Transcript Muon reconstruction and selection at experiment Roberto Crupi

Roberto Crupi
INFN Lecce & Physics Department,
University of Salento - Italy
on behalf of the
ATLAS Collaboration
Muon reconstruction and selection at
the last trigger level of the ATLAS
experiment
11th ICATPP Conference on Astroparticle,
Particle, Space Physics, Detectors and
Medical Physics Applications
Villa Olmo, Como 5-9 October 2009
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Outline
• Overview on the ATLAS Trigger System
• The Muon Event Filter
• Performance Studies
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ATLAS (A Toroidal LHC ApparatuS)
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The ATLAS Muon Spectrometer
The Muon Spectrometer has the standalone capability of measuring muon
momenta
pT/pT < 10% up to 1 TeV
Technologies deployed in the muon system:
Tracking Chambers:
• MDT (Monitored Drift Tubes)
||<2
• CSC (Cathode Strip Chambers)
2<||<2.7
ΔpT/pT of few % up to 100 GeV
Trigger Chambers:
• RPC (Resistive Plate Chambers)
||<1.05
• TGC (Thin Gap Chambers )
1.05<||<2.4
A magnetic field allows muon momentum measurements
MDT
RPC
TGC
CSC
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The ATLAS Trigger
To obtain the required event rate reduction, the trigger is structured in three levels:
• Level-1 : hardware based, with coarse granularity from calorimeters and muon
systems. It reduces the ~40MHz initial rate to 75 kHz in a maximum latency of 2.5
s. It looks for regions of activity in the detector (Regions of Interest, RoIs)
• Level-2 : software based, algorithms optimized for fast rejection. It reduces
Level-1 output rate to ~3 kHz with a 40 ms mean processing time. It accesses the
full granularity data inside a Level-1 RoI. BUNCH CROSSING
• Event Filter : software based. It uses
the same algorithms as in the offline
and runs after event building, then
it can access the complete event data.
It reduces Level-2 output rate to
~200 Hz with a 4s mean processing
time. It accesses the full granularity
data inside a Level-2 RoI.
Level-2 & EF are together referred as
High Level Trigger (HLT)
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RATE
40 MHz
~75 kHz
2.5 s
~3 kHz
40 ms
~200 Hz
4s
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Reconstruction and identification
in the muon system
• The Muon Spectrometer is a powerful tool for precise measurements of
high pT muons and identification of low pT muons
• The longer lever arm improves charge determination for high energy
muons
• Extrapolation to the Interaction Point (IP) can better discriminate from
secondary muons and the ones coming from the in-flight decay of π/K
mesons.
• The Inner Detector (ID) performs precise track reconstruction with high
efficiency for low pT muons
• Muon Identification is achieved by combining ID measurements with the
ones provided by MS
• The final momentum resolution takes advantage of both ID and MS
• Isolation criteria applied to muons before reaching the MS allows a good
muons discrimination from the ones deriving from jets
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Region of Interest selected
By Level-1
The Muon Trigger
Level-1
result
Level-2:
• muFast: standalone MS muon
reconstruction, fast muon pT estimation
via LUT
• muComb: combined ID/MS muon
reconstruction; improvement of pT
resolution wrt μFast
• muIso: Calorimeter and ID isolation
• muTile: Muon Tagging in Tile
Calorimeter
Event Filter (EF) performs muon
Identification and Reconstruction
thanks to dedicated offline packages
developed in the ATHENA framework
• Refines Level-2 estimates
• Confirms/Discards Level-2 Hypothesis
• For debug purposes EF could directly
receive a seed from Level-1 RoIs
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Level-2
tasks:
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• rejection of low pT muons
• rejection of secondary
muons deriving from the
decay in flight of π/k
• rejection of fake muon
tracks generated by hits due
to cavern background
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Muon Event Filter
Two algorithms have been implemented in muon EF:
TrigMuonEF and TrigMuGirl.
Both are wrappers of muon offline reconstruction tools.
They have an opposite strategy in muon reconstruction/identification:
TrigMuonEF starts from the reconstruction in the MS and performs a
backward extrapolation to the IP and track combination in the inner
detector.
TrigMuGirl starts muon reconstruction from the ID and extrapolates
tracks to MS.
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Muon Event Filter
Trigger selection strategy is based on the definition of:
• Trigger Elements (TEs) : abstract objects representing the
state of the reconstructions.
TE
• Chains: sequences of several TEs
• For each chain, the trigger steering runs the sequence of
algorithms configured to produce all the output TEs at
each step of the chain.
TrigMuonEF and TrigMuGirl are structured in :
• Feature EXtraction (FEX) algorithms: normally RoI seeded,
retrieve detector data inside RoI and look for feature in this
data.
FEX
HYPO
Updated
TE
• HYPOthesis (HYPO) algorithms: compare features
provided by the FEXs against some hypothesis and validate
or reject the TE output according to the success or failure
of the hypothesis
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TrigMuonEF Implementation
• TrigMuonEF is a wrapper of offline packages developed for the
purpose of muon reconstruction and identification:
TrigMuonEF is composed of:
• A sequence of four FEX
algorithms which compute physics
quantities.
• Four HYPO algorithms after each
FEX.
• Possible configurations:
– Seed from Level 2
(standard)
– Seed from Level 1 (for
debug)
– Unseeded (full event
reconstruction)
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TrigMuonEF Implementation
• Segment Finder: selects MDT precision hits to found a segment.
• Track Builder: Reconstructs muon tracks inside the MS taking
advantage of its high precision tracking system and provides a
precise measurement of the track parameter outside the calorimeter.
• TrackExtrapolator: performs a backward extrapolation to the
interaction region through the calorimeters.
• It takes into account the magnetic field and corrections for energy loss
and multiple scattering effects from all crossed material
• TrackCombiner: Extrapolated tracks are combined with the
corresponding matching tracks, if they exist, in the ID.
• MS and ID tracks are matched by forming a c2 built with the parameters
of both tracks
• use a global fit of all the hits collected in both ID and MS
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TrigMuGirl Implementation
• Muon candidate starts from an ID track inside
a Level-2 RoI, provided by EF ID algorithms.
• Tracks are extrapolated to the MS chambers.
• A road in eta and phi is created around the
extrapolated coordinates.
• The algorithm looks for hits around the
extrapolated track to fit segments.
EF ID Tracks
In
Muon RoI
CandidateTool
TrigMuGirl
ANNSelectionTool
GlobalFitTool
HypoAlg
• The best fitted segment is used to refine the
estimated track coordinates and road in the
current station.
• Hits and segments collected in the various stations of the MS chambers
allow TrigMuGirl to improve extrapolation and the to identify muon-like
candidate.
• Muon-like candidates are selected using precalibrated ANN functions.
• A global fit, icluding ID and MS hits is applied to tracks belonging to
identified muons for a further improvement of the momentum estimate.
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TrigMuGirl: Slow Particle Mode
Background Rates
• TrigMuGirl includes a dedicated tool to
trigger Stable Massive Particles (SMP, e.g. in
Susy R-parity violating scenario).
Source
1034 rate (Hz)
1031 rate(Hz)
L2_mu40
Slow
Slow
c-cbar
125
2.5
0.0024
b-bbar
123
1.65
0.0016
• b and mass reconstruction using RPC and
MDT technologies.
Wmunu
19
0.4
0.0004
t-tbar
0.2
small
0.0000
• Specific R-Hadrons selection for candidates
that don’t have an ID track.
zmumu
small
0.0000
• Reconstruction efficiency takes advantage
of hits collected from both the previous and
the following Bunch Crossing.
• Seed search in the MS from a Level-2 trigger
element.
SMP trigger runs in a separate chain
(Slow) in the trigger Menu for 1031
M=110 GeV
L1: mu10
Mtruth=110 GeV
M = 113.4 ± 9.7 GeV
L2: beta/mass reco in the barrel
EF: TrigMuGirl beta/mass reco
Estimated beta resolution and
reconstructed mass for single stau
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Muon EF Performance
EF efficiencies wrt Level-2
• Simulated t-tbar sample
• Association to true muon in a cone
of DR<0.002 for muon EF combined
algorithms.
• No pT thresholds have been
applied for TrigMuonEF while the
requirement of a 10GeV pT muon in
the event is applied for TrigMuGirl.
• The pT resolution as a function of
transverse momentum is shown for
ATLAS Preliminary
pT resolution vs pT
Extrapolator
Combiner
TrigMuGirl
with respect to Level-2 muComb
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Muon EF Performance
eta resolution vs pT
TrigMuonEF and TrigMuGirl:
Level-2:
muFast
muComb
•
improve significantly trigger rejection
power due to better pT resolution wrt
Level-2
• have similar performance in efficiency
and resolutions
Efficiency wrt MC Truth
phi resolution vs pT
Zμμ MC sample
TrigMuGirl
TrigMuonEF
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Results on 2008 cosmic data
• TrigMuonEF TrackBuilder algorithm: comparison of
eta () and phi (Φ) wrt to offline algorithm
=0.007, =17mrad
• Solenoidal and toroidal field on
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Conclusions
• The Muon Event Filter has been designed and
implemented to cope with the demanding requirements
of the ATLAS trigger system in the high luminosity and
background enviroment at LHC
• TrigMuonEF and TrigMuGirl have been successfully
integrated in the Muon EF and constantly tested and
validated
• TrigMuonEF and TrigMuGirl show very good and
similar performance
• At the startup muon selection will take advantage of
both algorithms running
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BACK UP SLIDES
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•
Center of Mass Energy (p-p)
– 14 TeV (design)
– 10 TeV (start-up)
Instantaneous Luminosity:
– L = 1031cm-2s-1
– L = 1034cm-2s-1
Integrated Luminosity / year (@ 1034)
–  Ldt  100 fb-1
•
•
•
The cross sections of interesting physics
processes are highly suppressed w.r.t. tot
–
–
•
40 MHz (BC frequency) x 23 interactions per BC
Max allowed acquisition rate ~200 Hz
–
–
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Calibrations and precision physics
• Leptonic W decay
New Physics
• Higgs boson production
Event rate at LHC (@ design lumi) ~1 GHz
–
•
LHC
Bandwidth ~ 300 MByte/s
Event Size ~ 1.5 MByte
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1031 Muon menu and rates
Total of muons: 1.7k -220 ~40
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Slow Particle Trigger
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TrigMuGirl for Slow Particles
At Level-2; StauHypo: pT> 40GeV, b < 0.097 and m>40 GeV
•
Enables to select candidates when segment
reconstruction is imperfect
•
Efficiency for low b is improved using TGC hits form
next BC
•
b estimation using RPC- as in Level-2
•
b estimation using MDT
•
•
Loop over possible b
•
Change TOF according to b –MDT radii change
accordingly
•
Create MDT segments from the re-time radii
•
Minimize segment c2 wrt b → estimate
Determine b and Mass
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
New mu20_slow is in trigger menu for 1031




L1 mu10 
L2 stau (B) | mu20 (EC) 
TrigMuGirlSlow
Estimated background rates
1031
Number of events
10 rate (Hz)
Source
rate(Hz)
EF_SLO L2_mu
Total
L2_mu40 EF_mu40 W
40
Slow
Slow
c-cbar
33800
1991
1537
39
125
2.5
0.0024
b-bbar
4958
76
37
1
123
1.65
0.0016
W
239947
43287
34433
930
19
0.4
0.0004
t1
9190
1679
1558
29
0.2
small
0.0000
z
15203
7216
6601
151
small
0.0000
34
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Trigger
type
R-hadrons m=300
Barrel
L2 MS
L2 Ms
Slow
End-cap
R-hadrons
m=1000
Barrel
0.99
End-cap
1.00
NA
0.92
Slepton
s β=0.6
Barrel
1.00
NA
0.93
0.99
0.58
0.58
L2
Comb
0.49
0.50
1.00
Muon
EF
0.24
0.28
0.39
Slow EF
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0.90
0.88
0.90
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0.88
0.90
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