Search Results (331 total)

Gravitational waves from the inspiral of a stellar-size black hole to a supermassive black hole can be accurately approximated by a point particle moving in a Kerr background. This paper presents progress on finding the electromagnetic and gravitational field of a point particle in a black-hole spacetime and on computing the self-force in a “radiation gauge.” The gauge is chosen to allow one to compute the perturbed metric from a gauge-invariant component ψ₀ (or ψ₄) of the Weyl tensor and follows earlier work by Chrzanowski, Cohen, and Kegeles (we correct a minor, but propagating, error in the Cohen-Kegeles formalism). The electromagnetic field tensor and vector potential of a static point charge and the perturbed gravitational field of a static point mass in a Schwarzschild geometry are found, surprisingly, to have closed-form expressions. The gravitational field of a static point charge in the Schwarzschild background must have a strut, but ψ₀ and ψ₄ are smooth except at the particle, and one can find local radiation gauges for which the corresponding spin ±2 parts of the perturbed metric are smooth. Finally a method for finding the renormalized self-force from the Teukolsky equation is presented. The method is related to the Mino, Sasaki, Tanaka and Quinn and Wald (MiSaTaQuWa) renormalization and to the Detweiler-Whiting construction of the singular field. It relies on the fact that the renormalized ψ₀ (or ψ₄) is a source-free solution to the Teukolsky equation; and one can therefore reconstruct a nonsingular renormalized metric in a radiation gauge.

Equilibria of binary neutron stars in close circular orbits are computed numerically in a waveless formulation: the full Einstein-relativistic-Euler system is solved on an initial hypersurface to obtain an asymptotically flat form of the 4-metric and an extrinsic curvature whose time derivative vanishes in a comoving frame. Two independent numerical codes are developed, and solution sequences that model inspiraling binary neutron stars during the final several orbits are successfully computed. The binding energy of the system near its final orbit deviates from earlier results of third post-Newtonian and of spatially conformally flat calculations. The new solutions may serve as initial data for merger simulations and as members of quasiequilibrium sequences to generate gravitational-wave templates, and may improve estimates of the gravitational-wave cutoff frequency set by the last inspiral orbit.

Two Fokker actions and corresponding equations of motion are obtained for two point particles in a post-Minkowski framework, in which the field of each particle is given by the half-retarded + half-advanced solution to the linearized Einstein equations. The first action is parametrization invariant, the second a generalization of the affinely parametrized quadratic action for a relativistic particle. Expressions for a conserved 4-momentum and angular momentum tensor are obtained in terms of the particles’ trajectories in this post-Minkowski approximation. A formal solution to the equations of motion is found for a binary system with circular orbits. For a bound system of this kind, the post-Minkowski solution is a toy model that omits nonlinear terms of relevant post-Newtonian order; and we also obtain a Fokker action that is accurate to first post-Newtonian order, by adding to the post-Minkowski action a term cubic in the particle masses. Curiously, the conserved energy and angular momentum associated with the Fokker action are each finite and well-defined for this bound 2-particle system despite the fact that the total energy and angular momentum of the radiation field diverge. Corresponding solutions and conserved quantities are found for two scalar charges (for electromagnetic charges we exhibit the solution found by Schild). For a broad class of parametrization-invariant Fokker actions and for the affinely parametrized action, binary systems with circular orbits satisfy the relation dE=ΩdL (a form of the first law of thermodynamics), relating the energy, angular velocity and angular momentum of nearby equilibrium configurations.

We present the first measurement of the A₂ and A₃ angular coefficients of the W boson produced in proton-antiproton collisions. We study W→eν_e and W→μν_μ candidate events produced in association with at least one jet at CDF, during Run Ia and Run Ib of the Tevatron at sqrt[s]=1.8  TeV. The corresponding integrated luminosity was 110  pb^-1. The jet balances the transverse momentum of the W and introduces QCD effects in W boson production. The extraction of the angular coefficients is achieved through the direct measurement of the azimuthal angle of the charged lepton in the Collins-Soper rest-frame of the W boson. The angular coefficients are measured as a function of the transverse momentum of the W boson. The electron, muon, and combined results are in good agreement with the standard model prediction, up to order α_s² in QCD.

We present a study of the dijet invariant mass distribution for the reaction p̅ p→two   jets+γ+X, at a center of mass energy of 1.8 TeV, using data collected by the CDF experiment. We compare the data to predictions for the production of a photon with two jets, together with the resonant processes p̅ p→W/Z+γ+X, in which the W and Z bosons decay hadronically. A fit is made to the dijet invariant mass distribution combining the nonresonant background and resonant processes. We use the result to establish a limit for the inclusive production cross section of W/Z+γ with hadronic decay of the W and Z bosons.

A search for the direct production of Higgs bosons in the di-tau decay mode is performed with 86.3±3.5  pb^-1 of data collected with the Collider Detector at Fermilab during the 1994–1995 data taking period of the Tevatron. We search for events where one tau decays to an electron plus neutrinos and the other tau decays hadronically. We perform a counting experiment and set limits on the cross section for supersymmetric Higgs boson production where tan⁡β is large and m_A is small. For a benchmark parameter space point where m_A⁰=100  GeV/c² and tan⁡β=50, we limit the production cross section multiplied by the branching ratio to be less than 77.9 pb at the 95% confidence level compared to the theoretically predicted value of 11.0 pb. This is the first search for Higgs bosons decaying to tau pairs at a hadron collider.

We present a study of the production of K_S⁰ and Λ⁰ in inelastic pp̅ collisions at sqrt[s]=1800 and 630 GeV using data collected by the CDF experiment at the Fermilab Tevatron. Analyses of K_S⁰ and Λ⁰ multiplicity and transverse momentum distributions, as well as of the dependencies of the average number and ⟨p_T⟩ of K_S⁰ and Λ⁰ on charged particle multiplicity, are reported. Systematic comparisons are performed for the full sample of inelastic collisions, and for the low and high momentum transfer subsamples, at the two energies. The p_T distributions extend above 8  GeV/c, showing a ⟨p_T⟩ higher than previous measurements. The dependence of the mean K_S⁰(Λ⁰) p_T on the charged particle multiplicity for the three samples shows a behavior analogous to that of charged primary tracks.

We present a new search for H⁰V production, where H⁰ is a scalar Higgs boson decaying into bb̅ with branching ratio β, and V is a Z⁰ boson decaying into e⁺e^-, μ⁺μ^-, or νν̅ . This search is then combined with previous searches for H⁰V where V is a W^± boson or a hadronically decaying Z⁰. The data sample consists of 106±4  pb^-1 of pp̅ collisions at sqrt[s]=1.8  TeV accumulated by the Collider Detector at Fermilab. Observing no evidence of a signal, we set 95% Bayesian credibility level upper limits on σ(pp̅ →H⁰V)×β. For H⁰ masses of 90, 110, and 130  GeV/c², the limits are 7.8, 7.2, and 6.6 pb, respectively.

We report the first largely model independent measurement of charged particle multiplicities in quark and gluon jets, N_q and N_g, 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 measurements are made for jets with average energies of 41 and 53 GeV by counting charged particle tracks in cones with opening angles of θ_c=0.28, 0.36, and 0.47 rad around the jet axis. The corresponding jet hardness Q=E_jetθ_c varies in the range from 12 to 25 GeV. At Q=19.2  GeV, the ratio of multiplicities r=N_g/N_q is found to be 1.64±0.17, where statistical and systematic uncertainties are added in quadrature. The results are in agreement with resummed perturbative QCD calculations.

We have measured the azimuthal angular correlation of bb̅ production, using 86.5  pb^-1 of data collected by Collider Detector at Fermilab (CDF) in pp̅ collisions at sqrt[s]=1.8  TeV during 1994–1995. In high-energy pp̅ collisions, such as at the Tevatron, bb̅ production can be schematically categorized into three mechanisms. The leading-order (LO) process is “flavor creation,” where both b and b̅ quarks substantially participate in the hard scattering and result in a distinct back-to-back signal in final state. The “flavor excitation” and the “gluon splitting” processes, which appear at next-leading-order (NLO), are known to make a comparable contribution to total bb̅ cross section, while providing very different opening angle distributions from the LO process. An azimuthal opening angle between bottom and antibottom, Δϕ, has been used for the correlation measurement to probe the interaction creating bb̅ pairs. The Δϕ distribution has been obtained from two different methods. One method measures the Δϕ between bottom hadrons using events with two reconstructed secondary vertex tags. The other method uses bb̅ →(J/ψX)(ℓX^′) events, where the charged lepton (ℓ) is an electron (e) or a muon (μ), to measure Δϕ between bottom quarks. The bb̅ purity is determined as a function of Δϕ by fitting the decay length of the J/ψ and the impact parameter of the ℓ. Both methods quantify the contribution from higher-order production mechanisms by the fraction of the bb̅ pairs produced in the same azimuthal hemisphere, f_toward. The measured f_toward values are consistent with both parton shower Monte Carlo and NLO QCD predictions.

The polarization of the W boson in t→Wb decay is unambiguously predicted by the standard model of electroweak interactions and is a powerful test of our understanding of the tbW vertex. We measure this polarization from the invariant mass of the b quark from t→Wb and the lepton from W→lν whose momenta measure the W decay angle and direction of motion, respectively. In this paper we present a measurement of the decay rate (f_V+A) of the W produced from the decay of the top quark in the hypothesis of V+A structure of the tWb vertex. We find no evidence for the nonstandard V+A vertex and set a limit on f_V+A < 0.80 at 95% confidence level. By combining this result with a complementary observable in the same data, we assign a limit on f_V+A < 0.61 at 95% CL. This corresponds to a constraint on the right-handed helicity component of the W polarization of f₊<0.18 at 95% CL. This limit is the first significant direct constraint on f_V+A in top decay.

The properties of three-jet events with total transverse energy greater than 320 GeV and individual jet energy greater than 20 GeV have been analyzed and compared to absolute predictions from a next-to-leading order (NLO) perturbative QCD calculation. These data, of integrated luminosity 86  pb^-1, were recorded by the CDF Experiment for pp̅ collisions at sqrt[s]=1.8 TeV. This study tests a model of higher order QCD processes that result in gluon emission and may give some indication of the magnitude of the contribution of processes higher than NLO. The total cross section is measured to be 466±3(stat.)_-70⁺²⁰⁷(syst.) pb. The differential cross section is furthermore measured for all kinematically accessible regions of the Dalitz plane, including those for which the theoretical prediction is unreliable. While the measured cross section is consistent with the theoretical prediction in magnitude, the two differ somewhat in shape in the Dalitz plane.

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 present a measurement of the isolated direct photon cross section in pp̅ collisions at sqrt[s]=1.8  TeV and |η|<0.9 using data collected between 1994 and 1995 by the Collider Detector at Fermilab (CDF). The measurement is based on events where the photon converts into an electron-positron pair in the material of the inner detector, resulting in a two track event signature. To remove π⁰→γγ and η→γγ events from the data we use a new background subtraction technique which takes advantage of the tracking information available in a photon conversion event. We find that the shape of the cross section as a function of photon p_T is poorly described by next-to-leading-order QCD predictions, but agrees with previous CDF measurements.

For comparison of inclusive jet cross sections measured at hadron-hadron colliders to next-to-leading order (NLO) parton-level calculations, the energy deposited in the jet cone by spectator parton interactions must first be subtracted. The assumption made at the Tevatron is that the spectator parton interaction energy is similar to the ambient level measured in minimum bias events. In this paper, we test this assumption by measuring the ambient charged track momentum in events containing large transverse energy jets at sqrt[s]=1800  GeV and sqrt[s]=630  GeV and comparing this ambient momentum with that observed both in minimum bias events and with that predicted by two Monte Carlo models. Two cones in η-ϕ space are defined, at the same pseudorapidity, η, as the jet with the highest transverse energy (E_T⁽¹⁾), and at ±90^o in the azimuthal direction, ϕ. The total charged track momentum inside each of the two cones is measured. The minimum momentum in the two cones is almost independent of E_T⁽¹⁾ and is similar to the momentum observed in minimum bias events, whereas the maximum momentum increases roughly linearly with the jet E_T⁽¹⁾ over most of the measured range. This study was carried out using data from the CDF detector taken during Run 1 (1994-1995). The study will help improve the precision of comparisons of jet cross section data and NLO perturbative QCD predictions. The distribution of the sum of the track momenta in the two cones is also examined for five different E_T⁽¹⁾ bins. The HERWIG and PYTHIA Monte Carlo generators are reasonably successful in describing the data, but neither can describe completely all of the event properties.

We used a Josephson junction as a radiation detector to look for evidence of the emission of electromagnetic radiation during magnetization avalanches in a crystal assembly of Mn₁₂ acetate. The crystal assembly exhibits avalanches at several magnetic fields in the temperature range from 1.8 to 2.6 K with durations of the order of 1 ms. Although a recent study shows evidence of electromagnetic radiation bursts during these avalanches [J. Tejada , Appl. Phys. Lett. 84, 2373 (2004)], we were unable to detect any significant radiation at well-defined frequencies. A control experiment with external radiation pulses allows us to determine that the energy released as radiation during an avalanche is less than one part in 10⁴ of the total energy released. In addition, our avalanche data indicates that the magnetization reversal process does not occur uniformly throughout the sample.

We use millimeter-wave radiation to manipulate the populations of the energy levels of a single crystal of molecular magnet Fe₈. When continuous-wave radiation is in resonance with the transition from the ground state to the first excited state, the equilibrium magnetization exhibits a peak or dip whose field position varies linearly with the radiation frequency. Our results provide a lower bound of 0.17 ns for transverse relaxation time and suggest the possibility that single-molecule magnets might be utilized for quantum computation.

We report results from a study of events with a double-Pomeron exchange topology produced in p̅ p collisions at sqrt[s]=1800  GeV. The events are characterized by a leading antiproton and a large rapidity gap on the outgoing proton side. We find that the differential production cross section agrees in shape with predictions based on Regge theory and factorization, and that the ratio of double-Pomeron exchange to single diffractive production rates is relatively unsuppressed as compared to the O(10) suppression factor previously measured in single diffractive production.

Two relations, the virial relation M_ADM=M_K and the first law in the form δM_ADM=ΩδJ, should be satisfied by a solution and a sequence of solutions describing binary compact objects in quasiequilibrium circular orbits. Here, M_ADM, M_K, J, and Ω are the Arnowitt-Deser-Misner (ADM) mass, Komar mass, angular momentum, and orbital angular velocity, respectively. δ denotes an Eulerian variation. These two conditions restrict the allowed formulations that we may adopt. First, we derive relations between M_ADM and M_K and between δM_ADM and ΩδJ for general asymptotically flat spacetimes. Then, to obtain solutions that satisfy the virial relation and sequences of solutions that satisfy the first law at least approximately, we propose a formulation for computation of quasiequilibrium binary neutron stars in general relativity. In contrast to previous approaches in which a part of the Einstein equation is solved, in the new formulation, the full Einstein equation is solved with maximal slicing and in a transverse gauge for the conformal three-metric. Helical symmetry is imposed in the near zone, while in the distant zone, a waveless condition is assumed. We expect the solutions obtained in this formulation to be excellent quasiequilibria as well as initial data for numerical simulations of binary neutron star mergers.

We present a measurement of the polar-angle distribution of leptons from W boson decay, as a function of the W transverse momentum. The measurement uses an 80±4 pb^-1 sample of pp̅ collisions at sqrt[s]=1.8 TeV collected by the CDF detector and includes data from both the W→e+ν and W→μ+ν decay channels. We fit the W boson transverse mass distribution to a set of templates from a Monte Carlo event generator and detector simulation in several ranges of the W transverse momentum. The measurement agrees with the standard model expectation, whereby the ratio of longitudinally to transversely polarized W bosons, in the Collins-Soper W rest frame, increases with the W transverse momentum at a rate of approximately 15% per 10 GeV/c.

We report on a search for anomalous production of events with at least two charged, isolated, like-sign leptons, each with p_T>11  GeV/c using a 107  pb^-1 sample of 1.8 TeV pp̅ collisions collected by the CDF detector. We define a signal region containing low background from standard model processes. To avoid bias, we fix the final cuts before examining the event yield in the signal region using control regions to test the Monte Carlo predictions. We observe no events in the signal region, consistent with an expectation of 0.63_-0.07^+0.84 events. We present 95% confidence level limits on new physics processes in both a signature-based context as well as within a representative minimal supergravity (tan⁡β=3) model.

We present a detailed examination of the heavy flavor properties of jets produced at the Fermilab Tevatron collider. The data set, collected with the Collider Detector at Fermilab, consists of events with two or more jets with transverse energy E_T>~15 GeV and pseudorapidity |η|<~1.5. The heavy flavor content of the data set is enriched by requiring that at least one of the jets (lepton-jet) contains a lepton with a transverse momentum larger than 8 GeV/c. Jets containing hadrons with heavy flavor are selected via the identification of secondary vertices. The parton-level cross sections predicted by the HERWIG Monte Carlo generator program are tuned within theoretical and experimental uncertainties to reproduce the secondary-vertex rates in the data. The tuned simulation provides new information on the origin of the discrepancy between the bb̅ cross section measurements at the Tevatron and the next-to-leading order QCD prediction. We also compare the rate of away-jets (jets recoiling against the lepton-jet) containing a soft lepton (p_T>~2 GeV/c) in the data to that in the tuned simulation. We find that this rate is larger than what is expected for the conventional production and semileptonic decay of pairs of hadrons with heavy flavor.