Search Results (298 total)

We present measurements of the top quark mass (m_t) in tt̅ candidate events with two final state leptons using 1  fb^-1 of data collected by the D0 experiment. Our data sample is selected by requiring two fully identified leptons or by relaxing one lepton requirement to an isolated track if at least one jet is tagged as a b jet. The top quark mass is extracted after reconstructing the event kinematics under the tt̅ hypothesis using two methods. In the first method, we integrate over expected neutrino rapidity distributions, and in the second we calculate a weight for the possible top quark masses based on the observed particle momenta and the known parton distribution functions. We analyze 83 candidate events in the data and obtain m_t=176.2±4.8(stat)±2.1(sys)  GeV and m_t=173.2±4.9(stat)±2.0(sys)  GeV for the two methods, respectively. Accounting for correlations between the two methods, we combine the measurements to obtain m_t=174.7±4.4(stat)±2.0(sys)  GeV.

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 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 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 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.

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 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.

The Hartree-Fock approximation to the many-fermion problem can break exact symmetries, and in some cases by changing a parameter in the interaction one can drive the Hartree-Fock minimum from a symmetry-breaking state to a symmetry-conserving state (also referred to as a “phase transition” in the literature). The order of the transition is important when one applies the random-phase approximation (RPA) to the of the Hartree-Fock wave function: if first order, RPA is stable through the transition, but if second-order, then the RPA amplitudes become large and lead to unphysical results. The latter is known as “collapse” of the RPA. While the difference between first- and second-order transitions in the RPA was first pointed out by Thouless, we present for the first time nontrivial examples of both first- and second-order transitions in a uniform model, the interacting shell-model, where we can compare to exact numerical results.

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 carry out an interacting shell-model study of binding energies and spectra in the sd-shell nuclei to examine the effect of truncation of the shell-model spaces. Starting with a Hamiltonian defined in a larger space and truncating to the sd shell, the binding energies are strongly affected by the truncation, but the effect on the excitation energies is an order of magnitude smaller. We then refit the matrix elements of the two-particle interaction to compensate for the space truncation and find that it is easy to capture 90% of the binding energy shifts by refitting a few parameters. With the full parameter space of the two-particle Hamiltonian, we find that both the binding energies and the excitation energy can be fitted with remaining residual error about 5% of the average error from the truncation. Numerically, the rms initial error associated with our Hamiltonian is 3.4 MeV and the remaining residual error is 0.16 MeV. This is comparable to the empirical error found in sd-shell interacting shell-model fits to experimental data [B. A. Brown and W. A. Richter, Phys. Rev. C 74, 034315 (2006)].

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.

We report on a first search for production of the lightest neutral CP-even Higgs boson (h) in the next-to-minimal supersymmetric standard model, where h decays to a pair of neutral pseudoscalar Higgs bosons (a), using 4.2  fb^-1 of data recorded with the D0 detector at Fermilab. The a bosons are required to either both decay to μ⁺μ^- or one to μ⁺μ^- and the other to τ⁺τ^-. No significant signal is observed, and we set limits on its production as functions of M_a and M_h.

We present a search for the standard model Higgs boson using hadronically decaying tau leptons, in 1  fb^-1 of data collected with the D0 detector at the Fermilab Tevatron pp̅ collider. We select two final states: τ^± plus missing transverse energy and b jets, and τ⁺τ^- plus jets. These final states are sensitive to a combination of associated W/Z boson plus Higgs boson, vector boson fusion, and gluon-gluon fusion production processes. The observed ratio of the combined limit on the Higgs production cross section at the 95% C.L. to the standard model expectation is 29 for a Higgs boson mass of 115 GeV.

We present a search for a narrow resonance in the inclusive diphoton final state using ∼2.7  fb^-1 of data collected with the D0 detector at the Fermilab Tevatron pp̅ Collider. We observe good agreement between the data and the background prediction, and set the first 95% C.L. upper limits on the production cross section times the branching ratio for decay into a pair of photons for resonance masses between 100 and 150 GeV. This search is also interpreted in the context of several models of electroweak symmetry breaking with a Higgs boson decaying into two photons.

We report on a study of the relative rates of B meson decays into ψ(2S) and J/ψ mesons 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 observe the channels B_s⁰→ψ(2S)ϕ, B_s⁰→J/ψϕ, B^±→ψ(2S)K^±, and B^±→J/ψK^± and we measure the relative branching fractions for these channels to be B(B_s⁰→ψ(2S)ϕ) / B(B_s⁰→J/ψϕ)=0.53±0.10(stat)±0.07(syst)±0.06(B),  B(B^±→ψ(2S)K^±) / B(B^±→J/ψK^±)=0.63±0.05(stat)±0.03(syst)±0.07(B),where the final error corresponds to the uncertainty in the J/ψ and ψ(2S) branching ratio into two muons.

We present the first observation of the Zγ→νν̅ γ process at the Fermilab Tevatron at 5.1 standard deviations significance, based on 3.6  fb^-1 of integrated luminosity collected with the D0 detector at the Fermilab Tevatron pp̅ Collider at sqrt[s]=1.96  TeV. The measured Zγ production cross section multiplied by the branching fraction of Z→νν̅ is 32±9(stat+syst)±2(lumi)  fb for the photon E_T>90  GeV. It is in agreement with the standard model prediction of 39±4  fb. We set limits on anomalous trilinear Zγγ and ZZγ gauge boson couplings, most of which are the most restrictive to date.

We describe a search for production of a charged Higgs boson, qq̅ ^′→H⁺, reconstructed in the tb̅ final state in the mass range 180≤M_H⁺≤300  GeV. The search was undertaken at the Fermilab Tevatron collider with a center-of-mass energy sqrt[s]=1.96  TeV and uses 0.9  fb^-1 of data collected with the D0 detector. We find no evidence for charged Higgs boson production and set upper limits on the production cross section in the types I, II, and III two-Higgs-doublet models (2HDMs). An excluded region in the (M_H⁺, tan⁡β) plane for type I 2HDM is presented.

First measurements of the differential cross sections d³σ/(dp_T^γdy^γdy^jet) for the inclusive production of a photon in association with a heavy quark (b, c) jet are presented, covering photon transverse momenta 30<p_T^γ<150  GeV, photon rapidities |y^γ|<1.0, jet rapidities |y^jet|<0.8, and jet transverse momenta p_T^jet>15  GeV. The results are based on an integrated luminosity of 1  fb^-1 in pp̅ collisions at sqrt[s]=1.96  TeV recorded with the D0 detector at the Fermilab Tevatron Collider. The results are compared with next-to-leading order perturbative QCD predictions.

We search for long-lived charged massive particles using 1.1  fb^-1 of data collected by the D0 detector at the Fermilab Tevatron pp̅ Collider. Time-of-flight information is used to search for pair produced long-lived tau sleptons, gauginolike charginos, and Higgsino-like charginos. We find no evidence of a signal and set 95% C.L. cross section upper limits for staus, which vary from 0.31 to 0.04 pb for stau masses between 60 and 300 GeV. We also set lower mass limits of 206 GeV (171 GeV) for pair produced charged gauginos (Higgsinos).

We present first evidence for WW+WZ production in lepton+jets final states at a hadron collider. The data correspond to 1.07  fb^-1 of integrated luminosity collected with the D0 detector at the Fermilab Tevatron in pp̅ collisions at sqrt[s]=1.96  TeV. The observed cross section for WW+WZ production is 20.2±4.5  pb, consistent with the standard model and more precise than previous measurements in fully leptonic final states. The probability that background fluctuations alone produce this excess is <5.4×10^-6, which corresponds to a significance of 4.4 standard deviations.

Epitaxial films of Mn²⁺-doped ZnO were deposited by pulsed laser deposition on α-Al₂O₃(0001) using targets created from Mn²⁺-doped ZnO nanoparticles. Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism, Mn(II) was found to substitute for Zn(II) in the würtzite ZnO lattice with only a paramagnetic dichroic component from the Mn and no magnetic component from either the O or Zn. The dichroism reveals that, while substitutional, the Mn²⁺ distribution in the ZnO lattice is not stochastic. Rather, Mn²⁺ has a tendency to substitute with higher effective local concentrations than anticipated from a stochastic doping model.

Anomalous Wtb couplings modify the angular correlations of the top-quark decay products and change the single top-quark production cross section. We present limits on anomalous top-quark couplings by combining information from W boson helicity measurements in top-quark decays and anomalous coupling searches in the single top-quark final state. We set limits on right-handed vector couplings as well as left-handed and right-handed tensor couplings based on about 1  fb^-1 of data collected by the D0 experiment.