Search Results (185 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 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 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.

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.

We present the first observation and cross section measurement of exclusive dijet production in p̅ p interactions, p̅ p→p̅ +dijet+p. Using a data sample of 310  pb^-1 collected by the Run II Collider Detector at Fermilab at sqrt[s]=1.96  TeV, exclusive cross sections for events with two jets of transverse energy E_T^jet≥10  GeV have been measured as a function of minimum E_T^jet. The exclusive signal is extracted from fits to data distributions based on Monte Carlo simulations of expected dijet signal and background shapes. The simulated background distribution shapes are checked in a study of a largely independent data sample of 200  pb^-1 of b-tagged jet events, where exclusive dijet production is expected to be suppressed by the J_z=0 total angular momentum selection rule. Results obtained are compared with theoretical expectations, and implications for exclusive Higgs boson production at the pp Large Hadron Collider at sqrt[s]=14  TeV are discussed.

We have performed a search for B_s⁰→μ⁺μ^- and B⁰→μ⁺μ^- decays in pp̅ collisions at sqrt[s]=1.96  TeV using 2  fb^-1 of integrated luminosity collected by the CDF II detector at the Fermilab Tevatron Collider. The observed number of B_s⁰ and B⁰ candidates is consistent with background expectations. The resulting upper limits on the branching fractions are B(B_s⁰→μ⁺μ^-)<5.8×10^-8 and B(B⁰→μ⁺μ^-)<1.8×10^-8 at 95% C.L.

We search for evidence of resonant top quark pair production in 955 pb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV recorded with the CDF II detector at the Fermilab Tevatron. For fully reconstructed candidate tt̅ events triggered on leptons with large transverse momentum and containing at least one identified b-quark jet, we compare the invariant mass spectrum of tt̅ pairs to the expected superposition of standard model tt̅ , non-tt̅ backgrounds, and a simple resonance model based on a sequential Z^′ boson. We establish upper limits for σ(pp̅ →Z^′)·Br(Z^′→tt̅ ) in the Z^′ mass interval from 450  GeV/c² to 900  GeV/c². A topcolor leptophobic Z^′ is ruled out below 720  GeV/c², and the cross section of any narrow Z^′-like state decaying to tt̅ is found to be less than 0.64 pb at 95% C.L. for M_Z^′ above 700  GeV/c².

Inclusive jet cross sections in Z/γ^* events, with Z/γ^* decaying into an electron-positron pair, are measured as a function of jet transverse momentum and jet multiplicity in pp̅ collisions at sqrt[s]=1.96   TeV with the upgraded Collider Detector at Fermilab in run II, based on an integrated luminosity of 1.7   fb^-1. The measurements cover the rapidity region |y^jet|<2.1 and the transverse momentum range p_T^jet>30  GeV/c. Next-to-leading order perturbative QCD predictions are in good agreement with the measured cross sections.

We present the first measurement of the production cross section of a W boson with a single charm quark (c) in pp̅ collisions at sqrt[s]=1.96  TeV, using soft muon tagging of c jets. In a data sample of ∼1.8  fb^-1, recorded with the Collider Detector at Fermilab II detector at the Fermilab Tevatron, we select events with W+1 or 2 jets. We use the charge correlation between the W and the muon from the semileptonic decay of a charm hadron to extract the Wc signal. We measure σ_Wc(p_Tc>20  GeV/c,|η_c|<1.5)×BR(W→ℓν)=9.8±3.2  pb, in agreement with theoretical expectations.

We present a search for the associated production of charginos and neutralinos in pp̅ collisions at sqrt[s]=1.96  TeV. The data were collected at the Collider Detector at Fermilab (CDF II) and correspond to integrated luminosities between 0.7 and 1.0  fb^-1. We look for final states with one high-p_T electron or muon, and two additional leptons. Our results are consistent with the standard model expectations, and we set limits on the cross section as a function of the chargino mass in three different supersymmetric scenarios. For a specific minimal supersymmetric standard model scenario with no slepton mixing, we set a 95% C.L. limit at 151  GeV/c².

We report the observation of two narrow resonances consistent with states of orbitally excited (L=1) B_s mesons using 1  fb^-1 of pp̅ collisions at sqrt[s]=1.96  TeV collected with the Collider Detector at Fermilab II detector at the Fermilab Tevatron. We use two-body decays into K^- and B⁺ mesons reconstructed as B⁺→J/ψK⁺, J/ψ→μ⁺μ^- or B⁺→D̅ ⁰π⁺, D̅ ⁰→K⁺π^-. We deduce the masses of the two states to be m(B_s1)=5829.4±0.7  MeV/c² and m(B_s2^*)=5839.6±0.7  MeV/c².

A direct measurement of the total decay width of the W boson Γ_W is presented using 350  pb^-1 of data from pp̅ collisions at sqrt[s]=1.96  TeV collected with the CDF II detector at the Fermilab Tevatron. The width is determined by normalizing predicted signal and background distributions to 230 185 W candidates decaying to eν and μν in the transverse-mass region 50<M_T<90  GeV and then fitting the predicted shape to 6055 events in the high-M_T region, 90<M_T<200  GeV. The result is Γ_W=2032±45_stat±57_syst  MeV, consistent with the standard model expectation.

We report the first top-quark mass measurement that uses a cross-section constraint to improve the mass determination. This measurement is made with a dilepton tt̅ event candidate sample collected with the Collider Detector II at Fermilab. From a data sample corresponding to an integrated luminosity of 1.2  fb^-1, we measure a top-quark mass of 170.7_-3.9^+4.2(stat)±2.6(syst)±2.4(theory)  GeV/c². The measurement without the cross-section constraint is 169.7_-4.9^+5.2(stat)±3.1(syst)  GeV/c².

We present a measurement of the cross section for W-boson production in association with jets in pp̅ collisions at sqrt[s]=1.96  TeV. The analysis uses a data sample corresponding to an integrated luminosity of 320  pb^-1 collected with the CDF II detector. W bosons are identified in their electron decay channel and jets are reconstructed using a cone algorithm. For each W+≥n-jet sample (n=1-4) we measure the differential cross section dσ(pp̅ →W+≥n-jet)/dE_T^nth-jet×B(W→eν) with respect to the transverse energy E_T of the nth-highest E_T jet above 20 GeV, and the total cross section σ(pp̅ →W+≥n-jet;E_T^nth-jet>25  GeV)×B(W→eν), for a restricted W→eν decay phase space. The cross sections, corrected for all detector effects, can be directly compared to particle level W+jet(s) predictions. We present here comparisons to leading order and next-to-leading order predictions.