Fast TracKing at ATLAS Why and How Jamie Saxon University of - - PowerPoint PPT Presentation

Fast TracKing at ATLAS

Why and How

Jamie Saxon University of Chicago

What is the FTK?

• Global tracking early in the HLT (100 µs).

(pT, η, φ, d₀, z₀, hits, and χ²)

• Dramatic step for the ID in the trigger.
• Currently in production and installation.
• Operating next year through 2022.

Why do this?

• A. Lower thresholds for b, τ triggers.
• Tracking currently limited to cones around

L1-objects.

• 3rd gen very interesting for Run II.
• B. Confront pileup through vertexing.
• Good for pileup corrections, multijet triggers,

and Ɇᵀ corrections/track Ɇᵀ.

• C. Exotic: displaced vertices, rare flavor,

unbiased sample (large µ), ‘mini-events.’

How? (Algorithmically)

φ2

φ1

φ2 φ1

Not a Track Track Not a Track Maybe!?

How? (Algorithmically)

φ2

φ1

φ2 φ1

Not a Track Track Not a Track Maybe!?

How? (Algorithmically)

• 1. Find patterns of

plausible candidates.

φ2

φ1

How? (Algorithmically)

• 1. Find patterns of

plausible candidates.

• 2. Evaluate χ², linearized in

small detector regions.

φ2

φ1

φ2 φ1

Track Maybe!?

How? (Algorithmically)

• 1. Find patterns of

plausible candidates.

• 2. Evaluate χ², linearized in

small detector regions.

• 3. ‘Guess’ missing hits.

φ2

φ1

φ2 φ1

Track Maybe!?

How? (Algorithmically)

• 1. Find patterns of

plausible candidates.

• 2. Evaluate χ², linearized in

small detector regions.

• 3. ‘Guess’ missing hits.
• 4. Linearized calculation of

helix parameters.

φ2

φ1

φ2 φ1

Track Maybe!?

At ATLAS…

Silicon Hits HLT

Photos: atlasftk.uchicago.edu/photos/

• Split pixel and SCT hits from the

detector into 64 parallel pipelines.

Data Formatter

Data Formatter

• Cluster hits, share them among regions.
• Finite beamspot, curvature at low-pT.

Silicon Hits

Auxiliary Card

Data Formatter

Auxiliary Card

• Coarse resolution hits to pattern matching.

Silicon Hits

Auxiliary Card Assoc. Memory Board

Data Formatter

Associative Memory Board

• Find coarse-resolution track candidates.
• Massive fanout to 64 custom chips.

AM Chip 06

• Loads hits in memory.
• All candidates matched

simultaneously (magic).

• Variable-width patterns.
• Optimized for low-power.

Silicon Hits

Auxiliary Card Assoc. Memory Board

Data Formatter

Auxiliary Card

• Linearized χ² for patterns matched in

the AMB, using 8 detector layers.

Silicon Hits

Auxiliary Card Assoc. Memory Board

Second Stage Board

• Extrapolate aux. fit to 12 layers.
• Calculate track parameters, new χ².

Second Stage Board

Data Formatter Silicon Hits

4 silicon layers

Auxiliary Card FTK- Level 2 Interface Card Assoc. Memory Board

Data Formatter

Second Stage Board

FTK-Level 2 Interface Card

• Restore ‘global’ hit coordinates.
• Package complete events for HLT.

Silicon Hits HLT

The System

• 322 9U boards, 640 mezzanine cards,

7 racks… 250B fits/second.

Silicon Hits HLT

Tracking Performance

• Efficiencies WRT offline > 90%.
• Excellent resolution for η, φ, d₀, z₀.
• Good pT resolution; ‘cheap’ to refit.

[GeV]

T

p 10 20 30 40 50 60 70 80 90 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1

muon pion

Simulation, no IBL ATLAS

Efficiency WRT Offline

Resolution at 10 GeV

pT 0.5 GeV η 0.0015 φ 0.001 d₀ 0.06 mm z₀ 0.25 mm

FTK TDR: cds.cern.ch/record/1552953

Physics Performance

d0 Significance

• 20
• 10

10 20 30 40 Normalized Entries

• 5

10

• 4

10

• 3

10

• 2

10

• 1

10 ATLAS Simulation, no IBL | < 1.1) η Barrel (| > = 69 µ <

Offline Light-Flavor Offline b-Jet FTK Light-Flavor FTK b-Jet

• Great discrimination on physics

quantities, e.g., σ(d₀)!

FTK TDR: cds.cern.ch/record/1552953

Trigger Impact: τ’s and b’s

• Efficiencies v. rates for b and τ triggers, WITH

and or WITHOUT FTK: significant gains.

• What can you do with global tracking?

Tau Triggers b-Jet Triggers

FTK TDR: cds.cern.ch/record/1552953

Take-Away

• Starting next year, FTK will provide global

tracking for every level 1 trigger at ATLAS.

• Good resolution of track parameters.
• > 90% efficient WRT offline.
• Threshold-less b and τ-tagging, PV

information, and ‘exotic’ applications.

• Exciting time: intellectual challenges,

hands-on work, and physics potential now.