taking the world average ${cal B}(D^+tomu^+ u_mu) = (3.74pm0.17)times10^{-4}$, we obtain $R_{tau/mu} =Gamma(D^+totau^+ u_tau)/Gamma(D^+tomu^+ u_mu) = 3.21pm0.64_{text{stat.}}pm0.43_{text{syst.}}$, which is consistent with the Standard Model expectation of lepton flavor universality. Using external inputs, our results give values for the $D^+$ decay constant $f_{D^+}$ and the CKM matrix element $|V_{cd}|$ that are consistent with, but less precise than, other determinations.
Using a sample of tagged D_s decays collected near the D^*_s D_s peak production energy in e+e- collisions with the CLEO-c detector, we study the leptonic decay D^+_s to tau^+ nu_tau via the decay channel tau^+ to e^+ nu_e bar{nu}_tau. We measure B(D^+_s to tau^+ nu_tau) = (6.17 +- 0.71 +- 0.34) %, where the first error is statistical and the second systematic. Combining this result with our measurements of D^+_s to mu^+ nu_mu and D^+_s to tau^+ nu_tau (via tau^+ to pi^+ bar{nu}_tau), we determine f_{D_s} = (274 +- 10 +- 5) MeV.
Using a data set of 6.32 fb$^{-1}$ of $e^+ e^-$ annihilation data collected with the BESIII detector at center-of-mass energies between 4178 and 4226 MeV, we have measured the absolute branching fraction of the leptonic decay $D_s^+ to tau^+ u_{tau}$ via $tau^+ to e^+ u_e bar{ u}_{tau}$ to be $mathcal{B}_{D_s^+ to tau^+ u_{tau}}=(5.27pm0.10pm0.12)times10^{-2}$, where the first uncertainty is statistical and the second is systematic. Combining with $f_{D_s^+}$ from Lattice quantum chromodynamics calculations or the $|V_{cs}|$ from the CKMfitter group, we extract $|V_{cs}|=0.978pm0.009pm0.012$ and $f_{D_s^+}= (251.1pm2.4pm3.0)$ MeV, respectively. These results are the most precise to date. Combining our result with the world averages of $mathcal{B}_{D_s^+ to tau^+ u_{tau}}$ and $mathcal{B}_{D_s^+ to mu^+ u_{mu}}$, we obtain the ratio of the branching fractions $mathcal{B}_{D_s^+ to tau^+ u_{tau}} / mathcal{B}_{D_s^+ to mu^+ u_{mu}} = 9.72pm0.37$, which is consistent with the standard model prediction of lepton flavor universality.
We report a measurement of the branching fraction of $B^+ to tau^+ u_tau$ decays using a data sample of $772 times 10^6 B bar{B}$ pairs, collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We reconstruct the accompanying $B$ meson in a semileptonic decay and detect the $B^+ to tau^+ u_tau$ candidate in the recoiling event. We obtain a branching fraction of ${cal B}(B^+ to tau^+ u_tau) = [1.25 pm 0.28 ({rm stat.}) pm 0.27({rm syst.})] times 10^{-4}$. This result is in good agreement with previous measurements and the expectation from calculations based on the Standard Model.
Using 13.53/fb of CLEO data, we have measured the ratios of the branching fractions R+(e) = BF(D+ --> K*0bar e+ nu_e) / BF(D+ --> K- pi+ pi+), R+(mu) = BF(D+ --> K*0bar mu+ nu_mu) / BF(D+ --> K- pi+ pi+) and the combined branching fraction ratio R+(l) = BF(D+ --> K*0bar l+ nu_l) / BF(D+ --> K- pi+ pi+). We find R+(e) = 0.74 +- 0.04 +- 0.05, R+(mu) = 0.72 +- 0.10 +- 0.06 and R+(l) = 0.74 +- 0.04 +- 0.05, where the first and second errors are statistical and systematic respectively. The known branching fraction BF(D+ --> K- pi+ pi+) leads to: BF(D+ --> K*0bar e+ nu_e) = (6.7 +- 0.4 +- 0.5 +- 0.4)%, BF(D+ --> K*0bar mu+ u_mu) = (6.5 +- 0.9 +- 0.5 +- 0.4)% and BF(D+ --> K*0bar l+ nu_l) = (6.7 +- 0.4 +- 0.5 +- 0.4)%, where the third error is due to the uncertainty in BF(D+ --> K- pi+ pi+).
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