Search Results (136 total)

We measured the lifetime and the mesonic and nonmesonic decay rates of the _Λ⁴He hypernucleus. The hypernuclei were created using a 750 MeV/c momentum K^- beam on a liquid ⁴He target by the reaction ⁴He (K^-,π^-)_Λ⁴He. The _Λ⁴He lifetime was directly measured using protons from Λp→np nonmesonic decay (also referred to as proton-stimulated decay) and was found to have a value of τ=245±24 ps. The mesonic decay rates were determined from the observed numbers of π^-'s and π^○'s as Γ_π^-/Γ_tot=0.270±0.024 and Γ_π^○/Γ_tot=0.564±0.036, respectively, and the values of the proton- and neutron-stimulated decay rates were extracted as Γ_p/Γ_tot=0.169±0.019 and Γ_n/Γ_tot≤0.032 (95% CL), respectively. The effects of final-state interactions and possible three-body ΛNN decay contributions were studied in the context of a simple model of nucleon-stimulated decay. Nucleon-nucleon coincidence events were observed and were used in the determination of the nonmesonic branching fractions. The implications of the results of this analysis were considered for the empirical ΔI=1 / 2 rule and the decay rates of the _Λ⁴H hypernucleus.

Using the CLEO II detector at the Cornell Electron Storage Ring we have measured the Ξ_c⁰ decay asymmetry parameter in the decay Ξ_c⁰→Ξ^-π⁺. We find α_Ξc⁰α_Ξ=0.26±0.18(stat)_-0.04^+0.05(syst); using the world average value of α_Ξ=-0.456±0.014 we obtain α_Ξc⁰=-0.56±0.39(stat)_-0.09^+0.10(syst). The physically allowed range of a decay asymmetry parameter is -1<α<+1. Our result prefers a negative value: α_Ξc⁰ is <0.1 at the 90% C.L. The central value occupies the middle of the theoretically expected range but is not yet precise enough to choose between models.

From a data sample of 29058 τ^±→π^±π⁺π^-π⁰ν_τ decays observed in the CLEO detector we derive a 95% confidence upper limit on the tau neutrino mass of 28 MeV.

Using a sample of 3.3×10⁶ Υ(4S)→BB̅ events collected with the CLEO II detector at the Cornell Electron Storage Ring (CESR), we measure B(B→ρlν), |V_ub|, and the partial rate (ΔΓ) in three bins of q²≡(p_B-p_ρ)². We find B(B⁰→ρ^-l⁺ν)=(2.69±0.41_-0.40^+0.35±0.50)×10^-4, |V_ub|=(3.23±0.24_-0.26^+0.23±0.58)×10^-3, ΔΓ(0<q²<7 GeV²/c⁴)=(7.6±3.0_-1.2^+0.9±3.0)×10^-2 ns^-1, ΔΓ(7<q²<14 GeV²/c⁴)=(4.8±2.9_-0.8^+0.7±0.7)×10^-2 ns^-1, and ΔΓ(14<q²<21 GeV²/c⁴)=(7.1±2.1_-1.1^+0.9±0.6)×10^-2 ns^-1. Here, l=e or μ, but not both, and the errors are statistical, systematic, and theoretical. The method is sensitive primarily to B→ρlν decays with leptons in the energy range above 2.3 GeV. Averaging with the previously published CLEO results for B→ρlν, we obtain B(B⁰→ρ^-l⁺ν)=(2.57±0.29_-0.46^+0.33±0.41)×10^-4 and |V_ub|=(3.25±0.14_-0.29^+0.21±0.55)×10^-3.

Based on a sample corresponding to 4.3×10⁶ produced τ-pair events, we have studied hadronic dynamics in the decay τ^-→ν_τπ^-π⁰π⁰ in data recorded by the CLEO II detector operating at the CESR e⁺e^- collider. The decay is dominated by the process τ^-→ν_τa₁^-(1260), with the a₁^- meson decaying to three pions predominantly via the lowest dimensional (mainly S-wave) ρ^-π⁰ Born amplitude. From model-dependent fits to the Dalitz plot and angular observables in bins of 3π mass, we find significant additional contributions from amplitudes for a₁ decay to σπ, f₀(1370)π, and f₂(1270)π, as well as higher dimensional a₁→ρπ and ρ^′π amplitudes. Notably, the squared σπ amplitude accounts for approximately 15% of the total τ^-→ν_τπ^-π⁰π⁰ rate in the models considered. The data are well described using couplings to these amplitudes that are independent of the 3π mass. These amplitudes also provide a good description for the τ^-→ν_τπ^-π⁺π^- Dalitz plot distributions. We have searched for additional contributions from τ^-→ν_τπ^′-(1300). We place 90% confidence level upper limits on the branching fraction for this channel of between 1.0×10^-4 and 1.9×10^-4, depending on the π^′ decay mode considered. The π^-π⁰π⁰ mass spectrum is parametrized by a Breit-Wigner form with a mass-dependent width which is specified according to the results of the Dalitz plot fits plus an unknown coupling to an a₁→K^*K amplitude. From a χ² fit using this parametrization, we extract the pole mass and width of the a₁, as well as the magnitude of the K^*K coupling. We have also investigated the impact of a possible contribution from the a₁^′(1700) meson on this spectrum. Finally, exploiting the parity-violating angular asymmetry in a₁→3π decay, we determine the signed value of the τ neutrino helicity to be h_ντ=-1.02±0.13 (stat)±0.03 (syst+model), confirming the left-handedness of the τ neutrino.

Using a sample of 4.7 fb^-1 integrated luminosity accumulated with the CLEO-II detector at the Cornell Electron Storage Ring (CESR), we have measured the ratios of the branching fractions B(τ^-→K^-h⁺π^-ν_τ)/B(τ^-→h^-h⁺h^-ν_τ)=(5.16±0.20±0.50)×10^-2, B(τ^-→K^-h⁺π^-π⁰ν_τ)/B(τ^-→h^-h⁺h^-π⁰ν_τ)=(2.54±0.44±0.39)×10^-2, B(τ^-→K^-K⁺π^-ν_τ)/B(τ^-→h^-h⁺h^-ν_τ)=(1.52±0.14±0.29)×10^-2, and the upper limit B(τ^-→K^-K⁺π^-π⁰ν_τ)/B(τ^-→h^-h⁺h^-π⁰ν_τ)<0.0154 at 95% C.L. Coupled with additional experimental information, we use our results to extract information on the structure of three-prong tau decays to charged kaons.

Using data recorded by the CLEO II detector at the Cornell Electron Storage Ring, we report evidence for two new charmed baryons, one decaying into Ξ_c⁺π⁺π^- via an intermediate Ξ_c^*0 and its isospin partner decaying into Ξ_c⁰π⁺π^- via an intermediate Ξ_c^*+. We measure the mass differences of the two states to be M(Ξ_c⁺π⁺π^-)-M(Ξ_c⁺)  =  348.6±0.6±1.0 MeV, and M(Ξ_c⁰π⁺π^-)-M(Ξ_c⁰)  =  347.2±0.7±2.0 MeV. We interpret these new states as the J^P  =  3 / 2^- Ξ_c1 particles, the charmed-strange analogs of the Λ_c1⁺(2625).

We report measurements of the D⁰, D⁺, and D_s⁺ meson lifetimes using 3.7 fb^-1 of e⁺e^- annihilation data collected near the ϒ(4S) resonance with the CLEO detector. The measured lifetimes of the D⁰, D⁺, and D_s⁺ mesons are 408.5±4.1_-3.4^+3.5 fs, 1033.6±22.1_-12.7^+9.9 fs, and 486.3±15.0_-5.1^+4.9 fs. The precisions of these lifetimes are comparable to those of the best previous measurements, and the systematic errors are very different. In a single experiment we find that the ratio of the D_s⁺ and D⁰ lifetimes is 1.19±0.04.

Using a sample of 3.3×10⁶ B-meson decays collected with the CLEO detector at the Cornell Electron Storage Ring, we have studied B^-→D⁰ℓ^-ν̅ and B̅ ⁰→D⁺ℓ^-ν̅ decays, where ℓ^- can be either e^- or μ^-. We distinguish B→Dℓν from other B semileptonic decays by examining the net momentum and energy of the particles recoiling against D-ℓ pairs. We find Γ(B→Dℓν)  =  (14.1±1.0±1.2) ns^-1 and derive branching fractions for B^-→D⁰ℓ^-ν̅ and B̅ ⁰→D⁺ℓ^-ν̅ of (2.32±0.17±0.20)% and (2.20±0.16±0.19)%, respectively, where the uncertainties are statistical and systematic. We also investigate the B→Dℓν form factor and the implication of the result for |V_cb|.

We report the results of a search for the rare baryonic decay modes B⁰→ΛΛ̅ , B⁺→Λ̅ p, B⁰→Λ̅ pπ^-, and B⁰→pp̅ (and their charge conjugate states) using 5.8×10⁶ BB̅ pairs collected with the CLEO detector. We see no statistically significant signals in any of these modes and set 90% confidence level upper limits on their branching fractions, B(B⁰→ΛΛ̅ )<3.9×10^-6, B(B⁺→Λ̅ p)<2.6×10^-6, B(B⁰→Λ̅ pπ^-)<1.3×10^-5, and B(B⁰→pp̅ )<7.0×10^-6.

We have observed four fully reconstructed B⁰→D^*+D^*- candidates in 5.8×10⁶ ϒ(4S)→BB̅ decays recorded with the CLEO detector. The background is estimated to be 0.31±0.10 events. The probability that the background could produce four or more signal candidates with the observed distribution among D^*+ and D^*- decay modes is 1.1×10^-4. The measured decay rate, B(B⁰→D^*+D^*-)  =  [6.2_-2.9^+4.0(stat)±1.0(syst)]×10^-4, is large enough for this decay mode to be of interest for the measurement of a time-dependent CP asymmetry.

We have searched for five decay modes of the τ lepton that simultaneously violate lepton and baryon number: τ^-→p̅ γ, τ^-→p̅ π⁰, τ^-→p̅ η, τ^-→p̅ 2π⁰, and τ^-→p̅ π⁰η. The data used in the search were collected with the CLEO II detector at the Cornell Electron Storage Ring. The integrated luminosity of the data sample is 4.7 fb^-1, corresponding to the production of 4.3×10⁶ τ⁺τ^- events. No evidence is found for any of the decays, resulting in much improved upper limits on the branching fractions for the two-body decays and first upper limits for the three-body decays.

Using a 4.19 fb^-1 data sample collected with the CLEO II detector at the Cornell Electron Storage Ring, we have searched for dipion transitions between pairs of Υ resonances at center of mass energies E_c.m.=10.58 GeV and E_c.m.=10.52GeV. We obtain the 90% confidence level upper limits B(Υ(4S)→Υ(2S)π⁺π^-)<3.9×10^-4 and B(Υ(4S)→Υ(1S)π⁺π^-)<1.2×10^-4. We also observe the transitions Υ(3S)→Υ(1S)π⁺π^-, Υ(3S)→Υ(2S)π⁺π^-, and Υ(2S)→Υ(1S)π⁺π^-, from which we measure the cross sections for the radiative processes e⁺e^-→Υ(3S)γ and e⁺e^-→Υ(2S)γ. We obtain σ_{ee→Υ(3S)γ}=(17.8±3.0±1.7) pb and σ_{ee→Υ(2S)γ}=(15.5±1.3±1.4) pb at E_c.m.=10.58 GeV, and σ_{ee→Υ(3S)γ}=(27.3±5.0±2.6) pb and σ_{ee→Υ(2S)γ}=(16.3±1.8±1.5) pb at E_c.m.=10.52 GeV, which we compare with theoretical predictions.

We report the first observation of two narrow charmed strange baryons decaying to Ξ_c⁺γ and Ξ_c⁰γ, respectively, using data from the CLEO II detector at the Cornell Electron Storage Ring. We interpret the observed signals as the Ξ_c^+′ (c{su}) and Ξ_c^0′ (c{sd}), the symmetric partners of the well-established antisymmetric Ξ_c⁺ (c[su]) and Ξ_c⁰ (c[sd]). The mass differences M(Ξ_c^+′)-M(Ξ_c⁺) and M(Ξ_c^0′)-M(Ξ_c⁰) have been measured to be 107.8±1.7±2.5 and 107.0±1.4±2.5 MeV/c², respectively.

We present new measurements of photon energies and branching fractions for the radiative transitions Υ(2S)→γχ_b(J=0,1,2)(1P). The masses of the χ_b states are determined from the measured radiative photon energies. The ratio of mass splittings between the χ_b substates, r≡(M_J=2-M_J=1)/(M_J=1-M_J=0), with M the χ_b mass, provides information on the nature of the bb̅ confining potential. We find r(1P)=0.542±0.022±0.024. This value is somewhat lower than the previous world average, but more consistent with the theoretical expectation that r(1P)<r(2P); i.e., that this mass splitting ratio is smaller for the χ_b(1P) states than for the χ_b(2P) states.

We report on a study of exclusive radiative decays of the ϒ(1S) resonance collected with the CLEO II detector operating at the Cornell Electron Storage Ring. We present the first observation of the radiative decays ϒ(1S)→γπ⁺π^- and ϒ(1S)→γπ⁰π⁰. For the dipion mass regime m_ππ>1.0  GeV, we obtain B(ϒ(1S)→γπ⁺π^-)  =  (6.3±1.2±1.3)×10^-5 and B(ϒ(1S)→γπ⁰π⁰)  =  (1.7±0.6±0.3)×10^-5.

The decay τ^-→K^*-ην_τ has been observed with the CLEO II detector. The K^*- is reconstructed in two decay channels, K^*-→K_Sπ^-→π^-π⁺π^- and K^*-→K^-π⁰. The η is reconstructed from the decay η→γγ. The measured branching fraction is B(τ^-→K^*-ην_τ)  =  (2.9±0.8±0.4)×10^-4. We also measure the inclusive branching fractions without requiring the K^* resonance, B(τ^-→K_Sπ^-ην_τ)  =  (1.10±0.35±0.11)×10^-4 and B(τ^-→K^-π⁰ην_τ)  =  (1.77±0.56±0.71)×10^-4. The results indicate that the K^*- resonance dominates the K_Sπ^- mass spectrum.

We have performed the first search for CP violation in tau lepton decay. CP violation in lepton decay does not occur in the minimal standard model but can occur in extensions such as the multi-Higgs doublet model. It appears as a characteristic difference between the τ^- and τ⁺ decay angular distributions for the semileptonic decay modes such as τ^-→K⁰π^-ν. We define an observable asymmetry to exploit this and find no evidence for any CP violation.

The CLEO II detector at the e⁺e^- storage ring CESR has been used to search for two-photon production of the f_J(2220) decaying into π⁺π^-. No evidence for a signal is found in 4.77 fb^-1 of data and a 95% C.L. upper limit on [Γ_γγB_π⁺π^-]_fJ(2220) of 2.5 eV is set. If this result is combined with the BES Collaboration's measurement of f_J(2220)→π⁺π^- in radiative J/ψ decay and the recent CLEO result for [Γ_γγB_KS⁰KS⁰]_fJ(2220), a 95% C.L. lower limit on the stickiness of 102 is obtained. This result for the stickiness provides further support for a substantial neutral parton content in the f_J(2220).

In this paper we describe a search for four radiative decay modes of the D⁰ meson: D⁰→φγ, D⁰→ωγ, D⁰→K^*γ, and D⁰→ρ⁰γ. We obtain 90% C.L. upper limits on the branching ratios of these modes of 1.9×10^-4, 2.4×10^-4, 7.6×10^-4, and 2.4×10^-4, respectively.

We report the first observation of B→η^′X transitions with high momentum η^′ mesons. We observe 39.0±11.6 B decay events with 2.0<p_η^′<2.7  GeV/c, the high momentum region where background from b→c processes is suppressed. We discuss the physical interpretation of the signal, including the possibility that it is due to b→sg^* transitions. Given that interpretation, we find B(B→η^′X_s)  =  [6.2±1.6(stat)±1.3(syst)_-1.5^+0.0(bkg)]×10^-4 for 2.0<p_η^′<2.7  GeV/c.

Using a 73.6 pb^-1 data sample of Υ(2S) events collected with the CLEO II detector at the Cornell Electron Storage Ring, we have investigated the hadronic transitions between the Υ(2S) and the Υ(1S). The dipion transition Υ(2S)→Υ(1S)π⁺π^- was studied using two different analysis techniques. Selecting events in which Υ(1S)→e⁺e^-,μ⁺μ^- (“exclusive” analysis), and using the Υ(1S) leptonic branching fractions world averages from the PDG review, we obtained B(Υ(2S)→Υ(1S)π⁺π^-)=0.189±0.004±0.010, while using a method allowing Υ(1S)→anything (“inclusive” analysis) we obtained B(Υ(2S)→Υ(1S)π⁺π^-)=0.196±0.002±0.010. The appropriate weighted average of the two measurements gives B(Υ(2S)→Υ(1S)π⁺π^-)=0.192±0.002±0.010. Combining the exclusive and inclusive results we derive the Υ(1S) leptonic branching fractions B_ee=0.0229±0.0008±0.0011 and B_μμ=0.0249±0.0008±0.0013. We also studied Υ(2S)→Υ(1S)π⁰π⁰ and obtained B(Υ(2S)→Υ(1S)π⁰π⁰)=0.092±0.006±0.008. Parameters of the ππ system (dipion invariant mass spectra, angular distributions) were analyzed and found to be consistent with current theoretical models. Lastly, we searched for the η and single π⁰ transitions and obtained the 90% confidence level upper limits B(Υ(2S)→Υ(1S)η)<0.0028 and B(Υ(2S)→Υ(1S)π⁰)<0.0011.

A measurement of the spin alignment of charged D^* mesons produced in continuum e⁺e^-→cc̅ events at sqrt[s]=10.5 GeV is presented. This study using 4.72 fb^-1 of CLEO II data shows that there is little evidence of any D^* spin alignment.