Speaker
Description
Summary
We present the Jet Trigger, an upgrade of the H1 experiment Liquid
Argon first level trigger.
After the luminosity upgrade of the HERA machine in the years
2000-2001 (HERA-2), a significant increase of the background rates
was expected and indeed observed. While parts of the H1 detector were
upgraded during the year 2001 as well, the H1 data logging rate to
permanent storage (about 10 Hz) remained a stringent constraint for
the data acquisition system. The aim of the upgrade of the digital
part of the LAr trigger, the Jet Trigger, was to complement the
existing global LAr calorimeter trigger with a system that performs
real-time clustering to avoid summing-up noise distributed over large
parts of the calorimeter, thus allowing for triggers on even lower
energy depositions while keeping the trigger rates within the
required bounds.
The Jet Trigger identifies the localized energy depositions of
electrons, photons and bundles of hadrons in the LAr calorimeter, and
uses these energy clusters ("jets"), including their topological
information, for a fast event selection. The "jets" are found by
identifying trigger towers with a local energy maximum. Around this
maximum the immediate neighboring towers are summed and added to the
center. The resulting local "jets" are the basis of the trigger
decision. Such a local concept improves the sensitivity for
low-energy depositions in the calorimeter. The "jets" are then sorted
by energy in decreasing order. The 16 highest energy "jets" are used
to provide flexible and optimized triggers based on discrimination of
individual jet energies, counting jets with energies above certain
thresholds, and determination of topological correlations between the
jets.
The realization of the above algorithm was implemented in the
following way. The input of the jet trigger is 1200 analog trigger
towers received at the 10 MHz HERA bunch crossing rate. The clock
generation is performed by a Clock Distribution and Configuration
Card with adjustable phases to minimize the overall system latency.
The ADC-Calculation-Storage unit digitizes the 1200 input towers to 8
bit accuracy each, transforms the energies into transverse energies,
and sums the electromagnetic and hadronic energies. The resulting 440
outputs are transferred via a bit-serial link to the so-called Bump
Finder Unit. This unit searches for local maxima of energy and sums
them with their immediate neighbors. This search and summing is done,
for each input tower, in a completely parallel fashion. The resulting
116 energy maxima are sorted by decreasing energy first
quadrant-wise, then detector-wise, by the Primary and Secondary
Sorting Units. The programmable Trigger Element Generator applies
conditions on the 16 highest energies and their locations. These
conditions are local (energy and polar angle criteria on each
individual jet, azimuthal and polar angle differences between jets),
and global (total energy and missing energy in the event).
In total, the Jet Trigger consists of about 550 FPGAs with 75 M
Gates, computing 300 G operations/s. The 12 GB/s raw data rate is
reduced to 16 trigger element bits per bunch cross, corresponding to
a data reduction factor of 600. Each unit performs its function
within 1 to 3 bunch crossings. The total latency is 800 ns.
The Jet Trigger operation started in the summer of 2006 and
accumulated about 100 pb-1 of luminosity until the end of the HERA-2
program in July 2007. It opened the phase space for events containing
a single forward jet of at least 8 GeV at low angle below 30 degrees.
The energy-sorted jet information was combined with track-based
triggers to successfully perform b-tagging with a track threshold of
1.5 GeV. The Jet Trigger was used to successfully decrease the
electron triggering threshold from 6 GeV down to 2 GeV and to perform
the world's first measurement of the longitudinal structure function
F_L of the proton.
