Search Results (1,258 total)

The B_c^± meson is observed through the decay B_c^±→J/ψπ^±, in data corresponding to an integrated luminosity of 2.4  fb^-1 recorded by the Collider Detector at Fermilab II detector at the Fermilab Tevatron. A signal of 108±15 candidates is observed, with a significance that exceeds 8σ. The mass of the B_c^± meson is measured to be 6275.6±2.9(stat)±2.5(syst)  MeV/c².

Molecular dynamics simulation of oligopeptide chains reveals configurational subdiffusion at equilibrium extending from 10^-12 to 10^-8  s. Trap models, involving a random walk with a distribution of waiting times, cannot account for the subdiffusion, which is found rather to arise from the fractal-like structure of the accessible configuration space.

We report on our search for decays proceeding via a tree-level b→c quark transition in which a gluon radiates into an ss̅ pair. We present observations of the decays B^-→D_s^(*)+K^-π^- and B̅ ⁰→D_s⁺K_S⁰π^- and evidence for B^-→D_s⁺K^-K^- and set upper limits on the branching fractions for B̅ ⁰→D_s^*+K_S⁰π^- and B^-→D_s^*+K^-K^- using 383×10⁶ Υ(4S)→BB̅ events collected by the BABAR detector at SLAC. We present evidence that the invariant mass distributions of D_s^(*)+K^- pairs from B^-→D_s^(*)+K^-π^- decays are inconsistent with the phase-space model, suggesting the presence of charm resonances lying below the D_s^(*)+K^- threshold.

We present partial branching fractions for inclusive charmless semileptonic B decays B̅ →X_uℓν̅ , and the determination of the Cabibbo-Kobayashi-Maskawa matrix element |V_ub|. The analysis is based on a sample of 383×10⁶ Υ(4S) decays into BB̅ pairs collected with the BABAR detector at the SLAC PEP-II e⁺e^- storage rings. We select events using the invariant mass M_X of the hadronic system, the invariant mass squared, q², of the lepton and neutrino pair, the kinematic variable P₊, or one of their combinations. We then determine partial branching fractions in limited regions of phase space: ΔB=(1.18±0.09_stat±0.07_syst±0.01_theor)×10^-3 (M_X<1.55  GeV/c²), ΔB=(0.95±0.10_stat±0.08_syst±0.01_theor)×10^-3 (P₊<0.66  GeV/c), and ΔB=(0.81±0.08_stat±0.07_syst±0.02_theor)×10^-3 (M_X<1.7  GeV/c², q²>8  GeV²/c⁴). Corresponding values of |V_ub| are extracted using several theoretical calculations.

We present an optical signature of a hybridization between a localized quantum dot state and a filled continuum. Radiative recombination of the negatively charged trion in a single quantum dot leaves behind a single electron. We show that in two regions of vertical electric field, the electron hybridizes with a continuum through a tunneling interaction. The hybridization manifests itself through an unusual voltage dependence of the emission energy and a non-Lorentzian line shape, features which we reproduce with a theory based on the Anderson Hamiltonian.

We present a measurement of the correlated bb̅ production cross section. The data used in this analysis were taken with the upgraded CDF detector (CDF II) at the Fermilab Tevatron collider, and correspond to an integrated luminosity of 742  pb^-1. We utilize muon pairs with invariant mass 5≤m_μμ≤80  GeV/c² produced by bb̅ double semileptonic decays. For muons with p_T≥3  GeV/c and |η|≤0.7, that are produced by b and b̅ quarks with p_T≥2  GeV/c and |y|≤1.3, we measure σ_{b→μ,b̅ →μ}=1549±133  pb. We compare this result with theoretical predictions and previous measurements. We also report the measurement of σ_{c→μ,c̅ →μ}, a by-product of the study of the background to bb̅ production.

We present for the first time a measurement of the weak phase 2β+γ obtained from a time-dependent Dalitz plot analysis of B⁰→D^∓K⁰π^± decays. Using a sample of approximately 347×10⁶ BB̅ pairs collected by the BABAR detector at the PEP-II asymmetric-energy storage rings and assuming the ratio r of the b→u and b→c decay amplitudes to be 0.3, we obtain 2β+γ=(83±53±20)° and the equivalent solution at +180°. The magnitudes and phases for the resonances associated with the b→c transitions are also extracted from the fit.

Ultraviolet-photoemission (UPS) measurements and supporting specific-heat, thermal-expansion, resistivity, and magnetic-moment measurements are reported for the magnetic shape-memory alloy Ni₂MnGa over the temperature range 100<T<250  K. All measurements detect clear signatures of the premartensitic transition (T_PM∼247  K) and the martensitic transition (T_M∼196  K). Temperature-dependent UPS shows a dramatic depletion of states (pseudogap) at T_PM located 0.3 eV below the Fermi energy. First-principles electronic structure calculations show that the peak observed at 0.3 eV in the UPS spectra for T>T_PM is due to the Ni d minority-spin electrons. Below T_M this peak disappears, resulting in an enhanced density of states at energies around 0.8 eV. This enhancement reflects Ni d and Mn d electronic contributions to the majority-spin density of states.

We present the results of a search for pair production of a new heavy toplike quark t^′ decaying to a W boson and another quark using the Collider Detector at Fermilab II detector in run II of the Tevatron pp̅ collider. Using a data sample corresponding to 760  pb^-1 of integrated luminosity, we fit the observed spectrum of total transverse energy and reconstructed t^′ quark mass to a combination of standard model processes and t^′ pair production. We see no evidence for t^′t̅ ^′ production, and we infer a lower limit of 256  GeV/c² on the mass of the t^′ at 95% C.L. assuming standard strong couplings for the t^′.

This Letter describes the first determination of bounds on the CP-violation parameter 2β_s using B_s⁰ decays in which the flavor of the bottom meson at production is identified. The result is based on approximately 2000 B_s⁰→J/ψϕ decays reconstructed in a 1.35  fb^-1 data sample collected with the CDF II detector using pp̅ collisions produced at the Fermilab Tevatron. We report confidence regions in the two-dimensional space of 2β_s and the decay-width difference ΔΓ. Assuming the standard model predictions of 2β_s and ΔΓ, the probability of a deviation as large as the level of the observed data is 15%, corresponding to 1.5 Gaussian standard deviations.

This paper describes the first measurement of b-quark fragmentation fractions into bottom hadrons in Run II of the Tevatron Collider at Fermilab. The result is based on a 360  pb^-1 sample of data collected with the CDF II detector in pp̅ collisions at sqrt[s]=1.96  TeV. Semileptonic decays of B̅ ⁰, B^-, and B̅ _s⁰ mesons, as well as Λ_b⁰ baryons, are reconstructed. For an effective bottom hadron p_T threshold of 7  GeV/c, the fragmentation fractions are measured to be f_u/f_d=1.054±0.018(stat)_-0.045^+0.025(sys)±0.058(B), f_s/(f_u+f_d)=0.160±0.005(stat)_-0.010^+0.011(sys)_-0.034^+0.057(B), and f_Λb/(f_u+f_d)=0.281±0.012(stat)_-0.056^+0.058(sys)_-0.087^+0.128(B), where the uncertainty B is due to uncertainties on measured branching ratios. The value of f_s/(f_u+f_d) agrees within one standard deviation with previous CDF measurements and the world average of this quantity, which is dominated by LEP measurements. However, the ratio f_Λb/(f_u+f_d) is approximately twice the value previously measured at LEP. The approximately 2σ discrepancy is examined in terms of kinematic differences between the two production environments.

We report a measurement of the branching fractions for B̅ →D^(*)(π)ℓ^-ν̅ _ℓ decays based on 341.1  fb^-1 of data collected at the Υ(4S) resonance with the BABAR detector at the SLAC PEP-II e⁺e^- storage rings. Events are tagged by fully reconstructing one of the B mesons in a hadronic decay mode. We obtain B(B^-→D⁰ℓ^-ν̅ _ℓ)=(2.33±0.09_stat±0.09_syst)%, B(B^-→D^*0ℓ^-ν̅ _ℓ)=(5.83±0.15_stat±0.30_syst)%, B(B̅ ⁰→D⁺ℓ^-ν̅ _ℓ)=(2.21±0.11_stat±0.12_syst)%, B(B̅ ⁰→D^*+ℓ^-ν̅ _ℓ)=(5.49±0.16_stat±0.25_syst)%, B(B^-→D⁺π^-ℓ^-ν̅ _ℓ)=(0.42±0.06_stat±0.03_syst)%, B(B^-→D^*+π^-ℓ^-ν̅ _ℓ)=(0.59±0.05_stat±0.04_syst)%, B(B̅ ⁰→D⁰π⁺ℓ^-ν̅ _ℓ)=(0.43±0.08_stat±0.03_syst)%, and B(B̅ ⁰→D^*0π⁺ℓ^-ν̅ _ℓ)=(0.48±0.08_stat±0.04_syst)%.

We report results of a search for CPT and Lorentz violation in B⁰-B̅ ⁰ oscillations using inclusive dilepton events from 232×10⁶ Υ(4S)→BB̅ decays recorded by the BABAR detector at the PEP-II B Factory at SLAC. We find 2.8σ significance, compatible with no signal, for variations in the complex CPT violation parameter z at the Earth’s sidereal frequency and extract values for the quantities Δa_μ in the general Lorentz-violating standard-model extension. The spectral powers for variations in z over the frequency range 0.26  yr^-1 to 2.1  solar day^-1 are also compatible with no signal.

Interprotein motions in low and fully hydrated carboxymyoglobin crystals are investigated using molecular dynamics simulation. Below ≈240  K, the calculated dynamic structure factor exhibits a peak arising from interprotein vibration. Above ≈240  K, the intermolecular fluctuations of the fully hydrated crystal increase drastically, whereas the low-hydration model exhibits no transition. Autocorrelation function analysis shows the transition to be dominated by the activation of diffusive intermolecular motion. The potential of mean force for the interaction remains quasiharmonic. The results indicate useful experimental avenues on protein:protein interactions to be explored using next-generation neutron sources.

We measure the mean lifetime τ=2/(Γ_L+Γ_H) and the decay-width difference ΔΓ=Γ_L-Γ_H of the light and heavy mass eigenstates of the B_s⁰ meson, B_sL⁰ and B_sH⁰, in B_s⁰→J/ψϕ decays using 1.7  fb^-1 of data collected with the CDF II detector at the Fermilab Tevatron pp̅ collider. Assuming CP conservation, a good approximation for the B_s⁰ system in the standard model, we obtain ΔΓ=0.076_-0.063^+0.059(stat)±0.006(syst)  ps^-1 and τ=1.52±0.04(stat)±0.02(syst)  ps, the most precise measurements to date. Our constraints on the weak phase and ΔΓ are consistent with CP conservation.

We measure the time dependence of the ratio of decay rates for the rare decay D⁰→K⁺π^- to the Cabibbo-favored decay D⁰→K^-π⁺. A signal of 12.7×10³ D⁰→K⁺π^- decays was obtained using the Collider Detector at Fermilab II detector at the Fermilab Tevatron with an integrated luminosity of 1.5  fb^-1. We measure the D⁰-D̅ ⁰ mixing parameters (R_D,y^′,x^′2), and find that the data are inconsistent with the no-mixing hypothesis with a probability equivalent to 3.8 Gaussian standard deviations.

We present the first observation and cross section measurement of exclusive dijet production in p̅ p interactions, p̅ p→p̅ +dijet+p. Using a data sample of 310  pb^-1 collected by the Run II Collider Detector at Fermilab at sqrt[s]=1.96  TeV, exclusive cross sections for events with two jets of transverse energy E_T^jet≥10  GeV have been measured as a function of minimum E_T^jet. The exclusive signal is extracted from fits to data distributions based on Monte Carlo simulations of expected dijet signal and background shapes. The simulated background distribution shapes are checked in a study of a largely independent data sample of 200  pb^-1 of b-tagged jet events, where exclusive dijet production is expected to be suppressed by the J_z=0 total angular momentum selection rule. Results obtained are compared with theoretical expectations, and implications for exclusive Higgs boson production at the pp Large Hadron Collider at sqrt[s]=14  TeV are discussed.

We present the results of a search for short-duration gravitational-wave bursts associated with 39 gamma-ray bursts (GRBs) detected by gamma-ray satellite experiments during LIGO’s S2, S3, and S4 science runs. The search involves calculating the crosscorrelation between two interferometer data streams surrounding the GRB trigger time. We search for associated gravitational radiation from single GRBs, and also apply statistical tests to search for a gravitational-wave signature associated with the whole sample. For the sample examined, we find no evidence for the association of gravitational radiation with GRBs, either on a single-GRB basis or on a statistical basis. Simulating gravitational-wave bursts with sine-Gaussian waveforms, we set upper limits on the root-sum-square of the gravitational-wave strain amplitude of such waveforms at the times of the GRB triggers. We also demonstrate how a sample of several GRBs can be used collectively to set constraints on population models. The small number of GRBs and the significant change in sensitivity of the detectors over the three runs, however, limits the usefulness of a population study for the S2, S3, and S4 runs. Finally, we discuss prospects for the search sensitivity for the ongoing S5 run, and beyond for the next generation of detectors.

We have performed a search for B_s⁰→μ⁺μ^- and B⁰→μ⁺μ^- decays in pp̅ collisions at sqrt[s]=1.96  TeV using 2  fb^-1 of integrated luminosity collected by the CDF II detector at the Fermilab Tevatron Collider. The observed number of B_s⁰ and B⁰ candidates is consistent with background expectations. The resulting upper limits on the branching fractions are B(B_s⁰→μ⁺μ^-)<5.8×10^-8 and B(B⁰→μ⁺μ^-)<1.8×10^-8 at 95% C.L.

We present a measurement of the branching fraction and photon-energy spectrum for the decay B→X_sγ using data from the BABAR experiment. The data sample corresponds to an integrated luminosity of 210  fb^-1, from which approximately 680 000 BB̅ events are tagged by a fully reconstructed hadronic decay of one of the B mesons. In the decay of the second B meson, an isolated high-energy photon is identified. We measure B(B→X_sγ)=(3.66±0.85_stat±0.60_syst)×10^-4 for photon energies E_γ above 1.9 GeV in the B rest frame. From the measured spectrum we calculate the first and second moments for different minimum photon energies, which are used to extract the heavy-quark parameters m_b and μ_π². In addition, measurements of the direct CP asymmetry and isospin asymmetry are presented.

We search for evidence of resonant top quark pair production in 955 pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV recorded with the CDF II detector at the Fermilab Tevatron. For fully reconstructed candidate tt̅ events triggered on leptons with large transverse momentum and containing at least one identified b-quark jet, we compare the invariant mass spectrum of tt̅ pairs to the expected superposition of standard model tt̅ , non-tt̅ backgrounds, and a simple resonance model based on a sequential Z^′ boson. We establish upper limits for σ(pp̅ →Z^′)·Br(Z^′→tt̅ ) in the Z^′ mass interval from 450  GeV/c² to 900  GeV/c². A topcolor leptophobic Z^′ is ruled out below 720  GeV/c², and the cross section of any narrow Z^′-like state decaying to tt̅ is found to be less than 0.64 pb at 95% C.L. for M_Z^′ above 700  GeV/c².

Inclusive jet cross sections in Z/γ^* events, with Z/γ^* decaying into an electron-positron pair, are measured as a function of jet transverse momentum and jet multiplicity in pp̅ collisions at sqrt[s]=1.96   TeV with the upgraded Collider Detector at Fermilab in run II, based on an integrated luminosity of 1.7   fb^-1. The measurements cover the rapidity region |y^jet|<2.1 and the transverse momentum range p_T^jet>30  GeV/c. Next-to-leading order perturbative QCD predictions are in good agreement with the measured cross sections.

The stochastic point processes formed by the zero crossings or extremal points of differentiable, stationary Gaussian processes are studied as a function of their autocorrelation function. The properties of these point processes are mapped to the space formed by the parameters appearing in the autocorrelation function, their adopted form being sensitive to the structure of the autocorrelation function principally in the vicinity of the origin. The distribution for the number of zeros occurring in an asymptotically large interval are approximately negative-binomial or binomial depending upon whether the relative variance or Fano factor is greater or less than unity. The correlation properties of the zeros are such that they are repelled from each other or are “antibunched” if the autocorrelation function of the Gaussian process is characterized by a single scale size, but occur in clusters if more than one characteristic scale size is present. The intervals between zeros can be interpreted in terms of the autocorrelation function of the zeros themselves. When bunching occurs the interval density becomes bimodal, indicating the interval sizes within and between the clusters. The interevent periods are statistically dependent on one another with densities whose asymptotic behavior is governed by that of the autocorrelation function of the Gaussian process at large delay times. Poisson distributed fluctuations of the zeros occur only exceptionally but never form a Poisson process.