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 event data is stored 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

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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ARCHITECTURE

40 MHz

Trigger DAQ

~1 PB/s(equivalent)

~ 200 Hz ~ 300 MB/sPhysics

Three logical levels

LVL1 - 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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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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Selected (inclusive) signatures

Process Level-1 Level-2

H0 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 GeV

ETmiss

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Central TriggerProcessor

Region-of-Interest Unit(Level-1/Level-2)

Level-2 TriggerFront-end Systems

Calor im eter Tr iggerP r ocessor

MuonTr igger

P r ocessor

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 Builder

EB

~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 Filter

EFPEFP

EFP

~ 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>30GeV

STEP 3

trackfinding

trackfinding

STEP 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 – can 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, U and 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 higher rate triggers (e.g. only accept every 10th

trigger of a particular type)• Can be used to include a low rate of calibration events

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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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Physics Letters B cover

ATLAS and CMS “Higgs discovery” papers published side by side inPhys. Lett. B716 (2012)

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2e2μ candidate with m2e2μ= 123.9 GeV

pT (e,e,μ,μ)= 18.7, 76, 19.6, 7.9 GeV, m (e+e-)= 87.9 GeV, m(μ+μ-) =19.6 GeV12 reconstructed vertices

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Evolution of the excess with time

Significance increase from 4th July to now from including 2012 data for H WW* search

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Evolution of the excess with time

Significance increase from 4th July to now from including 2012 data for H WW* search

Exotic Physics Search Summary

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SUSY Searches

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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

48SDX1|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

EM

e electron

g photon

MU mu muon

HA tau tau

FJfj

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 runningh coverage 2.1 to 3.8

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Hadronic Tau Trigger• W/Z ➝ t, 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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Jet Trigger in 2012

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L2 Single Jets – cone algo. in L1 RoI. L2 Multi-jets: • L2FS – fullscan anti-kT jets from L1Calo trigger tower info• L2PS – anti-kT jets in L1 & HLT RoI using cell-level info.

EF (single & multi-jets)- Fullscan anti-kT jets from topological clusters of cells

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Missing ET Trigger

• SUSY, Higgs• Level 1: ET

miss and ET calculated from all calorimeter towers

• Level 2: Initially only muon corrections possible. Later fetch energy sums from each part of calo ROS

• Event Filter: re-calculate from calorimeter cells and reconstructed muons

Level 15 GeV threshold

Level 120 GeV threshold

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Trigger Menus

• 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 each year to 2012

• 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!