Implementation of the HERA-B First Level Trigger

Paper: 393
Session: B (talk)
Speaker: Glaess, Joachim, Universitaet Mannheim
Keywords: parallelization, special architectures, trigger algorithms, trigger systems, massive parallel systems

Implementation of the HERA-B First Level Trigger

J. Gläss, A. Gröpl, C. Hähnel, T. Wolf, A. Wurz and R. Männer
University of Mannheim, Informatik V, D-68131 Mannheim, Germany

F. Saadi-Lüdemann and D. Ressing
DESY, D-22603 Hamburg, Germany

HERA-B Collaboration

The main goal of the HERA-B first level trigger is the identification of decays
in the B meson system to measure CP violation.
It reduces a 10 MHz input rate by a factor of 200 with a latency of 5 to 10 micro seconds.
Using Kalman filtering it searches possible lepton and hadron tracks from a location given
by particle identification devices through up to 7 layers of tracking chambers.
The trigger decision is based on the momenta and masses of track pairs.

The first level trigger is a parallel and pipelined hardware processor consisting of
60 track finding units, 4 track parameter units, and a trigger decision unit.
Tracking chamber data are transferred at a rate of 1 Terabit/s into memories distributed over
the track finding units.
A track finding unit is a synchronous pipeline that can process a track hit every 20 ns.
For the current bunch crossing, a coincidence matrix identifies track hits in the search region
using the 3 stereo views of the assigned tracking chamber section.
The improved search region, extrapolated to the next tracking chamber, is read from a lookup
table and transmitted to one or more track finding units assigned to the next tracking chamber
or, eventually, to the track parameter units.
A track parameter unit determines from the transmitted track information the kinematic
parameters of the particle candidate to accept or reject it.
Finally the tracks are collected in the trigger decision unit.
It computes the invariant mass of particle pairs of the same event and makes the
final trigger decision.
The first level trigger is in the prototype stage.