Triggering at the Frontier: Top Quark Physics at ATLAS

Transcription

1 Triggering at the Frontier: Top Quark Physics at ATLAS Institutskolloquium Technisch Universität Dresden 23. April 2009 Martin zur Nedden, Humboldt-Universität zu Berlin 1

2 Content Open Questions The Large Hadron Collider LHC Accelerator Proton-Proton interactions The ATLAS Experiment Overview Trigger System: search for the needle in the haystack Top-Quark Physics at ATLAS Production: top-pairs and single top How to find events with top quarks Physics goals 2

3 Content Open Questions The Large Hadron Collider LHC Accelerator Proton-Proton interactions The ATLAS Experiment Overview Trigger System: search for the needle in the haystack Top-Quark Physics at ATLAS Production: top-pairs and single top How to find events with top quarks Physics goals 3

4 The Standard Model Standard Model (SM matter: 3 generations of fermions interactions: 4 gauge bosons interactions described by gauge field theories Strong: QCD: SU(3) Electroweak: GSW: SU(2) x U(1) Highly successful so far predicted existence of W,Z, and top quark tested with high precision But many open questions No description of Gravity No Grand Unification of Forces: Do we know all symmetries? Are there more than 3 generations of matter? What is the origin of matter (CP-Violation)? Neutrino Masses? Do we know all particles (supersymmetrie)? Universe mostly unknown (Dark matter and energy) Breacking of electro-weack symmetry: W and Z are massive, γ is massless What is the origin of mass? fermion generations: up charm top down strange bottom e μ τ ν e ν μ ν τ gauge bosons: W, Z, γ, g new scalar boson: Higgs 4

5 SM Higgs Mass Bounds Higgs mass limits: - LEP II: m H > GeV (95% CL) - EW precision measurements favours m H < 153 GeV (95% CL) Constraint of Higgs mass very sensitive on top-quark and W-Boson mass.. 5

6 Open Questions for Top Physics at LHC top quark discovered at TEVATRON in 1995 Production as pair (QCD) or as single top (EW) Top decays exclusively via t bw electro-weak decay relevant coupling determined by CKM matrix Mass of top-quark ~35 times bigger than b-quark mass: Close to electro weak symmetry breaking scale Is the top quark mass generated by Higgs-mechanism? Is the top quark mass related to Yukawa coupling or more fundamental? Large discovery potential for new Physics: Non standard couplings of top quarks in production and decay LHC is a top-factory: Millions of top-pairs in 10 fb -1 Expected cross section measurement expected with < 20% accuracy for first data 6

7 Content Open Questions The Large Hadron Collider LHC Accelerator Proton-Proton interactions The ATLAS Experiment Overview Trigger System: search for the needle in the haystack Top-Quark Physics at ATLAS Production: top-pairs and single top How to find events with top quarks Physics goals 7

8 The Research Centre CERN at Genf 8

9 The Large Hadron Collider LHCb CMS ATLAS ALICE 9

10 Detectors at LHC s = 14TeV 10

11 Installation of the LHC the magnets are - transported over large distances - connected - intensively tested 11

12 Tests of the Magnets Installation rate: approx. 20 dipoles per week Installation finished beginning of successful high current tests - incident during high current test (12000 A) at the last sector in September 2008 Dipol-connections tests Dipole installation in the tunnel 12

13 Incident September 2008 Major repair and upgrade of security and diagnostic tools caused a delay of 1 year During high current tests: Small resistance (~ nω) between two magnets leads to a quench liquid helium evaporated to fast security valve to small about 3 tons of He was blown into to tunnel 13

14 pp-interaction: Discoveries s : total center of mass energy of proton-proton interaction Interaction only between partons (quarks and gluons) effective Center of Mass Energy of colliding partons is smaller: s x1 x2 s s Necessary energy, to produce new particles with massen up to 1 TeV (x<1): s 1 1 1TeV x x TeV 14

15 Process of Proton-Proton-Scattering σ = ij f ( x, μ 2 ) f ( x, μ 2 ) σˆ ( p, p, μ 2, μ 2 ) dx dx i 1 F j 2 F ij 1 2 R F 1 2 inner structure of the proton: Structure function f i,j (x,q 2 Interaction of partons ) Structure functions: universal, can be taken from other experiments (HERA) P 1 P f i f j (x 1 ) (x 2 ) p 2 p 2 = x P = x P 22 σ ^ (p ij 1, p 2 ) Q _ Q Hadronization: formation of new particles factorization scale (μ F ): important for theoretical description 15

16 Expected Cross Sections at LHC large bb production cross section: ~ 500 µb (~ 1 bb pair per 100 p-p collisions) high tt production cross section: ~830 pb (~ tt pairs per year) LHC is a top-factory Extensive tests of the SM in QCD and EW Search potential for new physics (SUSY, BSM) Most important standard candle for detector calibration, commissioning and trigger optimization Discovery potential for Standard model Higgs Super symmetric particles Luminosity phases: 2009: about 100 pb -1 if a luminosity of cm -2 s -1 will be reached Design: cm -2 s -1 (~100 fb -1 per year) Initial energy in 2009: 10 TeV 16

17 Contence Open Questions The Large Hadron Collider LHC Accelerator Proton-Proton interactions The ATLAS Experiment Overview Trigger System: search for the needle in the haystack Top-Quark Physics at ATLAS Production: top-pairs and single top How to find events with top quarks Physcis goals 17

18 ATALS: Overview Main office and meeting building: ATLAS: AToroidal LHC ApparatuS Diameter Barrel toroid length Total length Totale weight 25 m 26 m 46 m 7000 t 18

19 Point 1: Location of ATLAS 19

20 ATLAS-Detector: Schematic View 20

21 Detector compontens: Myon-spectrometer: toroid with drift tubes Calorimeter: -hadronic - electro magnetic Tracking (inner detector): - transition radiation detector - silicon semiconducting detector Particle-Identification at ATLAS 21

22 ATLAS: Construction Beginn

23 ATLAS: Construction Implementation of the muon-toroid magnet coils 23

24 ATLAS Detector at 2008 Inserting of tracking detectors and calorimeters 24

25 ATLAS Detector in 2008 Installation of the muonend caps 25

26 Particle Collisions at LHC and Selection Bunch spacing: 7.5 m 25 ns Hz 10 5 Hz selection of 1 out of events Highly selective and efficient trigger system needed 26

27 Looking for Interesting Events H ZZ μμee event on top of 23 additional overlaid interactions 27

28 Data Acquisition Restrictions adequate precision need small granularity detectors: many readout-channels A) Totally 140 million channels event size ~ 1.5 MB at 40 MHz: 1 PB/sec available bandwidth: 300 MB/sec storage rate: ~ 200 Hz: 3 PB/year for offline analysis B) Read-out takes time dead time of about 10 ms If read-out would be triggered just by randomly available events, probability to get events with rates of 10-5 Hz would be 0! 28

29 Requirements for the Trigger System σ interaction-rate storage-rate Background rate interactionrate: ~ 1 GHz bunchcrossingrate: 40 MHz storagerate: ~ 200 Hz online -reduction: % powerful and reliable trigger inevitable: discoveries selection of rare events out of the extremely high background LHC environment: Physics-Trigger: - high p T ( un-prescaled ) - low p T ( prescaled, excl.) E T Technical Trigger: - monitoring and calibration trigger 29

30 Trigger Concept von ATLAS Level 1: reduction from 1GHz to 75 khz (2.5 μs) triggering on (high) p T -objects L1-Calo and L1-Muon sends Regions of Interest (RoI) to LVL2 for e/γ/τ/jet/μ candidates for a certain energy threshold pure hardware-trigger, larger granularity, synchronuos to LHC bunch structure Level 2: from 75 khz to 1kHz (10 ms) usesl1-regions of Interest as seed of the reconstruction (full granularity) only data within the RoI are used: small data transfer: only ~2% of total event data combination of different detectorinformation within the RoIs. software-trigger, readout after L2-aceptance Event-Filter: from 1kHz to 200 Hz (1s) full event information, quasi- offline -algorithms pure software-trigger (high felexibility) 30

31 ATLAS Multi-Level-Trigger HLT <2.5 μs ~1-2 khz out ~10 ms ~100 Hz out ~1 s LEVEL 1 hard ware based: FPGAs, ASICs uses larger granularity of the calorimeter and muon information identify Regions of Interest for further processing reduction from 1 GHz to 75 khz latency of 2.2 μs LEVEL 2 full granularity within the RoI seeded by LVL1-trigger fast reconstruction only data within RoI processed combination of detectors within RoI reduction from 75 khz to 1 khz execution time of ~ 40 ms EVENT FILTER seeded by level 2 full event information available full granularity of detectors offline like algorithms reduction from 1kHz to 200 Hz averaged execution time of 4 s HLT (LVL2 + EF): software LVL1: hardware 31

32 LVL1 Trigger Overview Calorimeter trigger Pre-Processor (analogue E T ) Muon Barrel Trigger (RPC) Muon trigger Muon End-cap Trigger (TGC) Jet / Energy-sum Processor Cluster Processor (e/g, t/h) Muon-CTP Interface (MuCTPI) multiplicities of e/γ, τ/h, jet for 8 p T thresholds each; flags for ΣΕ Τ, ΣΕ Τ j, E T miss over thresholds Central Trigger Processor (CTP) multiplicities of μ for 6 p T thresholds L1A signal LVL1 Trigger-items: Calo-clusters or muon-candidates 32

33 HLT Selection Strategy LVL1-items are the start for HLT activity: step-wise processing and decision fast algorithms first increasing complexity of algorithms seeded reconstruction algorithms use results from previous steps initial seeds for LVL2 are LVL1 RoIs Chains can be split at beginning of new level example: di-electron trigger ATLAS trigger terminology: Trigger chain: whole decision sequence Trigger signature: intermediate result Trigger element: trigger object LVL2 confirms & refines LVL1 EF confirms & refines LVL2 Event read-out and building after LVL2 EF accept events according to physics selection early reject as soon a signature fails, all following connected chains at all levels are switched off 33

34 First Trigger Events Inner tracking system First beams in LHC (September 2008) Splash-events (beam-collimator collision) ATLAS event display 34

35 Cosmic Event Test measurements with cosmic muons: - good test for all detector (and trigger) systems in real time - important references for commissioning - successful tests from October to December

36 Triggering Top-Events Final state relevant for trigger: Pure hadronic decay (44%): 6 jets in final state semi leptonic decay (44%): 1 letpon and 1 neutrino, 4 jets Pure leptonic decay (11%): 2 leptons and 2 neutrinos, 2 jets Trigger objects: LVL1 EM trigger objects, example: e22i η < 2.5; Δη Δφ = Electron, muon, jet and E T miss trigger objects Rich topology of top events: large overlap between trigger signatures Good opportunity for trigger monitoring and efficiency determination e22i mu20 e22i top-pair sample single-top sample 36

37 Content Open Questions The Large Hadron Collider LHC Accelerator Proton-Proton interactions The ATLAS Experiment Overview Trigger System: search for the needle in the haystack Top-Quark Physics at ATLAS Production: top-pairs and single top How to find events with top quarks Physics goals 37

38 Quarks and Leptons Charge 0 masses in MeV -1 +2/3-1/3 38

39 Discovery of the Top-Quark 1995 at TEVATRON in proton antiproton interactions 39

40 Top-Quark Properties Mass measured at TEVATRON SM branching fraction Br(t Wb) ~ 100% Spin and charged still unmeasured! No t-hadrons: 25 1 τ t = 5 10 s < τ had = = Γ Λ t QCD 3 10 s

41 Top Pair Production and Decay Pair Production: QCD process σ tt 833± 100 pb at LHC gluon-interaction are dominating: ~ 90% of the cases Decay: electroweak process Final state: Hadronic decay: 2 additional Jets or Leptonic decay: Lepton + neutrino (E T miss ) Allways: 1 b-flavored Jet 41

42 Top-Decay Channels Top production at TEVATRON dominated by quark antiquark processes, At LHC by gluon-gluon interactions, decay channels are the same. 42

43 Top-Pair Reconstruction t W ± b W W ± ± qq ( Jet + lν ( l Jet) Lepton + E miss ) typical tagging signal (also for Trigger): lepton + missing energy only Jets in final state: dominant BG from QCD-multi-jet events t-mass reconstruction 43

44 Typical top Finger-Print: b-jets B hadrons are massive and long lived (cτ~460 μm) decay into lighter flavors with secondary vertices on same hemisphere as b-jet: Lxy > 0 Displaced tracks L xy > 0 Secondary vertex Tagged b jet Primary vertex Displaced tracks semileptonic decay: b c + l + υ l Jets with - impact parameter - relative transverse momentum Light Flavor Mistag Impact parameters and secondary vertices used to define b-weights for b-jet selection Prompt tracks Primary vertex L xy <0 Secondary vertex Tracks from light flavor jets can fake displaced vertex due to detector resolution ( Mistags ) mistag L xy distribution is symmetric; estimate from L xy < 0 Prompt tracks 44

45 Top Physics at ATLAS Masurement of top-quark mass - with m t = m jjb (m jj = m W ) - in semi-leptonic channel Measurement of top-properties - top charge reconstruction - polarisation studies - top-pair resonances Decay of top-quarks - branching ratios/copplings - rare decays: FNCN m t = ± 0.2GeV σ = 11.7 ± 0. 4GeV t g/γ/z q (q=u,c) ideal standard-candle top-pair resonance 5σ discovery potential - Detector calibration and commissioning, - Trigger optimization 45

46 Single Top Production Single top production is an electro weak process Source of single ~100% polarized top quarks: - test V-A structure of W-t-b vertex - access to the top quark spin Allows direct Measurement of CKM- Matrix Element V tb : - σ st / V tb 2 - indirect measurements: V tb ~0.99 (not yet well measured) -verify V ub2 + V cb2 + V tb2 = 1 46

47 Single-Top Physics Potential direct measurement of the coupling strength of t-w-b vertex: W-gluon-fusion: sensitive to modifications of couplings of the top-quarks to other SM-particles Wt: sensitive to FCNC W*: sensitive to excited W -bosons expected cross sections 90% lower as top-pair production at LHC W-gluon-fusion t-channel direct production, Wt-process W*-process s-channel σ NLO 246 ± 10 pb σ NLO 62 ± 1pb σ NLO 10 ± 1 pb 47

48 Conclusion Large challenges ahead of us Triggering to be established and to be understood Triggering the unknown Top physics at LHC is the key to Understanding the tools (detector and analysis frame work) Develop the experimental methods Test the Standard Model Constrain the Higgs Mass Enables sensitive test and search potentials for new physics 48

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