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:
Instructors: