What is the BaBar Trigger?

At the most basic level, the BaBar Trigger does the important job of deciding which events observed by the BaBar detector are interesting enough to keep and record, and then be later analyzed for physics content.

The Trigger consists of two levels: a hardware level (called Level 1) and a software level (called Level 3 -- a Level 2 trigger is used in some other high energy particle physics experiments, but was not needed for BaBar).

The Level 1 trigger system consists of four subsystems: the charged particle trigger (the Drift Chamber Trigger, or DCT), the neutral particle trigger (the Electromagnetic Calorimeter Trigger, or EMT), the cosmic trigger (the Instrumented Flux Return (IFR) Trigger, or IFT) and the Global Level Trigger (GLT).

The DCT and EMT receive information from the Drift Chamber and Calorimeter detectors, respectively, process it, and send condensed data to the Global Trigger. The GLT attempts to match the angular locations of calorimeter towers and drift chamber tracks, and flexibly generates Level 1 triggers and sends them on to the Fast Control and Timing system (FCTS), based on the results of the processing. The GLT also uses the IFT information to independently trigger on cosmic ray and mu-pair events. The Level 1 trigger has been upgraded since 2004 with a new DCT system which performs 3D tracking using stereo wire information from the Drift-Chamber to obtain ~4cm resolution in track Z (along beamline) coordinates of tracks to imporve background rejection. The Level 1 trigger rates are typically 2.5 Khz at a luminosity of of L=8x10³³ cm^-2s^-1. The various stages of the L1 system operate at 4Mhz to 15MHz intervals with a total L1 trigger latency of ~11 microseconds.

The Level 3 trigger then analyzes the event data from the Drift-Chamber and Calorimeter sub-systems in conjunction with the L1 trigger information to further reduce background events. Besides the physics filters. L3 trigger also performs Bhabha veto, selection of various calibration events and critical general online monitoring tasks. The L3 operates on an online farm which consists of 28 Dell 1650 (dual Pentium-III 1.4 GHz) computers with fast algorithms processing at ~4ms per event. The L1 triggers are reduced by typically a factor of ~10 after the L3 filtering, before logging to the data storage system.