arXiv subjects

K. Cho

Publications and source records attributed to K. Cho.

At least 505 records · Page 28Linked to original sources

arXiv2013

Measurements of branching fractions of leptonic and hadronic Ds+ meson decays and extraction of the Ds+ meson decay constant

We present measurements of absolute branching fractions of hadronic and leptonic Ds+ decays to K- K+ pi+, anti-K0 K+, eta pi+, mu+ nu_mu and tau+ nu_tau and report a search for the leptonic Ds+ to e+ nu_e decays. The results are obtained from a data sample of 913 fb^-1 collected at or near the Y(4S) and Y(5S) resonances with the Belle detector at the KEKB asymmetric-energy e+e- collider. The branching fractions of hadronic decays are measured to be Br(Ds+ -> K- K+ pi+) = (5.06 +- 0.15 +- 0.21)%, Br(Ds+ -> anti-K0 K+) = (2.95 +- 0.11 +- 0.09)%, and Br(Ds+ -> eta pi+) = (1.82 +- 0.14 +- 0.07)%, where the first and second uncertainties are statistical and systematic, respectively. The branching fractions of leptonic decays are measured to be Br(Ds+ -> mu+ nu_mu) = (0.531 +- 0.028 +- 0.020)%, and Br(Ds+ -> tau+ nu_tau) = (5.70 +- 0.21 +0.31-0.30)%, which are combined to determine the Ds+ meson decay constant f_Ds = (255.5 +- 4.2 +- 5.1) MeV. We find no significant signal for Ds+ -> e+ nu_e decays and set an upper limit of Br(Ds+ -> e+ nu_e) < 1.0 (0.83) x 10^-4 at 95% (90%) confidence level.

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arXiv2013

Search for the production of ZW and ZZ boson pairs decaying into charged leptons and jets in proton-antiproton collisions at sqrt(s)=1.96 TeV

We present a measurement of the production cross section for ZW and ZZ boson pairs in final states with a pair of charged leptons, from the decay of a Z boson, and at least two jets, from the decay of a W or Z boson, using the full sample of proton-antiproton collisions recorded with the CDF II detector at the Tevatron, corresponding to 8.9 fb^(-1) of integrated luminosity. We increase the sensitivity to vector boson decays into pairs of quarks using a neural network discriminant that exploits the differences between the spatial spread of energy depositions and charged-particle momenta contained within the jet of particles originating from quarks and gluons. Additionally, we employ new jet energy corrections to Monte Carlo simulations that account for differences in the observed energy scales for quark and gluon jets. The number of signal events is extracted through a simultaneous fit to the dijet mass spectrum in three classes of events: events likely to contain jets with a heavy-quark decay, events likely to contain jets originating from light quarks, and events that fail these identification criteria. We determine the production cross section to be 2.5 +2.0 -1.0 pb (< 6.1 pb at the 95% confidence level), consistent with the standard model prediction of 5.1 pb.

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arXiv2013

Angular analysis of $B^0 \to ϕK^{*}$ decays and search for CP violation at Belle

We report the measurements of branching fractions and CP violation asymmetries in $B^0 \to ϕK^{*}$ decays obtained in an angular analysis using the full data sample of $772 \times 10^6 B\bar{B}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We perform a partial wave analysis to distinguish among scalar [$B^0 \to ϕ(Kπ)^{*}_{0}$], vector [$B^0 \to ϕK^{*}(892)^{0}$] and tensor [$B^0 \to ϕK^{*}_{2}(1430)^{0}$] components, and determine the corresponding branching fractions to be $\mathcal{B}[B^0 \to ϕ(Kπ)^{*}_{0}] = (4.3 \pm 0.4 \pm 0.4) \times 10^{-6}$, $\mathcal{B}[B^0 \to ϕK^{*}(892)^{0}] = (10.4 \pm 0.5 \pm 0.6) \times 10^{-6}$ and $\mathcal{B}[B^0 \to ϕK^{*}_{2}(1430)^{0}] = (5.5 ^{+0.9}_{-0.7} \pm 1.0) \times 10^{-6}$. We also measure the longitudinal polarization fraction $f_L$ in $B^0 \to ϕK^{*}(892)^{0}$ and $B^0 \to ϕK^{*}_{2}(1430)^{0}$ 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 $B^0 \to ϕK^{*}$ system. No evidence for CP violation is found.

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arXiv2013

Search for new physics in trilepton events and limits on the associated chargino-neutralino production at CDF

We perform a search for new physics using final states consisting of three leptons and a large imbalance in transverse momentum resulting from proton-antiproton collisions at 1.96 TeV center-of-mass energy. We use data corresponding to 5.8 fb-1 of integrated luminosity recorded by the CDF II detector at the Tevatron collider. Our main objective is to investigate possible new low-momentum (down to 5 GeV/c) multi-leptonic final states not investigated by LHC experiments. Relative to previous CDF analyses, we expand the geometric and kinematic coverage of electrons and muons and utilize tau leptons that decay hadronically. Inclusion of tau leptons is particularly important for supersymmetry (SUSY) searches. The results are consistent with standard-model predictions. By optimizing our event selection to increase sensitivity to the minimal supergravity (mSUGRA) SUSY model, we set limits on the associated production of chargino and neutralino, the SUSY partners of the electroweak gauge bosons. We exclude cross sections up to 0.1 pb and chargino masses up to 168 GeV/c^2 at 95% CL, for a suited set of mSUGRA parameters. We also exclude a region of the two-dimensional space of the masses of the neutralino and the supersymmetric partner of the tau lepton, not previously excluded at the Tevatron.

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arXiv2013

Measurement of $e^+ e^- \to ωπ^0$, $K^{\ast}(892)\bar{K}$ and $K_2^{\ast}(1430)\bar{K}$ at $\sqrt{s}$ near 10.6 GeV

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 $e^+e^- \to ωπ^0$, $K^{\ast}(892)\bar{K}$, and $K_2^{\ast}(1430)\bar{K}$ (Charge-conjugate modes are included implicitly). Significant signals of $ωπ^0$, $K^{\ast}(892)^0\bar{K}^0$, 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 $R_{\rm VP} = \frac{σ_B(e^+e^-\to K^{\ast}(892)^0\bar K^0)} {σ_B(e^+e^-\to K^{\ast}(892)^-K^+)}>$ 4.3, 20.0, and 5.4, and $R_{\rm TP} = \frac{σ_B(e^+e^-\to K_2^{\ast}(1430)^0\bar K^0)} {σ_B(e^+e^-\to K_2^{\ast}(1430)^-K^+)}<$ 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.

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arXiv2013

Search for Bottomonium States in Exclusive Radiative Upsilon(2S) Decays

We search for bottomonium states in Upsilon(2S)-> (bb-bar) gamma decays with an integrated luminosity of 24.7fb^-1 recorded at the Upsilon(2S) resonance with the Belle detector at KEK, containing (157.8+-3.6) X 10^6 Upsilon(2S) events. The (bb-bar) system is reconstructed in 26 exclusive hadronic final states composed of charged pions, kaons, protons, and K^0_S mesons. We find no evidence for the state recently observed around 9975 MeV (X_(bb-bar)) in an analysis based on a data sample of 9.3 X 10^6 Upsilon(2S) events collected with the CLEO III detector. We set a 90 % confidence-level upper limit on the branching fraction B[Upsilon(2S)-> X_(bb-bar) gamma] X \sum_i{B[X_(bb-bar)-> h_i]}< 4.9 X 10^-6, summed over the exclusive hadronic final states employed in our analysis. This result is an order of magnitude smaller than the measurement reported with CLEO data. We also set an upper limit for the eta_b(1S) state of B[Upsilon(2S)-> eta_b(1S) gamma] X \sum_i{B[eta_b(1S)-> h_i]}< 3.7 X 10^-6.

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arXiv2013

First Observation of the Zb0(10610) in a Dalitz Analysis of Y(5S) -> Y(nS) pi0 pi0

We report the first observation of Y(5S) -> Y(nS) pi0 pi0 decays. The neutral partner of the Zb+-(10610), the Zb0(10610) decaying to Y(2,3S) pi0, is observed for the first time with a 6.5 sigma significance using a Dalitz analysis of Y(5S) -> Y(2,3S) pi0 pi0 decays. The results are obtained with a 121.4 fb^-1 data sample collected with the Belle detector at the Y(5S) resonance at the KEKB asymmetric-energy e+e- collider.

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arXiv2013

Evidence for a bottom baryon resonance Lambda_b* in CDF data

Using data from proton-antiproton collisions at Ecms=1.96 TeV recorded by the CDF II detector at the Fermilab Tevatron, evidence for the excited resonance state Lambda_b* is presented in its Lambda_b0 pi+ pi- decay, followed by the Lambda_b0 -->Lambda_c+ (-->proton K- pi+) pi- decays. The analysis is based on a data sample corresponding to an integrated luminosity of 9.6/fb collected by an online event selection based on charged-particle tracks displaced from the proton-antiproton interaction point. The significance of the observed signal is 3.5 Gaussian sigmas. The mass of the observed state is found to be 5919.22 +- 0.76 MeV in agreement with similar findings in proton-proton collision experiments.

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arXiv2013

Precision measurement of charged pion and kaon differential cross sections in electron-positron annihilation at Q = 10.52 GeV

Measurements of inclusive differential cross sections for charged pion and kaon production in electron-positron annihilation have been carried out at a center-of-mass energy of Q = 10.52 GeV. The measurements were performed with the Belle detector at the KEKB electron-positron collider using a data sample containing 113 million e+e- -> qqbar events, where q={u,d,s,c}. We present charge-integrated differential cross sections dσ_h+-/dz for h+- = pi+-, K+- as a function of the relative hadron energy z = 2*E_h / sqrt{s} from 0.2 to 0.98. The combined statistical and systematic uncertainties for pi+- (K+-) are 4% (4%) at z ~ 0.6 and 15% (24%) at z ~ 0.9. The cross sections are the first measurements of the z-dependence of pion and kaon production for z > 0.7 as well as the first precision cross section measurements at a center-of-mass energy far below the Z^0 resonance used by the experiments at LEP and SLC.

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arXiv2013

Measurement of the leptonic asymmetry in ttbar events produced in ppbar collisions at sqrt(s)=1.96 TeV

We measure the asymmetry in the charge-weighted rapidity of the lepton in semileptonic ttbar decays recorded with the CDF II detector using the full Tevatron Run II sample, corresponding to an integrated luminosity of 9.4/fb. A parametrization of the asymmetry as a function of the charge-weighted rapidity is used to correct for the finite acceptance of the detector and recover the production-level asymmetry. The result of afb(lep) = 0.094 +0.032 -0.029 is to be compared to the standard model next-to-leading-order prediction of afb(lep) = 0.038 +-0.003.

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arXiv2013

Combination of CDF and D0 W-Boson Mass Measurements

We summarize and combine direct measurements of the mass of the $W$ boson in $\sqrt{s} = 1.96 \text{TeV}$ proton-antiproton collision data collected by CDF and D0 experiments at the Fermilab Tevatron Collider. Earlier measurements from CDF and D0 are combined with the two latest, more precise measurements: a CDF measurement in the electron and muon channels using data corresponding to $2.2 \mathrm{fb}^{-1}$ of integrated luminosity, and a D0 measurement in the electron channel using data corresponding to $4.3 \mathrm{fb}^{-1}$ of integrated luminosity. The resulting Tevatron average for the mass of the $W$ boson is $\MW = 80\,387 \pm 16 \text{MeV}$. Including measurements obtained in electron-positron collisions at LEP yields the most precise value of $\MW = 80\,385 \pm 15 \text{MeV}$.

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arXiv2013

Measurement of the Wrong-Sign Decay D0 -> K+ pi- pi+ pi-

A measurement of the rate for the "wrong-sign" decay D0 -> K+ pi- pi+ pi- relative to that for the "right-sign" decay D0 -> K- pi+ pi+ pi- is presented. Using 791 fb-1 of data collected with the Belle detector, we obtain a branching fraction ratio of R_WS = [0.324 +- 0.008 (stat) +- 0.007 (sys)]%. Multiplying this ratio by the world average value for the branching fraction B(D0 -> K- pi+ pi+ pi-) gives a branching fraction B(D0 -> K+ pi- pi+ pi-) = (2.61 +- 0.06 +0.09 -0.08) x 10-4.

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arXiv2013

Evidence of a new narrow resonance decaying to $χ_{c1}γ$ in $B \to χ_{c1} γK$

We report measurements of $B \to χ_{c1} γK$ and $χ_{c2} γK$ decays using $772 \times 10^{6}$ $B\bar{B}$ events collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. Evidence of a new resonance in the $χ_{c1} γ$ final state is found with a statistical significance of $3.8 σ$. This state has a mass of $3823.1 \pm 1.8 {(stat)} \pm 0.7 {(syst)}$ MeV/$c^2$, a value that is consistent with theoretical expectations for the previously unseen $1^3 D_2$ $c\bar{c}$ meson. We find no other narrow resonance and set upper limits on the branching fractions of the $X(3872) \to χ_{c1} γ$ and $χ_{c2} γ$ decays.

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arXiv2013

Precise measurement of the branching fractions for Bs->Ds()+ Ds()- and first measurement of the Ds+ Ds- polarization using e+e- collisions

We have made a precise measurement of the absolute branching fractions of Bs->Ds(*)+ Ds(*)- decays using 121.4 fb-1 of data recorded by the Belle experiment running on the Upsilon(5S) resonance. The results are B(Bs0->Ds+ Ds-) = (0.58 +0.11 -0.09 +-0.13)%, B(Bs0->Ds*+- Ds-+) = (1.8 +-0.2 +-0.4)%, and B(Bs0->Ds*+ Ds*-) = (2.0 +-0.3 +-0.5)%; the sum is B(Bs0->Ds(*)+ Ds(*)- = (4.3 +-0.4 +-1.0)%. Assuming Bs->Ds(*)+ Ds(*)- saturates decays to CP-even final states, the branching fraction constrains the ratio Delta(Gamma_s)/cos(phi_s), where Delta(Gamma_s) is the difference in widths between the two Bs0-Bs0bar mass eigenstates, and phi_s is the CP-violating phase in Bs0-Bs0bar mixing. For phi_s=0 we obtain Delta(Gamma_s)/Gamma_s = 0.090 +-0.009 (stat.) +-0.023 (syst.), where Gamma_s is the mean decay width. We also measure for the first time the longitudinal polarization fraction of Bs->Ds*+ Ds*-; the result is 0.12 +0.30 -0.28 +0.04 -0.05.

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arXiv2013

Measurement of exclusive $Υ(1S)$ and $Υ(2S)$ decays into Vector-Pseudoscalar final states

Using samples of 102 million $Υ(1S)$ and 158 million $Υ(2S)$ events collected with the Belle detector, we study exclusive hadronic decays of these two bottomonium resonances to $\ks K^+ π^-$ and charge-conjugate (c.c.) states, $π^+ π^- π^0 π^0$, and $π^+ π^- π^0$, and to the two-body Vector-Pseudoscalar ($K^{\ast}(892)^0\bar{K}^0+ {\rm c.c.}$, $K^{\ast}(892)^-K^+ + {\rm c.c.}$, $ωπ^0$, and $ρπ$) final states. For the first time, signals are observed in the modes $Υ(1S) \to \ks K^+ π^- + {\rm c.c.}$, $π^+ π^- π^0 π^0$, and $Υ(2S) \to π^+ π^- π^0 π^0$, and evidence is found for the modes $Υ(1S)\to π^+ π^- π^0$, $K^{\ast}(892)^0 \bar{K}^0+ {\rm c.c.}$, and $Υ(2S) \to \ks K^+ π^- + {\rm c.c.}$ 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 $3σ$. The ratios of the branching fractions of $Υ(2S)$ and $Υ(1S)$ decays into the same final state are used to test a perturbative QCD prediction for OZI-suppressed bottomonium decays.

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arXiv2013

A signature-based search for delayed photons in exclusive photon plus missing transverse energy events from $p \bar{p}$ collisions with $\sqrt{s} = 1.96$ TeV

We present the first signature-based search for delayed photons using an exclusive photon plus missing transverse energy final state. Events are reconstructed in a data sample from the CDF II detector corresponding to $6.3 \text{fb}^{-1}$ of integrated luminosity from $\sqrt{s}=1.96$ TeV proton-antiproton collisions. Candidate events are selected if they contain a photon with an arrival time in the detector larger than expected from a promptly-produced photon. The mean number of events from standard model sources predicted by the data-driven background model based on the photon timing distribution is $286 \pm 24$. A total of 322 events are observed. A $p$-value of 12% is obtained, showing consistency of the data with standard model predictions.

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arXiv2013

Study of Exclusive B->X_u l nu Decays and Extraction of |V_{ub}| using Full Reconstruction Tagging at the Belle Experiment

We report the results of a study of the exclusive semileptonic decays B->pi0lnu, B->pi+lnu, B->rho0lnu, B->rho+lnu and B->omegalnu, where l represents an electron or a muon. The events are tagged by fully reconstructing a second B meson in the event in a hadronic decay mode. The measured branching fractions are Br(B->pi0lnu) = (0.80 +- 0.08 +- 0.04)x10^-4, Br(B->pi+lnu) = (1.49 +- 0.09 +- 0.07)x10^-4, Br(B->rho0lnu) = (1.83 +- 0.10 +- 0.10)x10^-4, Br(B->rho+lnu) = (3.22 +- 0.27 +- 0.24)x10^-4, and Br(B->omegalnu) =(1.07 +- 0.16 +- 0.07)x10^-4, where the first error is statistical and the second one is systematic. The obtained branching fractions are inclusive of soft photon emission. We also determine the branching fractions as a function of the 4-momentum transfer squared to the leptonic system q^2=(p_l+p_nu)^2, where p_l and p_nu are the lepton and neutrino 4-momenta, respectively. Using the pion modes, a recent LCSR calculation, lattice QCD results and a model-independent description of the hadronic form factor, a value of the CKM matrix element |V_{ub}| = (3.52 +- 0.29)x10^-3 is extracted. For the first time, a charmless state with invariant mass greater than 1 GeV/c^2, which might be dominated by the decay B->f2lnu, is observed. These results are obtained from a 711 1/fb data sample that contains 772 millions BB pairs, collected near the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^+ e^- collider.

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arXiv2013

Search for B -> h(*) nu nubar with the full Belle Y(4S) data sample

We report a search for the rare decays B -> h(*) nu nubar, where h(*) stands for K+, Ks0, K*+, K*0, pi+, pi0, rho+, rho0 and phi. The results are obtained from a 711 fb-1 data sample that contains 772 million BB pairs collected at the Y(4S) resonance with the Belle detector at the KEKB e+e- collider. We search for signal candidates by fully reconstructing a hadronic decay of the accompanying B meson and requiring a single h(*) meson left on the signal side. No significant signal is observed and we set upper limits on the branching fractions at 90% confidence level. The measurements of B+ -> K*+ nu nubar, B+ -> pi+ nu nubar, B0 -> pi0 nu nubar and B0 -> rho^0 nu nubar provide the world's currently most restrictive limits.

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