Quarkonia are meson type particles in which the quark and the anti-quark are
of the same type, e.g. the J/Psi consisting of a charm and an anti-charm quark
or the Upsilon(nS) made of a bottom and an anti-bottom quark. Quarkonium
production in proton-antiproton collisions provides an insight into the nature
of the strong force, one of the four fundamental forces and the one responsible
for holding together the quark constituents inside the nuclei of everyday
matter. The description of this process within the Quantum Chromodynamics (QCD,
theory used to describe the strong force) is factorized into two steps,
production of a heavy quark-antiquark pair followed by the two quarks forming a
particle of a color-singlet quarkonium state. The first step is relatively well
understood. The second is a subject of different model approaches, with
parameters tuned to the data.
The rate for direct production of quarkonium states at the Tevatron collider, reported by the CDF Collaboration and based on the Run I data, turned out to be more than an order of magnitude larger than expected. This observation generated a lot of interest and led to various theoretical attempts to describe the data. The new measurement of the Upsilon(1S) production by the D0 Collaboration extends studies of this process to the higher center of mass energy and a much wider range of rapidity, a variable related to the production angle with respect to the beamline.
Upsilon(1S) bottomonium states are detected by their decays into two muons.
Clean muon identification over a large angular range of the DØ
detector has been essential for this measurement. The sample of approximately
45000 Upsilon(1S) candidates, collected before the September 2003 Tevatron
collider shutdown, was used to determine the production rate as a function of
Upsilon(1S) momentum in three rapidity ranges. A good agreement is observed
between the data and some of the most recent theoretical calculations. The
precision of the new measurement will allow for a better determination of
parameters used in different theoretical approaches.