We present the result of a direct search for the decay K_S -> e+e-, obtained with a sample of e+e- -> phi -> K_S K_L events produced at DAFNE, the Frascati phi-factory, for an integrated luminosity ...of 1.9 fb^-1. The search has been performed using a pure K_S beam tagged by the simultaneous detection of a K_L interaction in the calorimeter. Background rejection has been optimized by using both kinematic and particle identification cuts. We find BR(K_S -> e+e-) < 9x10^-9 at 90% CL, which improves by an order of magnitude on the previous best limit.
We present measurements of the masses and decay widths of the baryonic states \(\Sigma_{c}(2455)^{0/++}\) and \(\Sigma_{c}(2520)^{0/++}\) using a data sample corresponding to an integrated luminosity ...of 711 fb\(^{-1}\) collected with the Belle detector at the KEKB \(e^{+}e^{-}\) asymmetric-energy collider operating at the \(\Upsilon(4S)\) resonance. We report the mass differences with respect to the \(\Lambda_{c}^{+}\) baryon \(M(\Sigma_{c}(2455)^{0})-M(\Lambda_{c}^{+}) = 167.29\pm0.01\pm0.02\) MeV/\(c^{2}\), \(M(\Sigma_{c}(2455)^{++})-M(\Lambda_{c}^{+}) = 167.51\pm0.01\pm0.02\) MeV/\(c^{2}\), \(M(\Sigma_{c}(2520)^{0})-M(\Lambda_{c}^{+}) = 231.98\pm0.11\pm0.04\) MeV/\(c^{2}\), \(M(\Sigma_{c}(2520)^{++})-M(\Lambda_{c}^{+}) = 231.99\pm0.10\pm0.02\) MeV/\(c^{2}\), and the decay widths \(\Gamma(\Sigma_{c}(2455)^{0}) = 1.76\pm0.04^{+0.09}_{-0.21}\) MeV/\(c^{2}\), \(\Gamma(\Sigma_{c}(2455)^{++}) = 1.84\pm0.04^{+0.07}_{-0.20}\) MeV/\(c^{2}\), \(\Gamma(\Sigma_{c}(2520)^{0}) = 15.41\pm0.41^{+0.20}_{-0.32}\) MeV/\(c^{2}\), \(\Gamma(\Sigma_{c}(2520)^{++}) = 14.77\pm0.25^{+0.18}_{-0.30}\) MeV/\(c^{2}\), where the first uncertainties are statistical and the second are systematic. The isospin mass splittings are measured to be \(M(\Sigma_{c}(2455)^{++})-M(\Sigma_{c}(2455)^{0})=0.22\pm0.01\pm0.01\) MeV/\(c^{2}\) and \(M(\Sigma_{c}(2520)^{++})-M(\Sigma_{c}(2520)^{0})=0.01\pm0.15\pm0.03\) MeV/\(c^{2}\). These results are the most precise to date.
We observe \(D^0-\bar{D}^0\) mixing in the decay \(D^0\rightarrow K^+\pi^-\) using a data sample of integrated luminosity 976 fb\(^{-1}\) collected with the Belle detector at the KEKB \(e^+e^-\) ...asymmetric-energy collider. We measure the mixing parameters \({x'}^2 = (0.09\pm0.22)\times 10^{-3}\) and \(y' = (4.6\pm3.4)\times 10^{-3}\) and the ratio of doubly Cabibbo-suppressed to Cabibbo-favored decay rates \(R_D = (3.53\pm0.13)\times 10^{-3}\), where the uncertainties are statistical and systematic combined. Our measurement excludes the no-mixing hypothesis at the 5.1 standard deviation level.
We report a measurement of the branching fraction of the decay \(B^0_s\to J/\psi\,\phi(1020)\), evidence and a branching fraction measurement for \(B^0_s\to J/\psi\,f'_2(1525)\), and the ...determination of the total \(B^0_s\to J/\psi\,K^+K^-\) branching fraction, including the resonant and non-resonant contributions to the \(K^+K^-\) channel. We also determine the \(S\)-wave contribution within the \(\phi(1020)\) mass region. The absolute branching fractions are \(\mathcal{B}B^0_s\to J/\psi\,\phi(1020)=(1.25 \pm 0.07\left(\mathrm{stat}\right)\pm 0.08\left(\mathrm{syst}\right)\pm 0.22\left(f_s\right))\times 10^{-3}\), \(\mathcal{B}B^0_s\to J/\psi\,f'_2(1525)=(0.26\pm 0.06\left(\mathrm{stat}\right)\pm 0.02\left(\mathrm{syst}\right) \pm 0.05\left(f_s\right))\times 10^{-3}\) and \(\mathcal{B}B^0_s\to J/\psi\,K^+K^- = (1.01\pm 0.09\left(\mathrm{stat}\right) \pm 0.10\left(\mathrm{syst}\right)\pm 0.18\left(f_s\right))\times 10^{-3}\), where the last systematic error is due to the branching fraction of \(b\bar{b}\to B^{(*)}_s B^{(*)}_s\). The branching fraction ratio is found to be \(\mathcal{B}B^0_s\to J/\psi\,f'_2(1525)/\mathcal{B}B^0_s\to J/\psi\,\phi(1020)=(21.5\pm 4.9\left(\mathrm{stat}\right) \pm2.6\left(\mathrm{syst}\right))%\). All results are based on a 121.4 fb\(^{-1}\) data sample collected at the \(\Upsilon(5S)\) resonance by the Belle experiment at the KEKB asymmetric-energy \(e^+e^-\) collider.
The cross sections of the processes \(e^+ e^- \to K^+ K^- J/\psi\) and \(K_S^0K_S^0J/\psi\) are measured via initial state radiation at center-of-mass energies between the threshold and 6.0~GeV using ...a data sample of 980~fb\(^{-1}\) collected with the Belle detector on or near the \(\Upsilon(nS)\) resonances, where \(n=\)1, 2, ..., 5. The cross sections for \(e^+ e^- \to K^+ K^- J/\psi\) are at a few pb level and the average cross section for \(e^+ e^- \to K_S^0K_S^0J/\psi\) is \(1.8\pm 0.6 (\rm stat.)\pm 0.3 (\rm syst.)\)~pb between 4.4 and 5.2~GeV. All of them are consistent with previously published results with improved precision. A search for resonant structures and associated intermediate states in the cross section of the process \(e^+ e^- \to K^+ K^- J/\psi\) is performed.
The lifetime of the \(\tau\)-lepton is measured using the process \(e^+e^-\rightarrow\tau^+\tau^-\), where both \(\tau\)-leptons decay to \(3\pi\nu_\tau\). 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 \(|\Delta\tau| / \tau < 7.0 \times 10^{-3}\) at 90% CL.
Using samples of 102 million \(\Upsilon(1S)\) and 158 million \(\Upsilon(2S)\) events collected with the Belle detector, we study exclusive hadronic decays of these two bottomonium resonances to ...\(\ks K^+ \pi^-\) and charge-conjugate (c.c.) states, \(\pi^+ \pi^- \pi^0 \pi^0\), and \(\pi^+ \pi^- \pi^0\), and to the two-body Vector-Pseudoscalar (\(K^{\ast}(892)^0\bar{K}^0+ {\rm c.c.}\), \(K^{\ast}(892)^-K^+ + {\rm c.c.}\), \(\omega\pi^0\), and \(\rho\pi\)) final states. For the first time, signals are observed in the modes \(\Upsilon(1S) \to \ks K^+ \pi^- + {\rm c.c.}\), \(\pi^+ \pi^- \pi^0 \pi^0\), and \(\Upsilon(2S) \to \pi^+ \pi^- \pi^0 \pi^0\), and evidence is found for the modes \(\Upsilon(1S)\to \pi^+ \pi^- \pi^0\), \(K^{\ast}(892)^0 \bar{K}^0+ {\rm c.c.}\), and \(\Upsilon(2S) \to \ks K^+ \pi^- + {\rm c.c.}\) Branching fractions are measured for all the processes, while 90% confidence level upper limits on the branching fractions are also set for the modes with a statistical significance of less than \(3\sigma\). The ratios of the branching fractions of \(\Upsilon(2S)\) and \(\Upsilon(1S)\) decays into the same final state are used to test a perturbative QCD prediction for OZI-suppressed bottomonium decays.
We report the measurements of branching fractions and CP violation asymmetries in \(B^0 \to \phi K^{*}\) decays obtained in an angular analysis using the full 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 perform a partial wave analysis to distinguish among scalar \(B^0 \to \phi(K\pi)^{*}_{0}\), vector \(B^0 \to \phi K^{*}(892)^{0}\) and tensor \(B^0 \to \phi K^{*}_{2}(1430)^{0}\) components, and determine the corresponding branching fractions to be \(\mathcal{B}B^0 \to \phi(K\pi)^{*}_{0} = (4.3 \pm 0.4 \pm 0.4) \times 10^{-6}\), \(\mathcal{B}B^0 \to \phi K^{*}(892)^{0} = (10.4 \pm 0.5 \pm 0.6) \times 10^{-6}\) and \(\mathcal{B}B^0 \to \phi K^{*}_{2}(1430)^{0} = (5.5 ^{+0.9}_{-0.7} \pm 1.0) \times 10^{-6}\). We also measure the longitudinal polarization fraction \(f_L\) in \(B^0 \to \phi K^{*}(892)^{0}\) and \(B^0 \to \phi K^{*}_{2}(1430)^{0}\) decays to be \(0.499 \pm 0.030 \pm 0.018\) and \(0.918 ^{+0.029}_{-0.060} \pm 0.012\), respectively. The first quoted uncertainties are statistical and the second are systematic. In total, we measure 26 parameters related to branching fractions, polarization and CP violation in the \(B^0 \to \phi K^{*}\) system. No evidence for CP violation is found.
Using data samples of 89 fb\(^{-1}\), 703 fb\(^{-1}\), and 121 fb\(^{-1}\) collected with the Belle detector at the KEKB asymmetric-energy \(e^+e^-\) collider at center-of-mass energies 10.52 GeV, ...10.58 GeV, and 10.876 GeV, respectively, we study the exclusive reactions \(e^+e^- \to \omega\pi^0\), \(K^{\ast}(892)\bar{K}\), and \(K_2^{\ast}(1430)\bar{K}\) (Charge-conjugate modes are included implicitly). Significant signals of \(\omega\pi^0\), \(K^{\ast}(892)^0\bar{K}^0\), and \(K_2^{\ast}(1430)^-K^+\) are observed for the first time at these energies, and the energy dependencies of the cross sections are presented. On the other hand, no significant excesses for \(K^{\ast}(892)^-K^+\) and \(K_2^{\ast}(1430)^0 \bar{K}^0\) are found, and we set limits on the cross section ratios \(R_{\rm VP} = \frac{\sigma_B(e^+e^-\to K^{\ast}(892)^0\bar K^0)} {\sigma_B(e^+e^-\to K^{\ast}(892)^-K^+)}>\) 4.3, 20.0, and 5.4, and \(R_{\rm TP} = \frac{\sigma_B(e^+e^-\to K_2^{\ast}(1430)^0\bar K^0)} {\sigma_B(e^+e^-\to K_2^{\ast}(1430)^-K^+)}<\) 1.1, 0.4, and 0.6, for center-of-mass energies of 10.52 GeV, 10.58 GeV, and 10.876 GeV, respectively, at the 90% C.L.
We have studied the e+e- -> omegapi0 cross section in the sqrt(s) interval 1000-1030 MeV using the pi+pi-pi0pi0 and pi0pi0gamma final states with a sample of ~600 pb^-1 collected with the KLOE ...detector at DAFNE. By fitting the observed interference pattern around M_phi for both final states, we extract the ratio of the decay widths Gamma(omega->pi0gamma)/Gamma(omega->pi+pi-pi0) = 0.0897 +- 0.0016 and derive the branching fractions BR(omega -> pi+pi-pi0)= (90.24 +- 0.19)%, BR(omega -> pi0gamma) = (8.09 +- 0.14)%. The parameters describing the e+e- -> omegapi0 reaction around M_\phi are also used to extract the branching fraction for the OZI and G-parity violating phi -> omegapi0 decay: BR(phi->omegapi0) = (4.4 +- 0.6)x10^-5.
