Probing Hot and Dense Matter with Charm and Bottom Measurements with   PHENIX VTX Tracker
  We present the first measurements of the nuclear modification factor (RAA) for flavor-separated b, c-quark electrons in Au+Au collisions at 200 GeV. The newly installed Silicon Vertex Tracker is used to measure the distance of closest approach distributions of electrons at midrapidity (|eta|< 0.35) over the transverse momentum range 1 < pt < 5 GeV/c. From this, the relative fraction of bottom (b) and charm (c) quarks is determined in both the p+p and Au+Au collision systems, which form the basis of the measured RAA . In p+p, we observe that a FONLL perturbative QCD calculation of b->e/(c->e+b->e) ratio is in good agreement with the data. In Au+Au, the data imply a large suppression of b->e or a large modification of B meson pT distributions, which implies very interesting physics of B mesons in Au+Au collisions. 
Rachid Nouicer (for the PHENIX Collaboration)
nucl-ex