B Physics at STAR Jens

In relativistic nucleus-nucleus collisions, heavy quarks (c,b) are produced in the early stage [ 2], at time scales t≃0.2-0.5 fm/c, in comparison to typical pion freeze-out time scales of t≃10 fm/c. In particular, b quarks are interesting for two reasons: (1) The mass of the c quark m(c)=1.40 GeV is about the same order of magnitude as Λ QCD ≃1 GeV, but m(b)=4.75 GeV is significantly larger. Thus, for the b quarks, pQCD based cross section predictions are more reliable than for c quarks. (2) For Au+Au collisions at RHIC, the primordial temperature might be high enough to create cc pairs thermally, but not bb pairs. For t=0.6 fm/c, a temperature of T ≃0.3 GeV can be estimated [ 3]. At such a high temperature, QCD predicts that gluons, which are massless at T =0, acquire an effective mass, which turns out to be in the same order of magnitude as the temperature itself, i.e. m gluon ≃0.3 GeV [ 4]. Thus the production process gg→cc might be enhanced thermally. However, for bb production, the Boltzmann suppression factor is /=exp[−2m(b)/2m(c)] ≃1/23, thus any thermal bb enhancement is unlikely. Thus, b quarks can be regarded as a pure probe of the early stage of the collision. However, bb events are rare. At RHIC, bb measurements are now feasible for first time, due to three reasons: (1) The bb production cross section at √ s=200 GeV (RHIC) is about a factor of ∼10 3 higher than at √ s=18 GeV (SPS). (2) High collider luminosity provides a recorded data sample of 10 6-10 7 events. (3) Recent technological and price development of fast CPUs created the basis for the possibility of event reconstruction in realtime for application as high level trigger on rare events. This paper describes the first attempt to observe bb production with the STAR experiment at RHIC. The main STAR detector is a large scale (R=2 m, L=4 m) TPC (Time Projection Chamber) [ 1] with 5×10 7 position pixels, each measuring the particle ionization dE/dx using a 10-bit ADC. For triggering on rare bb events, a Level-3 trigger CPU farm was used, described in Section 4. The bb-bound state Υ is specifically interesting, as the formation and re-breakup