2
Outline• Motivation• Few words on LHC• ATLAS trigger requirements• Basis of ATLAS trigger• Atlas detector picture• L1 Trigger
– Ex: L1 electromagnetic trigger
• High Level Trigger (HLT)– Ex: e
Some relevant open questions
1. Mass:Mass: What is the origin of mass?
- How is the electroweak symmetry broken ?
- Does the Higgs boson exist ?
2. Unification:Unification: What is the underlying fundamental theory ?
Motivation: Gravity not yet included; Standard Model as a low energy approximation
- Is our world supersymmetric ?
- Are there extra space time dimensions ?
- Other extensions ?
3. Flavour:Flavour: or the generation problem
- Why are there three families of matter?
- Neutrino masses and mixing?
- What is the origin of CP violation?
The role of Hadron Colliders 1. M1. Massass
- Search for the Higgs boson
2. Unification2. Unification
- Test of the Standard Model
- Search for Supersymmetry
- Possible link between SUSY and Dark Matter?
- Search for other Physics Beyond the SM
3. 3. FlavourFlavour
- B hadron masses and lifetimes
- Mixing of neutral B mesons
- CP violation Energy Explore the TeV energy domain Experiments must also be prepared for “the unexpected”
Precision Further tests of the Standard Model
5
The Large Hadron Collider
LHCb
CMS
ALICEATLA
S
proton-proton collisions at an energy of 14 TeV
26.7 km circumference
start: Fall 2008
6
Some LHC parameters• Bunch spacing is 25 ns
• Bunch-crossing rate is 40 MHz
• Total proton-proton cross-section ~ 100 mb– Interaction rate at L = 1034 cm-2s-1 is R ~ 109 Hz
• On average ~ 25 interactions per bunch crossing – Background activity that complicates analysis of what happened
in the interaction of interest
• Centre-of-mass energy s = 14 TeV for proton-proton collisions
• c.f. 2 TeV at Tevatron collider• Foreseen start-up: s ~ 10 TeV
• Luminosity– L = 1034 cm-2s-1 (for proton-proton collisions in ATLAS and CMS)
• c.f. L = 1032 cm-2s-1 at Tevatron• Foreseen start-up: L~1031 cm-2s-1
7
µµnn
pp
ee
Proton-Proton Collisions at the LHC
p pH
µ+
µ-
µ+
µ-
Z
Zp p
e- e
q
q
q
q
1
-
g~
~
20~
q~
10
~
7 TeV Proton Energy1034cm-2s-1 Luminosity2835 Bunches per Beam1011 Protons per Bunch
7.5 m 25ns
Bunch-Crossings 4x107 Hz
Proton-Proton Collisions 109 Hz
Quark/Gluon Collisions
Heavy particle production 10+3…-7 Hz(W, Z, t, Higgs, SUSY, BH,…)
8
ATLAS triggerWe can not afford to record all the LHC collisions• The trigger is the system that selects particle interactions
that are potentially of interest for physics analysis
• The ATLAS trigger has to provide the data for:– Standard Model precision measurements and searches for new
physics– Physics processes needed for commissioning, calibration,
background studies, monitoring, etc
• The ATLAS trigger has to have a high rejection power to keep the rates within the allocated bandwidth
• The trigger has to be fast and robust– E.g. robust against pile-up, misscalibration,misalignment, etc
9
Data that trigger should provide (I)
• Typical examples of new physics that trigger selection should cover:– Origin of electro-weak symmetry breaking (mW and mZ)
• Higgs boson (Standard Model and beyond)• Alternative schemes
– SUSY• With and without R-parity conservation
– Compositeness– Leptoquarks– W’ and Z’– Extra dimensions
• KK excitations, black holes
– The unpredicted!– LHC has an order of magnitude more centre-of-mass energy and
two orders of magnitude more luminosity compared to today’s most powerful machine
10
Data that trigger should provide (II)
• Standard Model production processes– W, Z, direct-photon production– Jet production (including multi-jet production)
• Interesting in their own right• Must be understood as backgrounds to new physics
• B-physics studies (especially in the early phases) • Standard Model precision measurements
– W mass and top mass• Important for consistency checks with Higgs studies
11
LHC environment• The trigger needs to reduce the event rate to a
manageable level for data recording and offline analysis– L = 1034 cm-2s-1, and ~ 100 mb 109 Hz interaction rate
• Even rate of events containing leptonic W and Z decays is O(100 Hz)
– The size of the events is very large, O(1) MByte• Huge number of detector channels, high particle multiplicity per
event
– Recording and subsequently processing offline, O(100) Hz event rate with O(1) MByte event size implies major computing resources!
– Hence, only a tiny fraction of proton–proton collisions can be selected
• Maximum fraction of interactions triggering at full luminosity O(10-7)
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Multiple interactions per BC (“pileup”)
Higgs → ZZ → 2e+2Higgs → ZZ → 2e+2 23 min bias events/bunch crossing~1725 particles produced
23 min bias events/bunch crossing~1725 particles produced
13
Trigger requirements summary
• Discovery physics– Vast range of predicted
processes with diverse signatures
• Very low signal rates expected in some cases
– Sensitive to unpredicted new physics
• Huge rate of Standard Model physics backgrounds
• Balance: maximising physics coverage and reaching affordable recording rates
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ATLAS trigger• L1
– Hardware based– < 2.5s. Output rate: ~100KHz– Coarse granularity– Calorimeter and muon syst.
• L2– Software based– ~40ms. Output rate: ~1kHz– RoI seeded by L1– Full detector granularity – Fast tracking and calorimetry
• EF– “Offline-like” algorithms– ~4s . Output rate: ~200Hz– RoI seeded by L2 (full detector possible)– Full aligment/calibration info
15
Trigger signatures
Trigger objects•Electron/Photon•Tau•Jets•Muon•B-Physics•Missing ET
•b-tagging•Minimum bias
e
jet
IDET ECAL HCAL MuDET
17
Trigger menus
• Different threshold values in selection cuts can be applied.– Ex:
• e15 electron ET > 15GeV• e15i: isolated e ET > 15GeV• e60: electron ET > 60GeV
• Different objects combined:– Ex: e15i+Missing ET
– Ongoing work to define combined signatures menu
Trigger objects•Electron/Photon•Tau•Jets•Muon•B-Physics•Missing ET
•b-tagging•Minimum bias
Remember you want to be sensitive to the unpredicted: be as inclusive as you can
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ATLAS LVL1 Trigger
Calorimeter trigger Muon trigger
Central Trigger Processor (CTP)
Timing, Trigger, Control (TTC)
Cluster Processor (e/, /h)
Pre-Processor (analogue ET)
Jet / Energy-sum Processor
Muon Barrel Trigger
Muon End-cap Trigger
Muon-CTP Interface (MUCTPI)
Multiplicities of for 6 pT thresholdsMultiplicities of e//h, jet
for 8 pT thresholds each; flags for ET, ET
jet, ETmiss over
thresholds; multiplicity of fwd jets
LVL1 Accept, clock, trigger-type to Front End systems, RODs, etc – RoI pointers
~7000 calorimeter trigger towers O(1M) RPC/TGC channels
ET values (0.20.2)EM & HAD
ET values (0.10.1)EM & HAD
pT, information onup to 2 candidates/sector(208 sectors in total)
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L1 e trigger
• ATLAS e/ trigger is based on 44 “overlapping, sliding windows” of trigger towers– Each trigger tower 0.10.1
in
• pseudo-rapidity, azimuth
– ~3500 such towers in each of the EM and hadronic calorimeters
• 8 sets of thresholds(+ 8 shared with )
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ATLAS LVL1 μ trigger
• ATLAS LVL1 μ trigger is based on coincidences among hits within “window” in layers of RPCs (TGCs).
Window size determines pT threshold.– Low pT trigger 3 thresholds
– High pT trigger 3 thresholds
21
High Level Trigger (HLT)
• Software based:– L2: Specific fast trigger algorithms– EF: Uses offline algorithms as much as
possible
• Key ATLAS trigger design concepts:– Region of Interest (RoI) mechanism– Early rejection
22
Region of Interest (RoI)
• L1 indicates the geographical location of candidate objects (,)
• L2 only access data from a detector subregion around (,): “Region of Interest” (RoI)
• Reduces L2 network bandwith• Reduces L2/EF processing
time
23
HLT general scheme• “Trigger chain”: each trigger signature (ex: e25i)
corresponds to a chain of algorithms that are executed sequentially– 2 types of algorithms:
• Feature extraction algorithms (FEX): create EDM objects (clusters, tracks, etc)
• Hypothesis algorithms: apply selection cuts
• “Trigger steering”: master algorithm that controls the HLT run– According to the loaded configuration (menu) establishes chains to be
executed– Calls sequentially the algorithms that have to be run– After each “Hypo” step verifies if trigger element is active
– Early rejection: As soon as a trigger requirement is not satisfied the execution of the corresponding chain is stopped
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Electronslice
T2CaloL2CaloHypoL2Tracking
L2CaloIDHypo
TrigCaloRecEF ID
TrigEgammaRec
EFTrackHypo
EFEgammaHypo
L2 Calo+Trk Match
T2Calo
L2CaloHypo
TrigCaloRec
TrigEgammaRec
EFEgammaHypo
L2Photon FEX
L1L1 PhotonsliceTrigEMCluster
TrigPhoton
egamma
TrigElectron
CaloClusterCaloCell
TrigInDetTrack
TrackParticle
•FEX algorithms: create EDM objects•Hypothesis alg.: apply selection cuts
L2: specific trigger algorithms EF: use of offline tools as possible
Example of a slice: e slice
25
e L2 CaloT2CaloEgamma:
• Performs calorimeter cluster reconstrunction.
• Full detector granularity• Builds shower shape
variables to discriminate electron/photon of jets
L2 Calo Hypo:• Applies selection cuts on
shower shape to discriminate electron/photon of jets
Current L2 calo selection:
26
• Look for additional maximum in sampling 1 (E1-E2)/(E1+E2). (~1 for electrons)
Ex. of L2 calo selection variable
0
E1
E2
27
e L2 tracking + calo combinationCurrently two alternative tracking algorithms implemented: IDSCAN:
• zFinder: Reconstruction of the z-position of the primary pp collision• hitFilter & groupCleaner: The main pattern recognition step• trackFitter: final track fit and removal of outliers
SiTrack:• Space point sorting• Track seeds formation• Primary vertex reconstruction• Track extension
TRT standalone is being improved
Cluster-Track matching to build L2 electron object
• match• match• ET/pT (currently not active)
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e Event FilterTrigCaloRec:
• Performs calorimeter cluster reconstruction• Wraps-up offline tools • Involved also in the tau and jet slices
EFID:• Based on offline tools in a seeded mode• Involved in the tau, b-physics, b-tagging and
muon slices also TrigEgammaRec
• Reconstructs the EDM egamma object• Wraps-up offline tools • Combines Inner Detector and Calorimeter
information• Includes possibility of bremstrahlung correction
e EF selection: currently uses isem (offline identification)For identification not ID information used
ET/pT without brem recoverywith brem recovery
29
Examples of electron slice performance studies
• Study trigger efficiency dependencies on individual cuts and ET, and .
• Compare electrons from single electron and from Zee, + pile-up effects
Ex: e25i signature
• Scan selection cuts thresholds• For a given rate maximum trigger efficiency
Example of L2 selection optimization
30
Developing methods to determine trigger efficiency from data (Zee)
• Control sample: reconstruct Z + 1e trigger
• Determine trigger efficiency checking if second electron has been triggered
Determine differential trigger efficiency (vs , and ET)
2 artificial inefficient regions in for L1
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Cannot rely on Monte Carlo models, especially in early days Very important to develop tools to measure trigger efficiency from data with as little reliance as possible on MC
31
Selection slices• Electron/Photon• Jets• Tau• Muon• B-Physics• Missing ET
• b-tagging• Minimum bias• Cosmics
Require detailed detector understanding, may not be available on day-1
Comparatively very low pT threshold, take advantage of low-luminosity era
Already started
Crucial for large part of physics programme and must be operational from the start
Too long to cover it in detailJust give the flavor with a few examples
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Tau slice
95% efficiency
lowest rate
Ex. L2 optimization
EF turn on curves
combined run Maymean 6.2 ms
tau 15itau 20itau 25itau 35i
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Jet SliceRetrieve cells in the RoI Fast cone algorithm (R=0.4)Calibrate the jet
ET cut
“Offline” jet cone (or fastKt) reconstruction - Uses offline tools and calibration
Calorimeter towers (or topo cluster)
Data unpacking
ET cut
34
ROIROI
Level-2 jet trigger• Level-1 RoI is passed to Level-2Level-1 RoI is passed to Level-2
• LVL2: LVL2: iterative (3 iter.) cone iterative (3 iter.) cone
algorithm calculates energy-algorithm calculates energy-weighted position (weighted position (,,..
Apply simple, robust, fast Apply simple, robust, fast calibration procedurecalibration procedure..
Half Width
L2 jetL2 jet
Niterationgrid element
ConeRadius
Main difficulty: jet energy scale calibration
•JES within 2% correct•Resolution fit to
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Muon slice
L2:• muFast: muon spectrometer stand alone reconstruction (η, φ and pT)
• Track reconstruction efficiency: ~99.5% barrel, ~100% endcap• muComb: refines muon tracks combining them with the Inner Detector
track.• muIso: Calorimeter isolation algorithm to reject muons from beauty and
charm semileptonic decays.EF:• Wraps offline reconstruction
L2 EF
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
36
Examples of muon slice performance studiesExtensive studies of efficiency and resolution for different thresholds, regions, micalibration and misalingment
6GeV
40GeV
8GeV
20GeV
Barrel EF MuId combined
Barrel
muComb
EC L2 resolutionComb recoversthe resolutionIn all regions
37
B-Physics slice
Grid samplephi massall eta values
Mean=1019.5Sigma = 4.92
First studies with pileup No pileup Pileup
L1_MU06 81 +/- 2 % 91 +/- 1.5 %
+ L2_DsPhiPi_RoI 68 +/- 2 % 79 +/- 2 %
+ L2_DsPhiPi_FullScan 74 +/- 2 % 79 +/- 2 %
J. Kirk
Triggers currently being studies & developed for:
– B->J/ XJeeXX
– B-> B->K*B->– B->K*B->– Bs->Ds, Bs->Dsa
– B->J/X, B->K*/ etc
• B-Physics strategies to keep rate within budget:• Low luminosity: lowest
possible muon pT threshold and full InnerDetector reconstruction
• High luminosity: higher pT threshold (mu6) and RoI based reconstruction
Example: Bs→Ds(Φπ)π L2 trigger
38
Missing ET
– RoI concept does not apply to global quantity– Data preparation is a major concern when accessing entire calorimeter
• L2– L1 Missing ET + all L2 Muons
• EF– default Algorithm = loop over all cells at EM-scale– alternative algorithm = loop over Ex/Ey sums in FEB header– + muons– simple hadronic calibration
Ex. resolution studies:
39
b-tagging
L1: use jet thresholds
HLT:• 3j/2b or 4j/3b
– b-tagging 70%eff.• (Offline b-tag 60%)
• L2 tracking• EF tracking• Hypo: likelihood based on impact parameter
• Under study:– Use cluster to get jet direction– Use more offline “tools”, ex.:
secondary vertex
Significance of longitudinal impact parameter
Rejection for u-jets vs b-tag trigger efficiency
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Summary
• ATLAS trigger challenges have been summarized:– Need high efficiency for selecting processes for physics
analysis• Selection should not have biases that affect physics results
– Need large reduction of rate (~O(1/107)) from unwanted high-rate processes
– System must be affordable and robust• e.g algorithms executed at high rate must be fast
• An overview of Atlas trigger system has been given– L1 & HLT have been outlined
• As an example e trigger has been explained wirh more detail
42
LHC uses existing CERN complex
• LHC is being built in the existing tunnel previously used for LEP
– Circumference = 27 km• Radius = 4.3 km
• Use existing accelerators as injection system
• Four major “experiments”– ATLAS and CMS are “general-
purpose” detectors optimised for exploring new physics in pp collisions
– LHCb is a specialized detector optimised for B-physics studies
– ALICE is a specialized detector optimised for heavy-ion physics
4343Chris Bee - CPPM – April 2007Chris Bee - CPPM – April 2007
LVL1
ATLAS Trigger/DAQATLAS Trigger/DAQATLAS Trigger/DAQATLAS Trigger/DAQ
40 MHz
75 kHz
~3 kHz
~ 200 Hz
120 GB/s
~ 300 MB/s
~2+4 GB/s
Trigger DAQ
2.5
s
Calo MuTrChOther detectors
FE Pipelines
RoI
Lvl1 acc = 75 kHz
40 MHz
Read-Out Sub-systems
Event Building N/work
DATAFLOW
EB
ROSH
L
T
LVL2~ 10 ms
SFI
SFO
EBN
EFN
Read-Out Drivers
Dataflow Manager
Sub-Farm Input
Event Filter N/work
ROIB
L2P
L2SV
L2N
Event Filter
DFM
EFPEFP
EFPEFP
RoI Builder
L2 SupervisorL2 N/work
L2 Proc Unit
RoI data = 1-2%
RoI requests
Lvl2 acc = ~3 kHz
~ sec
120 GB/s
~4
GB
/s
EFacc = ~0.2 kHz
Read-Out Buffers
Read-Out Links
Event Builder
DE
T ROD ROD ROD
ROB ROB ROB
44
High empty event rejection while high trigger efficiency for minbias can be obtained by cutting on SCT spacepoints
Track multiplicity cut at 5 tracks gives 93% minbias trigger efficiency while beamgas events are strongly supressed
Minimum Bias Trigger
R. Kwee
• Minimum bias trigger strategies:– Minimum bias trigger
scintillators (MBTS)– Cut on number of SCT
space points– Cut on number of tracks
R. Kwee
E. Feng
MBTS
45
Accidental Rate
• Noise spectrum fits well to a Gaussian with σ=2.52pC (left plot)• Beyond 9pC (almost 4σ), non-Gaussian behavior is possible• Accidental rate must be suppressed to ~Hz, as Event Filter can
only write 100Hz
46
MBTS Counters
• “Inner” counters: 2.83 ≤ η ≤ 3.85• “Outer” counters: 2.12 ≤ η ≤ 2.83• 8 inner, 8 outer on each of A and
C sides 32 total counters
47
Data Flow for the HLTLVL2Supervisor
Database
ROS
L2PU
Event Building
SFI/SFO
EFD
LVL1Result
LVL2Decision
LVL2Selection
EFSelection
DataRequest
EventFragments
EventFragments
DataRequest
LVL1 Result
LVL2 Decision
FullEvent
FullEvent
Full Event+ EF Result
SelectedFull Event
LVL2 Result
50
Global Requirements of TDAQ Systems at LHC
Rejection power for Higgs search 1013
No overlapping events / 25 ns 23 No particles in ATLAS / 25 ns 1400 Data throughput
At detectors (40 MHz) (equivalent to) PB/s
--> LVL1 Accepts 100 GB/s --> Mass storage 100 MB/s
51
Some LVL1-trigger design goals
• Need large reduction in physics rate already at the first level (otherwise readout system becomes unaffordable)– O(109) interaction rate less than 100 kHz in ATLAS and CMS
• Require complex algorithms to reject background while keeping signal
• An important constraint is to achieve a short latency– Information from all detector channels (O(108) channels!) has to be held
in local memory on detector pending the LVL1 decision• The pipeline memories are typically implemented in ASICs (Application
Specific Integrated Circuits), and memory size contributes to the cost
– Typical values are a few s (e.g. less than 2.5 s ATLAS, 3.2 s CMS)
• Require flexibility to react to changing conditions (e.g. wide luminosity range) and — hopefully — new physics– Algorithms must be programmable (adjustable parameters at least)
52
Inclusive Selection Signatures
Object Examples of physics coverage Nomenclature
ElectronsHiggs (SM, MSSM), new gauge bosons,
extra dimensions, SUSY, W, top e25i, 2e15i
Photons Higgs (SM, MSSM), extra dimensions, SUSY 60i, 220i
MuonsHiggs (SM, MSSM), new gauge bosons,
extra dimensions, SUSY, W, top 20i, 210
Jets SUSY, compositeness, resonances j360, 3j150, 4j100
Jet+missing ET SUSY, leptoquarks j60 + xE60
Tau+missing ET Extended Higgs models (e.g. MSSM), SUSY 30 + xE40_ __
ATLAS is a multipurpose experiment aiming at discovery and precision measurements of “expected” and “unexpected” Physics signals.
The selection of Physics signals requires the identification of objects that can be isolated from the high particle density environment.
The list must be flexible, extendable, non-biasing and general enough to account for the “unexpected”.
53
LEP requirements (1)
• Trigger had to:– Identify all events coming from e+e- annihilations with visible final
states• Including at LEP-I: Z hadrons, Z e+e-, Z +-, Z +-
• Including at LEP-II: WW, ZZ, single-boson
• Including cases where there is little visible energy
– e.g. in Standard Model: e+e- Z – e.g. in new particle searches such as e+e- (with small – mass difference), giving only low energy visible particles ( LSP)
– Retain some fraction of two-photon collision events• Used for QCD studies
– Identify Bhabha scatters • Needed for precise luminosity determination
54
LEP requirements (2)
• Could retain events with any significant activity in the detector– Even when running at Z peak, rate of Z decays was only O(1 Hz)
• Physics rate was not an issue
• Challenge was in maximising efficiency/acceptance of trigger– And also, making sure that the efficiency and acceptance could be
determined with very high precision• Absolute cross-section determination depends on knowing:
– Integrated luminosity (efficiency to trigger on Bhabha events)– Experimental efficiency and acceptance for process in question
(efficiency to trigger on physics process)» Events selected by many redundant triggers
(high efficiency; cross-checks)• A major achievement at LEP was to reach per-mil precision
• The trigger rates and also the DAQ data rates were modest
56
LVL1 signatures at hadron colliders• LVL1 triggers therefore search for
– High-pT muons• Identified beyond calorimeters; need pT cut to control rate from ,
K+ , as well as semi-leptonic beauty and charm decays
– High-pT photons• Identified as narrow EM calorimeter clusters; need cut on ET; cuts on isolation
and hadronic-energy veto reduce strongly rates from high-pT jets
– High-pT electrons• Same as photon (some experiments require matching track already at LVL1)
– High-pT taus (decaying to hadrons)• Identified as narrow cluster in EM+hadronic calorimeters
– High-pT jets• Identified as cluster in EM+hadronic calorimeter — need to cut at very high pT
to control rate (jets are dominant high-pT process)
– Large missing ET or total scalar ET
57
LVL1 selection criteria• Features that distinguish new physics from the bulk of the cross-
section for Standard Model processes at hadron colliders are– In general, the presence of high-pT particles (or jets)
• e.g. these may be the products of the decays of new heavy particles– In contrast, most of the particles produced in minimum-bias interactions
are soft (pT ~ 1 GeV or less)
– More specifically, the presence of high-pT charged leptons (e, ), photons and/or neutrinos
• e.g. the products (directly or indirectly) of new heavy particles– These give a clean signature c.f. low-pT hadrons in minimum-bias case,
especially if they are “isolated” (i.e. not inside jets)
– The presence of known heavy particles• e.g. W and Z bosons may be produced in Higgs particle decays
– Leptonic W and Z decays give a very clean signature» Also interesting for physics analysis and detector studies
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