Chapter 12
Triggering and High-Level Data Selection
W. H. Smith
12.1 Level-1 Trigger
12.1.1 Introduction
The data taken by a particle physics collider detector consists of events, which are
snapshots of the detector data at specific intervals in time. Usually these snapshots
are taken at the frequency of the crossing of the colliding beams. For HERA this
was 96 ns, for the Tevatron Run II this was 396 ns and for the LHC at design
luminosity this is 25 ns. An individual bunch crossing may contain either no, one
or many interactions between the particles in the colliding beams. The time during
which beam collisions take place during a beam crossing is 1–2 ns. Even if there
are multiple collisions in a single crossing the detector elements will make only
one recording and the events will be superimposed. Therefore, each bunch crossing
is individually evaluated. Not all of the detector data from an individual crossing
is available immediately. Some may be stored as charge and need digitization.
Other digital detector data may be inaccessible until further detector processing is
complete.
The selection of bunch crossings is a highly complex function that involves a
series of levels which take increasing amounts of time, process increasing amounts
of data, use increasingly complex algorithms and make increasingly more precise
determinations to reject increasing numbers of crossings. The first level(s) of the
series usually involve(s) specific custom high-speed electronics. The subsequent
level(s) involve more general CPU farms that run code similar to that found in
the offline reconstruction. Due to this structure, the first level of trigger decision
W. H. Smith ()
University of Wisconsin-Madison, Madison, WI, USA
e-mail: wsmith@hep.wisc.edu
© The Author(s) 2020
C. W. Fabjan, H. Schopper (eds.), Particle Physics Reference Library,
https://doi.org/10.1007/978-3-030-35318-6_12
533
Triggering and High-Level Data Selection
W. H. Smith
12.1 Level-1 Trigger
12.1.1 Introduction
The data taken by a particle physics collider detector consists of events, which are
snapshots of the detector data at specific intervals in time. Usually these snapshots
are taken at the frequency of the crossing of the colliding beams. For HERA this
was 96 ns, for the Tevatron Run II this was 396 ns and for the LHC at design
luminosity this is 25 ns. An individual bunch crossing may contain either no, one
or many interactions between the particles in the colliding beams. The time during
which beam collisions take place during a beam crossing is 1–2 ns. Even if there
are multiple collisions in a single crossing the detector elements will make only
one recording and the events will be superimposed. Therefore, each bunch crossing
is individually evaluated. Not all of the detector data from an individual crossing
is available immediately. Some may be stored as charge and need digitization.
Other digital detector data may be inaccessible until further detector processing is
complete.
The selection of bunch crossings is a highly complex function that involves a
series of levels which take increasing amounts of time, process increasing amounts
of data, use increasingly complex algorithms and make increasingly more precise
determinations to reject increasing numbers of crossings. The first level(s) of the
series usually involve(s) specific custom high-speed electronics. The subsequent
level(s) involve more general CPU farms that run code similar to that found in
the offline reconstruction. Due to this structure, the first level of trigger decision
W. H. Smith ()
University of Wisconsin-Madison, Madison, WI, USA
e-mail: wsmith@hep.wisc.edu
© The Author(s) 2020
C. W. Fabjan, H. Schopper (eds.), Particle Physics Reference Library,
https://doi.org/10.1007/978-3-030-35318-6_12
533