The L0 Calorimeter Trigger

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The L0 Calorimeter Trigger

• Basic principles
• Overall organization
• Front-End cards
• Validation cards

L0 trigger review, 4 July 2002
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Basic principles

• Identify hot spots
• A shower has a 'small' size
• Detect a high energy in a small surface
• Use a square of 2 x 2 cells area
• 8 x 8 cm2 in the central region of ECAL (may
  loose a few of the energy)
• more than 50 x 50 cm2 in the outer region of HCAL
• Select the particles with the highest ET
• Due to its high mass, a B particle decays into
  high PT particles
• 'High PT ' is a few GeV
• For the Level 0 decision, we need only the
  particle with the highest PT.
• Maybe also the second highest in HCAL, see later

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• One can then select locally the highest candidate
• Process further only these candidates
• Reduced complexity and cabling
• Only 200 for ECAL and 50 for HCAL starting from
  6000 and 1500 cells.
• Validate the candidates
• Electron, photon, ?0
• Electromagnetic nature using the PreShower,
  charge using the SPD
• Same granularity, projective
• Look only at the cells with the same number in
  the other detectors
• Hadron
• Would like to add the energy lost in ECAL, in
  front of the candidate
• Complex connectivity, expensive
• Useful only if the ECAL contribution is important
• If small, it can be ignored without too much harm
• Look only at ECAL candidates !
• Manageable number of connections

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• Select the highest validated candidate
• One wants simple decisions at this early level
• Using the second highest hadron was shown to
  improve marginally the efficiency in some cases.
• The studied implementation allows to produce this
  second highest.
• No need for a second highest electron or photon.
• The number of links, and consequently the cost is
  by far smaller if one reduces locally the number
  of candidates
• Processing entirely synchronous
• No dependence on occupancy and on history
• Easier to understand and to debug
• Pipeline processing at all stages.

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Hardware implementation

• Inputs
• About 6000 ECAL cells (same number for PreShower
  and SPD)
• Variable cell size, but identical structure for
  SPD/Prs/ECAL
• HCAL different, see later
• Front-end electronics located on top of the
  detector
• Order of 100 rads / year
• Use SEU immune components, I.e. ACTEL anti-fuse
  PGA
• 32 channels per card for ECAL/HCAL, 64 for
  Prs/SPD
• We want to minimise cabling complexity
• Integrate the first selection in the front-end
  card.
• Quantity to manipulate ET, converted from the 12
  bits ADC.
• 8 bits are OK, with full scale around 5 GeV.
• Use a dedicated backplane for as many connections
  as possible
• Use LVDS levels, multiplexed signals, as soon as
  there are several bits
• See Daniels talk for details.

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First selection

• Build the 2x2 sums
• Work inside a 32 channels (8x4) front-end card
• To obtain the 32 2x2 sums, one needs to get the 8
  1 4 neighbours
• Via the backplane (9) or dedicated point-to-point
  cables(4)

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• Select the local maximum in the card
• Simple comparison of the summed ET.
• Iterative, 16 ? 8 ? 4 ? 2 ? 1 comparisons in 21
  cycles
• Currently implemented in 3 ACTEL PGA's

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• In the Ecal/Hcal front-end card
• More details in Christophes talk

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ECAL Validation

• For each candidate, one needs to access the
  SPDPreShower information, i.e. 2 times 4 bits
  for the SPDPrs cells corresponding to the ECAL
  candidate.

• The address is sent from the ECAL to the
  PreShower FE card
• One PreShower card handles 64 channels, exactly 2
  ECAL cards.
• The 2x4 bits are extracted synchronously at each
  BX and sent to the Validation Card
• See Remis talk

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• Usage of the PreShower/SPD information
• SPD signs the charged particles.
• The PreShower detects early showers, i.e.
  electromagnetic particles
• Electrons give signal in SPD and PreShower.
• Photons give a signal in PreShower only.
• To remove pile-up signal, one look only at SPD
  cells matching a PreShower signal.
• One can also kill dirty candidates, for example
  if the four PreShower cells are fired.
• The decision will be in a Look-Up Table, 8 bits
  of address.
• Each output will be 3 identical bits, followed by
  triple voting for SEU protection.

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• From ECAL, produce 4 candidates
• Electrons and photons are just validated
  FE-candidates
• local ?0 are detected by a high total energy on
  the card.
• global ?0 are detected by summing the
  FE-candidates of two consecutive cards.
• No SPD/PreShower validation foreseen for the ?0,
  but this will be integrated in the card, in case
• This is just a few more output bits of the
  previous LUT, plus the validation of a register.
• Only the highest Et candidate is interesting
• We select the highest of the 8 on the Validation
  card
• Very similar to the selection on the FE-card.
• Output 4 candidates
• Each has 8 bits Et and 8 bits address, plus BX-ID.

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HCAL Validation

• Purpose Add the ECAL energy to HCAL candidate
• Frequently, a hadronic shower starts in ECAL.
• Ideal case Add the ECAL cells in front of the
  HCAL candidate.
• But this implies a lot of connections, at 40 MHz.
• But this addition is important only if the ECAL
  energy is important
• If important, it has a large chance to be a local
  maximum in the ECAL card.
• This is now a manageable problem
• About 200 ECAL local maximum, to send to 50 HCAL
  candidates
• But one could also sent the HCAL candidates to
  the ECAL ones,
• Less connections
• Some duplication
• But can use the ECAL Validation Card.
• One ECAL card matches only one HCAL card.
• One validation card receives at most four HCAL
  cards
• One HCAL card goes at most (30 of 50) to two
  Validation cards

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• Principle of operation
• Compare the addresses of the ECAL and HCAL
  candidates
• First select which HCAL card to use for this ECAL
  card
• This is a configuration parameter, in a
  multiplexer.
• Compare the 5 bits addresses
• 10 bits LUT, as this varies a lot from Validation
  card to Validation card
• Use 3 identical output bits, and a Triple Voting
  technique to be SEU immune.
• There may be several matches
• An HCAL candidate may be in front of more than
  one ECAL card !
• One should select the highest ECAL contribution
  matching
• The ECAL Et is added to the original HCAL Et
• No other changes
• One has no information of which ECAL energy was
  used.
• Each (modified) HCAL candidate is sent to the
  Selection Crate
• 5 bits address, 8 bits Et BX-ID
• Same format as ECAL candidates

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Overall Validation Card (prelim.)
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SPD Multiplicity

• Count the total multiplicity in the SPD
• New idea, under study.
• Reject dirty events, to cut tails in computing
  time distribution
• Should help L1, and reconstruction
• Biases to be understood.
• Re-use of the connections
• In the PreShower card, count the number of SPD
  bits fired
• Answer in the range 0-64 7 bits
• Send this number to the validation slot, using
  the same backplane links as in the ECAL crate
• The processing is simple Add the 8 numbers and
  send the 10 bits to the Selection Crate.
• This can be added on the (existing) SPD control
  card which will anyway exist on this slot.

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Selection Crate

• Receive several similar inputs
• 28 electrons, 28 photons, 28 local ?0, 28 global
  ?0.
• Build the final 14 bits address, select the
  highest, send it to the L0 Decision Unit
• Minor difference for the SPD multiplicity
• Should add the inputs on 13 bits, instead of
  selecting the highest
• HCAL processing more delicate
• Removal of pseudo-clones
• Same HCAL sent to two ECAL Validation
• Keep only the one with the highest energy.
  Address is the same.
• We want also the second highest, and the total Et
• Details in Umbertos presentation.

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Summary

• Relatively complex object
• 350 FE-cards, 28 (16) Validation cards,
  Selection crate.
• Many links, but a lot on a dedicated backplane
• About 500 cables inside and between the 12 racks
  on the Calorimeter Platform.
• Fully synchronous.
• More technical information
• PGA on the FE and Validation Card Christophe
  Beigbeder
• Backplane and links Daniel Charlet
• PreShower and SPD Remi Cornat
• Optical links and Selection Crate Umberto Marconi