2008 IEEE Nuclear Science Symposium Conference Record

Status of the ATLAS Trigger System for LHCStartup

Judita Mamuzic on behalf of the ATLAS TDAQ and Trigger community

N30-124

Abstract-The ATLAS experiment at Large Hadron Collider(LHC) has a goal to explore the mechanism of Electro Weaksymmetry breaking and search for new physics. The design of thedetector fulfills extreme requirements for high precision muonmomentum measurement, efficient tracking, very goodelectromagnetic calorimetry for photon and electronidentification. With LHC bunch crossing rate of 40 MHz andabout 23 interactions per bunch crossing, highly selective triggersystem is required to reduce the rate of 1 GHz interactions toabout 100 Hz for data storage. For better understanding oftrigger efficiencies, a number of software tools have been used,from trigger event selection to offline physics analysis. TheATLAS trigger system is based on three levels, Levell, hardwarebased, Level 2 and Event Filter, together called High LevelTrigger (HLT), software based. A number of tests have beenperformed to emulate data taking conditions and test the fullexperiment setup in combined operation and cosmic runs. Thesystem performed successfully in the first beam events. Differenttrigger menus are prepared for varying conditions, todemonstrate the readiness of ATLAS for data taking withcollisions.

I. INTRODUCTION

T HIS The Large Hadron Collider (LHC) Fig. 1, situatedin CERN, Geneva, has the aim to extend the frontier of

physics experiments by reaching extremely high energies andluminosity. Accelerated protons in both directions in LHCring will collide 40 million times per second, leading to 14TeV center of mass energy and design luminosity of 1034 cm­2S-1. The starting phase of LHC collisions will operate on 10TeV energies and luminosity of 1031 cm-2s·1

• It is foreseen thatLHC will also collide nuclei, at 5.5 TeV per nucleon pair, atdesign luminosity of 1027 cm-2s-1

•

Fig. 1. Large Hadron Collider with four experiments: ATLAS, CMS,Alice and LHCb

Manuscript received November 14,2008.1. Mamuzic is with the DESY, Zeuthen, Germany (telephone: +49 337627

7226, e-mail: judita.mamuzic@desy.de).The full list of authors is given in [1].

There have been 4 detectors designed for the LHCmachine: ATLAS (A Tororidal LHC Aparatus), CMS(Compact Muon Solenoid), Alice (A Large Ion ColliderExperiment) and LHCb (Large Collider Beauty Experiment).

ATLAS is a general purpose experiment, which willprovide a rich physics potential, ranging from more precisemeasurements of Standard Model to the search for newphysics phenomena [2]. High luminosity enables highprecision tests of QCD, electroweak interactions, and flavorphysics, while top quark will be produced with rates giving theopportunity to test its couplings and spin. ATLAS gives theopportunity for studies of Higgs boson, new heavy gaugebosons W' and Z' , supersymmetry and extra dimensions.LHC will produce a total rate of 109 inelastic events/s atdesign luminosity [2], resulting that every candidate event fornew physis will on average be followed by 23 inelastic eventsper bunch crossing. The nature of proton-proton collisionsimposes that QCD jet production cross-sections dominate overthe rare processes, Fig. 2.

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Fig. 2. Discovery potential in ATLAS

Having in mind these physics goals and the nature ofproton-proton collisions, detector needs to be designedaccordingly, Fig. 3. First of all it needs radiation hardelectronics and sensor elements, large acceptance inpseudorapidity and almost full azimuthal angle coverage, goodcharged-particle momentum resolution and reconstruction,

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very good electromagnetic calorimetry, good muonidentification and momentum resolution and finally, needs tobe highly efficient on triggering on low transverse momentumobjects with sufficient background rejection.

further to 2 kHz within around 10 ms latency. Event Filter,further receives the seed from Level 2, and using rermed andcomplete data performs the final online selection with bettercalibration and alignment, reducing the rate to around 100 Hzwith latency or around Is.

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HLT algorithms are performed in a stepwise, seeded andconsecutive way, Fig. 5, and are separated into two classes:Feature Extraction algorithms (FEX) to perform timeconsuming operations of data unpacking and calculating ofphysics quantities in objects, and hypothesis algorithms(Hypo) which perform quick selection cuts (energy, isolation)based on features.

The Data Acquisition system (DAQ), Fig. 6 [5], receivesand buffers the data from the detector read out electronics intoRead out Buffers. It transmits to Level 2 trigger the requesteddata, and further, for events fulfilling the Level 2 eventselection, performs event building. The assembled events aremoved by the DAQ to the event filter, and after the selection,built events are moved to the permanent storage. The HLTsystem currently runs on a Linux PC farm with a total of about6500 CPU cores.

Data recording

Fig. 4. Trigger system in ATLAS, Levell, Level 2 and Event Filter.

Fig. 5. Feature Extraction and Hypothesis algorithms, stepwise, seeded,consecutive way of algorithm execution in HLT.

At nominal operation, bunches containing about lOllprotons, will collide at 40 MHz, resulting in about 23 proton­proton interactions per bunch crossing, producing the outgoingrate of about IGHz. In addition, only a small fraction of theseevents will contain interesting physics processes. The Triggerand Data Acquisition (TDAQ) System of ATLAS needs toselect the manageable rate of these events for the massstorage, keeping the potentially interesting events for physicsanalysis. There will be about 1.6 MB per event recorded, atthe rate of about 200 Hz [3]. Therefore TDAQ needs to have ahigh rejection power, which is in ATLAS implementedthrough the so called Regions of Interest (RoI), and the threelevel Trigger system. This allows processing information fromthe potentially interesting parts of the detector, with highlysophisticated algorithm system for high rejection.

Fig. 3. The ATLAS detector.

II. ATLAS TRIGGER AND DATA ACQUISITION SYSTEM

Atlas has a design of three-level trigger system to keepthe bandwidth under control, it performs the mechanism ofearly rejection to minimize the decision time and hencemaximize the event rate, Fig. 4.

The first level trigger is implemented in customelectronics (mainly ASICs and FPGAs) [4], and has a task ofearly event selection. Event selection on this level is based oninclusive high PT objects(muons, electromagnetic/tau/hadronicclusters, jet clusters, missing and scalar transverse energysums) with configurable thresholds. During the decisionlatency of 2.5 ~s the data is collected in the pipelinememories. For accepted events, the geometrical locations ofthe objects, Regions of Interest (RoI) are sent to the secondlevel, and the data is moved to the Read-Out Buffers (ROB).This reduces the rate from 40 MHz to 75 kHz.

The High Level Trigger (HLT) is software based, andruns on commodity PC farms. It consists of Level 2 and EventFilter. Level 2 uses Level I RoI, geometrical position of theobject, and performs a sequence of algorithms using finegrained detector data, optimal calibration and tracking. Thealgorithms are highly optimized for speed, reducing the rate

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Fig. 8. Data Quality Framework.

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Fig. 9. Examples ofonline monitoring tools.

Highly important component of the system is triggeronline monitoring. This includes rate monitoring to ensurestabIe data taking, and also give feedback for the triggerconfiguration design. Since trigger runs on commodity PCfarms, it is of great importance to monitor it's reliability andstability. In addition, the monitoring system needs to provideinformation on physics quality, to ensure stable and goodselectivity. Trigger offline monitoring is designed to performthe same HLT configuration with full algorithms in order tooptimize HLT parameters and give feedback to onlinemonitoring and HLT configuration.

In the control room, the shifter and expert are providedwith a number of monitoring tools for the full system, Fig. 9.

Event Builder:full events@-3kHz

III. ONLINE AND OFFLINE MONITORING

Software framework and tools have been designed tomonitor trigger, detector and physics performance, in order toprovide complete coverage and consistent data qualityassessment. Therefore, the aim is to have a softwareframework more reliable than the system being monitored.Means for accessing data quality must be lightweight, highlyreliable and thorough.

Fig. 6. Data Acquisition System (DAQ).

full system is being performed, on detector components, HLT,computer infrastructure, etc. Automated analysis of monitoreddata is performed on great number of parameters, where userfriendly summary result is being produced as output. Thissystem provides the shifter with online monitored information.In addition, the system can be used offline as well, on thereconstructed data to assess the quality.

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UA() system selects events accordIng to trIgger cnterIa,and further Data Assessment is being performed, Fig. 7. [6].After the trigger selection, DAQ records the status to theCondition Data Base. Data is written into streams according totrigger composition. Calibration streams, which are selectedby subdetector groups, ensure that the calibration constants arealways update. Analysis data is processed in parallel, andfurther merged and assessed. Express stream, being theprescaled selection of many types of triggers but processedwith less accurate calibration, is calculated to provide rapidfeedback. After the run completition, express stream isprocessed once more, this time using updated calibration, toensure the calibration is correct. Physics objects streams areprocessed and accessed, the DAQ metrics used for assessmentis stored locally and on the Grid.

Data Quality Monitoring (DQM) Fig. 8. [7], is animportant part of data taking process. The monitoring of the

Fig. 7. Data Assessment process.

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through mode, and later using online HLT selection.Planned collisions in 2009 would start again with single

beam commissioning, and proceed with the aim to tune forbest detector performance, ie. include all the hardware andperform calibration on all the components. Also, aim is toachieve best Level 1 timing and large Level 1 accept rateoutput, to have well understood minimum bias trigger system,and make use of efficient online and offline monitoring.

REFERENCES

[1] "The ATLAS Trigger/DAQ Authorlist", ATL-DAQ-PUB-2008-004.[2] ATLAS Collabortion, "The ATLAS experiment at the CERN large

hadron collider", 2008 JINST 3 S08003, pp. 29-31.[3] K.Kordas on behalf or ATLAS Trigger and Data Acquisition system,

"The ATLAS Data Acquisition and Trigger: concept, design and status",unpublished.

[4] N.Berger, T.Bold, T.Eifert, G.Fisher, S.George, lHaller, A.Hocker,J.Masik, M zur Nedden, V.Perez Reale, C.Risler, C. Schiavi, lStelzer,X.Wu, "The High-Level-Trigger steering of the ATLAS experiment",2007, ATL-DAQ-CONF-2007-026, 08 October 2007

[5] ATLAS Collaboration, "Integration of the Trigger and Data AcquisitionSystems in ATLAS", unpublished.

[6] M.G.Wilson, for the ATLAS collaboration, "Assessment of data qualityin ATLAS", CHEP'07, Journal of Physics: Conference Series 119(2008)042034, doi: 10.1 088/1742-6596/119/4/042034.

Fig. 12. First beam event in ATLAS, Level I Calo stream.

On the LHC startup day, 10 September 2008 the so calledcollimator shots were used, Fig. 11. Bunches of protons in theLHC ring were accelerated first in clockwise direction, andlater in opposite. The bunches were shot on the closedcollimators, going from one LHC sector to another. Laterduring the day, circulating beam in one direction was alsoachieved by LHC. ATLAS was using RPC, Thin GapChamber detector (TGC) and Level 1 Calo detectors for Level1 trigger, which were already extensively used incommissioning runs. Additional Level 1 triggers wererequired for single beam, in the low angle range along thebeam line, using Beam Pickup signal (BPTX), Minimum BiasTrigger Scintilators (MBTS), Diamond Beam ConditioningMonitors (BCM) and LUCID (small-angle Cherenkovdetector). Startup trigger was prepared with Random (RNDM)and BPTX triggers, with lowest thresholds and open roads tobeam gas and beam halo events. Raw data streaming was usedwith dummy HLT. In the Fig. 12. the snapshot of the very firstbeam event in Level 1 Calo is shown.

high PT

Fig. 11. LHC collimator shots, moving from one point to the other andbeam in LHC ring.

o 5 10 15 m

Fig. 10. Detectors used in commissioning runs for Level 1 trigger:Re~istive Plate Chambers (RPC), Thin Gap Chambers (TGC), MuonDnft Tubes (MDT).

A number of tests have been pertormed on the detectorand TDAQ system prior to LHC start. Combined runs, inwhich the combination of detectors is to test variouscomponents, have been performed frequently during the lastyear. Several commissioning runs with cosmic rays have beenperformed to test the full trigger system. In these runs, forLevel 1 trigger, barrel muon and Resistive Plate Chambers(RPC) detectors have been used [9], Fig. 10. HLT was runonline, and the data was taken with full slice of muon triggerreadout chain in one RPC system, and data was taken to theLevel 2 trigger algorithms and the DAQ system.

IV. ONLINE AND OFFLINE MONITORING

Having the same reconstruction software framework usedin online and offiine analysis, the trigger menu can be highlyoptimized for different conditions. Since the experiment willstart with low luminosity and lower energies (1031 cm-2s-1 and10 TeV). Here having low number of bunches is ideal forcommissioning of trigger and detector systems. Therefore, thefirst data taking will focus on Standard Model processes, withlow PT thresholds, very loose selection criteria and passthrough trigger mode, primarily to provide betterunderstanding of the detector, HLT and the full system. Withevolution of LHC to higher luminosities and energies (1034

cm-2s-1 and 14 TeV), previously done observations with earlydata will allow optimization of trigger algorithms and menusand estimate rates of triggers for high luminosity. Here thehigher PT thresholds and tighter selections will be used toreduce the rate to required 100 Hz [8].

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[7] A. Corso-Radu, S. Kolos, H. Hadavand, R. Kehoe, M. Hauschild, "DataQuality Monitoring Framework for the ATLAS Experiment at theLHC", ATL-DAQ-CONF-2008-006, 27 February 2008.

[8] The ATLAS collaboration, "The ATLAS Trigger for Early Running",unpublished.

[9] 1. Boyd on behalf of the ATLAS TDAQ Collaboration, "The ATLASTrigger - Commissioning with cosmic rays", CHEP'07, Journal ofPhysics: Conference Series 119(2008) 022014, doi:l0.1088/1742­6596/119/2/022014.

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