Trigger Peter Jacobs, LBNL

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TriggerPeter Jacobs, LBNL
PbPb interaction rate 4-8 kHz Hard process
trigger 10 Hz ?need rejection factor 400-800
p0 10 Hz ? pT20 GeV/c Inclusive jets 10 Hz ?
ET50 GeV/c
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EMCal trigger enhancement factor relative
tominbias trigger TPC
s0 p0 vs p

• Live time
• DAQ dynamic scaledown of common triggers
  (central, minbias)
• Pierre v.d.Vyvre reasonable to expect 90
  livetime

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Why bother with jets in pp?

• Is ALICE irrelevant for jet studies in pp? Maybe
  not.
• pp at top luminosity 1034/cm2/s x 100 mb 109
  Hz
• 20 ns bunch spacing ? 20 minbias interactions per
  bunch crossing
• ALICE runs at L1031 ? much cleaner environment
  in low to intermediate pT region
• ALICE may have a unique niche in pp detailed
  studies of jet fragmentation down to low z ?
  important reference for AA program

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PHOS L1 Trigger
L1 (6 ms) 2x2 tower analog sum ? TRU ? 4x4 tower
peak finder EMCal 12 FEE/GTL bus ? 1232384
towers/TRU (Dh x df0.24 x 0.36)
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Charged jets (HLT study)
C. Loizides, FfM
ETchargedgtm
Charged jets poor energy resolution, slow
turn-on above trigger threshold, highly biased
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Conjecture better solution is Level 1 EMCal
HLT TPCEMCal

• Some rough numbers
• Minbias data rate 20 MB/evt4 KHz 80 GB/s
• HLT input bandwidth 15 GB/s
• Least-biased efficient trigger algorithm
• EMCAL_at_L1 mildly biased jet patch trigger to cut
  minbias rate by factor 10 8 GB/s
• do the rest in HLT incorporating charged tracks,
  neutral energy from emcal, dijet topologies (?),
  etc

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Jet patch trigger in pp
PYTHIA
0.21x0.21
Strong biases for ETmaxgt10 GeV
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Jet Patch Trigger Simulations
Andre Mischke (Utrecht)

• Pythia jet (ET50 GeV) HIJING background
• square candidate jet patches Dh x Df s x s
• sweep patch quasi-continuously over detector,
  find maximum ETmax

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Why a large-ish trigger patch in AA?

• Leading particle (p0) trigger strongly biased
• Fragmentation bias (prefer low ET jets
  fragmenting hard)
• Geometric bias (prefer lower than average energy
  loss surface emission)
• PYTHIA fragmentation requires relatively small
  patches (0.1 x 0.1) for efficient triggering
  (Bill, Chris)
• fragmentation fluctuations primarily in
  distribution of few hardest (colinear) hadrons
• But the physics we are after is the modification
  of the fragmentation, including potentially large
  jet broadening effects (and correspondingly
  smaller background fluctuations?)
• ? no really solid theory guidance
• ? prudent experimental design requires flexible
  patch trigger
• ? also for lighter systems than PbPb

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Jet patch trigger in PbPb
PYTHIAHIJING
0.21x0.21
ET cut for 80 trigger efficiency _at_ 50 GeV
Weighted by Nch ? data volume

• Centrality dependent pedestal (no surprise)
• Background and fragmentation fluctuations of
  similar magnitude

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L1 output data rate for 80 jet efficiency _at_ 50
GeV

• Details strongly dependent on models of signal
  background
• Qualitative conclusion nevertheless required L1
  rejection achievable with reasonable efficiency
  for 50 GeV jets
• requires centrality-dependent threshold

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Trigger efficiency I
PYTHIAHIJING
Good efficiency at 100 GeV Background
dependencies at 50 GeV
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Trigger efficiency II patch size dependence
0.3 x 0.3 apparently larger than optimal
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Trigger efficiency III quenching models
Strong model dependencies But models are only
first guesses ? prudent experimental design
requires flexible patch trigger
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PHOS trigger Trigger Regional Unit

• each TRU DhxDf0.24x0.36 (depends on how we
  cable)
• no inter-TRU communication ?large acceptance hit
  for jet patch, limited maximum patch size

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Candidate implementation TRU hierarchy
Jet patch trigger

Max aggregate input bandwidth 3 Gbit/s
latency 1-2 ms
lt 100 Mbit/s
100 bits/evt
13K towers 30 TRUs
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Centrality-dependent trigger threshold? V0
Uniform jet trigger efficiency across
centralities need to account for
centrality-correlated pedestal fluctuations V0
is only fast (L1) detector with sufficient
coverage
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V0 response to PbPb
Forward detector TRD fig 3.6

• Large generation of secondaries in beampipe but
  response is nicely linear

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V0 bits in jet patch summary TRU?
Centrality-dependent threshold?
Significant problem no L1 cross-correlation in
Central Trigger Processor (Orlando V-B)
few bits

V0
technically feasible (H. Muller, F. Fomenti, Y.
Zoccarato)
latency 1-2 ms
lt 100 Mbit/s

• System design issues non-locality of trigger
  logic (i.e. not in CTP), scalars,
• PHOS TRU Hans Muller has added 3 optical
  Gigabit ports for interconnectivity

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V0 Interface between CTP, LTU, EMCal and FEE
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General trigger issues
Interface with ALICE CTP for issuing and
receiving L0/L1 Decisions being made now, we need
to be involved