Search Results (719 total)

We present the first simultaneous measurement of the ratio of branching fractions, R=B(t→Wb)/B(t→Wq), with q being a d, s, or b quark, and the top-quark pair production cross section σ_tt̅ in the lepton plus jets channel using 0.9  fb^-1 of pp̅ collision data at sqrt[s]=1.96  TeV collected with the D0 detector. We extract R and σ_tt̅ by analyzing samples of events with 0, 1, and ≥2 identified b jets. We measure R=0.97_-0.08^+0.09(stat+syst) and σ_tt̅ =8.18_-0.84^+0.90(stat+syst)±0.50(lumi)  pb, in agreement with the standard model prediction.

We present the results of a search for the production of an excited state of the electron, e^*, in proton-antiproton collisions at sqrt[s]=1.96  TeV. The data were collected with the D0 experiment at the Fermilab Tevatron Collider and correspond to an integrated luminosity of approximately 1  fb^-1. We search for e^* in the process pp̅ →e^*e, with the e^* subsequently decaying to an electron plus photon. No excess above the standard model background is observed. Interpreting our data in the context of a model that describes e^* production by four-fermion contact interactions and e^* decay via electroweak processes, we set 95% C.L. upper limits on the production cross section ranging from 8.9 to 27 fb, depending on the mass of the excited electron. Choosing the scale for contact interactions to be Λ=1  TeV, excited electron masses below 756 GeV are excluded at the 95% C.L.

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 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.

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 present the first measurement of the integrated forward-backward charge asymmetry in top-quark–top-antiquark pair (tt̅ ) production in proton-antiproton (pp̅ ) collisions in the lepton+jets final state. Using a b-jet tagging algorithm and kinematic reconstruction assuming tt̅ +X production and decay, a sample of 0.9  fb^-1 of data, collected by the D0 experiment at the Fermilab Tevatron Collider, is used to measure the asymmetry for different jet multiplicities. The result is also used to set upper limits on tt̅ +X production via a Z^′ resonance.

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.

We present a study of μμμμ, eeee, and μμee events using 1  fb^-1 of data collected with the D0 detector at the Fermilab Tevatron pp̅ Collider at sqrt[s]=1.96  TeV. Requiring the lepton pair masses to be greater than 30 GeV, we observe one event, consistent with the expected background of 0.13±0.03 events and with the predicted standard model ZZ and Zγ^* production of 1.71±0.15 events. We set an upper limit on the ZZ and Zγ^* cross section of 4.4 pb at the 95% C.L. We also derive limits on anomalous neutral trilinear ZZZ and ZZγ^* gauge couplings. The one-parameter 95% C.L. coupling limits with a form-factor scale Λ=1.2  TeV are -0.28<f₄₀^Z<0.28, -0.31<f₅₀^Z<0.29, -0.26<f₄₀^γ<0.26, and -0.30<f₅₀^γ<0.28.

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 study the flavor-changing-neutral-current process c→uμ⁺μ^- using 1.3  fb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV recorded by the D0 detector operating at the Fermilab Tevatron Collider. We see clear indications of the charged-current mediated D_s⁺ and D⁺→ϕπ⁺→μ⁺μ^-π⁺ final states with significance greater than 4 standard deviations above background for the D⁺ state. We search for the continuum neutral-current decay of D⁺→π⁺μ⁺μ^- in the dimuon invariant mass spectrum away from the ϕ resonance. We see no evidence of signal above background and set a limit of B(D⁺→π⁺μ⁺μ^-)<3.9×10^-6 at the 90% C.L. This limit places the most stringent constraint on new phenomena in the c→uμ⁺μ^- transition.

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 present a measurement of the shape of the Z/γ^* boson transverse momentum (q_T) distribution in pp̅ →Z/γ^*→e⁺e^-+X events at a center-of-mass energy of 1.96 TeV using 0.98  fb^-1 of data collected with the D0 detector at the Fermilab Tevatron collider. The data are found to be consistent with the resummation prediction at low q_T, but above the perturbative QCD calculation in the region of q_T>30  GeV/c. Using events with q_T<30  GeV/c, we extract the value of g₂, one of the nonperturbative parameters for the resummation calculation. Data at large boson rapidity y are compared with the prediction of resummation and with alternative models that employ a resummed form factor with modifications in the small Bjorken x region of the proton wave function.

We search for decays of Kaluza-Klein excitations of the graviton in the Randall-Sundrum model of extra dimensions to e⁺e^- and γγ in 1  fb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV collected by the D0 detector at the Fermilab Tevatron. We set 95% confidence level upper limits on the production cross section times branching fraction, which translate into lower limits on the mass of the lightest excitation between 300 and 900 GeV for values of the coupling k/M̅ _Pl between 0.01 and 0.1.

We report on a search for gravitational waves from the coalescence of compact binaries during the third and fourth LIGO science runs. The search focused on gravitational waves generated during the inspiral phase of the binary evolution. In our analysis, we considered three categories of compact binary systems, ordered by mass: (i) primordial black hole binaries with masses in the range 0.35M_⊙<m₁, m₂<1.0M_⊙, (ii) binary neutron stars with masses in the range 1.0M_⊙<m₁, m₂<3.0M_⊙, and (iii) binary black holes with masses in the range 3.0M_⊙<m₁, m₂<m_max⁡ with the additional constraint m₁+m₂<m_max⁡, where m_max⁡ was set to 40.0M_⊙ and 80.0M_⊙ in the third and fourth science runs, respectively. Although the detectors could probe to distances as far as tens of Mpc, no gravitational-wave signals were identified in the 1364 hours of data we analyzed. Assuming a binary population with a Gaussian distribution around 0.75-0.75M_⊙, 1.4-1.4M_⊙, and 5.0-5.0M_⊙, we derived 90%-confidence upper limit rates of 4.9  yr^-1L₁₀^-1 for primordial black hole binaries, 1.2  yr^-1L₁₀^-1 for binary neutron stars, and 0.5  yr^-1L₁₀^-1 for stellar mass binary black holes, where L₁₀ is 10¹⁰ times the blue-light luminosity of the Sun.

We present a search for the decays B⁰→e⁺e^-, B⁰→μ⁺μ^-, and B⁰→e^±μ^∓ using data collected with the BABAR detector at the PEP-II e⁺e^- collider at SLAC. Using a data set corresponding to 384×10⁶ BB̅ pairs, we do not find evidence of any of the three decay modes. We obtain upper limit on the branching fractions, at 90% confidence level, of B(B⁰→e⁺e^-)<11.3×10^-8, B(B⁰→μ⁺μ^-)<5.2×10^-8, and B(B⁰→e^±μ^∓)<9.2×10^-8.

We report the direct observation of the excited L=1 state B_s2^* in fully reconstructed decays to B⁺K^-. The mass of the B_s2^* meson is measured to be 5839.6±1.1(stat)±0.7(syst)  MeV/c², and its production rate relative to the B⁺ meson is measured to be [1.15±0.23(stat)±0.13(syst)]%.

We report measurements of B-meson decays into two- and three-body final states containing two charmed baryons using a sample of 230×10⁶ Υ(4S)→BB̅ decays. We find significant signals in two modes, measuring branching fractions B(B^-→Λ_c⁺Λ̅ _c^-K^-)=(1.14±0.15±0.17±0.60)×10^-3 and B(B^-→Ξ_c⁰Λ̅ _c^-)×B(Ξ_c⁰→Ξ^-π⁺)=(2.08±0.65±0.29±0.54)×10^-5, where the uncertainties are statistical, systematic, and from the branching fraction B(Λ_c⁺→pK^-π⁺), respectively. We also set upper limits at the 90% confidence level on two other modes: B(B̅ ⁰→Ξ_c⁺Λ̅ _c^-)×B(Ξ_c⁺→Ξ^-π⁺π⁺)<5.6×10^-5 and B(B̅ ⁰→Λ_c⁺Λ̅ _c^-K̅ ⁰)<1.5×10^-3. We observe structure centered at an invariant mass of 2.93  GeV/c² in the Λ_c⁺K^- mass distribution of the decay B^-→Λ_c⁺Λ̅ _c^-K^-.

We present a new analysis of J/ψ production yields in deuteron-gold collisions at sqrt[s_NN]=200 GeV using data taken from the PHENIX experiment in 2003 and previously published in S. S. Adler [Phys. Rev. Lett 96, 012304 (2006)]. The high statistics proton-proton J/ψ data taken in 2005 are used to improve the baseline measurement and thus construct updated cold nuclear matter modification factors (R_dAu). A suppression of J/ψ in cold nuclear matter is observed as one goes forward in rapidity (in the deuteron-going direction), corresponding to a region more sensitive to initial-state low-x gluons in the gold nucleus. The measured nuclear modification factors are compared to theoretical calculations of nuclear shadowing to which a J/ψ (or precursor) breakup cross section is added. Breakup cross sections of σ_breakup=2.8_-1.4^+1.7 (2.2_-1.5^+1.6) mb are obtained by fitting these calculations to the data using two different models of nuclear shadowing. These breakup cross-section values are consistent within large uncertainties with the 4.2±0.5 mb determined at lower collision energies. Projecting this range of cold nuclear matter effects to copper-copper and gold-gold collisions reveals that the current constraints are not sufficient to firmly quantify the additional hot nuclear matter effect.