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Tracking Triggers. Fabrizio Palla INFN Pisa. The Tracker and the Trigger. Trigger rates control is extremely challenging in high luminosity hadron collider experiments As the luminosity increases, physics goals change in response to new discoveries and the detector ages.

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tracking triggers

Tracking Triggers

Fabrizio Palla


the tracker and the trigger
The Tracker and the Trigger
  • Trigger rates control is extremely challenging in high luminosity hadron collider experiments
    • As the luminosity increases, physics goals change in response to new discoveries and the detector ages.
  • It is thus essential that the trigger system be flexible and robust, and have redundancy and significant operating margin
    • Providing high quality track reconstruction over the full detector can be an important element in achieving these goals.
  • This has certainly been the case in the CDF experiment where the Silicon Vertex Trigger (SVT) has significantly extended the experiment’s physics capability
    • B-physics and Bs oscillations
    • Search for the Higgs boson
  • Even more challenging will be the trigger for the foreseen upgrade of LHC, the so-called SuperLHC (SLHC)
    • Luminosity increase of a factor 10 wrt “standard” LHC
main issues for tracker trigger
Main issues for Tracker Trigger
  • Data Rate in Tracker Volume at SLHC
    • About 12,000 tracks per bunch crossing (50 ns) in the Tracker volume |h|<2.5 and ~400 primary vertices/bx
      • Needs fairly high granular information
    • At 30 cm radius the rate is ~30 to 70 MHz cm-2 giving O(10K) links for 2.5 Gbps link speed
      • Need to reduce information for trigger purposes
  • Benefits to go to large radii
    • Lower occupancy
    • Better momentum measurement
    • Ideal to match with muon and calorimeter triggers
    • ~1 mm pointing resolution for z- larger than average 2 pileup interactions (~0.4 mm)
    • Still fairly nice impact parameter resolution in transverse plane (~100 mm for 10 GeV muons) to get rid of muons from p and K decays


Without pixel

With pixel


Without pixel

With pixel

tracker primitives from cdf svx approach
Tracker primitives from CDF SVX approach

Main problem: input Bandwidth

divide the detector in thin fsectors.

Each AM searches in a small Df


1 AM for each enough-small Df


Hits: position+time stamp

All patterns inside a single chip

N chips for N overlapping events

(identified by the time stamp)

Data links









trigger working model
Trigger working model
  • References
    • F. Palla, JINST 2:P02002,2007
    • F. Palla, F. Crescioli and F. Catastini, 15th IEEE Real Time Conference 2007 (RT 07)
    • G. Barbagli, G. Parrini and F. Palla TWEPP-07, Prague 2007
    • F. Palla, Vertex 2007 workshop
  • Subdivide the detector in many (~100) f sectors
    • Keep data volume limited in each sector
    • Match with the detector sizes
    • Well contained within the curvature of high pT tracks
  • Majority of at least 3 layers out of 5 in each trigger sector
  • Data transfer
    • Reduce the data rate for Trigger purpose
      • Use only a part for Trigger (data reduction on module)
        • Coarser resolution with compromise in momentum resolution
        • Stacked detector layout
        • Cluster shape based approach – see later
    • Use very high speed data links O(Gbps) to limit the no. of links to manageable level
  • Process the data using AM – Large data rate (up to hundreds of Gbps/sector)
    • Must distribute the data in parallel to many AM to accommodate bandwidth
      • Higher speed wrt CDF (asymptotically must go to 200 MHz)
      • Higher density patterns (90 or 65 nm technology)
      • Implement an efficient switch
  • Output of the Trigger
    • Tracks reconstructed above a given pT sector by sector
layouts under study
Layouts under study

10 sensors layers

Stacked 15 sensors layers

Trigger sector

Smaller a = greater pT

Smaller width = greater pT

cluster width discrimination
Cluster width discrimination
  • In the region above 50 cm, using 50µm pitch, about 5% of the total particles leave cluster sizes with ≤2 strips
    • No. of links (2.5Gbps) ~300 for whole tracker
  • Once reduced to ~100 KHz, it would only need few fast readout links to readout the entire Tracker

90 cm

70 cm


50 cm


30 cm

Discrimination of low pT tracks made directly on the strip detector by choosing suitable pitch values in the usual range for strip sensors.

am chips from 1992 to 2005




For both L1 & L2

AM chips from 1992 to 2005
  • (90’s) Full custom VLSI chip - 0.7mm (INFN-Pisa)
  • 128 patterns, 6x12bit words each
  • 32k roads / wedge
  • F. Morsani et al., “The AMchip: a Full-custom MOS VLSI Associative memory for Pattern Recognition”, IEEE Trans. on Nucl.Sci., vol. 39, pp. 795-797, (1992).

On the opposite side: FPGA for the same AMchip

P. Giannetti et al. “A Programmable Associative Memory for TrackFinding”, Nucl. Intsr. and Meth., vol. A413/2-3, pp.367-373,(1998).

In the middle: Standard Cell 0.18 mm

(INFN-Pisa) 5000 pattern/chip AMchip

L.Sartori, A. Annovi et al., “A VLSI Processor for Fast Track Finding Based on Content Addressable Memories”,

IEEE Transactions on Nuclear Science, Volume 53, Issue 4, Part 2, Aug. 2006 Page(s):2428 - 2433

Plan to increase the frequency to 200 MHz

and make use of 90 or 65 nmtechnology for AM chips

board dimensions
Board dimensions
  • Current board dimensions
    • The current AM for CDF holds ~5000 patterns/6 planes in 0.18mm technology
    • If developed in the 90 nm technology one could accommodate ~4 times more patterns/AM chip hence 30,000 for 4 planes
  • In order to evaluate the board dimensions need to define the granularity and the lowest pT threshold
    • For instance, a 50 mm pitch on 4 detector planes and a pT threshold of 10 GeV needs ~90,000 patterns (needs 3 AM chips)
      • It could of course go down in pT at the expenses of the no. of AM chips
electron gamma rates
Electron/gamma rates
  • HLT reduction is coming from pixel matching, E/p and isolation. Can it be done at L1?
    • Current E/g Trigger granularity DhDf= 0.087x 0.087 is too coarse for pixel matching
      • @4 cm (0.35x0.35) cm2 x 300 cm-2 MHz/20 MHz ~ 2 hits/bx
      • @11 cm (1x1) cm2 x 100 cm-2 MHz/20 MHz ~ 5 hits/bx
      • Exclude the possibility to have any possibility to have individual layer data reconstructed
    • Is there a possible way out?
      • Reduce the “search” window by a factor~10 – going to a granularity of 2x2 crystals or even lower.
      • Make use of “stacked” pixel layers at ~10 cm to make track stubs
        • Very hard
      • Explore starting the track stub at larger radii
        • Doable needs to define the radius
    • E/p and isolation at L1 will be comparable to the current HLT with the Tracker Trigger
      • Easy to implement in a AM model


170 KHz@1035

a first idea on selective readout from muons
A first idea on selective readout from muons
  • Provide a Muon Track Tag (MTT) to be associated with hits in outermost Tracker layers
    • Need a new detector for MTT yet to be defined (2 layers of RPC?)
  • Plug inside a AM and flag as interesting track
    • A. Montanari et al (CMS IN 2007-058)

Muon/ECAL pT, h, f



what about jets
What about jets?


  • Once tracks are formed, jets can be found quite easily.
    • Minimum reconstructable pT driven by several factors
      • In the AM approach it depends upon the size of the AM chips
    • However it is clear advantage to have jets reconstructed by tracks as shown in several CMS group meetings
      • High efficiency
      • Very good pointing resolution
      • To be combined with calorimeters jets to improve energy resolution
    • Help reducing the number of fake jets from the calorimeters
    • Could distinguish jets coming from different primary interactions (remember 400 primary vertices @1035)

P. Azzurri and D. Dobur

  • Tracker information helps reducing drastically the rate of uninteresting events
    • CDF SVT experience has become cornerstone for Tracker Trigger
      • B physics reach has been boosted after the usage of displaced vertex triggers and now Higgs search uses it
  • S-CMS will make use of tracking information at SLHC and has started to discuss several options
    • Triggering at SLHC is challenging due to fantastic data rate
    • SVT approach seems feasible, especially if complemented by a data reduction at the module level
    • The Tracker trigger will be highly beneficial for reducing the rate of Muons, Calorimeter jets and E/g.
      • Must develop a working model to how better integrate tracking and the Calo/Muons.
    • Next steps will involve simulating trigger with different layouts and are going to define which strategy will be used, stay tuned!
too large am 2 step approach


  • Then fit tracks inside roads.
  • Thanks to 1st step it is much easier
Too large AM? 2 step approach
  • Find low resolution track candidates called “roads”. Solve most of the pattern recognition

Super Bin (SB)

OTHER functions are needed inside SVT:

Hit Buffer + Track fitter +Hit Finder

on going am projects
Apart from SVT: Silicon Vertex Trigger @ CDF (L. Ristori et al.)

first AM application

proven to be easy to upgrade (1-2 years turn around time)

FTK: FastTracK @ ATLAS (University of Chicago, Laboratori Nazionali di Frascati, Harvard University, Univeristy of Illinois, University of Pisa and INFN Pisa, INFN Roma)

Full tracker reconstruction @ L2 @ full L1 out rate 100kHz

SLIM5: L1 tracking @ SuperB (F. Forti, M. Giorgi et al.)

R&D to develop MAPS sensor integrated with AM trigger

Ongoing R&D linked to CMS

Joint venture with CDF

CDF (Padova, Pisa, Siena) and CMS (Napoli, Perugia, Pisa, Roma)

Main goals:

Demonstrating AM-based Tracker Trigger also in conjunction with muon and electron triggering

Feasibility study of the switch using synergy with SLIM5 (INFN) developed boards

Funding status

Submitted proposal to Ministry of Research for FY 2008-09

On going AM projects
pattern matching in cdf m dell orso l ristori 1985

The pattern bank is flexible

  • set of pre-calculated patterns:
    • can account for misalignment
    • changing detector conditions
    • beam movement

The Pattern



Pattern matching in CDF (M. Dell’Orso, L. Ristori – 1985)

The Event

stacked readout for trigger
Stacked readout for Trigger
  • Angle  determines pT of track, assuming tracks coming from origin
    • Smaller a = greater pT
  • Pair of sensor planes at ~ mm distance for local pT estimate
    • Needs a correlator ASIC
    • Fast data links
      • If located at ~20 cm needs 3 Gbps
  • Stacks of 2 sensor planes at ~cm distance to be correlated (off detector) for pT measurement
  • Tight construction tolerances required for both sensors and their alignment

switch board numbers

  • All info here TBC with simulation and R&D!
  • 80 switch boards
    • 1 / -sector
  • 80 fibers / board
    • assume 5Gbps each
  • 40 AMchip / board
    • now we can fit 32 AMchips in one 4th of a 9U VME board
  • 4 FPGA switches (1/layer)
    • Each receiving ~20 fibers, i.e. ~100Gbps
    • 40 outputs: one per Amchip
    • Possible with today’s FPGAs

32 AMchips

  • GEANT4 simulation of tracking layers
    • Includes material budget and loopers

Current links in CMS Silicon Strip: 1300 @ 60 cm and 2600 @34 cm

  • ª average number on minimum bias events, 50 ns bx
  • *32 bits/hit
  • †for a data link speed of 5 Gbps
  • Pros: same fiber links for data and trigger
    • Simple and elegant
  • Cons : large number of links needed
    • Need >=5Gbps laser drivers: 90 nm technology required
    • Allow for fluctuations: increase by ~1.5 ?
    • Laser driver power: commercial range from 330mW/fiber (4Gbps) to 700 mW/fiber (10Gbps)
  • ~100 m fiber 300 ns [6 bx]
  • Switch + AM ~1ms [20 bx]
  • Sensor read-out latency budget should be less than ~20 bx
  • TOTAL ~ <50 bx [2.5 ms]