Search Results (197 total)

We have investigated the Mott transition in a quasi-two-dimensional Mott insulator EtMe₃P[Pd(dmit)₂]₂ with a spin-frustrated triangular-lattice in hydrostatic pressure and magnetic-field [Et and Me denote C₂H₅ and CH₃, respectively, and Pd(dmit)₂ (dmit=1,3-dithiole-  2-thione-4,5-dithiolate,dithiolate) is an electron-acceptor molecule]. In the pressure-temperature (P-T) phase diagram, a valence-bond solid phase is found to neighbor the superconductor and metal phases at low temperatures. The profile of the phase diagram is common to those of Mott insulators with antiferromagnetic order. In contrast to the antiferromagnetic Mott insulators, the resistivity in the metallic phase exhibits anomalous temperature dependence, ρ=ρ₀+AT^2.5.

The viscosity is measured for a Fermi liquid, a dilute ³He-⁴He mixture, under extremely high magnetic field/temperature conditions (B≤14.8  T, T≥1.5  mK). The spin-splitting energy μB is substantially greater than the Fermi energy k_BT_F; as a consequence the polarization tends to unity and s-wave quasiparticle scattering is suppressed for T≪T_F. Using a novel composite vibrating-wire viscometer an enhancement of the viscosity is observed by a factor of more than 500 over its low-field value. Good agreement is found between the measured viscosity and theoretical predictions based upon a t-matrix formalism.

We report the observation of strong electron dephasing in a series of disordered Cu₉₃Ge₄Au₃ thin films. A very short electron dephasing time possessing very weak temperature dependence around 6 K, followed by an upturn with further decrease in temperature below 4 K, is found. The upturn is progressively more pronounced in more disordered samples. Moreover, a ln⁡T-dependent, but high-magnetic-field-insensitive, resistance rise persisting from above 10 K down to 30 mK is observed in the films. These results suggest a nonmagnetic dephasing process which is stronger than any known mechanism and may originate from the coupling of conduction electrons to dynamic defects.

Measurements of the dc resistivity of surface-state electrons on liquid helium exposed to microwave radiation are reported. It is shown that the resonant microwave excitation of surface-state electrons is accompanied by a strong increase in their resistivity, which is opposite to the result expected from the previously used two-level model. We show that even a very small fraction of electrons excited to the first excited state and decaying back due to vapor-atom scattering strongly heat the electron system, causing a population of higher subbands. The calculated resistivity change is in good agreement with the observed data.

We present the results from our heat capacity experiments on two-dimensional liquid ³He on thin superfluid ⁴He films adsorbed to Nuclepore. Measurement of the specific heat at low temperatures (40<T<220 mK) over a ³He coverage range 0.02–0.93 atomic layers on 3.14 and 4.33 bulk-density atomic layer ⁴He films enables the determination of the ³He quasiparticle effective mass m^*. Combining these results with previous ³He NMR magnetic susceptibility measurements for the same substrate and ⁴He coverages permits the determination of the two-dimensional Landau Fermi-liquid parameters F₀^A and F₁^S.

We measure the hadronic branching ratios of the Z⁰ boson into heavy quarks: R_b=Γ_Z⁰→bb̅ /Γ_{Z⁰→hadrons} and R_c=Γ_Z⁰→cc̅ /Γ_{Z⁰→hadrons} using a multitag technique. The measurement was performed using about 400 000 hadronic Z⁰ events recorded in the SLC Large Detector experiment at SLAC between 1996 and 1998. The small and stable SLAC Linear Collider beam spot and the CCD-based vertex detector were used to reconstruct bottom and charm hadron decay vertices with high efficiency and purity, which enables us to measure most efficiencies from data. We obtain, R_b=0.21604±0.00098(stat.)±0.00073(syst.)∓0.00012(R_c) and, R_c=0.1744±0.0031(stat.)±0.0020(syst.)∓0.0006(R_b).

Exploiting the manipulation of the SLAC Linear Collider electron-beam polarization, we present precise direct measurements of the parity-violation parameters A_c and A_b in the Z-boson–c-quark and Z-boson–b-quark coupling. Quark-antiquark discrimination is accomplished via a unique algorithm that takes advantage of the precise SLAC Large Detector charge coupled device vertex detector, employing the net charge of displaced vertices as well as the charge of kaons that emanate from those vertices. From the 1996–1998 sample of 400 000 Z decays, produced with an average beam polarization of 73.4%, we find A_c=0.673±0.029(stat)±0.023(syst) and A_b=0.919±0.018(stat)±0.017(syst).

The results based on 1992–95 data (Run 1) from the CDF and D0 experiments on the measurements of the W boson mass and width are presented, along with the combined results. We report a Tevatron collider average M_W=80.456±0.059  GeV. We also report the Tevatron collider average of the directly measured W boson width Γ_W=2.115±0.105  GeV. We describe a new joint analysis of the direct W mass and width measurements. Assuming the validity of the standard model, we combine the directly measured W boson width with the width extracted from the ratio of W and Z boson leptonic partial cross sections. This combined result for the Tevatron is Γ_W=2.135±0.050  GeV. Finally, we use the measurements of the direct total W width and the leptonic branching ratio to extract the leptonic partial width Γ(W→eν)=224±13  MeV.

We report results from a study of events with a double-Pomeron exchange topology produced in p̅ p collisions at sqrt[s]=1800  GeV. The events are characterized by a leading antiproton and a large rapidity gap on the outgoing proton side. We find that the differential production cross section agrees in shape with predictions based on Regge theory and factorization, and that the ratio of double-Pomeron exchange to single diffractive production rates is relatively unsuppressed as compared to the O(10) suppression factor previously measured in single diffractive production.

We present improved measurements of the differential production rates of stable charged particles in hadronic Z⁰ decays, and of charged pions, kaons, and protons identified over a wide momentum range using the SLD Cherenkov ring imaging detector. In addition to flavor-inclusive Z⁰ decays, measurements are made for Z⁰ decays into light (u, d, s), c, and b primary flavors, selected using the upgraded vertex detector. Large differences between the flavors are observed that are qualitatively consistent with expectations based upon previously measured production and decay properties of heavy hadrons. These results are used to test the predictions of QCD in the modified leading logarithm approximation, with the ansatz of local parton-hadron duality, and the predictions of three models of the hadronization process. The light-flavor results provide improved tests of these predictions; the heavy-flavor results provide complementary model tests. In addition we have compared hadron and antihadron production in light quark (as opposed to antiquark) jets. Differences are observed at high momentum for all three charged hadron species, providing direct probes of leading particle effects, and stringent constraints on models.

We report on a search for direct Kaluza-Klein graviton production in a data sample of 84   pb^-1 of pp̅ collisions at sqrt[s]=1.8  TeV, recorded by the Collider Detector at Fermilab. We investigate the final state of large missing transverse energy and one or two high energy jets. We compare the data with the predictions from a (3+1+n)-dimensional Kaluza-Klein scenario in which gravity becomes strong at the TeV scale. At 95% confidence level (C.L.) for n=2, 4, and 6 we exclude an effective Planck scale below 1.0, 0.77, and 0.71 TeV, respectively.

We present the results of a search for pair production of scalar top quarks (t˜₁) in an R-parity violating supersymmetry scenario in 106   pb^-1 of pp̅ collisions at sqrt[s]=1.8  TeV collected by the Collider Detector at Fermilab. In this mode each t˜₁ decays into a τ lepton and a b quark. We search for events with two τ’s, one decaying leptonically (e or μ) and one decaying hadronically, and two jets. No candidate events pass our final selection criteria. We set a 95% confidence level lower limit on the t˜₁ mass at 122  GeV/c² for Br(t˜₁→τb)=1.

We have measured the number of like-sign (LS) and opposite-sign (OS) lepton pairs arising from double semileptonic decays of b and b̅ hadrons, pair produced at the Fermilab Tevatron collider. The data samples were collected with the Collider Detector at Fermilab during the 1992–1995 collider run by triggering on the existence of μμ or eμ candidates in an event. The observed ratio of LS to OS dileptons leads to a measurement of the average time-integrated mixing probability of all produced b-flavored hadrons which decay weakly, χ̅ =0.152±0.007 (stat)±0.011 (syst), that is significantly larger than the world average χ̅ =0.118±0.005.

We report on a search for a high mass, narrow width particle that decays directly to eμ, eτ, or μτ. We use approximately 110   pb^-1 of data collected with the Collider Detector at Fermilab from 1992 to 1995. No evidence of lepton flavor violating decays is found. Limits are set on the production and decay of sneutrinos with R-parity violating interactions.

Inclusive momentum distributions of charged particles in restricted cones around jet axes were measured in dijet events with invariant dijet masses in the range 80 to 600 GeV/c². Events were produced at the Fermilab Tevatron in pp̅ collisions with a center of mass energy of 1.8 TeV and recorded by the Collider Detector at Fermilab. The results were compared to perturbative QCD calculations carried out in the framework of the modified leading log approximation (MLLA) and assuming local parton-hadron duality. It was shown that the data follow theoretical predictions quite well over the whole range of the jet energies included in this analysis. We extracted the MLLA cutoff scale Q_eff and found a value of 230±40 MeV. The theoretical prediction of E_jetsin θ_c scaling, where θ_c is the cone opening angle, was experimentally observed for the first time. From the MLLA fits to the data, two more parameters were extracted: the ratio of parton multiplicities in gluon and quark jets, r=N_partons^g-jet/N_partons^q-jet=1.9±0.5, and the ratio of the number of charged hadrons to the number of predicted partons in a jet, K_LPHD^charged=N_hadrons^charged/N_partons=0.56±0.10.

We report a measurement of the fraction of events with a large pseudorapidity gap Δη within the pseudorapidity region available to the proton dissociation products X in p̅ +p→p̅ +X. For a final state p̅ of fractional momentum loss ξ_p̅ and 4-momentum transfer squared t_p̅ within 0.06<ξ_p̅ <0.09 and |t_p̅ |<1.0 [0.2] GeV² at sqrt[s]=1800 [630] GeV, the fraction of events with Δη>3 is found to be 0.246±0.001 (stat)±0.042 (syst) [0.184±0.001 (stat)±0.043 (syst)]. Our results are compared with gap fractions measured in minimum bias p̅ p collisions and with theoretical expectations.

We have searched for pair production of the supersymmetric partner of the top quark (stop) in 107   pb^-1 of pp̅ collisions at sqrt[s]=1.8   TeV collected by the Collider Detector at Fermilab (CDF). Each stop is assumed to decay into a lepton, bottom quark, and supersymmetric neutrino. Such a scenario would give rise to events with two leptons, two hadronic jets, and a substantial imbalance of transverse energy. No evidence of such a stop signal has been found. We exclude stop masses in the region (80≤m_t˜≤135   GeV/c²) in the mass plane of stop versus sneutrino.

We present a search for associated production of the Υ(1S) and a vector boson in 83   pb^-1 of pp̅ collisions at sqrt[s]=1.8   TeV collected by the CDF experiment in 1994–1995. We find no evidence of the searched signal in the data, and set upper limits to the production cross sections.

We present an improved direct measurement of the parity-violation parameter A_b in the Z boson–b-quark coupling using a self-calibrating track-charge technique applied to a sample enriched in Z→bb̅ events via the topological reconstruction of the B hadron mass. Manipulation of the Stanford Linear Collider electron-beam polarization permits the measurement of A_b to be made independently of other Z-pole coupling parameters. From the 1996–1998 sample of 400 000 hadronic Z decays, produced with an average beam polarization of 73.4%, we find A_b=0.906±0.022(stat)±0.023(syst).

We report a search for the production of long-lived charged massive particles in a data sample of 90   pb^-1 of sqrt[s]=1.8   TeV pp̅ collisions recorded by the Collider Detector at Fermilab. The search uses the muonlike penetration and anomalously high ionization energy loss signature expected for such a particle to discriminate it from backgrounds. The data are found to agree with background expectations, and cross section limits of O(1) pb are derived using two reference models, a stable quark and a stable scalar lepton.

Spin-echo experiments are reported for ³He-⁴He solutions under extremely high B/T conditions, B=14.75  T and T≥1.73  mK. The ³He concentration x₃ was adjusted close to the value x_c≈3.8% at which the spin-rotation parameter μM₀ vanishes. In this way the transverse and longitudinal spin-diffusion coefficients D_⊥,D_∥ were measured while keeping |μM₀|<1. It is found that the temperature dependence of D_⊥ deviates strongly from 1/T², with anisotropy temperature T_a=4.26_-0.44^+0.18  mK. This value is close to the theoretical prediction for dilute solutions and suggests that spin current relaxation remains finite as the temperature tends to zero.

We report the results of a search for a W^′ boson produced in pp̅ collisions at a center-of-mass energy of 1.8 TeV using a 106   pb^-1 data sample recorded by the Collider Detector at Fermilab. We observe no significant excess of events above background for a W^′ boson decaying to a top and bottom quark pair. In a model where this boson would mediate interactions involving a massive right-handed neutrino (ν_R) and have standard model strength couplings, we use these data to exclude a W^′ boson with mass between 225 and 536   GeV/c² at 95% confidence level for M_W^′≫M_νR and between 225 and 566   GeV/c² at 95% confidence level for M_W^′<M_νR.

We report a search for B_s⁰-B_s⁰̅ oscillations using a sample of 400 000 hadronic Z⁰ decays collected by the SLD experiment. The analysis takes advantage of the electron beam polarization as well as information from the hemisphere opposite that of the reconstructed B decay to tag the B production flavor. The excellent resolution provided by the pixel CCD vertex detector is exploited to cleanly reconstruct both B and cascade D decay vertices, and tag the B decay flavor from the charge difference between them. We exclude the following values of the B_s⁰-B_s⁰̅ oscillation frequency: Δm_s<4.9 ps^-1 and 7.9<Δm_s<10.3 ps^-1 at the 95% confidence level.

We have performed a search for radiative b-hadron decays using events produced in pp̅ collisions at sqrt[s]=1.8 TeV and collected by the Collider Detector at Fermilab. The decays we considered were B_d⁰→K^*0(→K^-π⁺)γ, B_s⁰→φ(→K⁺K^-)γ, Λ_b⁰→Λ(→pπ^-)γ, and their charge conjugates. Two independent methods to identify photons from such decays were employed. In the first method, the photon was detected in the electromagnetic calorimeter. In the second method, the photon was identified by an electron-positron pair produced through the external photon conversion before the tracking detector volume. By combining the two methods we obtain upper limits on the branching fractions for the B_d⁰, B_s⁰, and Λ_b⁰ radiative decays which, at the 95% confidence level, are found to be B(B_d⁰→K^*0γ)<1.4×10^-4, B(B_s⁰→φγ)<1.6×10^-4, and B(Λ_b⁰→Λγ)<1.9×10^-3.