Development of an Interaction Trigger in ATLAS

1. Development of an Interaction Trigger in ATLAS Regina Kwee CERN / Humboldt-Universität zu Berlin Gentner Day 18.November 2009, CERN

2. Outline • Motivation • Introductionto Minimum Bias Events • Designing an Interaction Trigger • Trigger Performance Studies • Summaryand Outlook Gentner Day at CERN

3. Motivation • Strategyat ATLAS • studyinelasticinteractionsatlowluminosities • L ≈ 1031 cm-2 s-1 • larger bunchpitchof 75 ns • → single pp-collisions • pp-interaction rate is <<1 • 10 % ofthebunch-crossingscontain a pp-interaction • → Minimum-Bias Trigger • selectsany pp-interaction • forpp-interaction rate ≈1 • → pure randomtrigger • Operation of LHC with pp-collisionsat (nominal) • √s = 14 TeV • L ≈ 1034 cm-2 s-1 • bunch-crossing rate of25 ns → highevent rate r = L x σ ≈ 109inel. pp-collisions/s • Pile-up • expect per bunch-crossing around 20 piled-up collisions → one of the biggest challenges at LHC: understanding QCD-background as basis for each physics analysis Gentner Day at CERN

4. Minimum Bias Measurments dNch/dη at η = 0 • Physicsprocessesaresoftinteractions: havelow Q2 (smallmomentumtransfer) • perturbative QCD is not applicableforsmall Q2 : differential cross-sectiondivergesforpT→ 0 → employphenomenologicalmodelstodescribe soft QCD → Monte-Carlo predictionsat LHC energiesarehighlyuncertain! CERN-OPEN-2008-020 pseudorapidity η = -ln(tan(θ/2)) 900 GeV 7 TeV 14 TeV firstmeasurements in ATLAS: kinematicspectraofcharged particlesdNch/dpTanddNch/dη Gentner Day at CERN

5. Minimum Bias Events • inelasticcross-sectioncanbe sub-dividedintoitssinglecontributions • predictionstronglydepend on thetheoretical model, hereshownforpythia 6.214 (phojet 1.12) at 14 TeV: • dominant: non-diffractiveevents • precisedefinitionofminimumbiasevents:givenbythetriggersystem! σndiff≈ 55 (68) mb σsdiff≈ 14 (11) mb σddiff≈ 10 (4) mb σcdiff≈ -/- (1) mb single-diffractivedissociation (sdiff) double diffractivedissociation (ddiff) central-diffractive dissociation (cdiff) non-diffractive dissociation (ndiff) Gentner Day at CERN

6. Event TopologiesPythia Generator Level at 14 TeV chargedstable primaryparticles pseudorapidity diffractiveeventsaremore forwarddirected transversemomentum ndiff sdiff ddiff ATLAS – work in progress ndiff sdiff ddiff ATLAS – work in progress trackmultiplicity mostlylow-pttracksfrom allinelasticprocesses ATLAS – work in progress ndiff sdiff ddiff lowermultiplicity in diffractiveevents (moreforward) Gentner Day at CERN

7. Minimum Bias Trigger ATLAS has 2 independentminimumbiastriggers: • Minimum Bias • Trigger Scintillators (MBTS) • relative simple detectorsystem: 16 scintillatorcounters per sidebetweenInnerDetector (trackingsystem) andLAr-calorimeter • cover 2.1< |η| < 3.8 • will beusedforcommissioning sub-detectorsystems • has a shortlifetime due toradiationdamages: 3-4 months at a luminosity of 5*1032 cm-2s-1 Gentner Day at CERN

8. Minimum Bias Trigger ATLAS has 2 independentminimumbiastriggers: • Minimum Bias • Trigger Scintillators (MBTS) • relative simple detectorsystem: 16 scintillatorcounters per sidebetweenInnerDetector (trackingsystem) andLAr-calorimeter • cover 2.1< |η| < 3.8 • will beusedforcommissioning sub-detectorsystems • has a shortlifetime due toradiationdamages: 3-4 months at a luminosity of 5*1032 cm-2s-1 • InnerDetectorbased • Minimum Bias Trigger • provide a complementaryapproach • cover centraldetectorregion |η| < 2.5 • triggerrequirements: • introduceaslittlebiasaspossible • suppressefficientlyemptybunch-crossingand beam-backgroundevents • → mywork! Gentner Day at CERN

9. Minimum Bias Trigger ATLAS has 2 independentminimumbiastriggers: • InnerDetectorbased • Minimum Bias Trigger • provide a complementaryapproach • cover centraldetectorregion |η| < 2.5 • triggerrequirements: • introduceaslittlebiasaspossible • suppressefficientlyemptybunch-crossingand beam-backgroundevents • → mywork! transitionradiation tracker (TRT) silicon strip detector (SCT) pixel detector Gentner Day at CERN

10. The ATLAS Trigger System • The systemhasbeendesignedtocopewith ~ 1.5 PB data per second • Itfollowstheconceptof an „earlyrejection“ → 3 triggerlevels • L1: hardware + internaltrigger • L1 latency 2.5 μs • L2: softwaretrigger • reconstructsphysicsobjects foundby L1, tracks. • L2 latency 10 ms • EF: softwaretrigger • usesmoredetailedcalibrationsconstantsas L2, else same as L2 • EF latency ~ 1 second Generally „region-of-interests“, RoI‘s, areprocessed –unlike in minimumbiastriggers! Gentner Day at CERN

11. ID based Minimum Bias Trigger • L1:randomtriggeron filledbunches → avoidsbias • L2:SpacePointCounter • forms 3d-detector hits in wholepixeland SCT (spacepoints) detector • operate just abovenoiselevel→ suppressemptybunch-crossingevents • needstobe robust against HV trips→ dynamicmaskingofnoisymodules • EF: TrackCounter • reconstructsiliconlow-pt-tracks • requireonetrackclosetothe IP → suppress beam-gas/-haloevents • Features/Status • algorithmsarehighlyconfigurable→ providesflexibilitytoreact on different beam backgroundconditions • a back-uptriggerusingonly TRT hitsat L2 hasbeendeveloped (TRT provedtobeverystableduringfirst beam operations) • all minbiastriggershavealreadytakenand still take (cosmic) data! Gentner Day at CERN

12. Performance Studies • triggerefficiencyat L2 • An SCT spacepointsisformedby a coincidenceof 2 strips • → intrinsicnoisesuppression SCT spacepoints ATLAS – work in progress ndiff sdiff ddiff empty ATLAS – work in progress cutat 3 SCT spacepointsat L2 suppresses all emptyevents Gentner Day at CERN

13. Performance Studies • triggerefficiencyat L2 • triggerefficiencyat EF ATLAS – work in progress ATLAS – work in progress cutat 3 SCT spacepointsat L2 + additional cutat 1 trackwith |z0| < 200 mm at EF cutat 3 SCT spacepointsat L2 suppresses all emptyevents Gentner Day at CERN

14. Performance Studies • triggerbias → change in distribution (e.g. η, pT, Nch) whichisintroducedbythetriggercondition (hereshownfor 3 SCT spacepoints) ID ndiff sdiff ddiff ndiff sdiff ddiff ATLAS – work in progress ATLAS – work in progress nobiasexpected in η (measurewithin ID range) morestudiesneededfor multiplicitydependence Gentner Day at CERN

15. Summary and Outlook • minimumbiasstudiesareneededto understand propertiesofinelasticinteractions→ they form dominant QCD background in LHC enviroment • minimumbiasevents will form pile-upathighluminosityruns → studyatlowluminosities, designed an interactiontrigger • Thesis plan (adaptedtonew LHC schedule) → preparefordetectorandtriggercommissioningwithminimumbiastriggers • moreperformancestudiesongoing→ will determinetheemploymentof (all) minbiastriggers • functionalityprovided, lookingforwardforcollsions! Gentner Day at CERN

16. Danksagung Manythankstomysupervisors Nick Ellis (CERN) and Hermann Kolanoski (Humboldt-University) andmyformersupervisor Klaus Mönig (DESY) as well astomy ATLAS colleguesand CMS friends andthe BMBF forcreatingthe Wolfgang-Genter-Program! Gentner Day at CERN