Search Results (426 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.

We report determination of branching fractions for the decays ψ(2S)→h+J/ψ, where h=any, π⁺π^-, π⁰π⁰, η, π⁰, and γγ through χ_c0,1,2. These measurements use 27M ψ(2S) decays produced in e⁺e^- collision data collected with the CLEO detector. The resulting branching fractions and ratios thereof improve upon previously achieved precision in all cases, and in combination with other measurements permit determination of B(χ_cJ→γJ/ψ) and B(ψ(2S)→light hadrons).

By using 1.8×10⁶ DD̅ pairs, we have measured B(D⁰→π^-e⁺ν_e)=0.299(11)(9)%, B(D⁺→π⁰e⁺ν_e)=0.373(22)(13)%, B(D⁰→K^-e⁺ν_e)=3.56(3)(9)%, and B(D⁺→K̅ ⁰e⁺ν_e)=8.53(13)(23)% and have studied the q² dependence of the form factors. By combining our results with recent lattice calculations, we obtain |V_cd|=0.217(9)(4)(23) and |V_cs|=1.015(10)(11)(106).

Using a sample of 1.8 million DD̅ mesons collected at the ψ(3770) with the CLEO-c detector, we study the semileptonic decays D⁰→π^-e⁺ν_e, D⁺→π⁰e⁺ν_e, D⁰→K^-e⁺ν_e, and D⁺→K̅ ⁰e⁺ν_e. For the total branching fractions we find B(D⁰→π^-e⁺ν_e)=0.299(11)(9)%, B(D⁺→π⁰e⁺ν_e)=0.373(22)(13)%, B(D⁰→K^-e⁺ν_e)=3.56(3)(9)%, and B(D⁺→K̅ ⁰e⁺ν_e)=8.53(13)(23)%, where the first error is statistical and the second systematic. In addition, form factors are studied through fits to the partial branching fractions obtained in five q² ranges. By combining our results with recent unquenched lattice calculations, we obtain |V_cd|=0.217(9)(4)(23) and |V_cs|=1.015(10)(11)(106), where the final error is theoretical.

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 exploit the quantum coherence between pair-produced D⁰ and D̅ ⁰ in ψ(3770) decays to study charm mixing, which is characterized by the parameters x and y, and to make a first determination of the relative strong phase δ between D⁰→K⁺π^- and D̅ ⁰→K⁺π^-. Using 281  pb^-1 of e⁺e^- collision data collected with the CLEO-c detector at E_cm=3.77  GeV, as well as branching fraction input and time-integrated measurements of R_M≡(x²+y²)/2 and R_WS≡Γ(D⁰→K⁺π^-)/Γ(D̅ ⁰→K⁺π^-) from other experiments, we find cos⁡δ=1.03_-0.17^+0.31±0.06, where the uncertainties are statistical and systematic, respectively. By further including other mixing parameter measurements, we obtain an alternate measurement of cos⁡δ=1.10±0.35±0.07, as well as xsin⁡δ=(4.4_-1.8^+2.7±2.9)×10^-3 and δ=(22_-12-11⁺¹¹⁺⁹)°.

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.

Using 281  pb^-1 of data collected with the CLEO-c detector, we present new measurements of Cabibbo-suppressed decays of D⁰ and D⁺ mesons to η and η^′ final states. We make first observations of D⁰→η^′π⁰, ηη, ηη^′, and ηπ⁺π^-, and find evidence for D⁺→ηπ⁺π⁰, D⁺→η^′π⁺π⁰, and D⁰→η^′π⁺π^-. We also report on improved measurements of D⁰→ηπ⁰, D⁺→ηπ⁺, and D⁺→η^′π⁺. Using the measured two-body Cabibbo-suppressed decays, we extract amplitudes for specific flavor topologies and compare them to those from Cabibbo-favored decays.

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.

Using 281  pb^-1 of data collected with the CLEO-c detector at the ψ(3770) resonance, we have studied Cabibbo-suppressed decays of D mesons to final states with two kaons. We present results for the absolute branching fractions of the modes D⁰→K⁺K^-, D⁰→K_S⁰K_S⁰, and D⁺→K⁺K_S⁰. We measure B(D⁰→K⁺K^-)=(4.08±0.08±0.09)×10^-3, B(D⁰→K_S⁰K_S⁰)=(1.46±0.32±0.09)×10^-4, and B(D⁺→K⁺K_S⁰)=(3.14±0.09±0.08)×10^-3. We also determine the ratio B(D⁰→K⁺K^-)/B(D⁰→π⁺π^-)=2.89±0.05±0.06. For each measurement, the first uncertainty is statistical and the second uncertainty is systematic.

First measurements of the breakdown threshold in a dielectric subjected to GV/m wakefields produced by short (30–330 fs), 28.5 GeV electron bunches have been made. Fused silica tubes of 100  μm inner diameter were exposed to a range of bunch lengths, allowing surface dielectric fields up to 27  GV/m to be generated. The onset of breakdown, detected through light emission from the tube ends, is observed to occur when the peak electric field at the dielectric surface reaches 13.8±0.7  GV/m. The correlation of structure damage to beam-induced breakdown is established using an array of postexposure inspection techniques.

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.

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.

Using e⁺e^-→D_s^*-D_s⁺ data collected near the peak D_s production energy, E_cm=4170  MeV, with the CLEO-c detector, we present the first observation of the decay D_s⁺→pn̅ . We measure a branching fraction B(D_s⁺→pn̅ )=(1.30±0.36_-0.16^+0.12)×10^-3. This is the first observation of a charmed meson decaying into a baryon-antibaryon final state.

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.

The branching fractions of D_s^± meson decays serve to normalize many measurements of processes involving charm quarks. Using 298  pb^-1 of e⁺e^- collisions recorded at a center of mass energy of 4.17 GeV, we determine absolute branching fractions for eight D_s^± decays with a double tag technique. In particular we determine the branching fraction B(D_s⁺→K^-K⁺π⁺)=(5.50±0.23±0.16)%, where the uncertainties are statistical and systematic, respectively. We also provide partial branching fractions for kinematic subsets of the K^-K⁺π⁺ decay mode.

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.

Using a sample of tagged D_s⁺ decays collected near the D_s^*±D_s^∓ peak production energy in e⁺e^- collisions with the CLEO-c detector, we study the leptonic decay D_s⁺→τ⁺ν_τ via the decay channel τ⁺→e⁺ν_eν̅ _τ. We measure B(D_s⁺→τ⁺ν_τ)=(6.17±0.71±0.34)%, where the first error is statistical and the second systematic. Combining this result with our measurements of D_s⁺→μ⁺ν_μ and D_s⁺→τ⁺ν_τ (via τ⁺→π⁺ν̅ _τ), we determine f_Ds=(274±10±5)  MeV.

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.

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.

We measure the time dependence of the ratio of decay rates for the rare decay D⁰→K⁺π^- to the Cabibbo-favored decay D⁰→K^-π⁺. A signal of 12.7×10³ D⁰→K⁺π^- decays was obtained using the Collider Detector at Fermilab II detector at the Fermilab Tevatron with an integrated luminosity of 1.5  fb^-1. We measure the D⁰-D̅ ⁰ mixing parameters (R_D,y^′,x^′2), and find that the data are inconsistent with the no-mixing hypothesis with a probability equivalent to 3.8 Gaussian standard deviations.