Search Results (124 total)

We present measurements of the branching fraction B and longitudinal polarization fraction f_L for B⁰→a₁(1260)⁺a₁(1260)^- decays, with a₁(1260)^±→π^-π⁺π^±. The data sample, collected with the BABAR detector at the SLAC National Accelerator Laboratory, represents 465×10⁶ produced BB̅ pairs. We measure B(B⁰→a₁(1260)⁺a₁(1260)^-)×[B(a₁(1260)⁺→π^-π⁺π⁺)]²=  (11.8±2.6±1.6)×10^-6 and f_L=0.31±0.22±0.10, where the first uncertainty is statistical and the second systematic. The decay mode is measured with a significance of 5.0 standard deviations including systematic uncertainties.

We present the result from a precision measurement of the mass of the τ lepton, M_τ, based on 423  fb^-1 of data recorded at the Υ(4S) resonance with the BABAR detector. Using a pseudomass endpoint method, we determine the mass to be 1776.68±0.12(stat)±0.41(syst)  MeV. We also measure the mass difference between the τ⁺ and τ^-, and obtain (M_τ⁺-M_τ^-)/M_AVG^τ=(-3.4±1.3(stat)±0.3(syst))×10^-4, where M_AVG^τ is the average value of M_τ⁺ and M_τ^-.

We present an analysis of the decays B⁰→K^*0(892)γ and B⁺→K^*+(892)γ using a sample of about 383×10⁶ BB̅ events collected with the BABAR detector at the PEP-II asymmetric energy B factory. We measure the branching fractions B(B⁰→K^*0γ)=(4.47±0.10±0.16)×10^-5 and B(B⁺→K^*+γ)=(4.22±0.14±0.16)×10^-5. We constrain the direct CP asymmetry to be -0.033<A(B→K^*γ)<0.028 and the isospin asymmetry to be 0.017<Δ_0-<0.116, where the limits are determined by the 90% confidence interval and include both the statistical and systematic uncertainties.

We observe the decays D_s1^*(2710)⁺→D^*K and D_sJ^*(2860)⁺→D^*K and measure their branching fractions relative to the DK final state. We also observe, in the D^*K mass spectrum, a new broad structure at a mass of (3044±8_stat(+30 / -5)_syst)  MeV/c² having a width Γ=(239±35_stat(+46 / -42)_syst)  MeV. To obtain this result we use 470  fb^-1 of data recorded by the BABAR detector at the PEP-II asymmetric-energy e⁺e^- storage rings at the Stanford Linear Accelerator Center running at center-of-mass energies near 10.6 GeV.

We study the decay B^-→DK^*- using a sample of 379×10⁶ Υ(4S)→BB̅ events collected with the BABAR detector at the PEP-II B factory. We perform a Gronau-London-Wyler analysis where the D meson decays into either a CP-even (CP+) eigenstate (K⁺K^-, π⁺π^-), CP-odd (CP-) eigenstate (K_S⁰π⁰, K_S⁰ϕ, K_S⁰ω) or a non-CP state (K^-π⁺). We also analyze D meson decays into K⁺π^- from a Cabibbo-favored D̅ ⁰ decay or doubly suppressed D⁰ decay [Atwood-Dunietz-Soni (ADS) analysis]. We measure observables that are sensitive to the Cabibbo-Kobayashi-Maskawa angle γ: the partial-rate charge asymmetries A_CP±, the ratios R_CP± of the B-decay branching fractions in CP± and non-CP decay, the ratio R_ADS of the charge-averaged branching fractions, and the charge asymmetry A_ADS of the ADS decays: A_CP+=0.09±0.13±0.06, A_CP-=-0.23±0.21±0.07, R_CP+=2.17±0.35±0.09, R_CP-=1.03±0.27±0.13, R_ADS=0.066±0.031±0.010, and A_ADS=-0.34±0.43±0.16, where the first uncertainty is statistical and the second is systematic. Combining all the measurements and using a frequentist approach yields the magnitude of the ratio between the Cabibbo-suppressed and favored amplitudes, r_B=0.31 with a one (two) sigma confidence level interval of [0.24, 0.38] ([0.17, 0.43]). The value r_B=0 is excluded at the 3.3 sigma level. A similar analysis excludes values of γ in the intervals [0, 7]°, [55, 111]°, and [175, 180]° ([85, 99]°) at the one (two) sigma confidence level.

We search for a light Higgs boson A⁰ in the radiative decay Υ(3S)→γA⁰, A⁰→τ⁺τ^-, τ⁺→e⁺ν_eν̅ _τ, or τ⁺→μ⁺ν_μν̅ _τ. The data sample contains 122×10⁶ Υ(3S) events recorded with the BABAR detector. We find no evidence for a narrow structure in the studied τ⁺τ^- invariant mass region of 4.03<m_τ⁺τ^-<10.10  GeV/c². We exclude at the 90% confidence level (C.L.) a low-mass Higgs boson decaying to τ⁺τ^- with a product branching fraction B(Υ(3S)→γA⁰)×B(A⁰→τ⁺τ^-)>(1.5–16)×10^-5 across the m_τ⁺τ^- range. We also set a 90% C.L. upper limit on the τ⁺τ^- decay of the η_b at B(η_b→τ⁺τ^-)<8%.

We measure the rate of D⁰-D̅ ⁰ mixing with the observable y_CP=(τ_Kπ/τ_KK)-1, where τ_KK and τ_Kπ are, respectively, the mean lifetimes of CP-even D⁰→K⁺K^- and CP-mixed D⁰→K^-π⁺ decays, using a data sample of 384  fb^-1 collected by the BABAR detector at the SLAC PEP-II asymmetric-energy B Factory. From a sample of D⁰ and D̅ ⁰ decays where the initial flavor of the decaying meson is not determined, we obtain y_CP=[1.12±0.26(stat)±0.22(syst)]%, which excludes the no-mixing hypothesis at 3.3σ, including both statistical and systematic uncertainties. This result is in good agreement with a previous BABAR measurement of y_CP obtained from a sample of D^*+→D⁰π⁺ events, where the D⁰ decays to K^-π⁺, K⁺K^-, and π⁺π^-, which is disjoint with the untagged D⁰ events used here. Combining the two results taking into account statistical and systematic uncertainties, where the systematic uncertainties are assumed to be 100% correlated, we find y_CP=[1.16±0.22(stat)±0.18(syst)]%, which excludes the no-mixing hypothesis at 4.1σ.

We have performed a search for the η_b(1S) meson in the radiative decay of the Υ(2S) resonance using a sample of 91.6×10⁶ Υ(2S) events recorded with the BABAR detector at the PEP-II B factory at the SLAC National Accelerator Laboratory. We observe a peak in the photon energy spectrum at E_γ=609.3_-4.5^+4.6(stat)±1.9(syst)  MeV, corresponding to an η_b(1S) mass of 9394.2_-4.9^+4.8(stat)±2.0(syst)  MeV/c². The branching fraction for the decay Υ(2S)→γη_b(1S) is determined to be [3.9±1.1(stat)_-0.9^+1.1(syst)]×10^-4. We find the ratio of branching fractions B[Υ(2S)→γη_b(1S)]/B[Υ(3S)→γη_b(1S)]=0.82±0.24(stat)_-0.19^+0.20(syst).

The ²³³U(n,f) cross section has been measured at the white neutron source n_TOF in a wide energy range with a dedicated fission ionization chamber. We report here the results from ~30 meV to 1 MeV neutron energy. The ²³³U(n,f) cross section has been determined relative to a reference sample of ²³⁵U(n,f) measured simultaneously with the same detector. The very high instantaneous neutron flux and the intrinsically low background of the n_TOF installation result in an accuracy around 3% in the whole energy range, while the energy resolution of the neutron beam allows for an accurate description of the fission cross section by means of R-matrix analysis over a wide energy range. The results are, in general, in good agreement with the most recent high-accuracy measurement of this fission cross section, over the more limited range of the previous measurements, and indicated that even the latest evaluations underestimate the cross section in the epithermal region. The present high-quality data provide the basis for a more precise evaluation of the ²³³U fission cross section and for improving the reliability of databases needed for the design of new energy systems based on the Th/U cycle.

We report the observation of the baryonic B-decay B̅ ⁰→Λ_c⁺p̅ K^-π⁺, excluding contributions from the decay B̅ ⁰→Λ_c⁺Λ̅ K^-. Using a data sample of 467×10⁶ BB̅ pairs collected with the BABAR detector at the PEP-II storage ring at SLAC, the measured branching fraction is (4.33±0.82_stat±0.33_syst±1.13_Λc⁺)×10^-5. In addition we find evidence for the resonant decay B̅ ⁰→Σ_c(2455)⁺⁺p̅ K^- and determine its branching fraction to be (1.11±0.30_stat±0.09_syst±0.29_Λc⁺)×10^-5. The errors are statistical, systematic, and due to the uncertainty in the Λ_c⁺ branching fraction. For the resonant decay B̅ ⁰→Λ_c⁺p̅ K̅ ^*0 we obtain an upper limit of 2.42×10^-5 at 90% confidence level.

We study the reaction e⁺e^-→e⁺e^-π⁰ in the single tag mode and measure the differential cross section dσ/dQ² and the γγ^*→π⁰ transition form factor in the momentum transfer range from 4 to 40  GeV². At Q²>10  GeV² the measured form factor exceeds the asymptotic limit predicted by perturbative QCD. The analysis is based on 442  fb^-1 of integrated luminosity collected at PEP-II with the BABAR detector at e⁺e^- center-of-mass energies near 10.6 GeV.

We present a search for decays of B mesons to final states with a b₁ meson and a ρ or K^*(892) meson. The search is based on a data sample consisting of 465 million BB̅ pairs collected by the BABAR detector at the SLAC National Accelerator Laboratory. We do not observe any statistically significant signal. The upper limits we set on the branching fractions range from 1.4 to 8.0×10^-6 at the 90% confidence level, including systematic uncertainties.

We have succeeded in increasing up to 150 K the Curie temperature in the Fe₆₄Ni₃₆ invar alloy by means of a severe mechanical treatment followed by a heating up to 1073 K. The invar behavior is still present as revealed by the combination of magnetic measurements with neutron and x-ray techniques under extreme conditions, such as high temperature and high pressure. The proposed explanation is based in a selective induced microstrain around the Fe atoms, which causes a slight increase in the Fe-Fe interatomic distances, thus reinforcing ferromagnetic interactions due to the strong magnetoelastic coupling in these invar compounds.

We search for evidence of a light scalar boson in the radiative decays of the Υ(2S) and Υ(3S) resonances: Υ(2S,3S)→γA⁰, A⁰→μ⁺μ^-. Such a particle appears in extensions of the standard model, where a light CP-odd Higgs boson naturally couples strongly to b quarks. We find no evidence for such processes in the mass range 0.212≤m_A⁰≤9.3  GeV in the samples of 99×10⁶ Υ(2S) and 122×10⁶ Υ(3S) decays collected by the BABAR detector at the SLAC PEP-II B factory and set stringent upper limits on the effective coupling of the b quark to the A⁰. We also limit the dimuon branching fraction of the η_b meson: B(η_b→μ⁺μ^-)<0.9% at 90% confidence level.

We present a study of the decays B⁰→D⁰K^*0 and B⁰→D̅ ⁰K^*0 with K^*0→K⁺π^-. The D⁰ and the D̅ ⁰ mesons are reconstructed in the final states f=K⁺π^-, K⁺π^-π⁰, K⁺π^-π⁺π^-, and their charge conjugates. Using a sample of 465×10⁶ BB̅ pairs collected with the BABAR detector at the PEP-II asymmetric-energy e⁺e^- collider at SLAC, we measure the ratio R_ADS≡[Γ(B̅ ⁰→[f]_DK̅ ^*0)+Γ(B⁰  →[f̅ ]_DK^*0)]/[Γ(B̅ ⁰→[f̅ ]_DK̅ ^*0)+Γ(B⁰→[f]_DK^*0)] for the three final states. We do not find significant evidence for a signal and set the following limits at 95% probability: R_ADS(Kπ)<0.244, R_ADS(Kππ⁰)<0.181, and R_ADS(Kπππ)<0.391. From the combination of these three results, we find that the ratio r_S between the b→u and the b→c amplitudes lies in the range [0.07,0.41] at 95% probability.

We present a study of B decays into semileptonic final states containing charged and neutral D₁(2420) and D₂^*(2460). The analysis is based on a data sample of 208  fb^-1 collected at the Υ(4S) resonance with the BABAR detector at the PEP-II asymmetric-energy B factory at SLAC. With a simultaneous fit to four different decay chains, the semileptonic branching fractions are extracted from measurements of the mass difference Δm=m(D^**)-m(D) distributions. Product branching fractions are determined to be B(B⁺→D₁⁰ℓ⁺ν_ℓ)×B(D₁⁰→D^*+π^-)=(2.97±0.17±0.17)×10^-3, B(B⁺→D₂^*0ℓ⁺ν_ℓ)×B(D₂^*0→D^(*)+π^-)=(2.29±0.23±0.21)×10^-3, B(B⁰→D₁^-ℓ⁺ν_ℓ)×B(D₁^-→D^*0π^-)=(2.78±0.24±0.25)×10^-3 and B(B⁰→D₂^*-ℓ⁺ν_ℓ)×B(D₂^*-→D^(*)0π^-)=(1.77±0.26±0.11)×10^-3. In addition we measure the branching ratio Γ(D₂^*→Dπ^-)/Γ(D₂^*→D^(*)π^-)=0.62±0.03±0.02.

We describe searches for B⁰ meson decays to the charmless final states π⁰K_S⁰K_S⁰, ηK_S⁰K_S⁰, and η^′K_S⁰K_S⁰. The data sample corresponds to 467×10⁶ BB̅ pairs produced in e⁺e^- annihilation and collected with the BABAR detector at the SLAC National Accelerator Laboratory. We find no significant signals and determine the 90% confidence level upper limits on the branching fractions, in units of 10^-7, B(B⁰→π⁰K_S⁰K_S⁰)<9, B(B⁰→ηK_S⁰K_S⁰)<10, and B(B⁰→η^′K_S⁰K_S⁰)<20.

We report an analysis of τ^- decaying into ωπ^-ν_τ with ω→π⁺π^-π⁰ using a data sample containing nearly 320×10⁶τ pairs collected with the BABAR detector at the PEP-II B-Factory. We find no evidence for second-class currents, and we set an upper limit of 0.69% at 90% confidence level for the fraction of second-class currents in this decay mode.

We search for the neutrinoless, lepton-flavor-violating tau decays τ^-→ℓ^-V⁰, where ℓ is an electron or muon and V⁰ is a vector meson reconstructed as ϕ→K⁺K^-, ρ→π⁺π^-, K^*→K⁺π^-, K̅ ^*→K^-π⁺. The analysis has been performed using 451  fb^-1 of data collected at an e⁺e^- center-of-mass energy near 10.58 GeV with the BABAR detector at the PEP-II storage rings. The number of events found in the data is compatible with the background expectation, and upper limits on the branching fractions are set in the range (2.6–19)×10^-8 at the 90% confidence level.

We present a measurement of the B⁰→Λ̅ pπ^- branching fraction performed using the BABAR detector at the PEP-II asymmetric e⁺e^- collider. Based on a sample of 467×10⁶ BB̅ pairs we measure B(B⁰→Λ̅ pπ^-)=[3.07±0.31 (stat)±0.23 (syst)]×10^-6. The measured differential spectrum as a function of the dibaryon invariant mass m(Λ̅ p) shows a near-threshold enhancement similar to that observed in other baryonic B decays. We study the Λ̅ polarization as a function of Λ̅ energy in the B⁰ rest frame (E_Λ̅ ^*) and compare it with theoretical expectations of fully longitudinally right-polarized Λ̅ at large E_Λ̅ ^*.

We report on a Dalitz plot analysis of B^-→D⁺π^-π^- decays, based on a sample of about 383×10⁶ Υ(4S)→BB̅ decays collected with the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. We find the total branching fraction of the three-body decay: B(B^-→D⁺π^-π^-)=(1.08±0.03±0.05)×10^-3. We observe the established D₂^*0 and confirm the existence of D₀^*0 in their decays to D⁺π^-, where the D₂^*0 and D₀^*0 are the 2⁺ and 0⁺cu̅ P-wave states, respectively. We measure the masses and widths of D₂^*0 and D₀^*0 to be: m_D2^*0=(2460.4±1.2±1.2±1.9)  MeV/c², Γ_D2^*0=(41.8±2.5±2.1±2.0)  MeV, m_D0^*0=(2297±8±5±19)  MeV/c², and Γ_D0^*0=(273±12±17±45)  MeV. The stated errors reflect the statistical and systematic uncertainties, and the uncertainty related to the assumed composition of signal events and the theoretical model.

We report the results of a search for Z(4430)^- decay to J/ψπ^- or ψ(2S)π^- in B^-,0→J/ψπ^-K^0,+ and B^-,0→ψ(2S)π^-K^0,+ decays. The data were collected with the BABAR detector at the SLAC PEP-II asymmetric-energy e⁺e^- collider operating at center-of-mass energy 10.58 GeV, and the sample corresponds to an integrated luminosity of 413  fb^-1. Each Kπ^- mass distribution exhibits clear K^*(892) and K₂^*(1430) signals, and the efficiency-corrected spectrum is well described by a superposition of the associated Breit-Wigner intensity distributions, together with an S-wave contribution obtained from the LASS I=1/2 Kπ^- scattering amplitude measurements. Each Kπ^- angular distribution varies significantly in structure with Kπ^- mass, and is represented in terms of low-order Legendre polynomial moments. We find that each J/ψπ^- or ψ(2S)π^- mass distribution is well described by the reflection of the measured Kπ^- mass and angular distribution structures. We see no significant evidence for a Z(4430)^- signal for any of the processes investigated, neither in the total J/ψπ^- or ψ(2S)π^- mass distribution, nor in the corresponding distributions for the regions of Kπ^- mass for which observation of the Z(4430)^- signal was reported. We obtain branching-fraction upper limits B(B^-→Z^-K̅ ⁰,Z^-→J/ψπ^-)<1.5×10^-5, B(B⁰→Z^-K⁺,Z^-→J/ψπ^-)<0.4×10^-5, B(B^-→Z^-K̅ ⁰,Z^-→ψ(2S)π^-)<4.7×10^-5, and B(B⁰→Z^-K⁺,Z^-→ψ(2S)π^-)<3.1×10^-5 at 95% confidence level, where the Z(4430)^- mass and width have been fixed to the reported central values.

We have performed a search for the rare leptonic decays B⁺→ℓ⁺ν_ℓ(l=e,μ), using data collected at the Υ(4S) resonance by the BABAR detector at the PEP-II storage ring. In a sample of 468×10⁶ BB̅ pairs we find no evidence for a signal and set an upper limit on the branching fractions B(B⁺→μ⁺ν_μ)<1.0×10^-6 and B(B⁺→e⁺ν_e)<1.9×10^-6 at the 90% confidence level, using a Bayesian approach.

We present measurements of the semileptonic decays B^-→D⁰τ^-ν̅ _τ, B^-→D^*0τ^-ν̅ _τ, B̅ ⁰→D⁺τ^-ν̅ _τ, and B̅ ⁰→D^*+τ^-ν̅ _τ, which are sensitive to non-standard model amplitudes in certain scenarios. The data sample consists of 232×10⁶  Υ(4S)→BB̅ decays collected with the BABAR detector at the PEP-II e⁺e^- collider. We select events with a D or D^* meson and a light lepton (ℓ=e or μ) recoiling against a fully reconstructed B meson. We perform a fit to the joint distribution of lepton momentum and missing mass squared to distinguish signal B̅ →D^(*)τ^-ν̅ _τ(τ^-→ℓ^-ν̅ _ℓν_τ) events from the backgrounds, predominantly B̅ →D^(*)ℓ^-ν̅ _ℓ. We measure the branching-fraction ratios R(D)≡B(B̅ →Dτ^-ν̅ _τ)/B(B̅ →Dℓ^-ν̅ _ℓ) and R(D^*)≡B(B̅ →D^*τ^-ν̅ _τ)/B(B̅ →D^*ℓ^-ν̅ _ℓ) and, from a combined fit to B^- and B̅ ⁰ channels, obtain the results R(D)=(41.6±11.7±5.2)% and R(D^*)=(29.7±5.6±1.8)%, where the uncertainties are statistical and systematic. Normalizing to measured B^-→D^(*)0ℓ^-ν̅ _ℓ branching fractions, we obtain B(B̅ →Dτ^-ν̅ _τ)=(0.86±0.24±0.11±0.06)% and B(B̅ →D^*τ^-ν̅ _τ)=(1.62±0.31±0.10±0.05)%, where the additional third uncertainty is from the normalization mode. We also present, for the first time, distributions of the lepton momentum, |p_ℓ^*|, and the squared momentum transfer, q².