Search Results (879 total)

We present the first measurement of dijet angular distributions in pp̅ collisions at sqrt[s]=1.96  TeV at the Fermilab Tevatron Collider. The measurement is based on a dataset corresponding to an integrated luminosity of 0.7  fb^-1 collected with the D0 detector. Dijet angular distributions have been measured over a range of dijet masses, from 0.25 TeV to above 1.1 TeV. The data are in good agreement with the predictions of perturbative QCD and are used to constrain new physics models including quark compositeness, large extra dimensions, and TeV^-1 scale extra dimensions. For all models considered, we set the most stringent direct limits to date.

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 provide the most precise measurement of the WW production cross section in pp̅ collisions to date at a center of mass energy of 1.96 TeV, and set limits on the associated trilinear gauge couplings. The WW→ℓνℓ^′ν (ℓ, ℓ^′=e, μ) decay channels are analyzed in 1  fb^-1 of data collected by the D0 detector at the Fermilab Tevatron Collider. The measured cross section is σ(pp̅ →WW)=11.5±2.1(stat+syst)±0.7(lumi)  pb. One- and two-dimensional 95% C.L. limits on trilinear gauge couplings are provided.

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 combine measurements of the top quark pair production cross section in pp̅ collisions in the ℓ+jets, ℓℓ, and τℓ final states (where ℓ is an electron or muon) at a center of mass energy of sqrt[s]=1.96  TeV in 1  fb^-1 of data collected with the D0 detector. For a top quark mass of 170  GeV/c², we obtain σ_tt̅ =8.18_-0.87^+0.98  pb in agreement with the theoretical prediction. Based on predictions from higher order quantum chromodynamics, we extract a mass for the top quark from the combined tt̅ cross section, consistent with the world average of the top quark mass. In addition, the ratios of tt̅ cross sections in different final states are used to set upper limits on the branching fractions B(t→H⁺b→τ⁺νb) and B(t→H⁺b→cs̅ b) as a function of the charged Higgs boson mass.

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).

We present a measurement of the W boson mass in W→eν decays using 1  fb^-1 of data collected with the D0 detector during Run II of the Fermilab Tevatron collider. With a sample of 499830 W→eν candidate events, we measure M_W=80.401±0.043  GeV. This is the most precise measurement from a single experiment.

Bose-Einstein correlations of charged kaons are used to probe Au+Au collisions at sqrt[s_NN]=200  GeV and are compared to charged pion probes, which have a larger hadronic scattering cross section. Three-dimensional Gaussian source radii are extracted, along with a one-dimensional kaon emission source function. The centrality dependences of the three Gaussian radii are well described by a single linear function of N_part^1/3 with a zero intercept. Imaging analysis shows a deviation from a Gaussian tail at r≳10  fm, although the bulk emission at lower radius is well described by a Gaussian. The presence of a non-Gaussian tail in the kaon source reaffirms that the particle emission region in a heavy-ion collision is extended, and that similar measurements with pions are not solely due to the decay of long-lived resonances.

We present a search for charged Higgs bosons in decays of top quarks, in the mass range 80<m_H^±<155  GeV, assuming the subsequent decay H⁺→τ⁺ν_τ (and its charge conjugate). Using 0.9  fb^-1 of lepton+jets data collected with the D0 detector at the Fermilab Tevatron pp̅ collider, operating at a center of mass energy sqrt[s]=1.96  TeV, we find no evidence for a H^± signal. Hence we exclude branching ratios B(t→H⁺b)>0.24 for m_H^±=80  GeV and B(t→H⁺b)>0.19 for m_H^±=155  GeV at the 95% C.L.

We report on a matched-filter search for gravitational wave bursts from cosmic string cusps using LIGO data from the fourth science run (S4) which took place in February and March 2005. No gravitational waves were detected in 14.9 days of data from times when all three LIGO detectors were operating. We interpret the result in terms of a frequentist upper limit on the rate of gravitational wave bursts and use the limits on the rate to constrain the parameter space (string tension, reconnection probability, and loop sizes) of cosmic string models. Many grand unified theory-scale models (with string tension Gμ/c²≈10^-6) can be ruled out at 90% confidence for reconnection probabilities p≤10^-3 if loop sizes are set by gravitational back reaction.

We present a direct measurement of trilinear gauge boson couplings at γWW and ZWW vertices in WW and WZ events produced in pp̅ collisions at sqrt[s]=1.96  TeV. We consider events with one electron or muon, missing transverse energy, and at least two jets. The data were collected using the D0 detector and correspond to 1.1  fb^-1 of integrated luminosity. Considering two different relations between the couplings at the γWW and ZWW vertices, we measure these couplings at 68% C.L. to be κ_γ=1.07_-0.29^+0.26, λ=0.00_-0.06^+0.06, and g₁^Z=1.04_-0.09^+0.09 in a scenario respecting SU(2)_L⊗U(1)_Y gauge symmetry and κ=1.04_-0.11^+0.11 and λ=0.00_-0.06^+0.06 in an “equal couplings” scenario.

We present a measurement of the mass difference between t and t̅ quarks in lepton+jets final states of tt̅ events in 1  fb^-1 of data collected with the D0 detector from Fermilab Tevatron Collider pp̅ collisions at sqrt[s]=1.96  TeV. The measured mass difference of 3.8±3.7  GeV is consistent with the equality of t and t̅ masses. This is the first direct measurement of a mass difference between a quark and its antiquark partner.

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.

According to general relativity a perturbed black hole will settle to a stationary configuration by the emission of gravitational radiation. Such a perturbation will occur, for example, in the coalescence of a black hole binary, following their inspiral and subsequent merger. At late times the waveform is a superposition of quasinormal modes, which we refer to as the ringdown. The dominant mode is expected to be the fundamental mode, l=m=2. Since this is a well-known waveform, matched filtering can be implemented to search for this signal using LIGO data. We present a search for gravitational waves from black hole ringdowns in the fourth LIGO science run S4, during which LIGO was sensitive to the dominant mode of perturbed black holes with masses in the range of 10M_⊙ to 500M_⊙, the regime of intermediate-mass black holes, to distances up to 300 Mpc. We present a search for gravitational waves from black hole ringdowns using data from S4. No gravitational wave candidates were found; we place a 90%-confidence upper limit on the rate of ringdowns from black holes with mass between 85M_⊙ and 390M_⊙ in the local universe, assuming a uniform distribution of sources, of 3.2×10^-5  yr^-1 Mpc^-3=1.6×10^-3  yr^-1L₁₀^-1,where L₁₀ is 10¹⁰ times the solar blue-light luminosity.

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 present inclusive charged hadron elliptic flow (v₂) measured over the pseudorapidity range |η|<0.35 in Au+Au collisions at sqrt[s_NN]=200 GeV. Results for v₂ are presented over a broad range of transverse momentum (p_T=0.2-8.0 GeV/c) and centrality (0–60%). To study nonflow effects that are correlations other than collective flow, as well as the fluctuations of v₂, we compare two different analysis methods: (1) the event-plane method from two independent subdetectors at forward (|η|=3.1-3.9) and beam (|η|>6.5) pseudorapidities and (2) the two-particle cumulant method extracted using correlations between particles detected at midrapidity. The two event-plane results are consistent within systematic uncertainties over the measured p_T and in centrality 0–40%. There is at most a 20% difference in the v₂ between the two event-plane methods in peripheral (40–60%) collisions. The comparisons between the two-particle cumulant results and the standard event-plane measurements are discussed.

We report the observation at the Relativistic Heavy Ion Collider of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron (γ-h) background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics, making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, I_AA, in central Au+Au collisions, is 0.32±0.12^stat±0.09^syst for hadrons of 3<p_T^h<5 in coincidence with photons of 5<p_T^γ<15 GeV/c. The suppression is comparable to that observed for high-p_T single hadrons and dihadrons. The direct photon associated yields in p+p collisions scale approximately with the momentum balance, z_T≡p_T^h/p_T^γ, as expected for a measurement of the away-side parton fragmentation function. We compare to Au+Au collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss.

We report on a search for gravitational waves from coalescing compact binaries, of total mass between 2 and 35M_⊙, using LIGO observations between November 14, 2006 and May 18, 2007. No gravitational-wave signals were detected. We report upper limits on the rate of compact binary coalescence as a function of total mass. The LIGO cumulative 90%-confidence rate upper limits of the binary coalescence of neutron stars, black holes and black hole-neutron star systems are 1.4×10^-2, 7.3×10^-4 and 3.6×10^-3  yr^-1 L₁₀^-1, respectively, where L₁₀ is 10¹⁰ times the blue solar luminosity.

We report observation of the electroweak production of single top quarks in pp̅ collisions at sqrt[s]=1.96  TeV based on 2.3  fb^-1 of data collected by the D0 detector at the Fermilab Tevatron Collider. Using events containing an isolated electron or muon and missing transverse energy, together with jets originating from the fragmentation of b quarks, we measure a cross section of σ(pp̅ →tb+X,tqb+X)=3.94±0.88  pb. The probability to measure a cross section at this value or higher in the absence of signal is 2.5×10^-7, corresponding to a 5.0 standard deviation significance for the observation.

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 search for a new light gauge boson, a dark photon, with the D0 experiment. In the model we consider, supersymmetric partners are pair produced and cascade to the lightest neutralinos that can decay into the hidden sector state plus either a photon or a dark photon. The dark photon decays through its mixing with a photon into fermion pairs. We therefore investigate a previously unexplored final state that contains a photon, two spatially close leptons, and large missing transverse energy. We do not observe any evidence for dark photons and set a limit on their production.

We report on a first search for resonant pair production of neutral long-lived particles (NLLP) which each decay to a bb̅ pair, using 3.6  fb^-1 of data recorded with the D0 detector at the Fermilab Tevatron collider. We search for pairs of displaced vertices in the tracking detector at radii in the range 1.6–20 cm from the beam axis. No significant excess is observed above background, and upper limits are set on the production rate in a hidden-valley benchmark model for a range of Higgs boson masses and NLLP masses and lifetimes.

This paper reports on an all-sky search for periodic gravitational waves from sources such as deformed isolated rapidly spinning neutron stars. The analysis uses 840 hours of data from 66 days of the fifth LIGO science run (S5). The data were searched for quasimonochromatic waves with frequencies f in the range from 50 to 1500 Hz, with a linear frequency drift f˙ (measured at the solar system barycenter) in the range -f/τ<f˙<0.1f/τ, for a minimum spin-down age τ of 1000 years for signals below 400 Hz and 8000 years above 400 Hz. The main computational work of the search was distributed over approximately 100 000 computers volunteered by the general public. This large computing power allowed the use of a relatively long coherent integration time of 30 hours while searching a large parameter space. This search extends Einstein@Home’s previous search in LIGO S4 data to about 3 times better sensitivity. No statistically significant signals were found. In the 125–225 Hz band, more than 90% of sources with dimensionless gravitational-wave strain tensor amplitude greater than 3×10^-24 would have been detected.