1. U. Marconi, D. Breton, S. Luitz
About the L1 Trigger
U. Marconi, D. Breton, S. Luitz
ETD/online session - Frascati Meeting
December 3rd 2009

2. SuperB ETD relies on a L1 trigger based on custom electronics.
L1 is an activity trigger (Umberto will explain this definition).
Event fragments will be transmitted from the L1-buffers to the ROM when the L1 trigger signal arrives with constant latency.
The system is fully synchronous up to this stage.
Size of the L1 latency buffer depends on the specific sub-detector: due to the size of the event fragment, and length of the time window required.
Depth of the L1 derandomizer has to be defined.
It is linked to the ratio between the maximum instantaneous and the mean trigger rate
It is centrally emulated by the FCTS to throttle, in case too many consecutive events happen to fire the trigger.

3. Detectors providing trigger primitives are: The drift chamber and the electromagnetic calorimeter (silicon vertex detector is being considered).
Drift chamber primitives consist of bit patterns: a single bit to set the status of each drift cell of the detector.
No bandwidth problems.
Calorimeter full granularity provides the maximum L1 trigger flexibility: to be pursued.

4. In the case of a full granularity EMT, the L1-buffer would not sit anymore on the FE boards. It would be located inside the trigger electronics itself instead, before data pursue its way towards the ROMs.
L1 accept
From the FCTS
ECM
EMT
L1 Buffer
ROM
Event
data
Event
data
Trigger Info
L1 Global Trigger
to the FCTS
EMT L1

5. In the case of a full granularity EMT, first estimation of the number of serial links required from the EMC to the EMT:
Serial link baseline bandwidth: 2 Gb/s
Number of EMC cells:
Forward: 3600 with 14MHz ADC => 560 links
Barrel: 5760 with 3.5 MHz ADC => 200links
Number of ADC bits: (range)

6. Number of links to the L1 Trigger: 760!
Possibility of performing a Bhabba veto with EMT thanks to the full granularity.
Output rate to the HLT ?
Number of links to the L1 Trigger: 760!
Is it reasonable ? (Especially compared to other subdetectors => between 12 and 58 each).
Is there another solution ?

7. Back to BaBar Barrel EMT
EMC front-ends read out untriggered
In ROM added up to ~24-cell “phi-strips” (3x8) and sent to EMT. Trigger granularity: 40x7 phi-strips
EMT adds phi-strips along theta to 40 “phi-sums”, using overlapping pairs of towers in phi to avoid splitting clusters  1-d projection
FIR time detection, threshold detection applied to projection  trigger primitives

8. BaBar EMT SuperB EMT (1)
Reduced-granularity trigger readout (barrel)
Can’t use phi-sums, need to improve tower granularity
Sum 3x4 crystals in the front-end electronics (instead of 3x8 in BaBar).
Overlapping 6x8 stamps at the trigger level. Apply FIR time detection and threshold detection to stamps  trigger primitives
Separate trigger and event readout paths (and fibers)
Reduces trigger readout data rate by a factor of 12
Triggered readout data reduction depends on # of samples. Factor of ~10 corresponds to ~1us window size (safety margin?)
Overall reduction of links: ~ factor of 5 compared to full-granularity
Front-ends more complex
Other tower sizes may be possible – but need to understand topology constraints
Similar considerations for the forward calorimeter

9. Full-Granularity EMT More flexibility at trigger level
Such as 3x3 towers that are overlapping by 1 strip in each direction?
Smarter clustering algorithms?
Upgradeable (within constraints)?
Per-cell pileup detection at trigger level?
Real trigger studies needed!