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
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.
Below is a simplified flowchart of the Level 1 algorithm:
Back to Trigger
Back to Operations
Back to BaBar detector home page
Mandeep S. Gill
Last modified: Sun Apr 24 15:40:08 PDT 2005