High Level Triggering
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Transcript of High Level Triggering
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High Level Triggering
Fred Wickens
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High Level Triggering (HLT)
• Introduction to triggering and HLT systems– What is Triggering– What is High Level Triggering – Why do we need it
• Case study of ATLAS HLT (+ some comparisons with other experiments)
• Summary
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Why do we Trigger and why multi-level• Over the years experiments have focussed on rarer processes
– Need large statistics of these rare events– DAQ system (and off-line analysis capability) under increasing
strain• limiting useful event statistics
• Aim of the trigger is to record just the events of interest– i.e. Trigger selects the events we wish to study
• Originally - only read-out the detector if Trigger satisfied– Larger detectors and slow serial read-out => large dead-time – Also increasingly difficult to select the interesting events
• Introduced: Multi-level triggers and parallel read-out– At each level apply increasingly complex algorithms to obtain better
event selection/background rejection• These have:
– Led to major reduction in Dead-time – which was the major issue– Managed growth in data rates – this remains the major issue
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Summary of ATLAS Data Flow Rates
• From detectors > 1014 Bytes/sec
• After Level-1 accept ~ 1011 Bytes/sec
• Into event builder ~ 109 Bytes/sec
• Onto permanent storage ~ 108 Bytes/sec
~ 1015 Bytes/year
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The evolution of DAQ systems
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TDAQ Comparisons
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Level 1• Time: few microseconds• Hardware based
– Using fast detectors + fast algorithms – Reduced granularity and precision
• calorimeter energy sums• tracking by masks
• During Level-1 decision time store event data in front-end electronics – at LHC use pipeline - as collision rate shorter than
Level-1 decision time• For details of Level-1 see Dave Newbold talk
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High Level Trigger - Levels 2 + 3• Level-2 : Few milliseconds (10-100)
– Partial events received via high-speed network– Specialised algorithms
• 3-D, fine grain calorimetry• tracking, matching• Topology
• Level-3 : Up to a few seconds– Full or partial event reconstruction
• after event building (collection of all data from all detectors)• Level-2 + Level-3
– Processor farm with Linux server PC’s– Each event allocated to a single processor, large farm of
processors to handle rate
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Summary of Introduction• For many physics analyses, aim is to obtain as high
statistics as possible for a given process– We cannot afford to handle or store all of the data a detector
can produce!• The Trigger
– selects the most interesting events from the myriad of events seen
• I.e. Obtain better use of limited output band-width• Throw away less interesting events• Keep all of the good events(or as many as possible)
– must get it right• any good events thrown away are lost for ever!
• High level Trigger allows:– More complex selection algorithms– Use of all detectors and full granularity full precision data
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Case study of the ATLAS HLT system
Concentrate on issues relevant forATLAS (CMS very similar issues), but
try to address some more general points
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Starting points for any Trigger system
• physics programme for the experiment– what are you trying to measure
• accelerator parameters– what rates and structures
• detector and trigger performance– what data is available– what trigger resources do we have to use it
• Particularly network b/w + cpu performance
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7 TeV Interesting events are buried in a seaof soft interactions
Higgs production
High energy QCD jet production
Physics at the LHC
B physics
top physics
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The LHC and ATLAS/CMS• LHC has
– Design luminosity 1034 cm-2s-1 • 2010: 1027 – 2x1032 ; 2011: up to 3.6x1033 ; 2012: up to 6x1033
– Design bunch separation 25 ns (bunch length ~1 ns)• Currently running with 50 ns
• This results in– ~ 23 interactions / bunch crossing (Already exceeded!)
• ~ 80 charged particles (mainly soft pions) / interaction • ~2000 charged particles / bunch crossing
• Total interaction rate 109 sec-1
– b-physics fraction ~ 10-3 106 sec-1
– t-physics fraction ~ 10-8 10 sec-1
– Higgs fraction ~ 10-11 10-2 sec-1
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Physics programme• Higgs signal extraction important - but very difficult • There is lots of other interesting physics
– B physics and CP violation– quarks, gluons and QCD– top quarks– SUSY– ‘new’ physics
• Programme evolving with: luminosity and HLT capacity– i.e. Balance between
• high PT programme (Higgs etc.)• b-physics programme (CP measurements)• searches for new physics
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Trigger strategy at LHC• To avoid being overwhelmed use signatures with
small backgrounds– Leptons– High mass resonances– Heavy quarks
• The trigger selection looks for events with: – Isolated leptons and photons, – -, central- and forward-jets – Events with high ET
– Events with missing ET
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Objects Physics signatures
Electron 1e>25, 2e>15 GeV Higgs (SM, MSSM), new gauge bosons, extra dimensions, SUSY, W, top
Photon 1γ>60, 2γ>20 GeV Higgs (SM, MSSM), extra dimensions, SUSY
Muon 1μ>20, 2μ>10 GeV Higgs (SM, MSSM), new gauge bosons, extra dimensions, SUSY, W, top
Jet 1j>360, 3j>150, 4j>100 GeV SUSY, compositeness, resonances
Jet >60 + ETmiss >60 GeV SUSY, exotics
Tau >30 + ETmiss >40 GeV Extended Higgs models, SUSY
Example Physics signatures
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ARCHITECTURE
40 MHzTrigger DAQ
~1 PB/s(equivalent)
~ 200 Hz ~ 300 MB/sPhysics
Three logical levelsLVL1 - Fastest:Only Calo and
MuHardwired
LVL2 - Local:LVL1 refinement
+track
associationLVL3 - Full
event:“Offline” analysis
~2.5 ms
~40 ms
~4 sec.
Hierarchical data-flow
On-detector electronics:
Pipelines
Event fragments buffered in
parallel
Full event in processor farm
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Selected (inclusive) signatures
Process Level-1 Level-2H0 2 em, ET>20 GeV 2 , ET>20 GeV
H0 Z Z* + – + – 2 em, ET>20 GeV2 µ, pT>6 GeV1 em, ET>30 GeV1 µ, pT>20 GeV
2 e, ET>20 GeV2 µ, ET>6 GeV, I1 e, ET>30 GeV1 µ, ET>20 GeV, I
Z+–+X 2 em, ET>20 GeV2 µ, pT>6 GeV1 em, ET>30 GeV1 µ, pT>20 GeV
2 e, ET>20 GeV2 µ, ET>6 GeV, I1 e, ET>30 GeV1 µ, ET>20 GeV, I
t t leptons+jets 1 em, ET>30 GeV1 µ, pT>20 GeV
1 e, ET>30 GeV1 µ, ET>20 GeV, I
W', Z' jets 1 jet, ET>150 GeV 1 jet, ET>300 GeVSUSY jets 1 jet, ET>150 GeV
ETmiss
3 jet, ET>150 GeVET
miss
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Central TriggerProcessor
Region-of-Interest Unit(Level-1/Level-2)
Level-2 TriggerFront-end Systems
Calorimeter TriggerProcessor
MuonTrigger
Processor
µ
Subtriggerinformation
Timing, trigger andcontrol distribution
JetET e /
Calorimeters Muon Detectors
Trigger design – Level-1• Level-1
– sets the context for the HLT– reduces triggers to ~75 kHz
• Limited detector data– Calo + Muon only– Reduced granularity
• Trigger on inclusive signatures
• muons; • em/tau/jet calo clusters;
missing and sum ET
• Hardware trigger– Programmable thresholds– CTP selection based on
multiplicities and thresholds
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Level-1 Selection• The Level-1 trigger
– an “or” of a large number of inclusive signals – set to match the current physics priorities and beam
conditions• Precision of cuts at Level-1 is generally limited• Adjust the overall Level-1 accept rate (and the
relative frequency of different triggers) by– Adjusting thresholds – Pre-scaling (e.g. only accept every 10th trigger of a
particular type) higher rate triggers• Can be used to include a low rate of calibration events
• Menu can be changed at the start of run – Pre-scale factors may change during the course of a run
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Trigger design - HLT strategy
• Level 2– confirm Level 1, some inclusive, some semi-
inclusive,some simple topology triggers, vertex reconstruction(e.g. two particle mass cuts to select Zs)
• Level 3– confirm Level 2, more refined topology selection,
near off-line code
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Trigger design - Level-2• Level-2 reduce triggers to ~4 kHz (was ~2 kHz)
– Note CMS does not have a physically separate Level-2 trigger, but the HLT processors include a first stage of Level-2 algorithms
• Level-2 trigger has a short time budget – ATLAS ~40 milli-sec average
• Note for Level-1 the time budget is a hard limit for every event, for the High Level Trigger it is the average that matters, so OK for a small fraction of events to take times much longer than this average
• Full detector data is available, but to minimise resources needed:– Limit the data accessed– Only unpack detector data when it is needed– Use information from Level-1 to guide the process– Analysis proceeds in steps - can reject event after each step– Use custom algorithms
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Regions of Interest
• The Level-1 selection is dominated by local signatures (I.e. within Region of Interest - RoI)– Based on coarse granularity
data from calo and mu only• Typically, there are
1-2 RoI/event• ATLAS uses RoI’s to reduce
network b/w and processing power required
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Trigger design - Level-2 - cont’d• Processing scheme
– extract features from sub-detectors in each RoI – combine features from one RoI into object – combine objects to test event topology
• Precision of Level-2 cuts– Limited (although better than at Level-1)– Emphasis is on very fast algorithms with
reasonable accuracy• Do not include many corrections which may be applied
off-line– Calibrations and alignment available for trigger not
as precise as ones available for off-line
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ARCHITECTURE
H
L
T
40 MHz
75 kHz
~4 kHz
~ 400 Hz
40 MHz
RoI data = 1-2%~2 GB/s
FE Pipelines2.5 ms
LVL1 accept
Read-Out DriversROD ROD ROD
LVL1 2.5 ms
CalorimeterTrigger
MuonTrigger
Event BuilderEB
~6 GB/s
ROS Read-Out Sub-systems
Read-Out BuffersROB ROB ROB
120 GB/s Read-Out Links
Calo MuTrCh Other detectors ~ 1 PB/s
Event FilterEFP
EFPEFP
~ 1 sec
EFN
~6 GB/s
~ 600 MB/s
~ 600 MB/s
Trigger DAQ
LVL2 ~ 10 ms
L2P
L2SV
L2NL2PL2P
ROIB
LVL2 accept
RoI requests
RoI’s
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CMS Event Building• CMS perform Event Building after Level-1• Simplifies the architecture, but places much
higher demand on technology:– Network traffic
~100 GB/s– 1st stage use
Myrinet – 2nd stage has
8 GbE slices
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t i m
e
e30i e30i +Signature
ecand ecand+Signature
e e +Signature
e30 e30+Signature
EM20i EM20i+Level1 seed
Cluster shape
Cluster shape STEP 1
Iso–lation
Iso–lationSTEP 4
pt>30GeV
pt>30GeVSTEP
3
trackfinding
trackfindingSTEP 2
HLT Strategy: Validate step-by-step Check intermediate signatures Reject as early as possible
Sequential/modular approach facilitates early rejection
LVL1 triggers on two isolated e/m clusters with pT>20GeV(possible signature: Z–>ee)
Example for Two electron trigger
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Trigger design - Event Filter / Level-3
• Event Filter reduce triggers to ~400 Hz – (was ~200 Hz)
• Event Filter budget ~ 4 sec average• Full event detector data is available, but to
minimise resources needed:– Only unpack detector data when it is needed– Use information from Level-2 to guide the process– Analysis proceeds in steps with possibility to reject
event after each step– Use optimised off-line algorithms
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Execution of a Trigger Chain
match?
L2 calorim.
L2 tracking
cluster?
track?
Level 2 seeded by Level 1• Fast reconstruction
algorithms • Reconstruction within RoI
Electromagneticclusters
EM ROI
Level1:Region of Interest is found and position in EM calorimeter is passed to Level 2
E.F.calorim.
E.F.tracking
track?
e/ OK?
e/ reconst.
Ev.Filter seeded by Level 2• Offline reconstruction
algorithms • Refined alignment and
calibration
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e/γ Trigger
• pT≈3-20 GeV: b/c/tau decays, SUSY
• pT≈20-100 GeV: W/Z/top/Higgs• pT>100 GeV: exotics
• Level 1: local ET maximum in ΔηxΔφ = 0.2x0.2 with possible isolation cut
• Level 2: fast tracking and calorimeter clustering – use shower shape variables plus track-cluster matching
• Event Filter: high precision offline algorithms wrapped for online running
L1 EM triggerpT > 5GeV
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• Discriminate against hadronic showers based on shower shape variables
• Use fine granularity of LAr calorimeter
• Resolution improved in Event Filter with respect to Level 2
R E37cells
E77cells
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80% acceptance due to support structures etc.
Muon Trigger• Low PT: J/Y, Uand B-physics• High PT: H/Z/W/τ μ, SUSY, exotics➝
• Level 1: look for coincidence hits in muon trigger chambers – Resistive Plate Chambers (barrel) and
Thin Gap Chambers (endcap)– pT resolved from coincidence hits in look-up
table
• Level 2: refine Level 1 candidate with precision hits from Muon Drift Tubes (MDT) and combine with inner detector track
• Event Filter: use offline algorithms and precision; complementary algorithm does inside-out tracking and muon reconstruction
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The Trigger Menu• Collection of trigger signatures• In LHC GPD’s menus there can be 100’s of algorithm
chains – defining which objects, thresholds and algorithms, etc should be used
• Selections set to match the current physics priorities and beam conditions within the bandwidth and rates allowed by the TDAQ system
• Includes calibration & monitoring chains• Principal mechanisms to adjust the accept rate (and
the relative frequency of different triggers)– Adjusting thresholds – Pre-scaling (e.g. only accept every 10th trigger of a
particular type) higher rate triggers• Can be used to include a low rate of calibration events
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L1 trigger items and estimated rates at 10^31 cm−2 s−1 for jets
Jet ET spectrum at 10^31 cm−2 s−1 before (dashed) and after (solid) pre-scaling at L1
Example use of thresholds/prescales at Level-1
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Trigger Menu cont’d• Basic Menu is defined at the start of a run
– Pre-scale factors can be changed during the course of a run• Adjust triggers to match current luminosity• Turn triggers on/off
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Trigger Evolution in ATLAS
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Matching problem• Ideally
– off-line algorithms select all the physics channel and no background
– trigger algorithms select all the physics accepted by the off-line selection (and no background)
• In practice, neither of these happen– Need to optimise the combined
selection• For this reason many trigger studies quote trigger efficiency wrt
events which pass off-line selection– BUT remember off-line can change algorithm, re-process and
recalibrate at a later stage• So, make sure on-line algorithm selection is well known, controlled
and monitored
Background
Physics channel
Off-line
On-line
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Other issues for the Trigger• Optimisation of cuts
– Balance background rejection vs efficiency• Efficiency and Monitoring
– In general need high trigger efficiency– Also for many analyses need a well known efficiency
• Monitor efficiency by various means– Overlapping triggers– Pre-scaled samples of triggers in tagging mode (pass-through)
• Final detector calibration and alignment constants not available for the trigger– keep as up-to-date as possible– allow for the lower precision in the trigger cuts
• Code used in trigger needs to be fast + very robust– low memory leaks, low crash rate
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Summary
• High-level triggers allow complex selection procedures to be applied as the data is taken– Thus allow large samples of rare events to be recorded
• The trigger stages - in the ATLAS example– Level 1 uses inclusive signatures (mu’s; em/tau/jet; missing and
sum ET)– Level 2 refines Level 1 selection, adds simple topology triggers,
vertex reconstruction, etc– Level 3 refines Level 2 adds more refined topology selection
• Trigger menus need to be defined, taking into account:– Physics priorities, beam conditions, HLT resources
• Include items for monitoring trigger efficiency and calibration• Try to match trigger cuts to off-line selection• Trigger efficiency should be as high as possible and well
monitored • Must get it right - events thrown away are lost for ever!• Triggering closely linked to physics analyses – so enjoy!
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ATLAS 2e2μ candidate with m2e2μ= 124.3 GeVpT (e+, e-, μ-, μ+)= 41.5, 26.5, 24.7, 18.3 GeVm (e+e-)= 76.8 GeV, m(μ+μ-) = 45.7 GeV
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4242
Excluded at 95% CL
Putting all channels together combined constraintsHγγ, H ττ H WW(*) lνlνH ZZ(*) 4l, H ZZ llννH ZZ llqq, H WWlνqqW/ZH lbb+X not included
Excluded at 99% CL
Expected if no signal
112.7 < mH < 115.5 GeV 131 <mH < 453 GeV, except 237-251 GeV
124.6-520 GeV
133 <mH < 230 GeV, 260 < mH < 437 GeV
LEP ATLAS+CMSCombination
ATLAStoday
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Additional Foils
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ATLAS HLT HardwareEach rack of HLT (XPU) processors contains- ~30 HLT PC’s (PC’s very similar to Tier-0/1 compute nodes)- 2 Gigabit Ethernet Switches- a dedicated Local File ServerFinal system will contain ~2300 PC’s
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47SDX1|2nd floor|Rows 3 & 2
CFS nodes
UPS for CFS
LFS nodes
XPUs
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Price to pay for the high luminosity: larger-than-expected pile-up
Z μμ
Period A: up to end August
Period B:Sept-Oct
Pile-up = number of interactions per crossing Tails up to ~20 comparable to design luminosity (50 ns operation; several machine parameters pushed beyond design)
LHC figures used over the last 20 years:~ 2 (20) events/crossing at L=1033 (1034)
Challenging for trigger, computing resources, reconstruction of physics objects (in particular ET
miss, soft jets, ..) Precise modeling of both in-time and out-of-time pile-up in simulation is essential
Event with 20 reconstructed vertices(ellipses have 20 σ size for visibility reasons)
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Naming ConventionFirst Level Trigger (LVL1) Signatures in
capitals e.g. LVL1 HLT type
EMe electron
g photon
MU mu muon
HA tau tau
FJ fj forward jet
JE je jet energy
JT jt jet
TM xe missing energy
HLT in lower case:
name
threshold
isolated
mu 20 i _ passEF
EF in tagging mode
name
threshold
isolated
MU 20 I
New in 13.0.30: • Threshold is cut value applied• previously was ~95% effic. point.
• More details : see :https://twiki.cern.ch/twiki/bin/view/Atlas/TriggerPhysicsMenu
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What is a minimum bias event ?
- event accepted with the only requirement being activity in the detector with minimal pT threshold [100 MeV] (zero bias events have no requirements) - e.g. Scintillators at L1 + (> 40 SCT S.P. or > 900 Pixel clusters) at L2
- a miminum bias event is most likely to be either: - a low pT (soft) non-diffractive event - a soft single-diffractive event - a soft double diffractive event(some people do not include the diffractive events in the definition !)
- it is characterised by: - having no high pT objects : jets; leptons; photons - being isotropic - see low pT tracks at all phi in a tracking detector - see uniform energy deposits in calorimeter as function of rapidity - these events occur in 99.999% of collisions. So if any given crossing has two interactions and one of them has been triggered due to a high pT component then the likelihood is that the accompanying event will be a dull minimum bias event.
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Phys.Lett.B 688, Issue 1, 2010
LHC collision rate (nb=4)
LHC collision rate (nb=2)
• Soft QCD studies• Provide control trigger on p-p collisions;
discriminate against beam-related backgrounds (using signal time)
• Minimum Bias Scintillators (MBTS) installed in each end-cap;
• Example: MBTS_1 – at least 1 hit in MBTS
• Also check nr. of hits in Inner Detector in Level-2
Minimum Bias Trigger
Minbias Trigger Scintillator: 32 sectors on LAr cryostatMain trigger for initial running coverage 2.1 to 3.8
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Hadronic Tau Trigger• W/Z ➝ , SM &MSSM Higgs, SUSY, Exotics
• Level 1: start from hadronic cluster – local maximum in ΔηxΔφ = 0.2x0.2 – possible to apply isolation
• Level 2: track and calorimeter information are combined – narrow cluster with few matching tracks
• Event Filter: 3D cluster reconstruction suppresses noise; offline ID algorithms and calibration used
• Typical background rejection factor of ≈5-10 from Level 2+Event Filter – Right: fake rate for loose tau trigger with pT > 12
GeV – aka tau12_loose– MC is Pythia with no LHC-specific tuning
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Jet Trigger• QCD multijet production, top, SUSY,
generic BSM searches
• Level 1: look for local maximum in ET in calorimeter towers of ΔηxΔφ = 0.4x0.4 to 0.8x0.8
• Level 2: simplified cone clustering algorithm (3 iterations max) on calorimeter cells
• Event Filter: anti-kT algorithm on calorimeter cells; currently running in transparent mode (no rejection)
Note in preparation
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Missing ET Trigger• SUSY, Higgs• Level 1: ET
miss and ET calculated from all calorimeter towers
• Level 2: only muon corrections possible (at present)
• Event Filter: re-calculate from calorimeter cells and reconstructed muons
Level 15 GeV threshold
Level 120 GeV threshold
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Example Level-1 Menu for 2x10^33
Level-1 signature Output Rate (Hz)
EM25i 12000
2EM15i 4000
MU20 800
2MU6 200
J200 200
3J90 200
4J65 200
J60 + XE60 400
TAU25i + XE30 2000
MU10 + EM15i 100
Others (pre-scaled, exclusive, monitor, calibration) 5000
Total ~25000
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End of pp trigger operations in 2010
Trigger group Trigger chain Rate[Hz]
Single-muon EF_mu13_tight 24Di-muon EF_2mu6 28
Single-electron EF_e15_medium 38
Di-electron EF_2e10_loose 2.4Single-photon EF_g40_loose 9
Di-photon EF_2g15_loose 2.1Single jet EF_L1J95_NoAlg 11
MET EF_xe40_noMu 6Single-tau EF_tau84_loose 6.8
Di-tau EF_2tau29_loose1 2.6Trigger Report
Run 167607 - record peak luminosity 2.1x1032cm-2s-1
For a given threshold tighten selection
Loose->medium->tightNon-isolation->isolation
Go higher in pT
Trigger evolution in 2010
L1 output 35kHz, L2 output 5kHz, EF output 400Hz
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3
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Other issues for the Trigger – cont’d• For details of the current ideas on ATLAS Menu evolution see
– https://twiki.cern.ch/twiki/bin/view/Atlas/TriggerPhysicsMenu• Gives details of menu since Startup and for 2011
• Corresponding information for CMS is at– https://twiki.cern.ch/twiki/bin/view/CMS/TriggerMenuDevelopment
• The expected performance of ATLAS for different physics channels (including the effect of the trigger) is documented in http://arxiv.org/abs/0901.0512 (beware - nearly 2000 pages)
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ATLAS works!
Top-pair candidate - e-mu + 2b-tag
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CMS works!