Search for neutral, long-lived particles decaying into two muons in pp[over] collisions at sqrt[s]=1.96 TeV

We present a search for a neutral particle, pair produced in pp[over] collisions at sqrt[s]=1.96 TeV, which decays into two muons and lives long enough to travel at least 5 cm before decaying. The analysis uses approximately 380 pb(-1) of data recorded with the D0 detector. The background is estimated to be about one event. No candidates are observed, and limits are set on the pair-production cross section times branching fraction into dimuons + X for such particles. For a mass of 10 GeV and lifetime of 4x10(-11) s, we exclude values greater than 0.14 pb (95% C.L.). These results are used to limit the interpretation of NuTeV's excess of dimuon events.     

Search for ZH → l+ l- bb production in 4.2 fb(-1) of pp collisions at sqrt[s] =1 .96 TeV

We present a search for the standard model Higgs boson produced in association with a Z boson in 4.2 fb(-1) of pp collisions, collected with the D0 detector at the Fermilab Tevatron at sqrt[s] =1 .96 TeV. Selected events contain one reconstructed Z → e+ e- or Z → μ+ μ- candidate and at least two jets, including at least one b-tagged jet. In the absence of an excess over the background expected from other standard model processes, limits on the ZH cross section multiplied by the branching ratios are set. The limit at M(H) = 115 GeV is a factor of 5.9 larger than the standard model prediction.     

Search for long-lived particles decaying into electron or photon pairs with the D0 detector

In this Letter we report on a search for long-lived particles that decay into final states with two electrons or photons. Such long-lived particles arise in a variety of theoretical models, such as hidden valleys and supersymmetry with gauge-mediated breaking. By precisely reconstructing the direction of the electromagnetic shower we are able to probe much longer lifetimes than previously explored. We see no evidence of the existence of such long-lived particles and interpret this search as a quasi model-independent limit on their production cross section, as well as a limit on a long-lived fourth generation quark.     

Simultaneous measurement of the ratio R=B(t --> Wb)/B(t --> Wq) and the top-quark pair production cross section with the D0 detector at sqrt(s) = 1.96 TeV

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 sigma(tt[over]) in the lepton plus jets channel using 0.9 fb(-1) of pp[over] collision data at sqrt(s)=1.96 TeV collected with the D0 detector. We extract R and sigma(tt[over]) by analyzing samples of events with 0, 1, and > or =2 identified b jets. We measure R=0.97(+0.09)/(-0.08)(stat+syst) and sigma(tt[over])=8.18(+0.09)(-0.84)(stat+syst) +/- 0.50(lumi) pb, in agreement with the standard model prediction.     

Measurement of the forward-backward charge asymmetry in top-quark pair production

We present the first measurement of the integrated forward-backward charge asymmetry in top-quark-top-antiquark pair (tt) production in proton-antiproton (pp) collisions in the lepton+jets final state. Using a b-jet tagging algorithm and kinematic reconstruction assuming tt + 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 tt+X production via a Z' resonance.     

Observation and properties of the orbitally excited B_(s2)(*) meson

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/c(2), and its production rate relative to the B+ meson is measured to be [1.15+/-0.23(stat)+/-0.13(syst)]%.     

Observation of ZZ production in p-p collisions at sqrt s=1.96 TeV

We present an observation for ZZ-->l+l-l'+l'- (l, l'=e or mu) production in p[over]p collisions at a center-of-mass energy of sqrt[s]=1.96 TeV. Using 1.7 fb(-1) of data collected by the D0 experiment at the Fermilab Tevatron Collider, we observe three candidate events with an expected background of 0.14(+0.03)_(-0.02) events. The significance of this observation is 5.3 standard deviations. The combination of D0 results in this channel, as well as in ZZ-->l+l- nu[over]nu, yields a significance of 5.7 standard deviations and a combined cross section of sigma(ZZ)=1.60+/-0.63(stat)+0.16_-0.17(syst) pb.     

Observation of the Bc meson in the exclusive decay Bc-->J/psipi

A fully reconstructed Bc-->J/psipi signal is observed with the D0 detector at the Fermilab Tevatron pp[over] collider using 1.3 fb(-1) of integrated luminosity. The signal consists of 54+/-12 candidates with a significance that exceeds 5 standard deviations, and confirms earlier observations of this decay. The measured mass of the Bc meson is 6300+/-14(stat)+/-5(syst) MeV/c2.     

Effects of ultraviolet irradiation during growth of ZnSe epilayers by atmospheric pressure metalorganic vapor-phase epitaxy using dimethyl zinc and H2Se

This work has investigated the effects of ultraviolet irradiation on the epitaxial growth process of undoped ZnSe by atmospheric-pressure metalorganic vapor phase epitaxy. Dimethyl zinc and H₂Se at a [VI]/[II] mole ratio of 20 were the source gases used for growth onto (100)-just oriented semi-insulating GaAs substrates. Hydrogen was used as the carrier gas. A 500 W Hg-Xe lamp irradiated the substrate at 300 nm wavelength during growth. Growth temperature was varied from 210 to 450°C. Epilayers grown in the presence of irradiation experienced a prominent decrease in growth rate, which occurred even at high temperatures. Through a combination of surface and vapor-phase reactions, UV irradiation also affected the photoluminescence properties, crystalline quality, and surface morphology of the epilayers.     

Response normalized liquid chromatography nanospray ionization mass spectrometry

The widely different LC-MS response observed for many structurally different compounds limits the use of LC-MS in full scan detection mode for quantitative determination of drugs and metabolites without using reference standard. The recently introduced nanospray ionization (NSI) technique shows comparable MS response for some compounds under non-LC-MS conditions. However, in the presence of numerous endogenous compounds commonly associated with biological samples such as urine, plasma, and bile, LC-MS is required to separate, detect, identify, and measure individual analytes. An LC-NSI-MS system was devised and the MS response obtained in this system for a variety of pharmaceutical drugs and their metabolites. The set-up involves two high-performance liquid chromatography (HPLC) systems, a chip-based NSI source and a quadrupole-time-of-flight (Q-TOF) mass spectrometer. Herein this is referred to as the response normalized-liquid chromatography NSI-MS (RNLC-NSI-MS) system. One HPLC unit performs the analytical separation, while the other unit adds solvent post-column with an exact reverse of the mobile phase composition such that the final composition entering the NSI source is isocratic throughout the entire HPLC run. The data obtained from four different structural classes of compounds [vicriviroc (VCV), desloratadine (DL), tolbutamide, and cocaine] and their metabolites indicate that by maintaining the solvent composition unchanged across the HPLC run, the influence of the solvent environment on the ionization efficiency is minimized. In comparison to responses obtained from radiochromatograms, responses from conventional LC-ESI-MS overestimated the VCV and DL responses, respectively, by 6- and 20-fold. Although VCV and DL responses obtained using LC-NSI-MS are within 2- to 6-fold from the respective radiochromatographic responses, the response normalization modification results in nearly uniform LC-NSI-MS response for all compounds evaluated.