Vertex detector upgrade plans for the PHENIX experiment at RHIC

The PHENIX experiment at the Relativistic Heavy Ion Collider has completed its baseline instrumentation for the start of the presently ongoing third physics run. In the next few years, PHENIX will focus on high-statistics measurements in this configuration. It is expected that the collider will reach full luminosity with heavy ion and polarized proton beams by the year 2006. An upgraded collider is intended for the second half of the decade, with a luminosity increase to about 20-40 times the design value of 8 multiplied by 10 **2**6 cm**-**2s**-**1 for Au+Au, and 2 multiplied by 10 **3**2 cm**- **2s**-**1 for polarized proton beams. The collision energies of root s//N//N=200 GeV will increase to root s//N//N=500 GeV for proton beams. The PHENIX collaboration plans to upgrade its experiment to exploit with an enhanced detector new physics then in reach. A silicon vertex detector comprising pixel and novel microstrip sensors in a new vertex spectrometer is the main new sub-system discussed. This paper overviews the physics motivation and the detector concept chosen, and explains the directions of the beginning research and development effort.