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.

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

We describe the design, construction and performance of the upgraded DØ muon system for Run II of the Fermilab Tevatron collider. Significant improvements have been made to the major subsystems of the DØ muon detector: trigger scintillation counters, tracking detectors, and electronics. The Run II central muon detector has a new scintillation counter system inside the iron toroid and an improved scintillation counter system outside the iron toroid. In the forward region, new scintillation counter and tracking systems have been installed. Extensive shielding has been added in the forward region. A large fraction of the muon system electronics is also new.

We determine the top quark mass m t using t t pairs produced in the DO " detector by ͱsϭ1.8 TeV pp collisions in a 125 pb Ϫ1 exposure at the Fermilab Tevatron. We make a two constraint fit to m t in t t→bW ϩ b W Ϫ final states with one W boson decaying to qq and the other to e or . Likelihood fits to the data yield m t (lϩjets)ϭ173.3Ϯ5.6 (stat) Ϯ 5.5 (syst) GeV/c 2 . When this result is combined with an analysis of events in which both W bosons decay into leptons, we obtain m t ϭ172.1Ϯ5.2 (stat) Ϯ 4.9 (syst) GeV/c 2 . An alternate analysis, using three constraint fits to fixed top quark masses, gives m t (lϩjets)ϭ176.0 Ϯ7.9 (stat)Ϯ 4.8 (syst) GeV/c 2 , consistent with the above result. Studies of kinematic distributions of the top quark candidates are also presented. ͓S0556-2821͑98͒06815-5͔

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