Computer-controlled trigger system of
the DIRAC experiment
A.Kulikov
on behalf of the DIRAC collaboration
DIRAC (DImeson Relativistic Atomic Complex) at PS CERN.
The purpose of the experiment is study of exotic atom-like bound states
of p and K mesons:
p+p-, p+K-, p-K+, [K+K-].
Such “atoms” are produced (with a very small probability) in interactions
of the proton beam with a nuclear target .
Measurement of the lifetime of these atoms (of the order of 10-15 s)
allows to obtain the values of pp, pK and KK scattering lengths.
These quantities are calculated within the Chiral Perturbation Theory
with a high precision but are not measured experimentally with
a good enough accuracy.
Production rate of the atoms is several orders of magnitude less
than of free pp pairs, therefore one needs a selective trigger.
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Method of the atom detection
Atoms consisting of pp (or pK, KK) mesons are very “fragile” and disintegrate
into a pair of free particles while passing through even thin layer of matter
of about 100 mm.
A specific feature of the pairs from the atom disintegration is very small relative
momentum of the two particles, Q < 3 MeV/c (while their laboratory momenta
are within the 2 GeV/c < P < 7 GeV/c interval).
Q < 3 MeV/c
Smallness of the relative momentum is a distinctive feature of such
“atomic” pairs which is used at the trigger level in order to select
useful events from a huge flux of hadron pairs.
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While planning and building the trigger system, the following
baseline principles were taken into account:
 the system should provide as
much as possible background suppression
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While planning and building the trigger system, the following
baseline principles were taken into account:
 the system should provide as much as possible background suppression
 selection criteria should
not cut useful events
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While planning and building the trigger system, the following
baseline principles were taken into account:
 the system should provide as much as possible background suppression
 selection criteria should not cut useful events
 parallel running of different triggers is required
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While planning and building the trigger system, the following
baseline principles were taken into account:
 the system should provide as much as possible background suppression
 selection criteria should not cut useful events
 parallel running of different triggers is required
 on-line and off-line monitoring of the trigger performance.
This is very important because improper trigger functioning may lead to
losses of useful events or to systematic biases in the collected data
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While planning and building the trigger system, the following
baseline principles were taken into account:
 the system should provide as much as possible background suppression
 selection criteria should not cut useful events
 parallel running of different triggers is required
 on-line and off-line monitoring of the trigger performance. This is
very important because improper trigger functioning may lead to
losses of useful events or to systematic biases in the collected data
 by requirements of the experiment, the trigger conditions should
alternate periodically. Therefore, it is strongly desirable that
changes of trigger could be easily done by any experimentalist
on shift, without presence of the experts in electronics.
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DIRAC setup
DC - drift chambers , VH – vertical hodoscopes, HH – horizontal
Upgraded
DIRAC
setup
hodoscopes,
Ch – nitrogen
Cherenkov
, PSh - preshower detectors,
Mu - muon detectors
24 GeV/c
MDC - microdrift gas chambers, SFD - scintillating fiber detector, IH – ionization hodoscope.
Modified parts
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1-st level trigger (T1)
Combining detector signals in different ways in coincidence/anticoincidence
schemes, so called “trigger primitives” in both arms were constructed:
p1 = VH1*HH1*Ch1
e1 = VH1*HH1*Ch1
p2 = VH2*HH2*Ch2
e1 = VH2*HH2*Ch2
K1 = p1*VH1cut*ChF1
K2 = p2*VH2cut*ChF2
………….
Further coincidence of “primitives” from both arms produced
first level trigger of different kinds:
p+pe+ep+Kp-K+
p-p (L –trigger)
3p (K-trigger)
4e
etc.
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The 1-st
level
trigger
is built
mainly of
commercially
available
modules of
CAEN and
LeCroy.
10
Methods of low relative momentum events selection
at the trigger level
1. Coplanarity selection: small Dy in the downstream part
Fast coplanarity processor evaluates data from the downstream
scintillation hodoscopes with horizontally oriented scintillators.
The event is accepted if a difference between the hit slab numbers
Dn ≤ 2.
This retains only events with a low Qy component.
1
Y
16
N1
N2
Dn =│N1-N2│
1
16
dedicated CAMAC module
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2. Small Dx in the upstream part (T2)
a) Selection using scintillating fiber detector: two hits
with Dx ≤ 9 mm.
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dedicated electronics
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2. Small Dx in the upstream part (T2)
a) Selection using scintillating fiber detector: two hits
with Dx ≤ 9 mm.
dedicated electronics
b) Selection using the upstream scintillation hodoscope (6 mm strip width):
either hits in adjacent strips are required or double ionization
in a single strip.
or
commercial CAMAC modules
This retains only events with a low Qx component.
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3. Limitation of the QL component (T3): dedicated FPGA based processor analyzes
the hit patterns in two downstream and the upstream scintillation hodoscopes.
dedicated CAMAC module
Rejection power – 2.0
Efficiency – 97% QL ≤ 30 MeV/c
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4. Neural network trigger
dedicated electronics
Magnet
Target
Neural network was trained to select
particle pairs with low relative momenta:
Qx ≤ 3 MeV/c, Qy ≤ 3 MeV/c, QL ≤ 30 MeV/c
to electronics of the neural network trigger
Rejection power – 2.0
Efficiency – 99% in the low momentum region.
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5. Drift chamber processor (T4)
reconstructs straight tracks in the X-projection and
analyzes them with respect to relative momentum.
dedicated CAMAC modules
selection:
Qx ≤ 3 MeV/c,
QL ≤ 30 MeV/c
Rejection power – 5.0
Efficiency > 99% in the low momentum region.
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Results of trigger selection by low relative momentum
Distribution on relative momentum Q
with different levels of trigger enabled
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Results of trigger selection by low relative momentum
Distribution on relative momentum Q
with different levels of trigger enabled
Efficiency as a function of Q
Same with expanded low Q region
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Trigger formation and operation
Using the modules of combinatory logics, dedicated coplanarity processor
and drift chamber processor (and some others at an early stage of the experiment)
a number of triggers was constructed:
Ap+pAK+pAK-p+
p+pe +e 2e+2eL→ p-p
K → 3p
main physics triggers
triggers for calibration and other physics
which can run in parallel, with individual prescaling factors.
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Trigger formation and operation
Formation of the 1-st level trigger
Two-level trigger scheme
Start ADC, TDC etc.
T1
‾ Clear
DC data
T4
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+ Readout
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Trigger formation and operation
The state of all electronics is given by the trigger file
which describes the structure of the trigger logic
and sets parameters of the front-end electronics.
Host
computer
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Trigger formation and operation
The state of all electronics is given by the trigger file
which describes the structure of the trigger logic
and sets parameters of the front-end electronics.
When the measurement cycle starts, the host computer
forms the load file from the trigger file and the electronic
configuration file (containing physical addresses of the modules).
Host
computer
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Trigger formation and operation
The state of all electronics is given by the trigger file
which describes the structure of the trigger logic
and sets parameters of the front-end electronics.
When the measurement cycle starts, the host computer
forms the load file from the trigger file and the electronic
configuration file (containing physical addresses of the modules).
The VME processor addresses the created load file and, using the program
library of CAMAC commands, provides loading of the parameters
into all controlled electronic modules.
Host
computer
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Trigger formation and operation
Trigger file
Hodoscopes
Cherenkov
counters
thresholds,
signal widths,
delays,
channel masking
….
Trigger types
trigger types enabled,
prescaling factors
Processor operation
….
processor activation,
loading of selection criteria
Trigger file consists of command files, each of them is a list of
commands to be sent to the modules.
A command file includes a group of commands which refer to
a definite detector or are united by some other common purpose.
As a rule, in order to change conditions of the data taking, it is needed
to change parameters of only part of the electronic modules, therefore
it is sufficient to modify only the corresponding command file(s).
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Trigger formation and operation
For any configuration of the trigger logic and front-end electronics to be
used in a beam time, the corresponding trigger file is prepared.
The needed file is selected from the list of files which opens on the screen
at the start of data taking.
Trigger files, command files and configuration file are text files
what is easy-to-use.
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Monitoring of the trigger system performance
Primary test is fulfilled at the loading of the trigger file:
1) comparison of the really detected modules with the content
of the configuration file;
2) readout of the loaded parameters and comparison of the read
data with the set data;
3) test of the drift chamber processor. This is automatically done
at the beginning of each measurement cycle in order to check
that “good” events are not rejected by the processor.
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Monitoring of the trigger system performance
Primary test is fulfilled at the loading of the trigger file:
1) comparison of the really detected modules with the content
of the configuration file;
2) readout of the loaded parameters and comparison of the read
data with the set data;
3) test of the drift chamber processor. This is automatically done
at the beginning of each measurement cycle in order to check
that “good” events are not rejected by the processor.
The results of the trigger file loading , including all set parameters,
are written in an electronic logbook.
e-logbook
This allows, if needed, to check at off-line data analysis which values
of the parameters were loaded to the modules.
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On-line monitoring
Measurement of the counting rates for each trigger
type at all trigger levels in each accelerator cycle.
These counts are displayed on the monitors and
recorded in the data flow.
Each event has an individual label of the trigger type,
therefore, information on the trigger type is available not only
integrally but for any separate event.
Accumulation of the histograms (hundreds!) from all the
detectors and from essential points of the trigger logic.
They can be controlled not only visually on the screen,
but also in the automatic mode when the monitoring
program compares the real spectra with the reference
spectra. If their difference exceeds some preset value,
the program informs the experimentalist on duty.
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Off-line trigger performance analysis
Test of the trigger processors
In order to test the efficiency of high level triggers the data are periodically
taken with only T1 as an active trigger, while the higher level triggers do not
control recording of data but evaluate the events and write the marks of their
positive or negative decisions.
At off-line express analysis, the relative momentum Q is calculated for
each event and therefore it can be specified as “good” (if Q is small) or
“bad” (if Q is large) event.
Then it is checked which mark was assigned to the event by the
processor – “good” or “bad” , and so the rejection power of the
processors and their efficiency are tested.
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Off-line trigger performance analysis
More labor-consuming but overall test of all trigger levels (including T1)
can be fulfilled using the minimum bias trigger.
In this mode triggering and recording of data is initiated by a signal from only
one detector (“VH”). It is possible that the signals of other detectors could be found in
the recorded event (though with a very small probability), and conditions for the 1st level trigger
could be fulfilled and, moreover, even for selection by the trigger processors.
In these cases the corresponding trigger marks are issued.
Off-line analysis of the recorded data allows to find the events where
the conditions for generation of trigger were fulfilled and therefore
the trigger marks should present. Then comparison with actually issued
trigger marks allows to estimate the trigger efficiency at all levels.
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Conclusions
The DIRAC trigger apparatus is multilevel hardware system
which provided data flow reduction using event selection
based on relative momentum.
All operations with trigger are fulfilled via computer,
without manual interference.
Performance of the trigger system is permanently under control.
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Conclusions
The DIRAC trigger apparatus is multilevel hardware system
which provided data flow reduction using event selection
based on relative momentum .
All operations with trigger are fulfilled via computer,
without manual interference.
Performance of the trigger system is permanently under control.
Thank you for your attention!
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Most of electronics of the experiment is in CAMAC and VME standard.
Nevertheless, some NIM modules are also used.
Those NIM modules (a little) which status should change at the change of
the data taking conditions, are included in electronic logic in a special way,
with use of the CAMAC output register. This provides possibility to modify
the function of the NIM module without manual operations.
output CAMAC register
output CAMAC register
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