Search Results (566 total)

We present a new method for studying high-p_T dilepton events (e^±e^∓, μ^±μ^∓, e^±μ^∓) and simultaneously extracting the production cross sections of pp̅ →tt̅ , pp̅ →W⁺W^-, and pp̅ →Z⁰→τ⁺τ^- at a center-of-mass energy of sqrt[s]=1.96  TeV. We perform a likelihood fit to the dilepton data in a parameter space defined by the missing transverse energy and the number of jets in the event. Our results, which use 360  pb^-1 of data recorded with the CDF II detector at the Fermilab Tevatron Collider, are σ(tt̅ )=8.5_-2.2^+2.7  pb, σ(W⁺W^-)=16.3_-4.4^+5.2  pb, and σ(Z⁰→τ⁺τ^-)=291_-46⁺⁵⁰  pb.

Data collected in run II of the Fermilab Tevatron are searched for indications of new electroweak scale physics. Rather than focusing on particular new physics scenarios, CDF data are analyzed for discrepancies with respect to the standard model prediction. A model-independent approach (Vista) considers the gross features of the data and is sensitive to new large cross section physics. A quasi-model-independent approach (Sleuth) searches for a significant excess of events with large summed transverse momentum and is particularly sensitive to new electroweak scale physics that appears predominantly in one final state. This global search for new physics in over 300 exclusive final states in 927  pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV reveals no such significant indication of physics beyond the standard model.

High-spin states of ¹⁵⁶Yb have been studied via the ¹⁴⁴Sm(¹⁶O,4n)¹⁵⁶Yb fusion-evaporation reaction at beam energy 102 MeV. The positive-parity yrast band and negative-parity cascade have been extended up to higher-spin states, respectively. In the present work, the negative-parity sequence above the 25^- state was found to be irregular and fragment into many parallel branches. This pattern may related to the excitation from the nucleon in the Z=64,N=82 core. The characteristics of alignment plot and E-GOS curve for the positive-parity yrast sequence in ¹⁵⁶Yb indicate that this nucleus may undergo an evolution from quasivibrational to quasirotational structure with increasing angular momentum. Based on a systematic summary of the available experimental alignments for the even-A ^{156,158,160,162,164}Yb isotopes, the structural evolutions induced by the increase in angular momentum, as well as by the change in neutron numbers, in these even-A Yb isotopes have been discussed in comparison with the cranked Woods-Saxon-Strutinsky calculations by means of total-Routhian-surface (TRS) methods.

We report on a search for narrow-width particles decaying to a top and antitop quark pair. The data set used in the analysis corresponds to an integrated luminosity of 680  pb^-1 collected with the Collider Detector at Fermilab in run II. We present 95% confidence level upper limits on the cross section times branching ratio. Assuming a specific top-color-assisted technicolor production model, the leptophobic Z^′ with width Γ_Z^′=0.012M_Z^′, we exclude the mass range M_Z^′<725  GeV/c² at the 95% confidence level.

The compound 10OHF has a partially inverted phase sequence, unique among the series of nOHF homologous compounds and all other known liquid crystals, with the smectic-C_FI2^∗ (SmC_FI2^∗) phase occurring at higher temperature than the smectic-C^∗ (SmC^∗) phase. We present ellipsometric data to identify the phase sequences of 9OHF, 10OHF, 11OHF, and 12OHF. Binary mixtures of 10OHF with C11, a compound with the typical phase sequence among the smectic phases, show that the unusual phase sequence of 10OHF stabilizes upon mixing and that SmC_FI2^∗ predominates over SmC^∗ throughout the entire mixing phase diagram. In thin films of some mixtures, surface interactions induce a reentrant SmC_FI2^∗-SmC^∗-SmC_FI2^∗ transition in the rest of the film.

We describe a measurement of the W boson mass m_W using 200  pb^-1 of sqrt[s]=1.96  TeV pp̅ collision data taken with the CDF II detector. With a sample of 63 964 W→eν candidates and 51 128 W→μν candidates, we measure m_W=[80.413±0.034(stat)±0.034(sys)=80.413±0.048]  GeV/c². This is the single most precise m_W measurement to date. When combined with other measured electroweak parameters, this result further constrains the properties of new unobserved particles coupling to W and Z bosons.

We studied the propagation of an electromagnetic (EM) wave in a defective multilayer microcavity with an artificial magnetic atom located at the edge of the defect layer. When the frequency of the defect state is tuned to the resonance frequency of the magnetic atom, strong coupling happens between this atom and EM waves. It creates a type of magnetic plasmon polariton (MPP) with Rabi-type splitting effect that results in the two branches of the MPP mode. The linewidth of the MPP and Rabi-type oscillation of magnetic field inside the atom are investigated in the simulations. A great enhancement of local fields can also be obtained from the MPP, which has a good application in nonlinear optics.

In spite of the absence of a macroscopic magnetic moment, an antiferromagnet is spin-polarized on an atomic scale. The electric current passing through a conducting antiferromagnet is polarized as well, leading to spin-transfer torques when the order parameter is textured, such as in antiferromagnetic noncollinear spin valves and domain walls. We report a first principles study on the electronic transport properties of antiferromagnetic systems. The current-induced spin torques acting on the magnetic moments are comparable with those in conventional ferromagnetic materials, leading to measurable angular resistances and current-induced magnetization dynamics. In contrast to ferromagnets, spin torques in antiferromagnets are very nonlocal. The torques acting far away from the center of an antiferromagnetic domain wall should facilitate current-induced domain wall motion.

We search for a third generation vector leptoquark (VLQ3) that decays to a b quark and tau lepton using the CDF II detector and 320  pb^-1 of integrated luminosity from the Fermilab Tevatron. Observing a number of events in agreement with standard model expectations, we obtain, assuming Yang-Mills (minimal) couplings, the most stringent upper limit on the VLQ3 pair production cross section of 344 fb (493 fb) and lower limit on the VLQ3 mass of 317  GeV/c² (251  GeV/c²) at 95% C.L.

We search for the standard model Higgs boson produced in association with an electroweak vector boson in events with no identified charged leptons, large imbalance in transverse momentum, and two jets where at least one contains a secondary vertex consistent with the decay of b hadrons. We use ∼1  fb^-1 integrated luminosity of pp̅ collisions at sqrt[s]=1.96  TeV recorded by the Collider Detector at Fermilab II experiment at the Tevatron. We find 268 (16) single (double) b-tagged candidate events, where 248±43 (14.4±2.7) are expected from standard model background processes. We observe no significant excess over the expected background and thus set 95% confidence level upper limits on the Higgs boson production cross section for several Higgs boson masses ranging from 110 to 140  GeV/c². For a mass of 115  GeV/c², the observed (expected) limit is 20.4 (14.2) times the standard model prediction.

An efficient first-principles method was developed to calculate spin-transfer torques in layered system with noncollinear magnetization. The complete scattering wave function is determined by matching the wave function in the scattering region with the Bloch states in the leads. The spin-transfer torques are obtained with the aid of the scattering wave function. We applied our method to the ferromagnetic spin valve and found that the material (Co, Ni, and Ni₈₀Fe₂₀) dependence of the spin-transfer torques could be well understood by the Fermi surfaces. Ni has much longer spin injection penetration length than Co. Interfacial disorder is also considered. It is found that the spin-transfer torques could be enhanced by the interfacial disorder in some system.

We report the first evidence of Z boson pair production at a hadron collider with a significance exceeding 4 standard deviations. This result is based on a data sample corresponding to 1.9  fb^-1 of integrated luminosity from pp̅ collisions at sqrt[s]=1.96  TeV collected with the Collider Detector at Fermilab II detector. In the ℓℓℓ^′ℓ^′ channel, we observe three ZZ candidates with an expected background of 0.096_-0.063^+0.092 events. In the ℓℓνν channel, we use a leading-order calculation of the relative ZZ and WW event probabilities to discriminate between signal and background. In the combination of ℓℓℓ^′ℓ^′ and ℓℓνν channels, we observe an excess of events with a probability of 5.1×10^-6 to be due to the expected background. This corresponds to a significance of 4.4 standard deviations. The measured cross section is σ(pp̅ →ZZ)=1.4_-0.6^+0.7(stat+syst)  pb, consistent with the standard model expectation.

We present the first measurement of two-particle momentum correlations in jets produced in pp̅ collisions at sqrt[s]=1.96  TeV. Results are obtained for charged particles within a restricted cone with an opening angle of 0.5 radians around the jet axis and for events with dijet masses between 66 and 563  GeV/c². A comparison of the experimental data to theoretical predictions obtained for partons within the framework of resummed perturbative QCD in the next-to-leading log approximation shows that the parton momentum correlations survive the hadronization stage of jet fragmentation, giving further support to the hypothesis of local parton-hadron duality. The extracted value of the next-to-leading-log-approximation parton shower cutoff scale Q_eff set equal to Λ_QCD is found to be (1.4_-0.7^+0.9)×100  MeV.

As predicted by quantum chromodynamics (QCD), at around T∼190  MeV in the early Universe, the QCD transition occurs during which the quarks are combined into the massive hadrons. This process reduces the effective relativistic degree of freedom and causes a change in the expansion behavior of the Universe. Similarly, the e⁺e^- annihilation occurred around T∼0.5  Mev and has the same kind of effect. Besides, the dark energy also drives the present stage accelerating expansion. We study these combined effects on the relic gravitational waves (RGWs). In our treatment, the QCD transition and the e⁺e^- annihilation are each, respectively, represented by a short period of expansion inserted into the radiation era. Incorporating these effects, the equation of RGWs is analytically solved for a spatially flat universe, evolving from the inflation up to the current acceleration, and the spectrum of RGWs is obtained, covering the whole range of frequency >10^-19  Hz. It is found that the QCD transition causes a reduction of the amplitude of RGWs by ∼20% in the range >10^-9  Hz, and the e⁺e^- annihilation causes a reduction of ∼10% in the range >10^-12  Hz. In the presence of the dark energy, the combination of the QCD transition and the e⁺e^- annihilation causes a larger reduction of the amplitude by ∼30% for the range >10^-9  Hz, which covers the bands of operation of LIGO and LISA. By analysis, it is shown that RGWs will be difficult to detect by the present LIGO, but can be tested by LISA for certain inflationary models.

The B_c^± meson is observed through the decay B_c^±→J/ψπ^±, in data corresponding to an integrated luminosity of 2.4  fb^-1 recorded by the Collider Detector at Fermilab II detector at the Fermilab Tevatron. A signal of 108±15 candidates is observed, with a significance that exceeds 8σ. The mass of the B_c^± meson is measured to be 6275.6±2.9(stat)±2.5(syst)  MeV/c².

Derbenev proposed producing a high quality flat beam of high-transverse-emittance ratio (HTER) with a linear accelerator. Kim also discussed the round-to-flat transformation of angular-momentum-dominated beam. Fermilab/NICADD Photoinjector Laboratory has performed many experiments on HTER beam production. Experiments and simulations, collectively, showed an S-shaped transverse distribution in the flat beam. In this paper, the source of this emittance deterioration in the transformation is identified as the nonlinear rf cavity focusing force; and a solution is proposed.

We present a measurement of the correlated bb̅ production cross section. The data used in this analysis were taken with the upgraded CDF detector (CDF II) at the Fermilab Tevatron collider, and correspond to an integrated luminosity of 742  pb^-1. We utilize muon pairs with invariant mass 5≤m_μμ≤80  GeV/c² produced by bb̅ double semileptonic decays. For muons with p_T≥3  GeV/c and |η|≤0.7, that are produced by b and b̅ quarks with p_T≥2  GeV/c and |y|≤1.3, we measure σ_{b→μ,b̅ →μ}=1549±133  pb. We compare this result with theoretical predictions and previous measurements. We also report the measurement of σ_{c→μ,c̅ →μ}, a by-product of the study of the background to bb̅ production.

A novel internal target has been developed, which will make electron scattering off short-lived radioactive nuclei possible in an electron storage ring. An “ion trapping” phenomenon in the electron storage ring was successfully utilized for the first time to form the target for electron scattering. Approximately 7×10⁶ stable ¹³³Cs ions were trapped along the electron beam axis for 85 ms at an electron beam current of 80 mA. The collision luminosity between the stored electrons and trapped Cs ions was determined to be 2.4(8)×10²⁵  cm^-2 s^-1 by measuring elastically scattered electrons.

We present the results of a search for pair production of a new heavy toplike quark t^′ decaying to a W boson and another quark using the Collider Detector at Fermilab II detector in run II of the Tevatron pp̅ collider. Using a data sample corresponding to 760  pb^-1 of integrated luminosity, we fit the observed spectrum of total transverse energy and reconstructed t^′ quark mass to a combination of standard model processes and t^′ pair production. We see no evidence for t^′t̅ ^′ production, and we infer a lower limit of 256  GeV/c² on the mass of the t^′ at 95% C.L. assuming standard strong couplings for the t^′.

This Letter describes the first determination of bounds on the CP-violation parameter 2β_s using B_s⁰ decays in which the flavor of the bottom meson at production is identified. The result is based on approximately 2000 B_s⁰→J/ψϕ decays reconstructed in a 1.35  fb^-1 data sample collected with the CDF II detector using pp̅ collisions produced at the Fermilab Tevatron. We report confidence regions in the two-dimensional space of 2β_s and the decay-width difference ΔΓ. Assuming the standard model predictions of 2β_s and ΔΓ, the probability of a deviation as large as the level of the observed data is 15%, corresponding to 1.5 Gaussian standard deviations.

An unusual nuclear magnetic resonance phenomenon is present to demonstrate the quantum size effect of ZnS clusters with its length scale properly estimated within 4  nm from the ⁶⁷Zn-NMR measurements. Strong quadrupole interaction induced from the quantum size effect is proposed to explain this phenomenon. A simple calculation of the electric field gradient by direct summation over all lattice points was performed to demonstrate this size effect phenomenally, and the result is in good agreement with the experimental observation.

This paper describes the first measurement of b-quark fragmentation fractions into bottom hadrons in Run II of the Tevatron Collider at Fermilab. The result is based on a 360  pb^-1 sample of data collected with the CDF II detector in pp̅ collisions at sqrt[s]=1.96  TeV. Semileptonic decays of B̅ ⁰, B^-, and B̅ _s⁰ mesons, as well as Λ_b⁰ baryons, are reconstructed. For an effective bottom hadron p_T threshold of 7  GeV/c, the fragmentation fractions are measured to be f_u/f_d=1.054±0.018(stat)_-0.045^+0.025(sys)±0.058(B), f_s/(f_u+f_d)=0.160±0.005(stat)_-0.010^+0.011(sys)_-0.034^+0.057(B), and f_Λb/(f_u+f_d)=0.281±0.012(stat)_-0.056^+0.058(sys)_-0.087^+0.128(B), where the uncertainty B is due to uncertainties on measured branching ratios. The value of f_s/(f_u+f_d) agrees within one standard deviation with previous CDF measurements and the world average of this quantity, which is dominated by LEP measurements. However, the ratio f_Λb/(f_u+f_d) is approximately twice the value previously measured at LEP. The approximately 2σ discrepancy is examined in terms of kinematic differences between the two production environments.

The immunity of microwave-induced magnetoresistance oscillations and corresponding zero resistance regions to the direction of (circular) polarization of microwaves is studied in this paper. We propose that a spontaneous circular motion of the whole electron fluid would stabilize the system and minimize the polarization sensitivity of the oscillatory dc resistance. Results of a self-consistent calculation capture the qualitative features of the experimental observation.

Using the particle-rotor model (PRM), in which two quasiparticles are coupled to a triaxial core, the negative-parity doublet bands in ¹⁰⁶Rh are studied. The quadrupole deformations β and γ for the πg_9/2⊗νh_11/2 configuration are calculated by configuration-fixed constrained triaxial relativistic mean-field (RMF) approaches, and these self-consistent deformation parameters are used as inputs for the PRM. The present calculated results well reproduce the observed energy spectra, energy staggering parameter, and electromagnetic transition ratios of the doublet bands, thus supporting the chiral interpretation of these doublet bands in ¹⁰⁶Rh. The effective angles between the angular momenta in the body-fixed frame are calculated for the first time. We demonstrate that rigid triaxial deformation can give rise to the experimentally observed constant energy separation between the yrast and side bands built on the asymmetrical πg_9/2⊗νh_11/2 configuration in the A~100 mass region.

We measure the mean lifetime τ=2/(Γ_L+Γ_H) and the decay-width difference ΔΓ=Γ_L-Γ_H of the light and heavy mass eigenstates of the B_s⁰ meson, B_sL⁰ and B_sH⁰, in B_s⁰→J/ψϕ decays using 1.7  fb^-1 of data collected with the CDF II detector at the Fermilab Tevatron pp̅ collider. Assuming CP conservation, a good approximation for the B_s⁰ system in the standard model, we obtain ΔΓ=0.076_-0.063^+0.059(stat)±0.006(syst)  ps^-1 and τ=1.52±0.04(stat)±0.02(syst)  ps, the most precise measurements to date. Our constraints on the weak phase and ΔΓ are consistent with CP conservation.