An Integrated Tracker for STAR

The STAR experiment at the Relativistic Heavy Ion Collider RHIC studies the new state of matter produced in relativistic heavy ion collisions and the spin structure of the nucleon in collisions of polarized protons. In order to improve the capabilities for heavy flavor measurements and the reconstruction of charged vector bosons an upgrade of the tracking system both in the central and the forward region is pursued. The integrated system providing high resolution tracking and secondary vertex reconstruction capabilities will use silicon pixel, strip and GEM technology. INTRODUCTION AND CURRENT CAPABILITIES The STAR experiment at RHIC studies the fundamental properties of the new state of strongly interacting matter produced in relativistic heavy ion collisions and investigates the spin structure of the proton in polarized p+ p collisions. A variety of results both in heavy ion collisions and polarized p+ p collisions have already been obtained. A key future step in these programs is the ability for direct reconstruction of particles containing charm and bottom quarks as well as flavor tagging of jets to allow precise measurements of the spectra, yields and flow of open charm and bottom and to determine spin dependent production asymmetries connected to the gluon polarization in the nucleon. The flavor dependence of the sea quark polarization will be determined by parity violating W production and decay in longitudinally polarized p+ p collisions at √ s = 500 GeV. STAR [1] is one of the two large detector systems at RHIC. Its main tracking detector is a large-volume time projection chamber (TPC) covering the pseudorapidity range |η|< 1.2. Additional vertex resolution for the reconstruction of secondary decay vertices is provided by the silicon vertex tracker (SVT, |η| < 1), a three–layer silicon drift detector, and the one–layer silicon strip detector (SSD). Tracking in the forward region is provided by the forward TPCs (FTPCs, 2.5 < |η|< 4.0). The barrel (BEMC) and endcap (EEMC) electromagnetic calorimeters cover −1 < η < 1 and 1 < η < 2, respectively. Additional small acceptance electromagnetic calorimetry at high rapidity is provided by the forward pion detector (FPD, 3.1 < |η| < 4.2). The current tracking capabilities are insufficient to address the future measurements outlined above. The planned integrated tracker is designed to provide the necessary vertex resolution to uniquely identify open charm and bottom and to provide precision tracking in the forward region to determine the charge sign of electrons from W and W− decays that are detected in the EEMC. Figure 1 shows an overview of the planned tracking upgrades for STAR. The two distinct areas of inner and forward tracking are driven by different physics motivations, outlined in the following sections together with the technology choices for the planned upgrades.