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

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We summarize a search for the top quark with the Collider Detector at Fermilab (CDF) in a sample of Pp collisions at Js =1.8 TeV with an integrated luminosity of 19.3 pb . We find 12 events consistent with either two 8' bosons, or a 8' boson and at least one b jet. The probability that the measured yield is consistent with the background is 0.26%. Though the statistics are too limited to establish firmly the existence of the top quark, a natural interpretation of the excess is that it is due to tt production.Under this assumption, constrained fits to individual events yield a top quark mass of 174+ 10 -)) GeV/c . The tt production cross section is measured to be 13.9 -+)II pb. PACS numbers: 14.65.Ha, 13.85.Ni, 13.85.gk The standard model has enjoyed outstanding success, yet the top quark, which is required as the weak-isospin partner of the bottom quark, has remained unobserved. Direct searches at the Fermilab Tevatron have placed a 95%%uo confidence level lower limit of M&,~& 131 GeV/c [1]. Global fits to precision electroweak measurements yield a favored mass of M&,~= 177-+|I+ -I9 GeV/c [2]. One expects that, at Tevatron energies, most top quarks are produced in pairs. For M,o&~85 GeV/c, each top quark decays to a real 8' boson and a b quark.The observed event topology is then determined by the decay mode of the two H bosons. About 5k of the time 226 VOLUME 73, NUMBER 2 PHYSICAL REVIEW LETTERS 11 JULY 1994 both W bosons decay to ev or p v (the "dilepton mode"), giving two high-P~leptons with opposite charge, two b jets, and large missing transverse energy (k"7. ) from the undetected neutrinos [3]. In another 30% of the cases one W boson decays to ev or p v, and the other to a qq' pair (the "lepton+jets mode"). This final state includes a high-Pz charged lepton, k"z, and jets from the 8' and the two b quarks. The remaining 65% of the final states involve the hadronic decays of both W bosons, or the decay of one or both of the 8'bosons into r leptons. These channels have larger backgrounds and are not considered here. This analysis is based on a sample of pp collisions at vs =1. 8 TeV with an integrated luminosity of 19.3