The HFT,a Heavy Flavor Tracker for STAR

Determining the degree of termalisation of the medium created in heavy ion collisions at RHIC remains highly debated. Elliptic flow (v₂) measurements at RHIC have already suggested the development of collectivity among partons before hadronization. If heavy flavor hadrons flow with the light flavor hadrons, this indicates frequent interactions between the light (u, d ,s) and heavy (c, b) quarks. Thus, thermalization of light quarks is likely to have been reached through partonic re-scattering. Experimentally this can be probed by making a direct measurement of D-mesons v₂ with sufficient precision at low transverse momentum. Using the Heavy Flavor Tracker (HFT) detector, the STAR experiment at RHIC is proposing to both study this process and also to directly reconstruct charmed hadrons (D⁰, D⁺, D_s, Λ_C, ...), using the displaced vertices of their decay products. The HFT is the first vertex detector to use a new and promising CMOS active pixel sensor technology. It will allow to build a relatively fast, accurate and radiation tolerant detector, while keeping the material budget low (∼0.3%X₀ per layer). Detector design and physics performance simulations are presented.