Introduction and Design Assumptions

A vertex detector, based on silicon strips¹, was designed for the lepton/photon spectrometer collaboration². The purposes of the vertex detector were to measure the number of charged particles per unit pseudo-rapidity () and their multiplicity in addition to finding the vertex. A large part of the R&D that went into this design should be useful for any RHIC experiment.

In order to cover the central rapidity () region, the measurement should cover from -3 to +3. The total charged particle multiplicity must be available for the first level trigger. When the multiplicity is low, an accurate measurement requires a detector which covers a large fraction of the total solid angle. For central Au+Au collisions at RHIC the expected³ number of charged particles in the range -3 < < 3 is around 5000. If the occupancy is to be kept to 10% or less, this implies that the detector will need at least 50K channels in each layer.

Finally, the vertex detector must find the vertex. This should be done approximately (to within 1cm) at the trigger level, with a more accurate determination (1mm) offline. Any vertex finding algorithm requires several charged particles in the detector, which is not a serious constraint for Au+Au collisions. However, in order to consistently find the vertex position for p+Au and p+p collisions, where the charged particle multiplicities can be much lower, a large fraction of the total solid angle must be covered.