ATLAS Trigger GroupPPD Staff Talk 23/5/05 1 Level-2 Trigger Overview When Level-1 accepts an event –Each part of ATLAS sends its data to the Read-Out Buffers

ATLAS Trigger Group PPD Staff Talk 23/5/05 1

Level-2 Trigger Overview

• When Level-1 accepts an event– Each part of ATLAS sends its data to the

Read-Out Buffers (ROB’s)– Level-1 sends Level-2 details of why the

event was accepted and where the “Regions of Interest” (RoI) are

• e.g. a high energy electron candidate• The data for each event is ~1.5 MB

– About the size of a picture in a digital camera

• Each event is passed to a Level-2 processor to refine Level-1 decision

– Level-2 accesses the data it needs across a network – but only for the RoI’s

– Only a few % of the data – reduces network size needed (~2 GB/s = 3 CD’s)

– Average Level-2 decision time ~ 10 ms– Level-2 accept rate < 2 kHz (i.e. reject

~98%, but keep the important ones!)

120 GB/s

Event Builder

~3 GB/s

RoI data

1-2% ~2 GB/s

Level-1 accept

ROD ROD ROD

Level-1

Calo MuTrChOther detectors

Level-2 accept

Data requests

RoI’s 120 GB/s

Level-2

ROB ROB ROB

L2ProcessorL2Processor

L2Processor

(75 kHz)

(<2 kHz)


Level-2 Software Structure

• The DataFlow components handle all I/O and communication with run control

• RAL group previously made major contribution to the L2PU DataFlow code – but activity now nearly all in Algorithms

• The Selection Code (Algorithms) sees an offline like framework (Gaudi/Athena)

• Algorithms can be developed/tested in offline environment reading data from files

• Algorithms can use various off-line components – if they are fast enough

• The Steering Controller provides the interface between online and offline worlds

Level-1 RoI’s

Data Flow

(L2PU)

Steering Controller

Algorithms

Level-2 decision

Request Response

Data

ROBROBROBROB


Level-2 Hardware

• Level-2 Infrastructure - hardware– Number of Level-2 PCs in final system ~ 500– ~30 in a rack + network switches– Rack mounted – each 1U (1.75 inches) high – Each processor has 2 network connections

• Data input at GbE (i.e. 10x speed of the PPD LAN) – 3 GHz Dual Processor (Xeon) is ~ x3 too slow

• Will they be fast enough in 2-3 years ?– Cooling is a major problem - ~10 kW/rack

• Other Tasks of FW– TDAQ Resources– UK Level-2 Project Leader– PPD Learning and Development– Contact for articles in HI-phi


The Level-2 (HLT) Group

• Fred Wickens– Level-2 Infrastructure (esp h/w)

• TDAQ Resources

• Julie Kirk – Level-2 b-physics + Track fitting efficiency

• John Baines– Trigger Selection s/w

• Level-2 Tracking s/w and b-physics

• Dmitry Emeliyanov– Level-2 Track fitting and data preparation

• Bill Scott – Tracking efficiency and b-physics

• Monika Wielers– (Level-2) Electron/photon selection

>


Trigger Selection Software

John Baines

The High Level Trigger (Level-2 and Event Filter) uses software running on a farm of PCs to perform:

Data Preparation: Take the raw data from the detectors and prepares it for the next step.

Pattern Recognition: Identify clusters of Energy in the Calorimeter, or find patterns of measurements representing a charged particles path in the Tracking detectors (Inner Detector and Muon detector).

Form “Trigger Elements”: Identify characteristic features in the events which separate interesting events e.g. containing new physics from uninteresting background events

The Level-2 Trigger :

Reduces the trigger rate down to about 2000 events per second

Works inside regions of the detector identified as interesting by the Level-1 trigger.


Data Preparation

Part of Semi Conductor tracker (SCT) Wafer

Part of Pixel Detector

Part of SCT Module (2 wafers)

See Dmitry’s Talk

One particle travelling through a wafer can cause signals on several adjacent strips or wafers.

Group Adjacent Strips or Pixels with charge as Cluster

Find the crossing point for SCT strips with charge in the two detectors of a module to form a Space-Point (3-dimensional measurement)


RECONSTRUCTED TRACK

RECONSTRUCTED TRACK

REGION OF INTERESTREGION OF INTEREST

HITS IN SemiConductor

Tracker

HITS IN SemiConductor

Tracker

• Level-1 Trigger finds calorimeter cluster

• Region Of Interest is a region of the detector

centred on the calorimeter cluster position

• Level-2 Trigger requests data from the Inner

Detector in this Region & performs Data

Preparation

• Level-2 algorithms reconstruct track(s)

Energy in Calorimeter

Hits in Transition Radiation Tracker

Hits in Pixel Detector

Track Reconstruction

See Dmitry’s & Bill’s Talks

Decay of Higgs Particle to 4 electrons


Hypothesis Algorithms

We identify features that are characteristic of interesting events. e.g.

• One or more isolated high energy electrons could signify a Higgs decay (as shown on previous slide) See Monika’s Talk

• An electron and positron pair, satisfying certain constraints,

could signify a decay of the J/ particle - used to select events for CP violation measurements. Recall Julie’s Talk

Bd J/ Ks

ee

SCT

TRT

Pixels

>


Dmitry Emeliyanov

• Brief Biography– Graduated from Moscow Institute of Physics and Technology

(M.Sc. in Applied Physics, 1992)

– Ph.D. in Control and Data Processing Systems, 1998

– Worked on tracking and vertexing for the HERA-B experiment (was DESY Fellow, 2000-2004)

– Joined PPD ATLAS group in 2004 as a fixed-term contract RA

• My work at ATLAS PPD group:– Development and optimization of the track reconstruction and

fitting algorithms for the Level 2 Trigger

– Currently working on timing measurements and optimization of the Level2 Trigger data preparation algorithms


Level 2 Tracking in Inner Detector

• Inner Detector comprises Pixel, Semiconductor (SCT) and Transition Radiation (TRT) trackers

• The tracking is always RoI-based: done only inside the Region-Of-Interests coming from Level 1 Trigger. Includes two major stages:

• Data preparation: – raw data from the detector are decoded and converted into a form usable for

the reconstruction – hits in Pixel, SCT and TRT– As each SCT module is a pair of single-sided silicon wafers, hits from both

sides are combined into the points in 3D space (“spacepoints”) where the particle passed through the detector

• Track reconstruction: – track finding – combining spacepoints into tracks – “joining the dots” – quite

challenging task because the number of “dots” is high – 50-300– track fitting – determine the particle parameters – direction of flight,

curvature of the trajectory in the magnetic field (to determine a momentum)


Track finding and fitting

• A number of software packages have been developed in ATLAS for Level 2 tracking. One of them is IDSCAN:– developed and supported by RAL/UCL collaboration

– fast track finding in Pixel and SCT: ~ 0.5 ms per track on single Pentium-4

– track fitting by dedicated algorithm based on the Kalman filtering mathematics, fast implementation takes about 0.1 ms per typical track

• Recently the IDSCAN has been extended to do track finding in the Transition Radiation Tracker:– tracks found in Pixel/SCT are extrapolated out to the TRT

– TRT hits are associated to the tracks using the “probabilistic data association filter” (PDAF) – an algorithm widely used in radar/sonar tracking applications

– takes about 1 ms per track to search the whole TRT

– calculates the number of transition radiation hits – used to distinguish electrons from pions in the trigger


Timing optimizations

• Level 2 algorithms are implemented in Athena – general ATLAS offline software framework

• As a result, many offline software components and solutions have been adopted by Level 2

• Some of them are not completely adequate to the Level 2 strict timing requirements – optimization/re-development needed

• Example: data preparation for Pixel and SCT:– due to the offline storage of clusters and spacepoints the overall timing was

around 60 ms/RoI – too slow for Level 2

– development and implementation of the alternative caching algorithm reduced this time down to ~ 6 ms/RoI

• The timing measurements provide an important input for assessment of the Level 2 software and further optimization.

>


Julie Kirk(Daphne Jackson Fellow)

• Daphne Jackson fellowships:– “Returning engineers and scientists to work after career breaks.”

– Fellowship is for two years, half-time with dual aspect of re-training plus new reasearch.

– The lab provides office/computer (“bench fees”), Daphne Jackson Trust cover salary and some extra expenses (e.g. conferences)

• After working at CERN on OPAL I took a career break for 8 years.

• In November 2004 I started a Daphne Jackson fellowship working on ATLAS level 2 trigger


B-physics triggers

• Triggering – finding and keeping events of interest (signal) to physicist while discarding the rest (background).

• ~1 per 100 interactions produce a bb pair. Rates for channels of interest to physicists are usually LOW.

• Finite cpu resources at level 2 mean that there is not enough CPU time to reconstruct the whole event.

• Strategy: use a level 1 trigger to indicate the area of the detector (Region of Interest or RoI) in which to do level 2 reconstruction.

• Example – bb μX

Bd(J/ψ(ee)K)

Use high momentum muon from one b to trigger the event at level1.

Look for electromagnetic ROI to seed track reconstruction and find electrons


Tracks show simulated particles in the detector.Yellow tracks - electrons from J/ψ

Example J/ψ→e+e- event

x

y

8 m

White cone illustrates a level1 EM RoI.

At level2 we only look inside the ROI to reconstruct tracks.Saves lots of cpu time by only considering a relatively small portion of the detector.


Event selection

True e+/e- tracks from J/ψ events

• Now use the reconstructed tracks to identify signal events.

•Assuming that they came from the decay of a particle we can combine pairs of reconstructed tracks to form a mass (plots).

•Make cuts on the mass (shown by arrow) to select signal events and reject most of background.

Background events

3 GeV 10 GeV

Reconstructed J/ψ mass

Reconstructed J/ψ mass

10 GeV3 GeV


Bill Scott

• Measure HLT Track Reconstruction Efficiencies for Low Energy Electrons using Simulated Data

– Tracks Found / Tracks Generated

– Two Algorithms: IDSCAN, Si-Track.

• Using Events that were already Generated– Single electrons: pT = 25 GeV, pT=15 GeV,

– Single electrons: pT=10 GeV

• Using Events Generated (by me) at RAL– Generate, Simulate and Digitise on CSF at RAL:

– Single electrons: pT=5 GeV

– Single muons: pT=5 GeV……..


Efficiency for High Energy Electrons

95% Constant Efficiecy :Problem No


Efficiency for Low Energy Electrons

Do to me) (for Work :Problems Some


Unitarity Triangle/CP-Violation

/JBs /JBs

:sb for . eg

Evolution. Time controlTriangles Unitarity .ie

""t

""c

""u



/JBs /JBs

:sb for . eg




/JBs /JBs

:sb for . eg


>


Monika Wielers

• Started last October at RAL

• Worked on electron and photon selection for many years now in ATLAS– Convenor of ATLAS e/gamma

trigger group

• Long experience on calorimetry– Matches well with tracking

activities here, since you need both to identify electrons and photons


Electron and Photon Triggers

• Number of events we ‘see’ per second: ~109

• Challenge to efficiently select interesting events and reject the enormous number of ‘uninteresting’ events– Not easy to find the interesting events

– We can only save ~1 out of 10 million events for further analysis

– Sometimes the events we are interested are very rare (H), so we have to make sure to collect all of those

– electrons and photons (and muons) are ‘easy’ to select

• Important physics events containing electrons/photons cover– whole range of physics topics we want to explore at the LHC

• From the known (Standard Model) to the unknown (Higgs, Susy, …)

– Whole spectrum of possible energies

• ‘Low’ pT: 5-15 GeV (B-physics)

• ‘High’ pT: 20-100 GeV (Higgs, Susy, top…)

• ‘Very high’ pT: 100-1000 GeV (new heavy W/Z bosons, excited electrons…)


Event Display

HZZ* 2e2

Electron and photon trigger selection

find interesting regions with lots of energy in the calorimeter at LVL1 (coarse granularity)

Now look in more detail at the information from the different detectors at LVL2 (calorimeter, inner detector)

if the object looks ‘like’ an electron go on to the EF

do something very similar but more ‘sophisticated’ at the EF (you have more time at this stage)

If you still think it’s an electron/photon save the event for the physicists to look at


Trigger Strategy

• Reject ‘uninteresting’ events as fast as possible– Achieved by sequential selection: after each step we look if we still want to

identify a given object as electron or photon

• HLT algorithms optimised in terms of:– Physics performance

• Signal efficiency

• Background rejection

• In the end we have to find the best compromise among all these constraints

– Signal performance

• Execution time

• Data requirements

Signal efficiency Physics reach

Rate latency No. of computers needed

Rate data size Size of network needed £


We are slowly getting there … 2007

Documents

ATLAS Trigger GroupPPD Staff Talk 23/5/05 1 Level-2 Trigger Overview When Level-1 accepts an event –Each part of ATLAS sends its data to the Read-Out Buffers