Triggering in Particle Physics Experiments
a short course at  IEEE Nuclear Science Symposium 2002

Course Abstract

A critical component of particle physics experiment design is
determining which events to store for further analysis and how to make
that decision.  The job of the Trigger is to quickly discard
uninteresting events while efficiently culling the most interesting
events in as unbiased a manner as possible.  In most experiments the
rate at which detector data is sampled, such as beam crossing rate for
a colliding beam experiment, is much higher than the rate of physics
interactions of primary interest.  At the same time the volume of data
from digitizing all readout channels is frequently too high to be
practically read-out by a data acquisition system (DAQ) for later
analysis let alone be fully reconstructed in real time.  Some
reduction of needed bandwidth can be achieved within the DAQ system by
suppressing channels with no interesting data or other data
compression methods.  While sparsification can reduce the needed
bandwidth by factors of 10 or 100, suppression by factors of a million
are often achieved with a combination of triggering and data
compression.  For example, the Run II trigger systems for the CDF and
D0 experiments at the Fermilab Tevatron collider each reduce an input of
about 10 TBytes/sec to an output of about 20 Mbytes/sec recorded on
tape.  This course will discuss the design of trigger systems for
particle physics ranging from cosmic ray experiments to future
colliding beam experiments such as those at the Large Hadron Collider.

The course will cover overall trigger system design with particular
attention to impact of beam environment and data acquisition design.
It will also cover the design of trigger subsystems which do fast
partial event reconstruction and pass information to more global
decision hardware.  This process is often referred to as generating
trigger primitives.  The focus will be on primitives that are common to
many modern HEP experiments: charge track reconstruction, calorimeter
and muon triggers.  Also covered will be systems to reconstruct tracks
from detached vertices which is a more recent and complicated task.
Specific examples from past, current and future experiments will be
used to illustrate the techniques of each topic and the progression of
those techniques with improving technology.  Comparisons will also be
made for different types of experiments (e.g. cosmic ray, fixed
target, colliding beam).

The overall system design of the trigger is very closely coupled to
the structure of the beam of the experiment.  The particle type, beam
energy and timing structure all have a large impact on the rate of
particle interactions both interesting (signal) and uninteresting
(background).  Beam environment can vary from neutrinos produced in
the sun (no timing structure) to proton-anti-proton collisions in
bunches separated by 25ns.  The complexity of the detector systems
also impact the trigger design: what types of events will the
experiment need to detect, how many channels are there?  The trigger
design is also intimately related to the DAQ architecture since the
DAQ must feed data to the trigger and the trigger must tell the DAQ
what to do with the data.  We will discuss how these issues impact the
trigger system design.  For example, how many decision levels are
needed, which levels will be implemented with hardware, which levels
with software and which as combination of hardware and software.  We
will show how these decisions have changed over time with improving
technology (impact of Moore's Law).

The design of subsystems to generate trigger primitives must be
closely connected to the design of the detector subsystems and the
front-end electronics which read them out.   These systems do fast
reconstruction of event data with a very focussed purpose.  To minimize
execution time, only certain classes of objects are reconstructed
(e.g. Tracks above a minimum momentum threshold).   We will
focus on reconstruction of physics objects in lower level parts of
trigger systems.  Since most Level 3 triggers are based on computing
farms running off-line type reconstruction code, the design
requirements are not particularly unique to the trigger application.
The most frequently used trigger primitives are from calorimeters for
electron, photon and jet reconstruction along with muons from muon
detectors.  These have provided and continue to provide standard
signatures for many types of particle decays.   We will cover these
along with the next most common trigger type from reconstruction of
charged particle tracks in tracking chambers.   These charged tracks
are used on their own or matched to objects found in calorimeters
(e.g. electrons) or muon detectors.  Recently, very powerful trigger
processors have been developed to exploit the long lifetime of heavy
quarks (b or c quarks) from the presence of displaced tracks or detached
track vertices.  These detached vertex triggers are very challenging but
are already revolutionizing triggering in hadron collider experiments.

Lectures in Course: