Search Results (670 total)

We present the results of a search for Higgs bosons predicted in two-Higgs-doublet models, in the case where the Higgs bosons decay to tau lepton pairs, using 1.8  fb^-1 of integrated luminosity of pp̅ collisions recorded by the CDF II experiment at the Fermilab Tevatron. Studying the mass distribution in events where one or both tau leptons decay leptonically, no evidence for a Higgs boson signal is observed. The result is used to infer exclusion limits in the two-dimensional space of tan⁡β versus m_A (the ratio of the vacuum expectation values of the two Higgs doublets and the mass of the pseudoscalar boson, respectively).

A combined mass and particle identification fit is used to make the first observation of the decay B̅ _s⁰→D_s^±K^∓ and measure the branching fraction of B̅ _s⁰→D_s^±K^∓ relative to B̅ _s⁰→D_s⁺π^-. This analysis uses 1.2  fb^-1 integrated luminosity of pp̅ collisions at sqrt[s]=1.96  TeV collected with the CDF II detector at the Fermilab Tevatron collider. We observe a B̅ _s⁰→D_s^±K^∓ signal with a statistical significance of 8.1σ and measure B(B̅ _s⁰→D_s^±K^∓)/B(B̅ _s⁰→D_s⁺π^-)=0.097±0.018(stat)±0.009(syst).

Free standing films of a liquid crystal compound with simple surface enhanced order were studied. The resultant phase diagram demonstrates that (1) the short helical pitch smectic-C_α^* phase disappears below a film thickness of 10 layers, and (2) the temperature window of a distorted 4 layer smectic-C_FI2^* phase increases dramatically upon decreasing film thickness. The experimental findings were attributed to the reduced dimensionality and enhanced surface effects in thin films. The results of the smectic-C_α^* phase are consistent with what have been reported for helically ordered magnetic thin films, with a noticeable difference due to the opposite effect of the surface on ordering in the two systems.

The rapid cycling synchrotron of the intense pulsed neutron source at Argonne National Laboratory normally operates at an average beam current of 14 to 15  μA, accelerating protons from 50 to 450 MeV 30 times per second. The beam current is limited by a single-bunch vertical instability that occurs in the later part of the 14 ms acceleration cycle. By analyzing turn-by-turn beam position monitor data, two cases of vertical beam centroid oscillations were discovered. The oscillations start from the tail of the bunch, build up, and develop toward the head of the bunch. The development stops near the bunch center and oscillations remain localized in the tail for a relatively long time (2–4 ms, 1–2×10⁴ turns). This vertical instability is identified as the cause of the beam loss. We compared this instability with a head-tail instability that was purposely induced by switching off sextupole magnets. It appears that the observed vertical instability is different from the classical head-tail instability.

In this work, we explore the cosmological consequences of the recently released Constitution sample of 397 Type Ia supernovae (SNIa). By revisiting the Chevallier-Polarski-Linder (CPL) parametrization, we find that, for fitting the Constitution set alone, the behavior of dark energy (DE) significantly deviates from the cosmological constant Λ, where the equation of state (EOS) w and the energy density ρ_Λ of DE will rapidly decrease along with the increase of redshift z. Inspired by this clue, we separate the redshifts into different bins, and discuss the models of a constant w or a constant ρ_Λ in each bin, respectively. It is found that for fitting the Constitution set alone, w and ρ_Λ will also rapidly decrease along with the increase of z, which is consistent with the result of CPL model. Moreover, a step function model in which ρ_Λ rapidly decreases at redshift z∼0.331 presents a significant improvement (Δχ²=-4.361) over the CPL parametrization, and performs better than other DE models. We also plot the error bars of DE density of this model, and find that this model deviates from the cosmological constant Λ at 68.3% confidence level (CL); this may arise from some biasing systematic errors in the handling of SNIa data, or more interestingly from the nature of DE itself. In addition, for models with same number of redshift bins, a piecewise constant ρ_Λ model always performs better than a piecewise constant w model; this shows the advantage of using ρ_Λ, instead of w, to probe the variation of DE.

We present a search for associated production of the standard model Higgs boson and a Z boson where the Z boson decays to two leptons and the Higgs decays to a pair of b quarks in pp̅ collisions at the Fermilab Tevatron. We use event probabilities based on standard model matrix elements to construct a likelihood function of the Higgs content of the data sample. In a CDF data sample corresponding to an integrated luminosity of 2.7  fb^-1 we see no evidence of a Higgs boson with a mass between 100  GeV/c² and 150  GeV/c². We set 95% confidence level upper limits on the cross section for ZH production as a function of the Higgs boson mass m_H; the limit is 8.2 times the standard model prediction at m_H=115  GeV/c².

Azimuthal single-spin asymmetries of leptoproduced pions and charged kaons were measured on a transversely polarized hydrogen target. Evidence for a naive-T-odd, transverse-momentum-dependent parton distribution function is deduced from nonvanishing Sivers effects for π⁺, π⁰, and K^±, as well as in the difference of the π⁺ and π^- cross sections.

We report the observation of the bottom, doubly-strange baryon Ω_b^- through the decay chain Ω_b^-→J/ψΩ^-, where J/ψ→μ⁺μ^-, Ω^-→ΛK^-, and Λ→pπ^-, using 4.2  fb^-1 of data from pp̅ collisions at sqrt[s]=1.96  TeV, and recorded with the Collider Detector at Fermilab. A signal is observed whose probability of arising from a background fluctuation is 4.0×10^-8, or 5.5 Gaussian standard deviations. The Ω_b^- mass is measured to be 6054.4±6.8(stat)±0.9(syst)  MeV/c². The lifetime of the Ω_b^- baryon is measured to be 1.13_-0.40^+0.53(stat)±0.02(syst)  ps. In addition, for the Ξ_b^- baryon we measure a mass of 5790.9±2.6(stat)±0.8(syst)  MeV/c² and a lifetime of 1.56_-0.25^+0.27(stat)±0.02(syst)  ps. Under the assumption that the Ξ_b^- and Ω_b^- are produced with similar kinematic distributions to the Λ_b⁰ baryon, we find σ(Ξ_b^-)B(Ξ_b^-→J/ψΞ^-) / σ(Λ_b⁰)B(Λ_b⁰→J/ψΛ)=0.167_-0.025^+0.037(stat)±0.012(syst) and σ(Ω_b^-)B(Ω_b^-→J/ψΩ^-) / σ(Λ_b⁰)B(Λ_b⁰→J/ψΛ)=0.045_-0.012^+0.017(stat)±  0.004(syst) for baryons produced with transverse momentum in the range of 6–20  GeV/c.

We present an analysis of the mass of the X(3872) reconstructed via its decay to J/ψπ⁺π^- using 2.4  fb^-1 of integrated luminosity from pp̅ collisions at sqrt[s]=1.96  TeV, collected with the CDF II detector at the Fermilab Tevatron. The possible existence of two nearby mass states is investigated. Within the limits of our experimental resolution the data are consistent with a single state, and having no evidence for two states we set upper limits on the mass difference between two hypothetical states for different assumed ratios of contributions to the observed peak. For equal contributions, the 95% confidence level upper limit on the mass difference is 3.6  MeV/c². Under the single-state model the X(3872) mass is measured to be 3871.61±0.16(stat)±0.19(syst)  MeV/c², which is the most precise determination to date.

We report on a search for the process pp̅ →γ+W/Z with W/Z→qq̅ in events containing two jets and a photon at the center-of-mass energy sqrt[s]=1.96  TeV, using 184  pb^-1 of data collected by the CDF II detector. A neural network event selection has been developed to optimize the rejection of the large QCD production background; it is shown that this method gives a significant improvement in both signal-to-noise ratio and signal sensitivity, as compared with an event selection based on conventional cuts. An upper limit is presented for the γ+W/Z production cross section with the W and Z decaying hadronically.

This paper investigates the radiation properties of two coupled split-ring resonators (SRRs). Due to electromagnetic coupling, two hybrid magnetic plasmon modes were induced in the structure. Our calculations show that the radiation loss of the structure was greatly suppressed by the hybridization effect. This led to a remarkable increase in the Q factor of the coupled system compared to the single SRR. By adjusting the distance between the two SRRs, the Q factor changed correspondingly due to different electromagnetic coupling strengths. This resulted in a coupled structure that functioned as a type of nanocavity with an adjustable Q factor.

We present a signature-based search for the anomalous production of events containing a photon, two jets, of which at least one is identified as originating from a b quark, and missing transverse energy (E̸_T). The search uses data corresponding to 2.0  fb^-1 of integrated luminosity from pp̅ collisions at a center-of-mass energy of sqrt[s]=1.96  TeV, collected with the CDF II detector at the Fermilab Tevatron. From 6.697 47×10⁶ events with a photon candidate with transverse energy E_T>25  GeV, we find 617 events with E̸_T>25  GeV and two or more jets with E_T>15  GeV, at least one identified as originating from a b quark, versus an expectation of 607±113 events. Increasing the requirement on E̸_T to 50 GeV, we find 28 events versus an expectation of 30±11 events. We find no indications of non-standard-model phenomena.

We have used the Collider Detector at Fermilab (CDF-II) to search for the flavor-changing neutral-current (FCNC) top-quark decay t→Zc using a technique employing ratios of W and Z production, measured in pp̅ data corresponding to an integrated luminosity of 1.52  fb^-1. The analysis uses a comparison of two decay chains, pp̅ →tt̅ →WbWb→ℓνbjjb and pp̅ →tt̅ →ZcWb→ℓℓcjjb, to cancel systematic uncertainties in acceptance, efficiency, and luminosity. We validate the modeling of acceptance and efficiency for lepton identification over the multiyear data set using another ratio of W and Z production, in this case the observed ratio of inclusive production of W to Z bosons. To improve the discrimination against standard model backgrounds to top-quark decays, we calculate the top-quark mass for each event with two leptons and four jets assuming it is a tt̅ event with one of the top quarks decaying to Zc. For additional background discrimination we require at least one jet to be identified as originating from a b quark. No significant signal is found and we set an upper limit on the FCNC branching ratio Br(t→Zc) using a likelihood constructed from the ℓℓcjjb top-quark mass distribution and the number of ℓνbjjb events. Limits are set as a function of the helicity of the Z boson produced in the FCNC decay. For 100% longitudinally-polarized Z bosons we find limits of 8.3% and 9.3% (95% C.L.) depending on the assumptions regarding the theoretical top-quark pair production cross section.

We present the first measurement of the mass of the top quark in a sample of tt̅ →ℓν̅ bb̅ qq̅ events (where ℓ=e,μ) selected by identifying jets containing a muon candidate from the semileptonic decay of heavy-flavor hadrons (soft muon b tagging). The pp̅ collision data used correspond to an integrated luminosity of 2  fb^-1 and were collected by the CDF II detector at the Fermilab Tevatron Collider. The measurement is based on a novel technique exploiting the invariant mass of a subset of the decay particles, specifically the lepton from the W boson of the t→Wb decay and the muon from a semileptonic b decay. We fit template histograms, derived from simulation of tt̅ events and a modeling of the background, to the mass distribution observed in the data and measure a top quark mass of 180.5±12.0(stat)±3.6(syst)  GeV/c², consistent with the current world average value.

We report on the first direct search for charged Higgs bosons decaying into cs̅ in tt̅ events produced by pp̅ collisions at sqrt[s]=1.96  TeV. The search uses a data sample corresponding to an integrated luminosity of 2.2  fb^-1 collected by the CDF II detector at Fermilab and looks for a resonance in the invariant mass distribution of two jets in the lepton+jets sample of tt̅ candidates. We observe no evidence of charged Higgs bosons in top quark decays. Hence, 95% upper limits on the top quark decay branching ratio are placed at B(t→H⁺b)< 0.1 to 0.3 for charged Higgs boson masses of 60 to 150  GeV/c² assuming B(H⁺→cs̅ )=1.0. The upper limits on B(t→H⁺b) are also used as model-independent limits on the decay branching ratio of top quarks to generic scalar charged bosons beyond the standard model.

High-spin states in ¹⁸⁹Pt have been studied experimentally using the ¹⁷⁶Yb(¹⁸O,5n) reaction at beam energies of 88 and 95 MeV. The level scheme of ¹⁸⁹Pt has been revised significantly and extended to high-spin states. Rotational bands have been analyzed in the framework of triaxial particle-rotor model, and a γ≈-30^° triaxial shape and a near-prolate shape have been proposed to the νi_13/2^-1 and νf_5/2(p_3/2) bands, respectively. Two ΔI=2 transition sequences with similar energies have been observed, and they have been proposed to be associated with the νi_13/2^-2νf_5/2(p_3/2) configuration. The structure built on the νi_13/2^-2νf_5/2(p_3/2) configuration could be interpreted as a pair of pseudospin partners. The level energies in the pseudospin partner bands are well reproduced by the theoretical calculations of the triaxial particle-rotor model if a near-oblate shape is assumed.

The angular dependence of high-order harmonic generation (HHG) from molecules lacking inversion symmetry (MLIS) is investigated using polarization gating method. The polarization gating with 5 fs, 750 nm laser pulses is an effective way to confine the recollision of electron wave packet within one half cycle. It is shown that the dramatic changes of HHG yields are observed when the MLISs are rotated by 180°. This feature implies that HHG can be manipulated by MLIS themselves. Moreover, polarization gating with 10 fs pulses are applicable for generating supercontinuous high harmonics from MLIS. Note that the pulse duration we used is twice as long as that for applying polarization gating method to atoms.

We present a search for a standard model Higgs boson produced in association with a W boson using 2.7  fb^-1 of integrated luminosity of pp̅ collision data taken at sqrt[s]=1.96  TeV. Limits on the Higgs boson production rate are obtained for masses between 100 and 150  GeV/c². Through the use of multivariate techniques, the analysis achieves an observed (expected) 95% confidence level upper limit of 5.6 (4.8) times the theoretically expected production cross section for a standard model Higgs boson with a mass of 115  GeV/c².

We present the first observation in hadronic collisions of the electroweak production of vector boson pairs (VV, V=W, Z) where one boson decays to a dijet final state. The data correspond to 3.5  fb^-1 of integrated luminosity of pp̅ collisions at sqrt[s]=1.96  TeV collected by the CDF II detector at the Fermilab Tevatron. We observe 1516±239(stat)±144(syst) diboson candidate events and measure a cross section σ(pp̅ →VV+X) of 18.0±2.8(stat)±2.4(syst)±1.1(lumi)  pb, in agreement with the expectations of the standard model.

We report the observation of single top-quark production using 3.2  fb^-1 of pp̅ collision data with sqrt[s]=1.96  TeV collected by the Collider Detector at Fermilab. The significance of the observed data is 5.0 standard deviations, and the expected sensitivity for standard model production and decay is in excess of 5.9 standard deviations. Assuming m_t=175  GeV/c², we measure a cross section of 2.3_-0.5^+0.6(stat+syst)  pb, extract the CKM matrix-element value |V_tb|=0.91±0.11(stat+syst)±0.07(theory), and set the limit |V_tb|>0.71 at the 95% C.L.

We have measured the differential cross section for the inclusive production of ψ(2S) mesons decaying to μ⁺μ^- that were produced in prompt or B-decay processes from pp̅ collisions at 1.96 TeV. These measurements have been made using a data set from an integrated luminosity of 1.1  fb^-1 collected by the CDF II detector at Fermilab. For events with transverse momentum p_T(ψ(2S))>2  GeV/c and rapidity |y(ψ(2S))|<0.6 we measure the integrated inclusive cross section σ(pp̅ →ψ(2S)X)·Br(ψ(2S)→μ⁺μ^-) to be 3.29±0.04(stat)±0.32(syst)  nb.

A search for a narrow diphoton mass resonance is presented based on data from 3.0  fb^-1 of integrated luminosity from pp̅ collisions at sqrt[s]=1.96  TeV collected by the CDF experiment. No evidence of a resonance in the diphoton mass spectrum is observed, and upper limits are set on the cross section times branching fraction of the resonant state as a function of Higgs boson mass. The resulting limits exclude Higgs bosons with masses below 106  GeV/c² at a 95% Bayesian credibility level for one fermiophobic benchmark model.

We compare the inclusive production of events containing a lepton (ℓ), a photon (γ), significant transverse momentum imbalance (E̸_T), and a jet identified as containing a b-quark, to SM predictions. The search uses data produced in proton-antiproton collisions at sqrt[s]=1.96  TeV corresponding to 1.9  fb^-1 of integrated luminosity taken with the CDF detector. We find 28 ℓγbE̸_T events versus an expectation of 31.0_-3.5^+4.1 events. If we further require events to contain at least three jets and large total transverse energy, the largest SM source is radiative top-quark pair production, tt̅ +γ. In the data we observe 16 tt̅ γ candidate events versus an expectation from SM sources of 11.2_-2.1^+2.3. Assuming the difference between the observed number and the predicted non-top-quark total of 6.8_-2.0^+2.2 is due to SM top-quark production, we estimate the tt̅ γ cross section to be 0.15±0.08  pb.

We present new limits on resonant tb̅ production in pp̅ collisions at sqrt[s]=1.96  TeV, using 1.9  fb^-1 of data recorded with the CDF II detector at the Fermilab Tevatron. We reconstruct a candidate tb̅ mass in events with a lepton, neutrino candidate, and two or three jets, and search for anomalous tb̅ production as modeled by W^′→tb̅ . We set a new limit on a right-handed W^′ with standard model-like coupling, excluding any mass below 800  GeV/c² at 95% C.L. The cross section for any narrow, resonant tb̅ production between 750 and 950  GeV/c² is found to be less than 0.28 pb at 95% C.L. We also present an exclusion of the W^′ coupling strength versus W^′ mass over the range 300–950  GeV/c².