1. Fake trigger background simulations
JEM-EUSO detector is basically a large field of view UV camera, pointing toward the earth atmosphere, to detect and measure fluorescence light imprint produced by development at speed of light of Extensive Air Showers. For a 1020eV EAS, a few thousands photons are expected on detector focal surface (FS). However, the background photons are much more than those of signal. Therefore background reduction is essential for such observatory EECR.
It is aim of the trigger to try to extract the signal from the background sea. Electronics will have to reject as much counts as possible without rejecting the signal itself. Fortunately signal has some peculiar characteristics that can be used to distinguish it. The shower generate a spot moving on focal surface. On the other hand, the background is distributed randomly. But it is necessary to assess, if the random processes do not produce fake patterns, which could be mistakenly interpreted as EECR events.
For this purpose huge amount of measurements with only background events have to be simulated. Obtained results would be consequently analysed by several pattern recognition algorithms to verify the probability of registration a fake trigger events in several trigger conditions.

2. The triggering philosophy
The JEM-EUSO trigger philosophy is at the core of the concept of the instrument.
The goal of the trigger system is to detect the occurrence of a scientifically valuable signal among the background noise detected by the JEM-EUSO telescope.
Since the total number of pixels in the array is very large (~ 2×105), a multi-level trigger scheme was developed.
This trigger scheme relies on the partitioning of the Focal Surface in subsections, named PDM (Photo Detector Module), which are large enough to contain a substantial part of the imaged track under investigation (this depends on the energy of air shower and the zenith angle).

3. JEM-EUSO FOCAL SURFACE LAYOUT
Typical air-shower seen by JEM-EUSO
FS is covered by large numbers of photo-detector tubes structured in series of similar pieces, the one embedded in the others. Largest piece is PDM -> photodetector module. The whole FS is made of 137 such PDM’s. PDM structure is itself squared matrix of 3x3 smaller elements called elementary cells EC. Each EC is a squared matrix of 2 × 2 multianode photomultipliers. An EC is a 12 × 12 pixel matrix, corresponding to 144 channels. PDM is a 36 × 36 pixel matrix corresponding to 1296 channels. Each PDM probe a squared pad of 27km x 27km, enough large to contain a substantial part of the imaged trace under study
JEM-EUSO

5. Outline of noise reduction capability.
The general JEM-EUSO trigger philosophy asks for a System Trigger organized into two main trigger-levels.
The two levels of trigger work on the statistical properties of the incoming photon flux in order to detect the physical events hindered in the background, basing on their position and time correlation.
The trigger is issued in accordance with two different stages:
Outline of noise reduction capability.
Level
Rate of signals/triggers at PDM level
Rate of signals/triggers at FS level
1st level trigger (PDM)
Photon trigger
~9.2 × 108 Hz
~1.4 × Hz
Counting trigger
~7.1 × 105 Hz
~1.1 × 108 Hz
Persistency trigger
~7 Hz
~103 Hz
2nd level trigger (PDM cluster)
~6.7 × Hz
~0.1 Hz
Expected rate of cosmic ray events
~6.7 × Hz
~10-3 Hz

6. The Table gives a synthetic idea of the expected rate of signals at each stage, and the expected rejection power.
The numbers here reported give a first rough estimation of the requirements.
The exact power rejection of each trigger level will be optimized in future.
The experience with the balloon measurement would provide us very useful information for tuning these parameters.
The last row gives also a reference number on the expected rate of cosmic ray events, which could fluctuate by around one order of magnitude depending on the effective threshold of the detector.
The previous table shows how important is the capability to cope with the nightglow background to reduce the data rate. The most critical level is the 1st one where the power reduction is of 8 orders of magnitude.

7. Based on the following assumptions:
FIRST + SECOND LEVEL TRIGGER CONCEPT
Based on the following assumptions:
PIXELS ABOVE <BACKGROUND> . For each Elementary Cell (EC) pixels, digitalized anode pulses (pe) are counted within a GTU(2.5 µs) and compared with a pre-set digital threshold N. At every GTU the counters C1, one for each pixel, are reset. For each C1, if the counts are greater than the pre-set threshold , the successive pulses are conveyed to a second counter C2, one for each pixel, and a signal L, one for each pixel, flags the pixel as active. All the L signals are OR-ed.
ELEMENTARY CELL ACTIVITY CHECK . A counter C3 (persistency counter), only one per EC, is increased at each GTU if the output signal O of the OR-ed L signals is active else it is reset.
SPACE-TIME CORRELATION OF PIXELS ABOVE THRESHOLD . The C3 counts are compared with a pre-set digital threshold P. If the C3 counts reach the P threshold a signal is issued to the adder A that holds the C2 counters 2x2 (or 3x3) grouped. The resulting addition is then compared with the pre-set value S corresponding to the total number of pe requested in the 2x2 (or 3x3) grouped pixels. If the condition is met, an EC trigger is then generated.
Obviously read-out of data is based on “free running method”: pixels counts recorded on memories of suitable depth are reading out at the occurrence of a trigger.

8. FIRST LEVEL TRIGGER
The 1st trigger level mainly operates to remove most of the background fluctuations by requiring a locally persistent signal above over a few GTU’s duration.
GTU is gate time unit of the value 2.5μs -> temporal time resolution of detector electronics.
In 1st level trigger -> PTT (Persistency Track Trigger) are pixels grouped in boxes of 3 × 3. Trigger is issued if for 5 consecutive GTU’s there is at least one pixel in box with an activity higher than a preset threshold and the total number of detected photoelectrons in the box is higher than a preset value.
These two values are set as a function of the average noise level in order to keep the rate of triggers on fake events at a few Hz per PDM.

9. SECOND LEVEL TRIGGER
Role of 2nd trigger level - Linear Track Trigger (LTT) is to find some track segments in 3dim
from the list of pixels provided by the first level, for each GTU time bin. The track speed has to be compatible with a point travelling at speed of light in whatever direction it propagates. So it follows the movement of the EAS spot inside the PDM over some predefined time, to distinguish this unique pattern of an EAS from the background. From a PTT trigger, the PDM electronics will send a starting point, which contains the pixel coordinates and the GTU which generated the trigger. The LTT algorithm will then define a small box around it, move the box from GTU to GTU and integrate the photon counting values. When excess of integrated value above the background exceeds the threshold, an LTT trigger will be issued. It is foreseen to have a total of 67 starting points for the integration, which are distributed equally over time and position around this box. Each integration will be performed over ±7 GTU’s for a predefined set of directions. The background-dependent threshold on the total number of counts inside the track is defined to reduce the level of fake events to a rate of 0.1 Hz per FS.
These two trigger levels combined together reduce therefore the rate of signals on the level of 109 at PDM level.

10. 1st level
PDM trigger
2nd level
CCB trigger

11. Motivation

12. Contribution/Activity: Computing facility – JEM-EUSO cluster
16 node Supermicro® SuperServer AS-1042G-MTF
Configuration of node :
- 4x Opteron 6134 (2,3GHz)
- 16GB RAM
- 600GB SATAII HDD (WD VelociRaptor)
2x master/disk server Supermicro® SuperServer AS- 1042G-MTF
Configuration od server:
- 1x Opteron 6134 (2,3GHz)
- 4x 2TB SATAII HDD (WD RE4)
All together:
CPU: GHz
Cores:
RAM: 264 GB (4GB / CPU)
Disk space: 16x600GB + 8x2TB = 25,2TB

13. LTT threshold for M64

14. M64 config

15. Simulations

16. Stored information

17. Next step
Fake trigger identification – pattern recognition method to disentangle signal from background:
Hough transformation
Clustering
Results from simulated UV background – optimize triggering and reconstruction algorithm in ESAF (EUSO simulation and analysis framework)

18. Pattern Recognition
Hough transformation or clustering to disentangle signal
from background…
…and to determine the geometrical parameters of the track. .

19. ESAF Euso Simulation and Analysis Framework
Simulation, Reconstruction, Visulation, Analysis
Simulates process from shower generation to light production, transport, detector response, reconstruction and analysis
A package of C++, C and F77 programs organized as shared object
Libraries, based on ROOT using its libraries with high modularity
Air showers: CORSIKA, CONEX, built-in Slast shoower generator