Search Results (64 total)

The results based on 1992–95 data (Run 1) from the CDF and D0 experiments on the measurements of the W boson mass and width are presented, along with the combined results. We report a Tevatron collider average M_W=80.456±0.059  GeV. We also report the Tevatron collider average of the directly measured W boson width Γ_W=2.115±0.105  GeV. We describe a new joint analysis of the direct W mass and width measurements. Assuming the validity of the standard model, we combine the directly measured W boson width with the width extracted from the ratio of W and Z boson leptonic partial cross sections. This combined result for the Tevatron is Γ_W=2.135±0.050  GeV. Finally, we use the measurements of the direct total W width and the leptonic branching ratio to extract the leptonic partial width Γ(W→eν)=224±13  MeV.

Anomalous quartic couplings between the electroweak gauge bosons may contribute to the νν̅ γγ and qq̅ γγ final states produced in e⁺e^- collisions. This analysis uses the LEP2 OPAL data sample at center-of-mass energies up to 209 GeV. Event selections identify νν̅ γγ and qq̅ γγ events in which the two photons are reconstructed within the detector acceptance. The cross section for the process e⁺e^-→qq̅ γγ is measured. Averaging over all energies, the ratio of the observed e⁺e^-→qq̅ γγ cross section to the standard model expectation is R(data/SM)=0.92±0.07±0.04, where the errors represent the statistical and systematic uncertainties respectively. The νν̅ γγ and qq̅ γγ data are used to constrain possible anomalous W⁺W^-γγ and ZZγγ couplings. Combining with previous OPAL results from the W⁺W^-γ final state, the 95% confidence level limits on the anomalous coupling parameters a₀^Z, a_c^Z, a₀^W and a_c^W are found to be -0.007 GeV^-2<a₀^Z/Λ²<0.023 GeV^-2, -0.029 GeV^-2<a_c^Z/Λ²<0.029 GeV^-2, -0.020 GeV^-2<a₀^W/Λ²<0.020 GeV^-2, -0.052 GeV^-2<a_c^W/Λ²<0.037 GeV^-2, where Λ is the energy scale of the new physics. Limits found when allowing two or more parameters to vary are also presented.

We present the first experimental results based on the jet boost algorithm, a technique to select unbiased samples of gluon jets in e⁺e^- annihilations, i.e. gluon jets free of biases introduced by event selection or jet finding criteria. Our results are derived from hadronic Z⁰ decays observed with the OPAL detector at the LEP e⁺e^- collider at CERN. First, we test the boost algorithm through studies with HERWIG Monte Carlo events and find that it provides accurate measurements of the charged particle multiplicity distributions of unbiased gluon jets for jet energies larger than about 5 GeV, and of the jet particle energy spectra (fragmentation functions) for jet energies larger than about 14 GeV. Second, we apply the boost algorithm to our data to derive unbiased measurements of the gluon jet multiplicity distribution for energies between about 5 and 18 GeV, and of the gluon jet fragmentation function at 14 and 18 GeV. In conjunction with our earlier results at 40 GeV, we then test QCD calculations for the energy evolution of the distributions, specifically the mean and first two nontrivial normalized factorial moments of the multiplicity distribution, and the fragmentation function. The theoretical results are found to be in global agreement with the data, although the factorial moments are not well described for jet energies below about 14 GeV.

We present a measurement of the mass difference m(D_s⁺)-m(D⁺), where both the D_s⁺ and D⁺ are reconstructed in the φπ⁺ decay channel. This measurement uses 11.6 pb^-1 of data collected by CDF II using the new displaced-track trigger. The mass difference is found to be m(D_s⁺)-m(D⁺)=99.41±0.38(stat)±0.21(syst) MeV/c².

We present the results of a search in pp̅ collisions at sqrt[s]=1.8 TeV for anomalous production of events containing a photon with large transverse energy and a lepton (e or μ) with large transverse energy, using 86 pb^-1 of data collected with the Collider Detector at Fermilab during the 1994–1995 collider run at the Fermilab Tevatron. The presence of large missing transverse energy (E_T), additional photons, or additional leptons in these events is also analyzed. The results are consistent with standard model expectations, with the possible exception of photon-lepton events with large E_T, for which the observed total is 16 events and the expected mean total is 7.6±0.7 events.

We present a search for the flavor-changing neutral current decay B_s→μ⁺μ^-φ in pp̅ collisions at sqrt[s]=1.8 TeV, using 91 pb^-1 of data collected at the Collider Detector at Fermilab (CDF). We find two candidate events for this decay, which are consistent with the background estimate of one event, and set an upper limit on the branching fraction of B(B_s→μ⁺μ^-φ)<6.7×10^-5 at a 95% confidence level. This is the first limit on the branching fraction of this decay.

We search for standard model single-top-quark production in the W-gluon fusion and W^⋆ channels using 106 pb^-1 of data from pp̅ collisions at sqrt[s]=1.8 TeV collected with the Collider Detector at Fermilab. We set an upper limit at 95% confidence level (C.L.) on the combined W-gluon fusion and W^⋆ single-top cross section of 14 pb, roughly six times larger than the standard model prediction. Separate 95% C.L. upper limits in the W-gluon fusion and W^⋆ channels are also determined and are found to be 13 and 18 pb, respectively.

The growth and development of “charged particle jets” produced in proton-antiproton collisions at 1.8 TeV  are studied over a transverse momentum range from 0.5 GeV/c to 50 GeV/c. A variety of leading (highest transverse momentum) charged jet observables are compared with the QCD Monte Carlo models HERWIG, ISAJET, and PYTHIA. The models describe fairly well the multiplicity distribution of charged particles within the leading charged jet, the size of the leading charged jet, the radial distribution of charged particles and transverse momentum around the leading charged jet direction, and the momentum distribution of charged particles within the leading charged jet. The direction of the leading “charged particle jet” in each event is used to define three regions of η-φ space. The “toward” region contains the leading “charged particle jet,” while the “away” region, on the average, contains the away-side jet. The “transverse” region is perpendicular to the plane of the hard 2-to-2 scattering and is very sensitive to the “underlying event” component of the QCD Monte Carlo models. HERWIG, ISAJET, and PYTHIA with their default parameters do not describe correctly all the properties of the “transverse” region.

We report on measurements of the ϒ(1S), ϒ(2S), and ϒ(3S) differential cross sections (d²σ/dp_Tdy)_|y|<0.4, as well as on the ϒ(1S) polarization in pp̅ collisions at sqrt[s]  =  1.8 TeV using a sample of 77±3 pb^-1 collected by the collider detector at Fermilab. The three resonances were reconstructed through the decay ϒ→μ⁺μ^-. The measured angular distribution of the muons in the ϒ(1S) rest frame is consistent with unpolarized meson production.

We present a study of pp̅ collisions at sqrt[s]=1800 and 630 GeV collected using a minimum bias trigger by the CDF experiment in which the data set is divided into two classes corresponding to “soft” and “hard” interactions. For each subsample, the analysis includes measurements of the multiplicity, transverse momentum (p_T) spectrum, and the average p_T and event-by-event p_T dispersion as a function of multiplicity. A comparison of results shows distinct differences in the behavior of the two samples as a function of the center of mass (c.m.) energy. We find evidence that the properties of the soft sample are invariant as a function of c.m. energy.

We report a measurement of the diffractive structure function F_jj^D of the antiproton obtained from a study of dijet events produced in association with a leading antiproton in p̅ p collisions at sqrt[s]  =  630 GeV at the Fermilab Tevatron. The ratio of F_jj^D at sqrt[s]  =  630 GeV to F_jj^D obtained from a similar measurement at sqrt[s]  =  1800 GeV is compared with expectations from QCD factorization and other theoretical predictions. We also report a measurement of the ξ ( x-Pomeron) and β ( x of parton in Pomeron) dependence of F_jj^D at sqrt[s]  =  1800 GeV. In the region 0.035<ξ<0.095, |t|<1 GeV², and β<0.5, F_jj^D(β,ξ) is found to be of the form β^-1.0±0.1ξ^-0.9±0.1, which obeys β-ξ factorization.

We present a detailed examination of the heavy flavor content of the W+jet data sample collected with the Collider Detector at Fermilab during the 1992–1995 collider run at the Fermilab Tevatron. Jets containing heavy flavor quarks are selected via the identification of secondary vertices or semileptonic decays of b and c quarks. There is generally good agreement between the rates of secondary vertices and soft leptons in the data and in the standard model simulation including single and pair production of top quarks. An exception is the number of events in which a single jet has both a soft lepton and a secondary vertex tag. In W+2,3 jet data, we find 13 such events where we expected 4.4±0.6 events. The kinematic properties of this small sample of events are statistically difficult to reconcile with the simulation of standard model processes.

We have searched for evidence of physics beyond the standard model in events that include an energetic photon and an energetic b-quark jet, produced in 85 pb^-1 of p̅ p collisions at 1.8 TeV at the Tevatron Collider at Fermilab. This signature, containing at least one gauge boson and a third-generation quark, could arise in the production and decay of a pair of new particles, such as those predicted by supersymmetry, leading to a production rate exceeding standard model predictions. We also search these events for anomalous production of missing transverse energy, additional jets and leptons (e, μ and τ), and additional b quarks. We find no evidence for any anomalous production of γb or γb+X events. We present limits on two supersymmetric models: a model where the photon is produced in the decay χ̃₂⁰→γχ̃₁⁰, and a model where the photon is produced in the neutralino decay into the gravitino LSP, χ̃₁⁰→γG̃. We also present our limits in a model-independent form and test methods of applying model-independent limits.

We present measurements of the B⁺ meson total cross section and differential cross section dσ/dp_T. The measurements use a 98±4 pb^-1 sample of pp̅ collisions at sqrt[s]=1.8 TeV collected by the CDF detector. Charged B meson candidates are reconstructed through the decay B^±→J/ψK^± with J/ψ→μ⁺μ^-. The total cross section, measured in the central rapidity region |y|<1.0 for p_T(B)>6.0 GeV/c, is 3.6±0.6(stat⊕ syst) μb. The measured differential cross section is substantially larger than typical QCD predictions calculated to next-to-leading order.

We present the first general search for new heavy particles, X, which decay via X→WZ⁰→eν+jj as a function of M_X and Γ(X) in pp̅ collisions at sqrt[s]  =  1.8 TeV. No evidence is found for production of X in 110 pb^-1 of data collected by the Collider Detector at Fermilab. General cross section limits are set at the 95% C.L. as a function of mass and width of the new particle. The results are further interpreted as mass limits on the production of new heavy charged vector bosons which decay via W^′→WZ⁰ in an extended gauge model as a function of the width, Γ(W^′), and mixing factor between the W^′ and the standard model W bosons.

We report a study of the decays B⁰→J/ψK^(*)0π⁺π^-, which involve the creation of a uu̅ or dd̅ quark pair in addition to a b̅ →c̅ (cs̅ ) decay. The data sample consists of 110 pb^-1 of pp̅ collisions at sqrt[s]  =  1.8 TeV collected by the CDF detector at the Fermilab Tevatron collider during 1992–1995. We measure the branching fractions to be B(B⁰→J/ψK^*0π⁺π^-)  =  (6.6±1.9±1.1)×10^-4 and B(B⁰→J/ψK⁰π⁺π^-)  =  (10.3±3.3±1.5)×10^-4. Evidence is seen for contributions from ψ(2S)K^(*)0, J/ψK⁰ρ⁰, J/ψK^*+π^-, and J/ψK₁(1270).

We report a measurement of the strong coupling constant, α_s(M_Z), extracted from inclusive jet production in pp̅ collisions at sqrt[s]  =  1800 GeV. The QCD prediction for the evolution of α_s with jet transverse energy E_T is tested over the range 40<E_T<450 GeV using E_T for the renormalization scale. The data show good agreement with QCD in the region below 250 GeV. The value of α_s at the mass of the Z⁰ boson averaged over the range 40<E_T<250 GeV is found to be α_s(M_Z)  =  0.1178±0.0001(stat)_-0.0095^+0.0081(expt. syst). The associated theoretical uncertainties are mainly due to the choice of renormalization scale ({+6%}{-4%}) and input parton distribution functions (5%).

We have performed a search for gluinos (g̃) and scalar quarks (q̃) in a data sample of 84 pb^-1 of pp̅ collisions at sqrt[s]  =  1.8 TeV, recorded by the Collider Detector at Fermilab. We investigate the final state of large missing transverse energy and three or more jets, a characteristic signature in R-parity-conserving supersymmetric models. The analysis has been performed “blind,” in that the inspection of the signal region is made only after the predictions from standard model backgrounds have been calculated. Comparing the data with predictions of constrained supersymmetric models, we exclude gluino masses below 195 GeV/c² ( 95% C.L.), independent of the squark mass. For the case m_q̃≈m_g̃, gluino masses below 300 GeV/c² are excluded.

We present a measurement of the cross section and the first measurement of the heavy flavor content of associated direct photon + muon events produced in hadronic collisions. These measurements come from a sample of 1.8 TeV pp̅ collisions recorded with the Collider Detector at Fermilab. Quantum chromodynamics (QCD) predicts that these events are primarily due to the Compton scattering process cg→cγ, with the final-state charm quark producing a muon. The cross section for events with a photon transverse momentum between 12 and 40 GeV/c is measured to be 46.8±6.3±7.5 pb, which is two standard deviations below the most recent theoretical calculation. A significant fraction of the events in the sample contain a final-state bottom quark. The ratio of charm to bottom production is measured to be 2.4±1.2, in good agreement with QCD models.

We present results of the first search for like-sign dilepton ( e^±e^±, e^±μ^±, μ^±μ^±) events associated with multijets and large missing energy using 106 pb^-1 of data in pp̅ collisions at sqrt[s]  =  1.8 TeV collected during 1992–1995 by the CDF experiment. Finding no events that pass our selection, we examine pair production of gluinos ( g̃) and squarks ( q̃) in a constrained framework of the minimal supersymmetric standard model. At tanβ  =  2 and μ  =  -800 GeV/c², we set 95% confidence level limits of M_g̃>221 GeV/c² for M_g̃≃M_q̃, and M_g̃>168 GeV/c² for M_q̃≫M_g̃, both with small variation as a function of μ.

We report the first observation of diffractive J/ψ(→μ⁺μ^-) production in p̅ p collisions at sqrt[s]  =  1.8 TeV. Diffractive events are identified by their rapidity gap signature. In a sample of events with two muons of transverse momentum p_T^μ>2 GeV/c within the pseudorapidity region |η|<1.0, the ratio of diffractive to total J/ψ production rates is found to be R_J/ψ  =  [1.45±0.25]%. The ratio R_J/ψ(x) is presented as a function of x-Bjorken. By combining it with our previously measured corresponding ratio R_jj(x) for diffractive dijet production, we extract a value of 0.59±0.15 for the gluon fraction of the diffractive structure function of the proton.

We present searches for quark-lepton compositeness and a heavy W^′ boson at high electron-neutrino transverse mass. We use ∼110 pb^-1 of data collected in pp̅ collisions at sqrt[s]  =  1.8 TeV by the CDF Collaboration during 1992–1995. The data are consistent with standard model expectations. Limits are set on the quark-lepton compositeness scale Λ, the ratio of partial cross sections σ(W^′→eν)/σ(W→eν), and the mass of a W^′ boson with standard model couplings. We exclude Λ<2.81 TeV and a W^′ boson with mass below 754 GeV/c² at the 95% confidence level. Combining with our previously published limit obtained using the muon channel, we exclude a W^′ boson with mass below 786 GeV/c² at the 95% confidence level.