The DØ experiment enjoyed a very successful data-collection run at the Fermilab Tevatron collider between 1992 and 1996. Since then, the detector has been upgraded to take advantage of improvements to the Tevatron and to enhance its physics capabilities. We describe the new elements of the detector, including the silicon microstrip tracker, central fiber tracker, solenoidal magnet, preshower detectors, forward muon detector, and forward proton detector. The uranium/liquid-argon calorimeters and central muon detector, remaining from Run I, are discussed briefly. We also present the associated electronics, triggering, and data acquisition systems, along with the design and implementation of software specific to DØ.

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Observation and Properties of the X(3872) Decaying to J/ψ π + π − in pp Collisions at √ s= 1.96 TeV

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La section efficace totale inélastique pp est reconstruite pour des valeurs de l'énergie cinétique dans le laboratoire comprises entre 0,3 et 400 GeV. Elle est obtenue par un fit global de la somme des sections efficaces totales pour les canaux inélastiques exclusifs, la somme des sections efficaces topologiques ainsi que des résultats des mesures directes indépendantes. La section efficace inélastique np est reconstruite jusqu'a 4,2 GeV en utilisant la somme des canaux individuels exclusifs et les mesures directes. Les sections efficaces sont décrites par des séries de polynômes de Laguerre généralisés assurant un comportement correct au seuil. Les résultats des fits sont présentés dans des tableaux et sur des figures où ils sont comparés aux données existantes. A partir des dépendances en énergie obtenues pour les sections efficaces totales, on peut déterminer la section efficace totale dans l'état d'isospin I = 0 et tester les prédictions d'invariance d'isospin pour les différents types de réactions. L'accord des données avec les prédictions est étonnamment faible dans le cas des réactions du type NN => NN03C0.

An energy dependent phase-shift analysis is reported for pp, np and pn elastic scattering between 10 and 800 MeV. For the pp case a unique solution smoothly varying with energy is found. A resonant behaviour is observed only in the 1D2 phase shift near the mass 2.14 GeV. For np and pn scattering only a few complex polarization parameters are available ; therefore the solution is less precise, especially above 500 MeV. A fixed-energy analysis at 1 GeV is also reported providing a unique phase-shift set. The formalism, data base and compatibility with other analyses are discussed

Increasing luminosity at the Fermilab Tevatron collider has led the D0 collaboration to make improvements to its detector beyond those already in place for Run IIa, which began in March 2001. One of the cornerstones of this Run IIb upgrade is a completely redesigned level-1 calorimeter trigger system. The new system employs novel architecture and algorithms to retain high efficiency for interesting events while substantially increasing rejection of background. We describe the design and implementation of the new level-1 calorimeter trigger hardware and discuss its performance during Run IIb data taking. In addition to strengthening the physics capabilities of D0, this trigger system will provide valuable insight into the operation of analogous devices to be used at LHC experiments.