Transcript Tracking at the ATLAS LVL2 Trigger Nikos Konstantinidis University College London Athens – HEP2003

Tracking at the
ATLAS LVL2 Trigger
Nikos Konstantinidis
University College London
Athens – HEP2003
Outline

Introduction
• ATLAS Trigger Strategy
• Tracking at LVL2

The IDScan tracking package
• The algorithms
• Performance

Conclusions – Outlook
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Triggering at the LHC

Challenge 1:
• Bunch crossing every 25ns => rate: 40MHz
• Data storage capability ~100Hz
Must select online a couple events in a million!!!
Online background rejection of ~6 orders of magnitude!

Challenge 2:
• Peak luminosity: 2x1033(low)
1034 (high)
 ~5
~25 pp interactions per bunch crossing
 Luminosity falls by a factor ~2 over a fill (~10hours)
Interesting (high pT) pp interaction complicated by pile-up
Very annoying for tracking (increases combinatorics)
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ATLAS Trigger Strategy
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ATLAS Trigger – Overview

LVL1
• Uses Calorimeters &
Muon Trigger Stations
(coarse granularity)

LVL2
• Uses LVL1 Regions of
Interest (RoI), so only
a small fraction of the
event data is accessed
• InDet tracking avail.
• Combines sub-dets
• Full granularity

Event filter
• Refined, offline-type
reconstruction, with
access to calibration &
alignment data
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Region of Interest
x-y view
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r-z view
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Tracking @ LVL2

Tracking is needed for
• Single, high-pT electron/muon identification
 Match tracks to info from outer detectors
• B Physics (at low lumi? budget permitting?)
 Exclusive reconstruction of golden decays (e.g. B–>pp)
• b-jet tagging (e.g. in MSSM H –>hh –> bbbb)

All must be done in ~10ms
• Must deal with combinatorics
• At high luminosity: ~20K space points in the Si Trackers
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The Si Trackers of ATLAS
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The IDScan algorithms


A sequence of four algorithms for pattern
recognition & track reconstruction using 3D
space points.
Basic idea:
• Find z-position of the interesting (high-pT) pp interaction
before any track reconstruction
• Select only groups of space points consistent with the
above z
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ZFinder

Relies on:
• Tracks are straight lines in r–z. Using the (r,z) from a pair
of space points of a track, you can determine its z0 by
simple linear extrapolation
• High-pT tracks are almost straight lines in r–f.

Steps:
• Make very thin slices in f (0.2-0.3 degrees)
• In each slice, make all pairs of space points from different
layers, calculate z0 by linear extrapolation and fill a 1D
histogram with this
• The bin with the max. number of entries corresponds to
the z0 you are looking for
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ZFinder – Example
Jet RoI from
WH(120GeV)
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HitFilter


Given this z0, all space points of a track
originating from z0 will have the same h
Steps:
• Put all space points in a 2D histogram in (h,f)
• Accept all space points in a bin if this bin contains space
points in at least 5 (out of 7) different layers
• Reject all other space points


No combinatorics => linear time behaviour
Returns groups of hits
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HitFilter – Example
x-y
view
r-z view
h-f histogram
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Performance (I)
Single e (pT=40GeV) RoI
at high L (DhxDf = 0.2x0.2)
•
•
•
•

<# space points> ~ 250
<execution time> ~1ms*
ZFinder resolution ~ 180mm
Efficiency ~98%
B physics (low L) full Si
Trackers reconstruction
• <execution time> ~20ms*
Execution Time (ms)

30
20
10
Linear scaling
with occupancy
0
*projected to CPU speed of 4GHz
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2000
4000
6000
8000 10000
# of space-points in Event
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Performance (II)

Using 2 rather than 3 pixel barrel layers
• Only necessary to change the min. number of space points
required to make a track, from 5 (out of ~7) to 4(out of 6)
BS
DS(fp)p
(3)
(2)
Signal Efficiency (%)
68.7
68.0
Eff (wrt offline) (%)
78.4
78.5
Bkg Efficiency (%)
3.5
3.8
Algorithms conceptually simple => Flexible => Robust
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Example – Electron RoI
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IDScan – Virtues

Modular –> flexible –> robust

Fast and linear: t ~ (# of space points)

Suitable for all tracking needs of LVL2
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No DetDescr dependence: only space points

Uniform treatment of barrel/endcaps

Uniform treatment of pixel/SCT
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Summary – Outlook




Triggering has a central role at the LHC; the physics
reach of ATLAS (and CMS) depends on it critically
Tracking at LVL2 is a real challenge, especially at
high luminosity
Determining the z-position of the interesting pp
interaction prior to any track reconstruction and
then rejecting all space points that cannot be due to
tracks from that z seems to work best
Still space for novel ideas to improve the ATLAS
physics potential and exploit the physics at the LHC
optimally
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