tracking at the atlas lvl2 trigger athens – hep2003 nikos konstantinidis university college london

Tracking at the Tracking at the ATLAS LVL2 TriggerATLAS LVL2 Trigger

Athens – HEP2003

Nikos KonstantinidisNikos Konstantinidis

University College LondonUniversity College London

Nikos KonstantinidisNikos Konstantinidis Tracking at the ATLAS LVL2 TriggerTracking at the ATLAS LVL2 Trigger 2

OutlineOutline

IntroductionIntroduction• ATLAS Trigger StrategyATLAS Trigger Strategy• Tracking at LVL2Tracking at LVL2

The IDScan tracking packageThe IDScan tracking package• The algorithmsThe algorithms• Performance Performance

Conclusions – Outlook Conclusions – Outlook


Triggering at the LHCTriggering at the LHC

Challenge 1:Challenge 1:• Bunch crossing every 25ns => rate: 40MHzBunch crossing every 25ns => rate: 40MHz• Data storage capability ~100HzData storage capability ~100Hz

Must select online a couple events in a million!!!Must select online a couple events in a million!!!

Online background rejection of ~6 orders of Online background rejection of ~6 orders of magnitude!magnitude!

Challenge 2:Challenge 2:• Peak luminosity: 2x10Peak luminosity: 2x103333(low) 10(low) 1034 34 (high)(high)

~5 ~25 pp interactions per bunch crossing~5 ~25 pp interactions per bunch crossing Luminosity falls by a factor ~2 over a fill (~10hours)Luminosity falls by a factor ~2 over a fill (~10hours)

Interesting (high pInteresting (high pTT) pp interaction complicated by ) pp interaction complicated by pile-uppile-up

Very annoying for tracking (increases combinatorics) Very annoying for tracking (increases combinatorics)


ATLAS Trigger StrategyATLAS Trigger Strategy


ATLAS Trigger – OverviewATLAS Trigger – Overview LVL1LVL1

• Uses Calorimeters & Uses Calorimeters & Muon Trigger Stations Muon Trigger Stations (coarse granularity)(coarse granularity)

LVL2LVL2• Uses LVL1 Regions of Uses LVL1 Regions of

Interest (RoI), so only Interest (RoI), so only a small fraction of the a small fraction of the event data is accessed event data is accessed

• InDet tracking avail.InDet tracking avail.• Combines sub-detsCombines sub-dets• Full granularityFull granularity

Event filterEvent filter• Refined, offline-type Refined, offline-type

reconstruction, with reconstruction, with access to calibration & access to calibration & alignment dataalignment data


Region of InterestRegion of Interest

x-y viewx-y view -z view-z view


Tracking @ LVL2Tracking @ LVL2

Tracking is needed for Tracking is needed for • Single, high-pSingle, high-pTT electron/muon identification electron/muon identification

Match tracks to info from outer detectorsMatch tracks to info from outer detectors• B Physics (at low lumi? budget permitting?)B Physics (at low lumi? budget permitting?)

Exclusive reconstruction of golden decays (e.g. B–>Exclusive reconstruction of golden decays (e.g. B–>))• b-jet tagging (e.g. in MSSM H –>hh –> bbbb)b-jet tagging (e.g. in MSSM H –>hh –> bbbb)

All must be done in ~10msAll must be done in ~10ms• Must deal with combinatoricsMust deal with combinatorics• At high luminosity: ~20K space points in the Si TrackersAt high luminosity: ~20K space points in the Si Trackers


The Si Trackers of ATLASThe Si Trackers of ATLAS


The IDScan algorithmsThe IDScan algorithms A sequence of four algorithms for pattern A sequence of four algorithms for pattern

recognition & track reconstruction using 3D recognition & track reconstruction using 3D space points. space points.

Basic idea:Basic idea:• Find z-position of the interesting (high-pFind z-position of the interesting (high-pTT) pp interaction ) pp interaction

before any track reconstructionbefore any track reconstruction• Select only groups of space points consistent with the Select only groups of space points consistent with the

above zabove z


ZFinderZFinder Relies on:Relies on:

• Tracks are straight lines in Tracks are straight lines in –z. Using the (–z. Using the (,z) from a pair ,z) from a pair of space points of a track, you can determine its zof space points of a track, you can determine its z00 by by simple linear extrapolationsimple linear extrapolation

• High-pHigh-pTT tracks are almost straight lines in tracks are almost straight lines in ––..

Steps:Steps:• Make very thin slices in Make very thin slices in (0.2-0.3 degrees) (0.2-0.3 degrees)• In each slice, make all pairs of space points from different In each slice, make all pairs of space points from different

layers, calculate zlayers, calculate z00 by linear extrapolation and fill a 1D by linear extrapolation and fill a 1D histogram with thishistogram with this

• The bin with the max. number of entries corresponds to the The bin with the max. number of entries corresponds to the zz00 you are looking for you are looking for


ZFinder – ExampleZFinder – Example

Jet RoI from Jet RoI from

WH(120GeV)WH(120GeV)


HitFilterHitFilter Given this zGiven this z00, all space points of a track , all space points of a track

originating from zoriginating from z00 will have the same will have the same

Steps:Steps:• Put all space points in a 2D histogram in (Put all space points in a 2D histogram in (,,))• Accept all space points in a bin if this bin contains space Accept all space points in a bin if this bin contains space

points in at least 5 (out of 7) different layerspoints in at least 5 (out of 7) different layers• Reject all other space points Reject all other space points

No combinatorics => linear time behaviourNo combinatorics => linear time behaviour Returns groups of hits Returns groups of hits


HitFilter – ExampleHitFilter – Example

-z view-z view

x-yx-y

viewview

histogramhistogram


Performance (I)Performance (I)

Single e (pSingle e (pTT=40GeV) RoI at =40GeV) RoI at high high LL ( (xx == 0.20.2xx0.2)0.2)• <# space points> ~ 250<# space points> ~ 250• <execution time> ~1ms<execution time> ~1ms**• ZFinder resolution ~ 180ZFinder resolution ~ 180mm• Efficiency ~98%Efficiency ~98%

B physics (low B physics (low LL) full Si ) full Si Trackers reconstructionTrackers reconstruction• <execution time> ~20ms<execution time> ~20ms**

Execu

tion T

ime (

ms) 30

0 2000 4000 6000 8000 10000

# of space-points in Event

20

10Linear scaling with occupancy

0

**projected to CPU speed of 4GHzprojected to CPU speed of 4GHz


Performance (II)Performance (II) Using 2 rather than 3 pixel barrel layersUsing 2 rather than 3 pixel barrel layers

• Only necessary to change the min. number of space points 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)required to make a track, from 5 (out of ~7) to 4(out of 6)

BS DS() (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 Algorithms conceptually simple => Flexible => Robust


Example – Electron RoIExample – Electron RoI


IDScan – VirtuesIDScan – Virtues Modular –> flexible –> robustModular –> flexible –> robust

Fast and linear: t ~ (# of space points) Fast and linear: t ~ (# of space points)

Suitable for all tracking needs of LVL2Suitable for all tracking needs of LVL2

No DetDescr dependence: only space pointsNo DetDescr dependence: only space points

Uniform treatment of barrel/endcapsUniform treatment of barrel/endcaps

Uniform treatment of pixel/SCTUniform treatment of pixel/SCT


Summary – Outlook Summary – Outlook Triggering has a central role at the LHC; the physics Triggering has a central role at the LHC; the physics

reach of ATLAS (and CMS) depends on it criticallyreach of ATLAS (and CMS) depends on it critically

Tracking at LVL2 is a real challenge, especially at Tracking at LVL2 is a real challenge, especially at high luminosityhigh luminosity

Determining the z-position of the interesting pp Determining the z-position of the interesting pp interaction interaction prior to any track reconstructionprior to any track reconstruction and and then rejecting all space points that cannot be due to then rejecting all space points that cannot be due to tracks from that z seems to work besttracks from that z seems to work best

Still space for novel ideas to improve the ATLAS Still space for novel ideas to improve the ATLAS physics potential and exploit the physics at the LHC physics potential and exploit the physics at the LHC optimallyoptimally

tracking at the atlas lvl2 trigger athens – hep2003 nikos konstantinidis university college london

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