Search Results (299 total)

In the collider run II, the Tevatron operates with 36 high intensity bunches of 980 GeV protons and antiprotons. Particles not captured by the Tevatron rf system pose a threat since they can quench the superconducting magnets during acceleration or at beam abort. We describe the main mechanisms for the origination of this uncaptured beam, and present measurements of its main parameters by means of a newly developed diagnostics system. The Tevatron electron lens is effectively used in the collider run II operation to remove uncaptured beam and keep its intensity in the abort gaps at a safe level.

The Tevatron in Collider Run II (2001–present) is operating with 6 times more bunches, many times higher beam intensities and luminosities than in Run I (1992–1995). Electromagnetic long-range and head-on interactions of high intensity proton and antiproton beams have been significant sources of beam loss and lifetime limitations. We present observations of the beam-beam phenomena in the Tevatron and results of relevant beam studies. We analyze the data and various methods employed in operations, predict the performance for planned luminosity upgrades, and discuss ways to improve it.

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.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

Inclusive momentum distributions of charged particles in restricted cones around jet axes were measured in dijet events with invariant dijet masses in the range 80 to 600 GeV/c². Events were produced at the Fermilab Tevatron in pp̅ collisions with a center of mass energy of 1.8 TeV and recorded by the Collider Detector at Fermilab. The results were compared to perturbative QCD calculations carried out in the framework of the modified leading log approximation (MLLA) and assuming local parton-hadron duality. It was shown that the data follow theoretical predictions quite well over the whole range of the jet energies included in this analysis. We extracted the MLLA cutoff scale Q_eff and found a value of 230±40 MeV. The theoretical prediction of E_jetsin θ_c scaling, where θ_c is the cone opening angle, was experimentally observed for the first time. From the MLLA fits to the data, two more parameters were extracted: the ratio of parton multiplicities in gluon and quark jets, r=N_partons^g-jet/N_partons^q-jet=1.9±0.5, and the ratio of the number of charged hadrons to the number of predicted partons in a jet, K_LPHD^charged=N_hadrons^charged/N_partons=0.56±0.10.

We report the results of a search for a W^′ boson produced in pp̅ collisions at a center-of-mass energy of 1.8 TeV using a 106   pb^-1 data sample recorded by the Collider Detector at Fermilab. We observe no significant excess of events above background for a W^′ boson decaying to a top and bottom quark pair. In a model where this boson would mediate interactions involving a massive right-handed neutrino (ν_R) and have standard model strength couplings, we use these data to exclude a W^′ boson with mass between 225 and 536   GeV/c² at 95% confidence level for M_W^′≫M_νR and between 225 and 566   GeV/c² at 95% confidence level for M_W^′<M_νR.

We have performed a search for radiative b-hadron decays using events produced in pp̅ collisions at sqrt[s]=1.8 TeV and collected by the Collider Detector at Fermilab. The decays we considered were B_d⁰→K^*0(→K^-π⁺)γ, B_s⁰→φ(→K⁺K^-)γ, Λ_b⁰→Λ(→pπ^-)γ, and their charge conjugates. Two independent methods to identify photons from such decays were employed. In the first method, the photon was detected in the electromagnetic calorimeter. In the second method, the photon was identified by an electron-positron pair produced through the external photon conversion before the tracking detector volume. By combining the two methods we obtain upper limits on the branching fractions for the B_d⁰, B_s⁰, and Λ_b⁰ radiative decays which, at the 95% confidence level, are found to be B(B_d⁰→K^*0γ)<1.4×10^-4, B(B_s⁰→φγ)<1.6×10^-4, and B(Λ_b⁰→Λγ)<1.9×10^-3.

The exclusive γE̸_T signal has a small standard model cross section and is thus a channel sensitive to new physics. This signature is predicted by models with a superlight gravitino or with large extra spatial dimensions. We search for such signals at the Collider Detector at Fermilab, using 87   pb^-1 of data at sqrt[s]=1.8   TeV, and extract 95% C.L. limits on these processes. A limit of 221 GeV is set on the scale |F|^1/2 in supersymmetric models. For 4, 6, and 8 extra dimensions, model-dependent limits on the fundamental mass scale M_D of 0.55, 0.58, and 0.60 TeV, respectively, are found. We also specify a “pseudo-model-independent” method of comparing the results to theoretical predictions.

The inclusive cross section for J/ψ production times the branching ratio B(J/ψ→μ⁺μ^-) has been measured in the forward pseudorapidity region: B×dσ[p̅ +p→J/ψ(p_T>10 GeV/c,2.1<|η|<2.6)+X]/dη=192±9(stat)±29(syst) pb. The results are based on 74.1±5.2 pb^-1 of data collected by the CDF Collaboration at the Fermilab Tevatron Collider. The measurements extend earlier measurements of the D0 Collaboration to higher p_T^J/ψ. In the kinematic range where the experiments partially overlap, these data are in good agreement with previous measurements.

We report on measurements of the branching ratios of the decays B⁺→χ_c1⁰(1P)K⁺ and B⁺→J/ψK⁺π⁺π^-, where χ_c1⁰(1P)→J/ψγ and J/ψ→μ⁺μ^- in pp̅ collisions at sqrt[s]=1.8 TeV. Using a data sample from an integrated luminosity of 110 pb^-1 collected by the Collider Detector at Fermilab we measure the branching ratios to be BR(B⁺→χ_c1⁰(1P)K⁺)=15.5±5.4(stat)±1.5(syst)±1.3(br)×10^-4 and BR(B⁺→J/ψK⁺π⁺π^-)=6.9±1.8(stat)±1.1(syst)±0.4(br)×10^-4 where (br) is due to the finite precision on BR(B⁺→J/ψK⁺), BR(χ_c1⁰(1P)→J/ψγ) is used to normalize the signal yield, and (syst) encompasses all other systematic uncertainties.

We report a measurement of the ratio of the bottom quark production cross section in antiproton-proton collisions at sqrt[s]=630 GeV to 1800 GeV using bottom quarks with transverse momenta greater than 10.75 GeV identified through their semileptonic decays and long lifetimes. The measured ratio σ(630)/σ(1800)=0.171±.024±.012 is in good agreement with next-to-leading order quantum chromodynamics.

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 the results of a search in pp̅ collisions at sqrt[s]  =  1.8 TeV for anomalous production of events containing a photon and a lepton ( e or μ), both 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 have measured the cross sections d²σ/dP_Tdη for production of isolated direct photons in pp̅ collisions at two different center-of-mass energies, 1.8 TeV and 0.63 TeV, using the Collider Detector at Fermilab. The normalization of both data sets agrees with the predictions of quantum chromodynamics for a photon transverse momentum (P_T) of 25 GeV/c, but the shapes versus photon P_T do not. These shape differences lead to a significant disagreement in the ratio of cross sections in the scaling variable x_T(≡2P_T/sqrt[s]). This disagreement in the x_T ratio is difficult to explain with conventional theoretical uncertainties such as scale dependence and parton distribution parametrizations.

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 present an improved measurement of b-meson lifetimes using fully reconstructed B-decays produced in pp̅ collisions at sqrt[s]=1.8 TeV, using 114 pb^-1 of data collected at the Collider Detector at Fermilab. We obtain τ(B⁺)=1.636±0.058(stat)±0.025(syst) ps, τ(B⁰)=1.497±0.073(stat)±0.032(syst) ps and for the lifetime ratio τ(B⁺)/τ(B⁰)=1.093±0.066(stat)±0.028(syst).

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.