Search Results (135 total)

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.

We report a measurement of the ratio of branching fractions of the decays B^±→J/ψπ^± and B^±→J/ψK^± using the CDF II detector at the Fermilab Tevatron Collider. The signal from the Cabbibo-suppressed B^±→J/ψπ^± decay is separated from B^±→J/ψK^± using the B^±→J/ψK^± invariant mass distribution and the kinematical differences of the hadron track in the two decay modes. From a sample of 220  pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV, we observe 91±15 B^±→J/ψπ^± events together with 1883±34 B^±→J/ψK^± events. The ratio of branching fractions is found to be B(B^±→J/ψπ^±)/B(B^±→J/ψK^±)=(4.86±0.82(stat)±0.15(syst))%.

Alloy segregation is believed to be an important factor in the growth of the wetting layer and subsequent formation of islands during heteroepitaxy. We report simulated annealing results that determine the lowest-energy configuration of a strained system. Our results indicate that in addition to vertical segregation, there is also an energetic driving force for lateral segregation and formation of subsurface features before island formation. We believe that these subsurface features play a crucial role in the nucleation of islands.

Models of maximal flavor violation (MxFV) in elementary particle physics may contain at least one new scalar SU(2) doublet field Φ_FV=(η⁰,η⁺) that couples the first and third generation quarks (q₁, q₃) via a Lagrangian term L_FV=ξ₁₃Φ_FVq₁q₃. These models have a distinctive signature of same-charge top-quark pairs and evade flavor-changing limits from meson mixing measurements. Data corresponding to 2  fb^-1 collected by the Collider Dectector at Fermilab II detector in pp̅ collisions at sqrt[s]=1.96  TeV are analyzed for evidence of the MxFV signature. For a neutral scalar η⁰ with m_η⁰=200  GeV/c² and coupling ξ₁₃=1, ∼11 signal events are expected over a background of 2.1±1.8 events. Three events are observed in the data, consistent with background expectations, and limits are set on the coupling ξ₁₃ for m_η⁰=180–300  GeV/c².

We present the first measurement of σ(gg→tt̅ )/σ(pp̅ →tt̅ ). We use 0.96  fb^-1 of sqrt[s]=1.96  TeV pp̅ collision data recorded with the CDF II detector at Fermilab. Using charged particles with low transverse momentum in tt̅ events, we find σ(gg→tt̅ )/σ(pp̅ →tt̅ )=0.07±0.14(stat)±0.07(syst), corresponding to a 95% confidence level upper limit of 0.33, in agreement with the standard model next-to-leading-order prediction of 0.15±0.05.

We search for pair production of doubly charged Higgs particles (H^±±) followed by decays into electron-tau (eτ) and muon-tau (μτ) pairs using data (350  pb^-1) collected from p̅ p collisions at sqrt[s]=1.96  TeV by the CDF II experiment. We search separately for cases where three or four final-state leptons are detected, and combine results for exclusive decays to left-handed eτ (μτ) pairs. We set an H^±± lower mass limit of 114(112)  GeV/c² at the 95% confidence level.

We study the effect of strain on the vertical and lateral self-organization of nanoscale patterns and stacked quantum dots during epitaxial growth. The computational approach is based on the level set method in combination with an atomistic strain code. Strain changes the energetics of microscopic parameters during growth, and thus determines the nucleation sites and the growth of islands and dots. Our results show that strain can lead to vertical alignment as well as lateral organization. Moreover, our simulations suggest that there is an optimal thickness of the capping layer to get the best alignment and most uniform size distribution of stacked quantum dots, and that its variation can be used to control the formation of interesting structures.

We present a search for standard model Higgs boson production in association with a W boson in proton-antiproton collisions (pp̅ →W^±H→ℓνbb̅ ) at a center of mass energy of 1.96 TeV. The search employs data collected with the CDF II detector which correspond to an integrated luminosity of approximately 1  fb^-1. We select events consistent with a signature of a single lepton (e^±/μ^±), missing transverse energy, and two jets. Jets corresponding to bottom quarks are identified with a secondary vertex tagging method and a neural network filter technique. The observed number of events and the dijet mass distributions are consistent with the standard model background expectations, and we set 95% confidence level upper limits on the production cross section times branching ratio ranging from 3.9 to 1.3 pb for Higgs boson masses from 110 to 150  GeV/c², respectively.

We search for pair production of supersymmetric top quarks (t˜₁), followed by R-parity violating decay t˜₁→τb with a branching ratio β, using 322  pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV collected by the upgraded Collider Detector at Fermilab. Two candidate events pass our final selection criteria, consistent with the standard model expectation. We set upper limits on the cross section σ(t˜₁t˜̅ ₁)×β² as a function of the top-squark mass m(t˜₁). Assuming β=1, we set a 95% confidence level limit m(t˜₁)>153  GeV/c². The limits are also applicable to the case of a third-generation scalar leptoquark (LQ₃) decaying LQ₃→τb.

We report the results of a search for the anomalous production of a massive particle decaying to four electrons via two Z⁰ bosons in 1.1  fb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV collected by the CDF II detector at Fermilab. We employ optimized electron identification criteria to maximize acceptance and efficiency. We estimate the backgrounds in the invariant mass range 500–1000  GeV/c² to be 0.028±0.009(stat)±0.011(syst) events. We observe zero events in this search region. Assuming a Randall-Sundrum graviton production model, we set 95% C.L. limits on σ×BF(G→Z⁰Z⁰)<4–6  pb, depending on the graviton mass.

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.

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.

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 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.

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 mathematical learning models—predictions of student’s knowledge vs amount of instruction—that are based on assumptions motivated by various theories of learning: tabula rasa, constructivist, and tutoring. These models predict the improvement (on the post-test) as a function of the pretest score due to intervening instruction and also depend on the type of instruction. We introduce a connectedness model whose connectedness parameter measures the degree to which the rate of learning is proportional to prior knowledge. Over a wide range of pretest scores on standard tests of introductory physics concepts, it fits high-quality data nearly within error. We suggest that data from MIT have low connectedness (indicating memory-based learning) because the test used the same context and representation as the instruction and that more connected data from the University of Minnesota resulted from instruction in a different representation from the test.

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.

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.

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.

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.