Transcript Document
Architecture of the ATLAS
High Level Triggers
Event Selection Software
Nikos Konstantinidis (UCL)
on behalf of the ATLAS HLT group
Frontier Detectors for Frontier Physics
Elba, May 25-31, 2003
Outline
Introduction
• The Trigger/DAQ challenges of the LHC
• The ATLAS Trigger
Event selection @ High Level Triggers (HLT)
• Event selection Strategy
• Software Architecture – Components
Conclusions – Outlook
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Trigger/DAQ at the LHC
CoM Energy
14 TeV
Design Luminosity
1034 cm-2 s-1
Interactions per x-ing
23
Bunch spacing
25 ns
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• High rate (40 MHz)
• High granularity large
event size (1-2 MBytes)
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ATLAS Trigger Strategy
HLT
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LVL1
> Latency: 2.5ms (max)
> Hardware based (FPGA, ASIC)
> Calo/Muon (coarse granularity)
LVL2
>
>
>
>
>
Latency: ~10 ms (average)
Software (specialised algs)
Uses LVL1 Regions of Interest
All sub-dets, full granularity
Emphasis on early rejection
EF
ATLAS Trigger/DAQ – Overview
>
>
>
>
Latency: few sec (average)
Offline-type algorithms
Full calibration/alignment info
Access to full event possible
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HLT Strategy
Event Selection relies on:
• Processing in Steps
Alternate steps of feature extraction / hypothesis testing
Events can be rejected at any step if features do not fulfil
certain criteria (signatures)
Emphasis on early event rejection
• Reconstruction in Regions of Interest (RoIs)
RoI size/position derived from previous step(s)
Emphasis on minimising
a. Processing time
b. Network traffic
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HLT Strategy – Example
LVL1 claims two isolated
e/m clusters with pT>20GeV
(possible signature: Z–>ee)
STEP 4
Signature
Strategy at HLT:
STEP 3
> Validate step-by-step
> Check intermediate signatures
> Reject as early as possible
Signature
STEP 2
e30
e
+
Cluster
shape
Level1 seed
EM20i
e30
pt>
30GeV
+
track
finding
STEP 1
e30i
Iso–
lation
pt>
30GeV
ecand
The ATLAS HLT Event Selection Software
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Iso–
lation
Signature
Sequential/modular approach
facilitates tuning for early rejection
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e30i
e
track
finding
+
time
Signature
ecand
Cluster
shape
+
EM20i
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HLTSSW – Design
HLT DataFlow
Software
Event Filter
Processing
Application
Package
HLTSSW
HLT Selection Software
Interface
Dependency
HLT Core Software
Level2
Steering
HLT Algorithms
Processing
Application
HLT
Algorithms
Data
Manager
ROBData
Collector
Event
DataModel
HLTSSW runs on the L2/EF processors
Several external dependencies
(Monitoring Svc, MetaData Svc, offline…)
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Core Components
Steering
• Controls the order in which HLT algorithms should run, given
the result of the previous triggering step
All possible signatures form the Trigger Menu
All possible sequences form the Sequence Table
Data Manager
• Handles the event data during processing
Provides feature extraction algorithms with access to
data within an RoI
Stores extracted data objects and can pass them to
hypothesis testing algorithms
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Data access by an HLT algo
Algorithm
HLT
Algorithm
Region
Selector
Transient
EventStore
Region
Selector
Data Access
Byte
Stream
Converter
Trans.
Event
Store
Data source
organized
by ROB
region
list DetElem IDs
list DetElem IDs
Data arranged
by DetElems
list DetElem IDs
Data arranged
by DetElems
ROB ID
raw event
data
DetElems: Detector Elements (e.g. Pixel wafers)
IDs: Identifiers – Allow access to Geometry and mapping to ROBs
For the Event Filter: data already in the TES
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Event Data and Algs
Closely coupled to offline software
• Common class definitions (Track, Cluster etc)
Facilitate code migration between LVL2/EF/Offline
Saves effort in development and maintenance
Makes comparisons and performance studies easier
• Same arguments for data access mechanism
However, special online requirements (esp. LVL2)
• Timing
• Multi-threaded running
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Conclusions – Outlook
ATLAS HLT Event Selection designed to be
• Flexible (to allow easy revision/tuning of trigger menus)
• Robust (to cope with the unexpected – machine/detector)
• Inclusive (to account for the unexpected new physics)
Minimise CPU/network resources by
• Analysing only Regions of Interest (only a few % of full event)
• Stepwise processing
Exploit synergies with offline software amap
• Algorithms / Data objects (EDM) / Data access; Easier to:
Compare Trigger vs. Offline performance
Import offline algs to Event Filter
Study LVL2 / Event Filter boundary
Evaluation of various design choices in progress
• Detailed conclusions in the HLT-DAQ TDR (due end of June)
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