The STAR Forward Silicon Tracker

TL;DR

The Forward Silicon Tracker (FST) at STAR is a compact, high-resolution silicon detector designed to improve forward tracking and charge sign identification in high-energy nuclear collisions.

Contribution

This paper presents the design, construction, and integration of the FST, including sensor technology, readout electronics, and cooling systems, as part of STAR's forward upgrade.

Findings

Achieved momentum resolution better than 30% for 0.2 < p_T < 2 GeV/c

Successfully integrated silicon sensors with APV25-S1 readout chips

Implemented effective cooling with NOVEC 7200 coolant

Abstract

The Forward Silicon Tracker (FST) is a pivotal component of the forward upgrade of the Solenoidal Tracker at RHIC (STAR), designed to discern hadron charge signs with a momentum resolution better than 30% for $0.2 < p_T < 2$ GeV/c in the $2.5 < \eta < 4$ pseudorapidity range. Its compact design features three disks along the beam direction, minimized material budget, and scattering effects. The FST uses Hamamatsu's p-in-n silicon strip sensors with a double metal layer that enables efficient signal routing to the readout electronics, enhancing overall detector performance. The flexible hybrid boards, essential for the readout system, are constructed with Kapton and copper layers to optimize signal handling and power distribution. These boards connect silicon strips to analogue pipeline ASIC APV25-S1 chips, which read up to 128 channels each. A cooling system with nonconducting, volatile NOVEC 7200 coolant at 22.2{\deg}C mitigates ASIC-generated heat. The FST enhances forward tracking performance at STAR as an integral part of the forward upgrade.