The ATLAS Experiment

1. The ATLAS Experiment Starting Operation (post 10 September)and Analysis of Boosted Top Events Try to focus less on technology and more on performance and physics

2. Outline of the seminar Strong personal bias: HLT tracking and boosted tops • ATLAS in Operation (>= 10 Sept.) • ATLAS Reminder • 10 September • After that… • HLT Tracking with Cosmics • Tracking in the trigger • Validation with cosmics • Analysis of boosted top event • New physics and boosted tops • Experimental challenge • Different methods • Conclusions and outlook q W q t b Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

3. ATLAS Reminder: Detector Muon Spectrometer: MDT, CSC, RPC, TGC Inner Detector: Pixel, SCT, TRT EM Calorimeter: LAr Hadron Calorimeter: Tile, LAr Magnets: Endcap/Barrel Toroid, Solenoid Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

4. ATLAS Reminder: Trigger-DAQ System Level 1: • Hardware trigger (40 MHz to 75 kHz) • Only based on Muons and Calorimeters • Store data in pipeline memory until L1 decision returns to the detector front-end Level 2: • Software trigger (75 kHz to 3.5 kHz) • Seeded by Regions of Interest from L1 • Uses data from detector fragments Event Filter (EF): • Software trigger (3.5 kHz to 200 Hz) • Uses built events • Can use (partly) same software as offline reconstruction Data (streamed according to trigger category) Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

5. 10 September: ATLAS Detector • A lot of detector work was focusing on being ready the 10th September • ATLAS was in a very close to complete state! • Muon system: (MDT, RPC, TGC) ON, at reduced HV • Calorimeter: LAr (-FCAL HV), Tile ON • Tracker: TRT on, SCT reduced HV, Pixel off • Aux: BCM, LUCID, MinBias Scint. (MBTS), Beam pickups (BPTX) • Trigger: L1 trigger processor, DAQ up and running, HLT available • (but used for streaming only) tertiary collimators 140 m BPTX 175 m Only about 10 shots expected! Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

6. 10 September: and ATLAS caught them (no beam had passed ATLAS before) ! • Splash Events: • Did rely on small radius triggers with well • defined cosmics timing (L1Calo, MBTS) • Of 11 splash events 9 were caught • Through-going beam: • Time in and trigger on BPTX signal • (the ATLAS timing reference) LHC bunch seen by the ATLAS BPTX Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

7. After 10 September Then the accident… (more details can be found for example in Lyn Evans talk at the 96th LHCC open session) • A relatively simple transformer repair gave a window of a few days to finish the tests for 5.5 TeV running of the last sector 3-4 (all other 7 sectors had passed without complications before the 10 September) • Approximate description of the event: • When ramping up the current to 9.3kA a defect in a bus-bar connection between two • magnets caused electrical resistance and a voltage which triggered the event • Within one second, an electrical arc developed and punctured the helium enclosure, • leading to release of helium into the insulation vacuum of the cryostat • This led to large pressure forces displacing dipoles in the subsectors affected from their • cold internal supports, knocked the Short Straight Section cryostats housing the • quadrupoles and vacuum barriers from their external support jacks and in some locations • breaking their anchors in the concrete floor of the tunnel • Estimated number of magnets to be removed for repair and cleaning is O(30) • Spare parts and manpower is available, however, details about what has to be done and how is still under investigation Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

8. Combined ATLAS Running • Left ATLAS only with cosmic muons, however, many studies of the • detector are still possible! • ATLAS was collecting cosmics data as one combined system until 3 Nov • Experience of operating the combined detector and data taking routines • (start/stop run, change configurations, shifter tasks…) • Collect large sample of ID tracks (especially for alignment studies) • Collect large sample of Muon spectrometer tracks • Timing in of the trigger-readout system • As well as cross-detector studies, debugging data and reconstruction…. • From November on, sub-system commissioning and various detector work Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

9. Combined Cosmics Data Collected and quite a lot of cosmics data has been recoded (ca 1 Pb) ! Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

10. Data Reconstruction and Distribution As a part of the combined running, the data have been reconstructed at the Tier-0 at CERN and the Event Summary Data (ESD) was distributed to the different Tier-1 The data can then be requested to be transferred to a specific Tier-2 or in many cases also analysis grid jobs were allowed to be submitted to the Tier-1 In addition a Central Analysis Facitlity (CAF) exist at CERN where one can access smaller amounts of RAW data, however, these jobs are normally managed centrally Raw data Tier-0 (CERN) Reco: ESD Tier-1 Reco: AOD Tier-1 Reco: AOD Tier-2 Keep: AOD Outflow of reconstructed data worked OK! (MC for analysis distributed in similar way) Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

11. HLT Tracking with Cosmics Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

12. Inner Detector 10 September: The SCT was running at reduced HV and the Pixel detector was OFF only due to safety reasons Combined runs with Cosmics: All detectors have been included in the same runs both w and w/o magnetic field A few module have been out for technical reasons Cooling problems experienced earlier seems to be under control [ATLAS Collaboration, JINST 3 S08003] Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

13. HLT Tracking with Cosmics • Tracking information only available in the high level trigger (L2 + EF) • L2: custom made track reconstruction algorithms (IDSCAN, SiTrack, TRTSegmentFinder) • EF: use the offline track reconstruction algorithms within the trigger framework • Tracking is requested within the algorithm sequence of specific trigger • “signatures”, e.g. e10 = EM7 + L2_e10 + EF_e10 • (L1 Calo) (Calo + Trk + Hypo) (Calo + Trk + Hypo) • Both L2 and EF • Perform tracking within x regions of interest (RoI) from previous trigger levels • Have several independent configurations for different physics triggers • (e.g. electron, muon…) • Combined cosmics running allow us to exercise the HLT tracking • with real data from the three detectors • in the real online HLT system • based on real L1 triggers (either any L1 or RPC) Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

14. ATLAS (New)Tracking • Will not go into details! • Main reconstruction sequence • (InSideOut tracking) • SCT Clustering • Pixel Clustering • SpacePoint formation • SP Seeded track finding • Ambiguity solver • TRT Extension • Extension Processor • But there are also a few others • available, e.g. TRT only [ATL-SOFT-PUB-2007-007] Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

15. EF Tracking • Example: t t event with four RoIs • With electron slice configuration • Inside out tracking • Within 0.3x0.3 window around • the RoIs • Track pT > 1 GeV • Cosmics is of course very • different from collision tracks • (very large impact parameters…) • We still use the reconstruction • for collisions at the EF in order • to test it as much as possible, • however, with a special relaxed • configuration Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

16. EF Tracking: Cosmics • Since the combined running was more • or less the first time to test the EF tracking, • it took quite a few iterations to get all • working • However, the difficult tracking conditions • have been a very good test environment • study the tracking performance and it • boundaries (away from the interaction point) • test what is needed from the monitoring • some problems would be hard to spot with • collisions • Both L2 and EF have by now reached a high • efficiency and general performance, but the • detailed studies have just started EF wrt Offline (precale of 4) EF wrt Offline Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

17. HLT Tracking with Cosmics Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

18. Analysis of Boosted Top Event and New Physics Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

19. Why Studying Boosted Top Events? • Many models of new physics predicts high-pT events with tops • However, the main context of high-pT tops is within models predicting • heavy resonances decaying into tops • Several examples: • Extra dimensions, e.g. GKK or gKK t t • Little Higgs, e.g. (heavy quark) U th or tZ • Technicolor, e.g. Z’ t t • Topcolor ? • Even SM t t production at the LHC will have a significant fraction • at high-pT. e.g. cross section with at least one top with pT > 1 TeV • tt = O(50 fb) • QCD = O(10 pb) Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

20. Personal Favorite: Extra Dimensions (of course!) Randall-Sundrum model non-factorizable geometry • Original model (SM on the brane): • SM fields on the TeV brane • Gravity in the bulk, but localized • at Planck brane • Would explain the weakness of gravity • Recent model (SM in bulk): • Only Higgs have to be localized near • TeV brane • Other SM fields in the bulk • Mass depends on location in the bulk • (near TeV brane imply large mass) • In addition, would explain the fermion • mass hierarchy (!) Gravity exponentially damped with distance (rc ||) away from Planck brane Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

21. Personal Favorite: Extra Dimensions • RS model with SM on the brane: • Predicts heavy resonance from the first KK • mode of the graviton • Graviton couples with equal strength to all • SM fields • RS model with SM in the bulk: • Lightest KK modes are the gluon and the • graviton (mG ~ 1.5 x mg) • The gKK and GKK couples differently to SM • particles depending on location in the bulk • Given by the overlap of their wavefunctions, • which favours particles close to the TeV • brane • Since the top is heaviest it is closest to the • TeV brane “Invisible” momentum modes  KK-mass tower Main search channel for RS model with SM in the bulk is a heavy resonance decaying to t t Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

22. Personal Favorite: Extra Dimensions Since this is a strong process, the cross section can be large Also, since all KK gauge bosons would primarily decay to tops the signatures do not become cleaner by searching for KK Z or W that decay into leptons LHC will produce enough gKK events to discover a resonance at several TeV BUT this is heavily depending on how well boosted tops can be identified [B.Lillie et al., hep-ph0701166] pT > 500 GeV Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

23. Boosted Tops: Experimental Challenge As the top pT increases above 500 GeV, the decay products are often too collimated for standard top reconstruction techniques to work Often all three partons are contained within the cone size of the ATLAS jet reconstruction and form so-called boosted top mono-jets q W Top mass Inside/outside cone q t b Parton shower Underlying event The reconstructed mass of the mono jet should be the top mass, but also get significant contributions from showering/UE (increase value) and from decay products falling outside of the cone (decrease value), both depend strongly on the cone size Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

24. Boosted Top: (G*) Events Example of boosted top event characteristics (process implemented in Pythia 8) Tops W b R between W and b R between W and b mG = 1 TeV Mean = 0.9 RMS = 0.6 mG = 2 TeV Mean = 0.5 RMS = 0.4 Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

25. Calorimeter and Jets • Inputs to Jet Reconstruction • Towers: Sum contributing cells in • x = 0.1x0.1 bins • Topological cell clusters: Sum up cells in 3D • based on the energy deposited wrt noise level • Or other four vectors, e.g. MC truth… • Jet Algorithms • Cone Jets (1 GeV seed): R = 0.4 or 0.7 • kT-Jets: R = 0.4 or 0.6 • Jet Calibration • Global (H1): Cells weighted according to MC • simulations of jets (algorithm specific) • Local: Calibrate individual cell clusters • (likely to be better, but need data analysis) • Initially H1Tower jets will be used, but in the long term one should be able to use LCtopoCluster to reconstruct jets at the analysis level Example: EM Calorimeter At least initially calorimeter granularity is constrained tox = 0.1x0.1 for jet reconstruction Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

26. Boosted Tops: Methods YSplitter[G. Brooijmans et al., ATL-PHYS-CONF-2008-008] In addition to the jet mass and pT, use the mass scale (aka YScale) at which the reconstructed jet resolves into two or more jets The YScales obtained during the jet reconstruction (YScale1-2, YScale2-3, Yscale3-4) will be stored in the analysis objects Due to the sub-structure of the top jet These should differ significantly from general QCD events In this study: (hadronic tops) Signal is Z’ with, m = 2/3 TeV, decaying to tops Using ATLAS (H1) Kt-0.6-Tower jets Top jets mt/2 QCD jets Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

27. Boosted Tops: Methods YSplitter Can then perform a multi-variate selection based on, pT, mass, Yscale1-2, YScale2-3, YScale3-4 Top jets mW/2 QCD jets Jet selection efficiency: (pT = 0.5 / 1.5 TeV) Top: 5.6% / 74% Bkg: 0.1% / 10% non-optimal only 2D cuts Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

28. Boosted Tops: Methods Sideband analysis wrt QCD background shape [G. Perez et al., arXiv:0810.0934] Calculated the (QCD) jet mass spectrum from first principles Fit this function to a signal free sideband region Search for significant top peak deviation wrt the QCD spectrum using a likelihoodratio method In this study: (hadronic tops) Signal is SM top events Selecting (SIS)Cone Jets with pT > 1 TeV and using R = 0.7 truth Jets with detector smearing Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

29. What about Tracking? Could tracking improve the granularity in order to resolve dense jets? Example: Top with pT = 1 TeV nr final state particles = O(100) Medium high track momentum Very rare to be larger than about 40 GeV Constituents pT of leading jet Top with pT = 1 TeV Jet R = 0.4 Mean = O(10 GeV) Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

30. Tracking Performance In general the tracking have both good spatial and momentum resolution Assuming charged pions in the top jet with an average pT of about 10-50 GeV But the track density is of course very high inside the top mono-jets Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

31. Tracking in High Density • Tracking have been studied mainly in the context • of b-tagging in high pT top events • [M. Vos et al., ATL-PHYS-CONF-2008-016] • Increasing ET implies higher track density • General characteristics • High reconstruction efficiency • Relatively many tracks • Definitely worth to investigate in further detail! • But much remains to be investigated in the • context of track based jet reconstruction • Tracking resolution • Fake rates • Final track and track-calo jet reconstruction • performance etc.. Pions in High pT Jets Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

32. Boosted Tops: Methods Given either the initial R granularity or with some improvements (either from the calorimeter, tracker or both) Could high-pT tops be identified by resolving an initial “big jet” in a similar way to the proposal for H to bb ? [J.Butterworth et al., arXiv:08082.2470] Boosted j0 j2 j1 • Schematic description of algorithm: • Reconstruct inclusive jets with a large R~1 (j0) • Resolve candidate jets into 2 exclusive sub-jets (ji=1-2) • Select as candidate if, • m0 >  x m1 (most massive sub-jets) • y > yCUT (y is a R related variable) • If not, remove the min pT sub-jet and iterate a fixed nr of times • If selected, reconstruct inclusively with small Rfilt value • and add three hardest sub-jets for the final mass Rely on a well separated mass hierarchy between J0 and the sub-jets (J1-2) Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

33. Mass Hierarchy Generator Level! J0 mass (R=1) J1 mass (most massive of 3 excl) Top jets, pT ~ 500 GeV (hadronic decays) J1 mass (most massive of 3 excl) J0 mass (R=1) QCD jets, pT ~ 500 GeV (di-jet events) For boosted tops the mass drop is not so different between signal and background (e.g. top pT ~ 1 TeV implies a mean mQCD > mtop) Difficult to separate top signal from QCD background using this (H to bb) procedure Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

34. New Version Within the H to bb approach, the quantities that seem to differ most between signal and background are top mass and R between leading subjets Top jets J0 mass Top jets R12 Their means are not so well separated, but they have fairly separated distributions Instead of using large mass steps, try to maximize resolution for mReco and R12 QCD jets J0 mass QCD jets R12 Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

35. New Version • New “Resolution” Algorithm: • Reconstruct jets with a large R0 (R0=1) • Reconstruct 3 exclusive jet from the J0 • to obtain R12 • Inclusive reconstruction within J0 with • small R1 (R1=0.1 here) and obtain top • mass (m1234) from 4 leading pT jets • R0 and R1 are tunable parameters • Try to: R1 R0 Minimise the QCD contribution by using a small R1 Minimised contribution by using sufficiently large R0 to catch the top constituents Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

36. Final Top Candidate Selection • Signal mass distribution not • improved, but background • distribution is significantly • pushed towards lower values • Top jet candidate selection: • j0 with pT > 500 GeV • Selection based on P value Top jets J0 mass Top jets m1234 QCD jets J0 mass QCD jets m1234 Works to some extent Since the main improvement come from suppressing the bkg, the procedure still seem more sensitive to R0 than R1 Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund

37. Conclusions • ATLAS is working very well as a combined system • Almost all sub-systems have been operated together during Cosmics data taking • and reconstructed data have been produced continuously • The whole ID has been used during Cosmic running and the tracking in the high • level trigger has been commissioned online • The tracking performance has reached a high level, but several detailed studies • have just started • When high energy collisions arrive a resonance in the top invariant mass spectrum • will be an important signature to look for new physics • In some cases, like the gKK process, only a modest amount of data might be • required for a discovery • It is, however, crucial to be able to reconstruct and identify boosted top jets • Several approaches have been studied with greatly improved results • However, there are still many options to be studied further Stefan Ask (ATLAS - Opertaion and Boosted Top Analysis) 27 Nov 2008, Lund