Search Results (277 total)

Electronic properties of the narrow-gap semiconductor FeSb₂ have been studied by the magnetization and ^121∕123Sb nuclear quadrupole resonance measurements. In addition to the susceptibility, the spin-lattice relaxation rate has revealed a fully opened gap at the Fermi level. The relaxation process is dominated by the magnetic contribution at high temperature T, while the quadrupole contribution becomes dominant below 70  K. Electronic field gradient at Sb sites shows anisotropic T dependence, reflecting most probably the anisotropic thermal expansion.

Propagation of femtosecond light pulses in a resonant absorption medium is studied. The propagation time of the light pulses was measured in a dye solution by the optical-Kerr-gate method. Nonadherence to the conventional group velocity dω∕dk, which is defined in weak absorption region, was found. The observed wavelength dependences of the group delay and the spectral change in the anomalous dispersion region are qualitatively in good agreement with the theoretical prediction obtained from the new definitions of group velocity, which can be clearly defined even in strong absorption region.

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 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.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

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 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.

The observation of slow-wave sustained (SW) discharge in a whistler- or helicon-wave range of frequency is made using high-frequency and very-high-frequency bands of rf. The SW discharge occurs at an extremely low rf power and plasma density, which are lower than a capacitive-coupling discharge region.

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 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.

The exclusive γE̸_T signal has a small standard model cross section and is thus a channel sensitive to new physics. This signature is predicted by models with a superlight gravitino or with large extra spatial dimensions. We search for such signals at the Collider Detector at Fermilab, using 87   pb^-1 of data at sqrt[s]=1.8   TeV, and extract 95% C.L. limits on these processes. A limit of 221 GeV is set on the scale |F|^1/2 in supersymmetric models. For 4, 6, and 8 extra dimensions, model-dependent limits on the fundamental mass scale M_D of 0.55, 0.58, and 0.60 TeV, respectively, are found. We also specify a “pseudo-model-independent” method of comparing the results to theoretical predictions.

The inclusive cross section for J/ψ production times the branching ratio B(J/ψ→μ⁺μ^-) has been measured in the forward pseudorapidity region: B×dσ[p̅ +p→J/ψ(p_T>10 GeV/c,2.1<|η|<2.6)+X]/dη=192±9(stat)±29(syst) pb. The results are based on 74.1±5.2 pb^-1 of data collected by the CDF Collaboration at the Fermilab Tevatron Collider. The measurements extend earlier measurements of the D0 Collaboration to higher p_T^J/ψ. In the kinematic range where the experiments partially overlap, these data are in good agreement with previous measurements.

We report on measurements of the branching ratios of the decays B⁺→χ_c1⁰(1P)K⁺ and B⁺→J/ψK⁺π⁺π^-, where χ_c1⁰(1P)→J/ψγ and J/ψ→μ⁺μ^- in pp̅ collisions at sqrt[s]=1.8 TeV. Using a data sample from an integrated luminosity of 110 pb^-1 collected by the Collider Detector at Fermilab we measure the branching ratios to be BR(B⁺→χ_c1⁰(1P)K⁺)=15.5±5.4(stat)±1.5(syst)±1.3(br)×10^-4 and BR(B⁺→J/ψK⁺π⁺π^-)=6.9±1.8(stat)±1.1(syst)±0.4(br)×10^-4 where (br) is due to the finite precision on BR(B⁺→J/ψK⁺), BR(χ_c1⁰(1P)→J/ψγ) is used to normalize the signal yield, and (syst) encompasses all other systematic uncertainties.

We report a measurement of the ratio of the bottom quark production cross section in antiproton-proton collisions at sqrt[s]=630 GeV to 1800 GeV using bottom quarks with transverse momenta greater than 10.75 GeV identified through their semileptonic decays and long lifetimes. The measured ratio σ(630)/σ(1800)=0.171±.024±.012 is in good agreement with next-to-leading order quantum chromodynamics.

We present the results of a search in pp̅ collisions at sqrt[s]=1.8 TeV for anomalous production of events containing a photon with large transverse energy and a lepton (e or μ) with large transverse energy, using 86 pb^-1 of data collected with the Collider Detector at Fermilab during the 1994–1995 collider run at the Fermilab Tevatron. The presence of large missing transverse energy (E_T), additional photons, or additional leptons in these events is also analyzed. The results are consistent with standard model expectations, with the possible exception of photon-lepton events with large E_T, for which the observed total is 16 events and the expected mean total is 7.6±0.7 events.

We present the results of a search in pp̅ collisions at sqrt[s]  =  1.8 TeV for anomalous production of events containing a photon and a lepton ( e or μ), both with large transverse energy, using 86 pb^-1 of data collected with the Collider Detector at Fermilab during the 1994–1995 collider run at the Fermilab Tevatron. The presence of large missing transverse energy ( E_T), additional photons, or additional leptons in these events is also analyzed. The results are consistent with standard model expectations, with the possible exception of photon-lepton events with large E_T, for which the observed total is 16 events and the expected mean total is 7.6±0.7 events.

We have measured the cross sections d²σ/dP_Tdη for production of isolated direct photons in pp̅ collisions at two different center-of-mass energies, 1.8 TeV and 0.63 TeV, using the Collider Detector at Fermilab. The normalization of both data sets agrees with the predictions of quantum chromodynamics for a photon transverse momentum (P_T) of 25 GeV/c, but the shapes versus photon P_T do not. These shape differences lead to a significant disagreement in the ratio of cross sections in the scaling variable x_T(≡2P_T/sqrt[s]). This disagreement in the x_T ratio is difficult to explain with conventional theoretical uncertainties such as scale dependence and parton distribution parametrizations.