Search

The data processing model for the CDF experiment is described. Data processing reconstructs events from parallel data streams taken with different combinations of physics event triggers and further splits the events into datasets of specialised physics interests. The design of the processing control system makes strict requirements on bookkeeping records, which trace the status of data files and event contents during processing and storage. The computing architecture was updated to meet the mass data flow of the Run II data collection, recently upgraded to a maximum rate of 40 MByte/sec. The data processing facility consists of a large cluster of Linux computers with data movement managed by the CDF data handling system to a multi-petaByte Enstore tape library. The latest processing cycle has achieved a stable speed of 35 MByte/sec (3 TByte/day). It can be readily scaled by increasing CPU and data-handling capacity as required. [ABSTRACT FROM AUTHOR]

The mass of the W boson, a mediator of the weak force between elementary particles, is tightly constrained by the symmetries of the standard model of particle physics. The Higgs boson was the last missing component of the model. After observation of the Higgs boson, a measurement of the W boson mass provides a stringent test of the model. We measure the W boson mass, MW, using data corresponding to 8.8 inverse femtobarns of integrated luminosity collected in proton-antiproton collisions at a 1.96 tera–electron volt center-of-mass energy with the CDF II detector at the Fermilab Tevatron collider. A sample of approximately 4 million W boson candidates is used to obtain MW ¼ 80;433:5 T 6:4stat T 6:9syst ¼ 80;433:5 T 9:4 MeV=c2, the precision of which exceeds that of all previous measurements combined (stat, statistical uncertainty; syst, systematic uncertainty; MeV, mega–electron volts; c, speed of light in a vacuum). This measurement is in significant tension with the standard model expectation. [ABSTRACT FROM AUTHOR]

The observation of the structure in decays produced in collisions at TeV is reported with a statistical significance greater than 5 standard deviations. A fit to the mass spectrum is performed assuming the presence of a Breit-Wigner resonance. The fit yields a signal of resonance events, and resonance mass and width of and , respectively. The parameters of this resonance-like structure are consistent with values reported from an earlier CDF analysis. [ABSTRACT FROM AUTHOR]

We report on a search for charged massive resonances decaying to top (t) and bottom (b) quarks in the full data set of proton-antiproton collisions at a center-of-mass energy of √s = 1.96 TeV collected by the CDF II detector at the Tevatron, corresponding to an integrated luminosity of 9.5 fb-1. No significant excess above the standard model background prediction is observed. We set 95% Bayesian credibility mass-dependent upper limits on the heavy charged-particle production cross section times branching ratio to tb. Using a standard model extension with a W'→tb and left-right-symmetric couplings as a benchmark model, we constrain the W' mass and couplings in the 300-900 GeV/c2 range. The limits presented here are the most stringent for a charged resonance with mass in the range 300-600 GeV/c2 decaying to top and bottom quarks. [ABSTRACT FROM AUTHOR]

Combined constraints from the CDF and D0 Collaborations on models of the Higgs boson with exotic spin J and parity P are presented and compared with results obtained assuming the standard model value Jp = 0+. Both collaborations analyzed approximately 10 fb-1 of proton-antiproton collisions with a center-of-mass energy of 1.96 TeV collected at the Fermilab Tevatron. Two models predicting exotic Higgs bosons with Jp = 0- and Jp = 2+ are tested. The kinematic properties of exotic Higgs boson production in association with a vector boson differ from those predicted for the standard model Higgs boson. Upper limits at the 95% credibility level on the production rates of the exotic Higgs bosons, expressed as fractions of the standard model Higgs boson production rate, are set at 0.36 for both the Jp = 0- hypothesis and the Jp = 2+ hypothesis. If the production rate times the branching ratio to a bottom-antibottom pair is the same as that predicted for the standard model Higgs boson, then the exotic bosons are excluded with significances of 5.0 standard deviations and 4.9 standard deviations for the Jp = 0- and Jp = 2+ hypotheses, respectively. [ABSTRACT FROM AUTHOR]

A search for particles with the same mass and couplings as those of the standard model Higgs boson but different spin and parity quantum numbers is presented. We test two specific alternative Higgs boson hypotheses: a pseudoscalar Higgs boson with spin-parity Jp = 0- and a gravitonlike Higgs boson with Jp = 2+, assuming for both a mass of 125 GeV/c2. We search for these exotic states produced in association with a vector boson and decaying into a bottom-antibottom quark pair. The vector boson is reconstructed through its decay into an electron or muon pair, or an electron or muon and a neutrino, or it is inferred from an imbalance in total transverse momentum. We use expected kinematic differences between events containing exotic Higgs bosons and those containing standard model Higgs bosons. The data were collected by the CDF experiment at the Tevatron proton-antiproton collider, operating at a center-of-mass energy of √s = 1.96 TeV, and correspond to an integrated luminosity of 9.45 fb-1. We exclude deviations from the predictions of the standard model with a Higgs boson of mass 125 GeV/c2 at the level of 5 standard deviations, assuming signal strengths for exotic boson production equal to the prediction for the standard model Higgs boson, and set upper limits of approximately 30% relative to the standard model rate on the possible rate of production of each exotic state. [ABSTRACT FROM AUTHOR]

We report final measurements of direct CP-violating asymmetries in charmless decays of neutral bottom hadrons to pairs of charged hadrons with the upgraded Collider Detector at the Fermilab Tevatron. Using the complete √s = 1.96 TeV proton-antiproton collisions data set, corresponding to 9.3 fb-1 of integrated luminosity, we measure Α(Ab0 → pΚ-) = +0.06 ± 0.07 (stat) ±0.03 (syst) and A(A0b → pK-) = -0.10 ± 0.08(stat) ± 0.04(syst), compatible with no asymmetry. In addition we measure the CP-violating asymmetries in Bos → K-π+ and Bo → K+ π- decays to be A(B0S →K-π+) = +0.22 ± 0.07(stat) ± 0.02(syst) and A(B0 → K+π-) = -0.083 ± 0.013(stat) ± 0.004(syst), respectively, which are significantly different from zero and consistent with current world averages. [ABSTRACT FROM AUTHOR]

We measure the inclusive forward–backward asymmetry of the charged-lepton pseudorapidities from top-quark pairs produced in proton–antiproton collisions and decaying to final states that contain two charged leptons (electrons or muons). The data are collected with the Collider Detector at Fermilab and correspond to an integrated luminosity of 9.1 fb-1. We measure the leptonic forward–backward asymmetry, AℓFB, to be 0.072±0.060 and the leptonic pair forward–backward asymmetry, AℓℓFB, to be 0.076±0.082. The measured values can be compared with the standard model predictions of AℓFB=0.038±0.003 and AℓℓFB=0.048±0.004, respectively. Additionally, we combine the AℓFB result with a previous determination from a final state with a single lepton and hadronic jets and obtain AℓFB=0.090+0.028-0.026. [ABSTRACT FROM AUTHOR]

We report evidence for s-channel single-top-quark production in proton-antiproton collisions at center-of-mass energy √s=1.96 TeV using a data set that corresponds to an integrated luminosity of 9.4 fb−1 collected by the Collider Detector at Fermilab. We select events consistent with the s-channel process including two jets and one leptonically decaying W boson. The observed significance is 3.8 standard deviations with respect to the background-only prediction. Assuming a top-quark mass of 172.5 GeV/c2, we measure the s-channel cross section to be 1.41−0.42+0.44 pb. [ABSTRACT FROM AUTHOR]

The first search for single-top-quark production from the exchange of an s-channel virtual W boson using events with an imbalance in the total transverse energy, b-tagged jets, and no identified leptons is presented. Assuming the electroweak production of top quarks of mass 172.5 GeV/c2 in the s channel, a cross section of 1.12−0.57+0.61 (stat+syst) pb with a significance of 1.9 standard deviations is measured. This measurement is combined with the result obtained from events with an imbalance in total transverse momentum, b-tagged jets, and exactly one identified lepton, yielding a cross section of 1.36−0.32+0.37 (stat+syst) pb, with a significance of 4.2 standard deviations. [ABSTRACT FROM AUTHOR]

We report the first observation of single-top-quark production in the s channel through the combination of the CDF and D0 measurements of the cross section in proton-antiproton collisions at a center-of-mass energy of 1.96 TeV. The data correspond to total integrated luminosities of up to 9.7 fb−1 per experiment. The measured cross section is σs=1.29−0.24+0.26 pb. The probability of observing a statistical fluctuation of the background to a cross section of the observed size or larger is 1.8×10−10, corresponding to a significance of 6.3 standard deviations for the presence of an s-channel contribution to the production of single-top quarks. [ABSTRACT FROM AUTHOR]

We present a measurement of the ratio of the top-quark branching fractions R=B(t→Wb)/B(t→Wq), where q represents any quark flavor, in events with two charged leptons, imbalance in total transverse energy, and at least two jets. The measurement uses proton-antiproton collision data at center-of-mass energy 1.96 TeV, corresponding to an integrated luminosity of 8.7 f-1 collected with the Collider Detector at Fermilab during Run II of the Tevatron. We measure R to be 0.87±0.07, and extract the magnitude of the top-bottom quark coupling to be ∣Vtb∣=0.93±0.04, assuming three generations of quarks. Under these assumptions, a lower limit of ∣Vtb∣>0.85(0.87) at 95% (90%) credibility level is set. [ABSTRACT FROM AUTHOR]

A search for forbidden and exotic Z boson decays in the diphoton mass spectrum is presented for the first time in hadron collisions, based on data corresponding to 10.0 fb-1 of integrated luminosity from proton-antiproton collisions at √s = 1.96 TeV collected by the CDF experiment. No evidence of signal is observed, and 95% credibility level Bayesian upper limits are set on the branching ratios of decays of the Z boson to a photon and neutral pion (which is detected as a photon), a pair of photons, and a pair of neutral pions. The observed branching ratio limits are 2.01 × 10-5 for Z → π0γ, 1.46 × 10-5 for Z → γγ, and 1.52 × 10-5 for Z → π0π0. The Z → π0γ and Z → γγ limits improve the most stringent results from other experiments by factors of 2.6 and 3.6, respectively. The Z → π0π0 branching ratio limit is the first experimental result on this decay. [ABSTRACT FROM AUTHOR]

We present a measurement of the total decay width of the top quark using events with top-antitop quark pair candidates reconstructed in the final state with one charged lepton and four or more hadronic jets. We use the full Tevatron run II data set of √s=1.96 TeV proton-antiproton collisions recorded by the CDF II detector. The top quark mass and the mass of the hadronically decaying W boson are reconstructed for each event and compared with distributions derived from simulated signal and background samples to extract the top quark width (Γtop) and the energy scale of the calorimeter jets with in situ calibration. For a top quark mass Mtop=172.5  GeV/c2, we find 1.10<Γtop<4.05  GeV at 68% confidence level, which is in agreement with the standard model expectation of 1.3 GeV and is the most precise direct measurement of the top quark width to date. [ABSTRACT FROM AUTHOR]

We report a measurement of the differential cross section dσ/d(cos θt) for top-quark pair production as a function of the top-quark production angle in proton-antiproton collisions at √s=1.96 TeV. This measurement is performed using data collected with the CDF II detector at the Tevatron, corresponding to an integrated luminosity of 9.4 fb-1. We employ the Legendre polynomials to characterize the shape of the differential cross section at the parton level. The observed Legendre coefficients are in good agreement with the prediction of the next-to-leading-order standard-model calculation, with the exception of an excess linear-term coefficient a1=0.40±0.12, compared to the standard-model prediction of a1=0.15-0.03+0.07. [ABSTRACT FROM AUTHOR]

We report on a measurement of the cross section for direct-photon production in association with a heavy quark using the full data set of √s=1.96  TeV proton-antiproton collisions corresponding to 9.1  fb-1 of integrated luminosity collected by the CDF II detector at the Fermilab Tevatron. The measurements are performed as a function of the photon transverse momentum, covering a photon transverse momentum between 30 and 300 GeV, photon rapidities ∣yγ∣<1.0, a heavy-quark-jet transverse momentum pTjet>20  GeV, and jet rapidities ∣yjet∣<1.5. The results are compared with several theoretical predictions. [ABSTRACT FROM AUTHOR]

We present a search for the pair production of a narrow nonstandard-model strongly interacting particle that decays to a pair of quarks or gluons, leading to a final state with four hadronic jets. We consider both nonresonant production via an intermediate gluon as well as resonant production via a distinct nonstandard-model intermediate strongly interacting particle. We use data collected by the CDF experiment in proton-antiproton collisions at √s = 1.96 TeV corresponding to an integrated luminosity of 6.6 fb-1.We find the data to be consistent with nonresonant production. We report limits on σ(p → jjjj) as a function of the masses of the hypothetical intermediate particles. Upper limits on the production cross sections for nonstandard-model particles in several resonant and nonresonant processes are also derived. [ABSTRACT FROM AUTHOR]

We present a search for chargino-neutralino associated production using like electric charge dilepton events collected by the CDF II detector at the Fermilab Tevatron in proton-antiproton collisions at √s = 1.96 TeV. One lepton is identified as the hadronic decay of a tau lepton, while the other is an electron or muon. In data corresponding to 6.0 fb-1 of integrated luminosity, we obtain good agreement with standard model predictions and set limits on the chargino-neutralino production cross section for simplified gravity- and gauge-mediated models. As an example, assuming that the chargino and neutralino decays to taus dominate, in the simplified gauge-mediated model we exclude cross sections greater than 300 fb at 95% credibility level for chargino and neutralino masses of 225 GeV/c². This analysis is the first to extend the LHC searches for electroweak supersymmetric production of gauginos to high tanβ and slepton next-to-lightest supersymmetric particle scenarios. [ABSTRACT FROM AUTHOR]

We present a search for new particles in an extension to the standard model that includes a heavy Higgs boson (H0), a lighter charged Higgs boson (H±), and an even lighter Higgs boson h0, with decays leading to a W-boson pair and a bottom-antibottom quark pair in the final state. We use events with exactly one lepton, missing transverse momentum, and at least four jets in data corresponding to an integrated luminosity of 8.7 fb-1 collected by the CDF II detector in proton-antiproton collisions at √s=1.96 TeV. We find the data to be consistent with standard model predictions and report the results in terms of a simplified Higgs-cascade-decay model, setting 95% confidence level upper limits on the product of cross section and branching fraction from 1.3 pb to 15 fb as a function of H0 and H± masses for mh0=126 GeV/c2. [ABSTRACT FROM AUTHOR]

This Letter reports a search for a narrow resonant state decaying into two W bosons and two b quarks where one W boson decays leptonically and the other decays into a quark-antiquark pair. The search is particularly sensitive to top-antitop resonant production. We use the full data sample of proton-antiproton collisions at a center-of-mass energy of 1.96 TeV collected by the CDF II detector at the Fermilab Tevatron, corresponding to an integrated luminosity of 9.45 fb-1. No evidence for resonant production is found, and upper limits on the production cross section times branching ratio for a narrow resonant state are extracted. Within a specific benchmark model, we exclude a Z' boson with mass, MZ', below 915 GeV/c2 decaying into a top-antitop pair at the 95% credibility level assuming a Z' boson decay width of ΓZ'=0.012MZ'. This is the most sensitive search for a narrow q̄q -initiated t̄t resonance in the mass region below 750 GeV/c2. [ABSTRACT FROM AUTHOR]

The first observation of the production of a W boson with a single charm quark (c) jet in pp collisions at ∞s=1.96 TeV is reported. The analysis uses data corresponding to 4.3 fb-1, recorded with the CDF II detector at the Fermilab Tevatron. Charm quark candidates are selected through the identification of an electron or muon from charm-hadron semileptonic decay within a hadronic jet, and a Wc signal is observed with a significance of 5.7 standard deviations. The production cross section σWc(pTc>20 GeV/c,ηc<1.5)×B(W→ lv) is measured to be 13.6-3.1+3.4 pb and is in agreement with theoretical expectations. From this result the magnitude of the quark-mixing matrix element Vcs is derived, Vcs=1.08±0.16 along with a lower limit of Vcs>0.71 at the 95% confidence level, assuming that the Wc production through c to s quark coupling is dominant. [ABSTRACT FROM AUTHOR]

We present the first direct measurement of the top-quark mass using it events decaying in the hadronic τ+ jets decay channel. Using data corresponding to an integrated luminosity of 2.2 fb-1 collected by the CDF II detector in pp collisions at js = 1.96 TeV at the Fermilab Tevatron, we measure the ti cross section, σ√ and the top-quark mass, Mmp, We extract Mlop from a likelihood based on per-event probabilities calculated with leading-order signal and background matrix elements. We measure &x-t = 8.8 ± 3.3(stat) ± 2.2(syst) pb and Mtop = 172.7 ± 9.3(stat) ± 3.7(syst) GeV/c2 [ABSTRACT FROM AUTHOR]

We report a measurement of the bottom-strange meson mixing phase βs using the time evolution of Bs0→J/ψ(→μ+μ-)ϕ(→K+K-) decays in which the quark-flavor content of the bottom-strange meson is identified at production. This measurement uses the full data set of proton-antiproton collisions at √s=1.96 TeV collected by the Collider Detector experiment at the Fermilab Tevatron, corresponding to 9.6 fb-1 of integrated luminosity. We report confidence regions in the two-dimensional space of βs and the Bs0 decay-width difference Δγs and measure βs∈[-π/2,-1.51]∪[-0.06,0.30]∪[1.26,π/2] at the 68% confidence level, in agreement with the standard model expectation. Assuming the standard model value of βs, we also determine Δγs=0.068±0.026(stat)±0.009(syst) ps-1 and the mean Bs0 lifetime τs=1.528±0.019(stat)±0.009(syst)  ps, which are consistent and competitive with determinations by other experiments. [ABSTRACT FROM AUTHOR]

Abstract: A search for a narrow Higgs boson resonance in the diphoton mass spectrum is presented based on data corresponding to of integrated luminosity collected by the CDF experiment from proton–antiproton collisions at . To increase the sensitivity of the search, we employ a multivariate discriminant technique for the first time in this channel at CDF. No evidence of signal is observed, and upper limits are set on the cross section times branching ratio of the resonant state as a function of the Higgs boson mass. The limits are interpreted in the context of the standard model with an expected (observed) limit on the cross section times branching ratio of 9.9 (17.0) times the standard model prediction at the 95% credibility level for a Higgs boson mass of . Moreover, a Higgs boson with suppressed couplings to fermions is excluded for masses below at the 95% credibility level. [Copyright &y& Elsevier]

Abstract: A measurement of the top quark mass () in the all-hadronic decay channel is presented. It uses 5.8 fb−1 of data collected with the CDF II detector at the Fermilab Tevatron Collider. Events with six to eight jets are selected by a neural network algorithm and by the requirement that at least one of the jets is tagged as a b-quark jet. The measurement is performed with a likelihood fit technique, which simultaneously determines and the jet energy scale (JES) calibration. The fit yields a value of . [Copyright &y& Elsevier]

We search for annihilation decay modes of neutral b mesons into pairs of charmless charged hadrons with the upgraded Collider Detector at the Fermilab Tevatron. Using a data sample corresponding to 6 fb-1 of integrated luminosity, we obtain the first evidence for the Bs0 → π+ π- decay, with a significance of 3.7σ, and a measured branching ratio B(Bs0 → π+ π-) = (0.57 ± 0.15(stat) ± 0.10(syst)) X 10-6. A search for the B0 → K+ K- mode in the same sample yields a significance of 2.0σ, and a central value estimate B(B0 → K+ K-) = (0.23 ± 0.10(stat) ± 0.10(syst)) X 10-6. [ABSTRACT FROM AUTHOR]

This Letter reports a measurement of the cross section of prompt isolated photon pair production in pj' collisions at a total energy s2 = 1.96 TeV using data of 5.36 th integrated luminosity collected with the CDF II detector at the Fermilab Tevatron. The measured cross section, differential in basic kinematic variables, is compared with three perturbative QCD predictions, a leading order parton shower calculation and two next-to-leading order calculations. The next-to-leading order calculations reproduce most aspects of the data. By including photon radiation from quarks before and after hard scattering, the parton shower prediction becomes competitive with the next-to-leading order predictions. [ABSTRACT FROM AUTHOR]

The observation of the bottom, strange baryon Ξob through the decay chain Ξ+c where Ξ Ξ+c → Air, and Λ → p ¶ is reported by using data corresponding to an integrated luminosity of 4.2 th' from pp collisions at s2 = 1.96 TeV recorded with the Collider Detector at Fermilab. A signal of 25.3+5.6-5.4 candidates is observed whose probability of arising from a background fluctuation is 3.6 X 1012. corresponding to 6.8 Gaussian standard deviations. The Ξob mass is measured to be 5787.8 ± 5.O(stat) ± l.3(syst) MeV/c2. In addition, the baryon is observed through the process Ξ+c where Ξ Ξ+c →, where Ξ+c →,. and Λ → p ¶. [ABSTRACT FROM AUTHOR]

Abstract: We report on a CDF measurement of the total cross section and rapidity distribution, , for events in the Z boson mass region () produced in collisions at with of integrated luminosity. The measured cross section of and distribution are compared with Next-to-Leading-Order (NLO) and Next-to-Next-to-Leading-Order (NNLO) QCD theory predictions with CTEQ and MRST/MSTW parton distribution functions (PDFs). There is good agreement between the experimental total cross section and measurements with theoretical calculations with the most recent NNLO PDFs. [ABSTRACT FROM AUTHOR]

Abstract: We present the result of a search for a massive color-octet vector particle, (e.g. a massive gluon) decaying to a pair of top quarks in proton–antiproton collisions with a center-of-mass energy of 1.96 TeV. This search is based on of data collected using the CDF detector during Run II of the Tevatron at Fermilab. We study events in the lepton+jets channel with at least one b-tagged jet. A massive gluon is characterized by its mass, decay width, and the strength of its coupling to quarks. These parameters are determined according to the observed invariant mass distribution of top quark pairs. We set limits on the massive gluon coupling strength for masses between 400 and 800 GeV and width-to-mass ratios between 0.05 and 0.50. The coupling strength of the hypothetical massive gluon to quarks is consistent with zero within the explored parameter space. [Copyright &y& Elsevier]

The European Physical Journal C—Particles and Fields—publishes scientific manuscripts of relevance to the scientific community following careful and strict peer reviewing and, whenever appropriate and necessary, through discussion with the authors, so as to optimise scientific content and style of presentation prior to publication. In some cases significant disagreement between authors and referees (and/or editors) of the journal cannot be resolved despite all efforts and best of intentions. While the journal—notwithstanding any appeals—retains the right to reject such manuscripts, the editors of this journal may decide, in cases deemed of exceptional interest and potential significance for the field, to accept the manuscript for publication, to amend it by “comments” of the editor(s) in charge and, if appropriate, by a “reply” of the authors of the commented manuscript. The present comment is on “Study of multi-muon events produced in $p\bar{p}$ interactions at $\sqrt{s}=1.96$ TeV” by T. Aaltonen et al. (the CDF Collaboration, Eur. Phys. J. C, , doi:). [ABSTRACT FROM AUTHOR]

We report the results of a study of multi-muon events produced at the Fermilab Tevatron collider and acquired with the CDF II detector using a dedicated dimuon trigger. The production cross section and kinematics of events in which both muon candidates are produced inside the beam pipe of radius 1.5 cm are successfully modeled by known processes which include heavy flavor production. In contrast, we are presently unable to fully account for the number and properties of the remaining events, in which at least one muon candidate is produced outside of the beam pipe, in terms of the same understanding of the CDF II detector, trigger, and event reconstruction. [ABSTRACT FROM AUTHOR]

We report a search for narrow resonances, produced in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV, that decay into muon pairs with invariant mass between 6.3 and 9.0 GeV/ c2. The data, collected with the CDF II detector at the Fermilab Tevatron collider, correspond to an integrated luminosity of 630 pb−1. We use the dimuon invariant mass distribution to set 90% upper credible limits of about 1% to the ratio of the production cross section times muonic branching fraction of possible narrow resonances to that of the ϒ(1S) meson. [ABSTRACT FROM AUTHOR]

Abstract: Fully reconstructed events are used to determine the fractions of right-handed () and longitudinally polarized () W bosons produced in top-quark decays. The helicity fractions are sensitive to the couplings and the Dirac structure of the Wtb vertex. This Letter reports measurements of the W-boson helicity fractions from two different methods using data corresponding to an integrated luminosity of of collisions at a center-of-mass energy of 1.96 TeV collected by the CDF II detector operating at the Fermilab Tevatron. Combining the results from the two methods, we find under the assumption that , and with fixed to the theoretically expected value of 0.70. Model-independent fits are also performed and simultaneously determine and . All these results are consistent with standard model expectations. [Copyright &y& Elsevier]

Abstract: We present a search for events with a tau lepton in the final state. The data sample corresponds to an integrated luminosity of 194 pb−1 collected with the CDF II detector from collisions at a center of mass energy of 1.96 TeV. We observe two events with an expected signal of events and a background of events. We determine a 95% confidence level upper limit on , the ratio of the measured rate of to the expectation, of 5.2. [Copyright &y& Elsevier]