Upgrade of the STAR Silicon Detectors

Abstract

a new micro-vertex detector called the Heavy Flavor Tracker (HFT) was installed in the center of the Solenoidal Tracker At RHIC (STAR) experiment for the 2014 heavy ion run of the Relativistic Heavy Ion Collider (RHIC). The HFT has been designed and built to extend the measurement capabilities of STAR in the heavy flavor domain by significantly improving its vertex resolution. The HFT consists of three silicon detectors arranged in four concentric cylinders around the STAR interaction point. The outermost layer, called Silicon Strip Detector (SSD), is located at the radius of 22 cm and is based on double-sided silicon strips. The second HFT layer, located at 14 cm, is the Intermediate Silicon Tracker (IST) based on the silicon pad technology. The two innermost layers at 8 and 2.8 cm radii are provided by the PXL detector, which is based on state-of-the-art CMOS monolithic active pixel sensors (MAPS) technology featured for the first time in an accelerator experiment. The excellent pointing resolution of the HFT is driven by the exceptional characteristics of the PXL, which include high granularity and low material budget that limits the Multiple Coulomb Scattering in the detector layers. The very high granularity provided by the 20.7 µm pixel pitch is combined with a matching mechanical stability. The low material budget of 0.4% radiation length per layer was achieved by using sensors thinned to 50 µm and their low power dissipation that allows for simple air-cooling. A novel mechanical approach to detector insertion enables effective installation and integration of the PXL detector within a 12 hour shift during the on-going STAR Run. In this paper we will describe the HFT detector with a particular focus on the PXL detector design and implementation followed by the detector status during the 2014 200 GeV Au+Au run. We will report on a preliminary estimate of the HFT performance that meets the design requirements and summarize experience from the construction and operation of the PXL detector.