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.

The DONUT experiment has analyzed 203 neutrino interactions recorded in nuclear emulsion targets. A decay search has found evi-

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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

The DONuT experiment collected data in 1997 and published first results in 2000 based on four observed ντ charged-current (CC) interactions. The final analysis of the data collected in the experiment is presented in this paper, based on 3.6 × 10 17 protons on target using the 800 GeV Tevatron beam at Fermilab. The number of observed ντ CC interactions is 9, from a total of 578 observed neutrino interactions. We calculated the energy-independent part of the tau-neutrino CC cross section (ν +ν), relative to the well-known νe and νµ cross sections. The ratio σ(ντ )/σ(νe,µ) was found to be 1.37±0.35±0.77. The ντ CC cross section was found to be 0.72±0.24±0.36×10 −38 cm 2 GeV −1 . Both results are in agreement with expectations from the Standard Model.