Measurement of the p(p)over-bar -> WZ+X cross section at root s=1.96 TeV and limits on WWZ trilinear gauge couplings

We present measurements of the process p (P) over bar -> WZ + X -> l 'nu(l ')l (l) over bar at root s = 1:96 TeV,where l and l ' are electrons or muons. Using 1 fb(-1) of data from the D0 experiment, we observe 13 candidates with an expected background of 4.5 +/- 0.6 events and measure a cross section sigma(WZ) = 2.7(-1.3)(+1.7) pb. From the number of observed events and the Z boson transverse momentum distribution, we limit the trilinear WWZ gauge couplings to -0: 17 <= lambda(Z) <= 0.21 (Delta k(Z) <= 0.29(lambda(Z) = 0) at the 95% C.L. for a form factor scale Lambda = 2 TeV. Further, assuming that Delta g(1)(Z) = Delta k(Z), we find -0.12 <= Delta k(Z) <= 0.29(lambda(Z) = 0) at the 95% C. L. These are the most restrictive limits on the WWZ couplings available to date.

Measurement of inclusive differential cross sections for Upsilon(1S) production in p(p)over-bar collisions at root s=1.96 TeV

We present measurements of the inclusive production cross sections of the Upsilon(1S) bottomonium state in p (p) over bar collisions at root s=1.96 TeV. Using the Upsilon(1S)->mu(+)mu(-) decay mode for a data sample of 159 +/- 10 pb(-1) collected by the D0 detector at the Fermilab Tevatron collider, we determine the differential cross sections as a function of the Upsilon(1S) transverse momentum for three ranges of the Upsilon(1S) rapidity: 0 <\y(Upsilon)\<= 0.6, 0.6 <\y(Upsilon)\<= 1.2, and 1.2 <\y(Upsilon)\<= 1.8.

Measurement of the t(t)over-bar production cross section in p(p)over-bar collisions at root s=1.96 TeV using lepton plus jets events with lifetime b-tagging

We present a measurement of the top quark pair (t (t) over bar) production cross section (sigma(t (t) over bar)) in pp collisions at root s = 1.96 TeV using 230 pb(-1) of data collected by the DO experiment at the Fermilab Tevatron Collider. We select events with one charged lepton (electron or muon), missing transverse energy, and jets in the final state. We employ lifetime-based b-jet identification techniques to further enhance the t F purity of the selected sample. For a top quark mass of 175 GeV, we measure sigma(t (t) over bar) 8.6(-1.5)(+1.6) (stat. + syst.) +/- 0.6(lumi.) pb, in agreement with the standard model expectation. (c) 2005 Published by Elsevier B.V.

Measurement of the t(t)over-bar production cross section in p(p)over-bar collisions at root s=1.96 TeV using kinematic characteristics of lepton plus jets events

We present a measurement of the top quark pair (t (t) over bar) production cross section (sigma(t (t) over bar)) in pp collisions at a center-of-mass energy of 1.96 TeV using 230 pb(-1) of data collected by the DO detector at the Fermilab Tevatron Collider. We select events with one charged lepton (electron or muon), large missing transverse energy, and at least four jets, and extract the t (t) over bar content of the sample based on the kinematic characteristics of the events. For a top quark mass of 175 GeV, we measure sigma(t (t) over bar) 6.7(-1.3)(+1.4)(stat)(-1.1)(+1.6)(syst) +/- 0.4(lumi) pb, in good agreement with the standard model prediction. (c) 2005 Elsevier B.V. All rights reserved.

Measurement of the t(t)over-bar production cross section in p(p)over-bar collisions at root s=1.96 TeV using secondary vertex b tagging

We report a new measurement of the t (t) over bar production cross section in p (beta) over bar collisions at a center-of-mass energy of 1.96 TeV using events with one charged lepton (electron or muon), missing transverse energy, and jets. Using 425 pb(-1) of data collected using the D0 detector at the Fermilab Tevatron Collider, and enhancing the t (t) over bar content of the sample by tagging b jets with a secondary vertex tagging algorithm, the t (t) over bar production cross section is measured to be sigma(p (t) over bar -> t (t) over bar +X)=6.6 +/- 0.9(stat+syst) +/- 0.4(lum) pb. This cross section is the most precise D0 measurement to date for t (t) over bar production and is in good agreement with standard model expectations.

Observing an invisible Higgs boson

Given its weak coupling to bottom quarks and tau leptons, the Higgs boson may predominantly decay into invisible particles like gravitinos, neutralinos, or gravitons. We consider the manifestation of such an invisibly decaying Higgs boson in weak boson fusion at the CERN LHC. Distinctive kinematic distributions of the two quark jets of the signal as compared to Zjj and Wjj backgrounds allow to restrict the Higgs branching ratio to 'invisible' final states to some 13% with 10 fb(-1) of data, provided events with two energetic forward jets of high dijet invariant mass and with substantial missing transverse momentum can be triggered efficiently. It is also possible to discover these particles with masses up to 480 GeV in weak boson fusion, at the 5 sigma level, provided their invisible branching ratio is close to 100%. (C) 2000 Elsevier B.V. B.V. All rights reserved.

Pair term in the electromagnetic current within the Front-Form dynamics: spin-0 case

The frame and scale dependence of the pair-term contribution to the electromagnetic form factor of a spin-zero composite system of two-fermions is studied within the Light Front. The form factor is evaluated from the plus-component of the current in the Breit frame, using for the first time a nonconstant, symmetric ansatz for the Bethe-Salpeter amplitude. The frame dependence is analyzed by allowing a nonvanishing plus component of the momentum transfer, while the dynamical scale is set by the masses of the constituents and by mass and size of the composite system. A transverse momentum distribution, associated with the Bethe-Salpeter amplitude, is introduced which allows to define strongly and weakly relativistic systems. In particular, for strongly relativistic systems, the pair term vanishes for the Drell-Yan condition, while is dominant for momentum transfer along the light-front direction. For a weakly relativistic system, fitted to the deuteron scale, the pair term is negligible up to momentum transfers of 1 (GeV/c)(2). A comparison with results obtained within the Front-Form Hamiltonian dynamics with a fixed number of constituents is also presented. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Measurement of the isolated photon cross section in p(p)over-bar collisions at root s=1.96 TeV

The cross section for the inclusive production of isolated photons has been measured in p (p) over bar collisions at root s = 1.96 TeV with the DO detector at the Fermilab Tevatron Collider. The photons span transverse momenta 23 to 300 GeV and have pseudorapidity vertical bar n vertical bar < 0.9. The cross section is compared with the results from two next-to-leading order perturbative QCD calculations. The theoretical predictions agree with the measurement within uncertainties. (c) 2006 Elsevier B.V. All rights reserved.

On dynamical gluon mass generation

The effective gluon propagator constructed with the pinch technique is governed by a Schwinger-Dyson equation with special structure and gauge properties, that can be deduced from the correspondence with the background field method. Most importantly the non-perturbative gluon self-energy is transverse order-by-order in the dressed loop expansion, and separately for gluonic and ghost contributions, a property which allows for a meanigfull truncation. A linearized version of the truncated Schwinger-Dyson equation is derived, using a vertex that satisfies the required Ward identity and contains massless poles. The resulting integral equation, subject to a properly regularized constraint, is solved numerically, and the main features of the solutions are briefly discussed.

Tightly bound gap solitons in a Fermi gas

Within the framework of the mean-field hydrodynamic model of a degenerate Fermi gas ( DFG), we study, by means of numerical methods and variational approximation ( VA), the formation of fundamental gap solitons ( FGSs) in a DFG ( or in a BCS superfluid generated by weak interaction between spin- up and spin- down fermions), which is trapped in a periodic optical- lattice ( OL) potential. An effectively one- dimensional ( 1D) con. guration is considered, assuming strong transverse confinement; in parallel, a proper 1D model of the DFG ( which amounts to the known quintic equation for the Tonks- Girardeau gas in the OL) is considered too. The FGSs found in the first two bandgaps of the OL- induced spectrum ( unless they are very close to edges of the gaps) feature a ( tightly bound) shape, being essentially confined to a single cell of the OL. In the second bandgap, we also find antisymmetric tightly bound subfundamental solitons ( SFSs), with zero at the midpoint. The SFSs are also confined to a single cell of the OL, but, unlike the FGSs, they are unstable. The predicted solitons, consisting of similar to 10(4) - 10(5) atoms, can be created by available experimental techniques in the DFG of Li-6 atoms.

Search for pair production of scalar bottom quarks in p(p)over-bar collisions at root s = 1.96 TeV

A search for direct production of scalar bottom quarks ((b) over bar) is performed with 310 pb(-1) of data collected by the D0 experiment in p (p) over bar collisions at root s = 1.96 TeV at the Fermilab Tevatron Collider. The topology analyzed consists of two b jets and an imbalance in transverse momentum due to undetected neutralinos (chi(0)(1)), with chi(0)(1) assumed to be the lightest supersymmetric particle. We find the data consistent with standard model expectations, and set a 95% C.L. exclusion domain in the (m(b), m(chi 1)(0)) mass plane, improving significantly upon the results from run I of the Tevatron.

Measurement of the top quark mass in the lepton plus jets channel using the ideogram method

A measurement of the top quark mass using events with one charged lepton, missing transverse energy, and jets in the final state, collected by the D0 detector from p (p) over bar collisions at root s=1.96 TeV at the Fermilab Tevatron collider, is presented. A constrained fit is used to fully reconstruct the kinematics of the events. For every event a top quark mass likelihood is calculated taking into account all possible jet assignments and the probability that an event is signal or background. Lifetime-based identification of b jets is employed to enhance the separation between t (t) over bar signal and background from other physics processes and to improve the assignment of the observed jets to the quarks in the t (1) over bar hypothesis. We extract a multiplicative jet energy scale (JES) factor in situ, greatly reducing the systematic effect related to the jet energy measurement. In a data sample with an integrated luminosity of 425 pb(-1), we observe 230 candidate events, with an estimated background of 123 events, and measure m(t)=173.7 +/- 4.4(stat+JES)(-2.0)(+2.1)(syst) GeV. This result represents the first application of the ideogram technique to the measurement of the top quark mass in lepton+jets events.

Testing color evaporation in photon-photon production of J/psi at CERN LEP II

The DELPHI Collaboration has recently reported the measurement of J/psi production in photon-photon collisions at CERN LEP II. These newly available data provide additional proof of the importance of colored c (c) over bar pairs for the production of charmonium, because these data can be explained only by considering resolved photon processes. We show here that the inclusion of color octet contributions to J/psi production in the framework of the color evaporation model is able to reproduce these data. In particular, the transverse-momentum distribution of the J/psi mesons is well described by this model.

Robust CERN LHC Higgs boson search in weak boson fusion

We demonstrate that a CERN LHC Higgs boson search in weak boson fusion production with subsequent decay to weak boson pairs is robust against extensions of the standard model or minimal supersymmetric standard model involving a large number of Higgs doublets. We also show that the transverse mass distribution provides unambiguous discrimination of a continuum Higgs signal from the standard model.

Measurement of dijet azimuthal decorrelations at central rapidities in p(p)over-bar collisions at root s=1.96 TeV

Correlations in the azimuthal angle between the two largest transverse momentum jets have been measured using the D0 detector in p (p) over bar collisions at a center-of-mass energy root s=1.96 TeV. The analysis is based on an inclusive dijet event sample in the central rapidity region corresponding to an integrated luminosity of 150 pb(-1). Azimuthal correlations are stronger at larger transverse momenta. These are well described in perturbative QCD at next-to-leading order in the strong coupling constant, except at large azimuthal differences where contributions with low transverse momentum are significant.