Design and Performance of the Level 1 Calorimeter Trigger for the BABAR Detector

TL;DR

This paper describes the design, implementation, and performance evaluation of the Level 1 Calorimeter Trigger system used in the BABAR detector at PEP-II, highlighting its FPGA-based digital architecture and real-time monitoring capabilities.

Contribution

It introduces a flexible, FPGA-based fixed latency trigger system with real-time monitoring for high-rate collider experiments, optimized for the BABAR detector.

Findings

Successful real-time operation during BABAR's first year

Effective background rejection at high collision rates

Trigger system's performance aligns with design expectations

Abstract

Since May 1999 the BABAR detector has been taking data at the PEP-II asymmetric electron-positron collider at the Stanford Linear Accelerator Center, California. This experiment requires a very large data sample and the PEP-II accelerator uses intense beams to deliver the high collision rates needed. This poses a severe challenge to the BaBar trigger system, which must reject the large rate of background signals resulting from the high beam currents whilst accepting the collisions of interest with very high efficiency. One of the systems that performs this task is the Level 1 Calorimeter Trigger, which identifies energy deposits left by particles in the BABAR calorimeter. It is a digital, custom, fixed latency system which makes heavy use of high-speed FPGA devices to allow flexibility in the choice of data filtering algorithms. Results from several intermediate processing stages are read out, allowing the selection algorithm to be fully analysed and optimized offline. In addition, the trigger is monitored in real time by sampling these data and cross-checking each stage of the trigger calculation against a software model. The design, implementation, construction and performance of the Level 1 Calorimeter Trigger during the first year of BABAR operation are presented.