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Eduardo Rodrigues NIKHEF Seminar at CPPM, Marseille, 4 th May 2007

LHCb: a stroll from Trigger to Physics. Eduardo Rodrigues NIKHEF Seminar at CPPM, Marseille, 4 th May 2007. Contents LHCb: the must know Detector and simulation Trigger system Reconstruction and PID Analysis: tagging and event selection Physics sensitivity studies.

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Eduardo Rodrigues NIKHEF Seminar at CPPM, Marseille, 4 th May 2007

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  1. LHCb: a stroll from Trigger to Physics Eduardo Rodrigues NIKHEF Seminar at CPPM, Marseille, 4th May 2007

  2. Contents • LHCb: the must know • Detector and simulation • Trigger system • Reconstruction and PID • Analysis: tagging and event selection • Physics sensitivity studies Disclaimer: no complete review of all aspects; emphasis put on personal work LHCb: a stroll from Trigger to Physics

  3. Jura Geneva 9 km diameter LHC CERN LHCb: a stroll from Trigger to Physics

  4. I. LHCb: the must know LHCb Goal: B-physics studies CP violation rare B-decays Acceptance: 1.8 < h < 4.9 Luminosity: 21032 cm-2 s-1 Nr of B’s / year: 1012 How forward is LHCb? • B hadrons mainly produced in forward region(s) • both B’s tend to be correlated • Unique forward spectrometer • at high energy! • Excellent tracking • Excellent PID LHCb: a stroll from Trigger to Physics

  5. LHC(b) environment • LHC environment • pp collisions at ECM = 14 TeV • tbunch = 25 ns  bunch crossing rate = 40 MHz • <L> = 2x1032 cm-2 s-1 @ LHCb interaction region  10-50 times lower than for ATLAS/CMS Cross sections Physical quantity Value Event rate Yield / year s total ~ 100 mb s visible ~ 60 mb ~ 12 MHz s (c-cbar) ~ 3.5 mb ~ 700 kHz ~ 7x1012 pairs s (b-bbar) ~ 0.5 mb ~ 100 kHz ~ 1012 pairs • Expected B-signal rates • branching ratios ~ 10-9 – 10-4  10 – 106 events / year ? B-hadrons are heavy and long-lived ! LHCb: a stroll from Trigger to Physics

  6. II. LHCb detector Ring Imaging Cherenkov Calorimeters 250/300 mrad Acceptance 10 mrad pp collision (side view) Muon System « Tracking » detectors LHCb: a stroll from Trigger to Physics

  7. LHCb cavern LHCb: a stroll from Trigger to Physics

  8. Simulation LHCb Monte Carlo simulation software: Pythia, EvtGen, GEANT4 and Gaudi-based reconstruction • Detailed detector and material description (GEANT) • Pattern recognition, trigger simulation and offline event selection • Implements detector inefficiencies, noise hits, effects of events from the previous bunch crossings LHCb: a stroll from Trigger to Physics

  9. III. The LHCb Trigger • Trigger overview and strategy • Level-0: first level trigger • HLT: High Level Trigger LHCb: a stroll from Trigger to Physics

  10. 12 MHz 1 MHz ~2 kHz Trigger overview pp collisions • custom hardware • high ET particles • partial detector information Level-0 • CPU farm -> software trigger • high ET / IP particles • full detector information HLT • event size ~35 kb Storage LHCb: a stroll from Trigger to Physics

  11. Trigger Strategy Be alert ! • Two-level Trigger • L0high ET / PT particles • hardware trigger, sub-detector specific implementation • pipelined operation, fixed latency of 4 ms • (minimum bias) rate reduction ~12 MHz -> 1 MHz • HLT:high ET/PT & high Impact Param. particles & displaced vertices & B-mass & … • algorithms run on large PC farm with ~1800 nodes • several trigger streams to exploit and refine L0 triggering information • software reconstruction on part/all of the data  tracking / vertexing with accuracy close to offline • selection and classification of interesting physics events  inclusive / exclusive streams • rate reduction 1 MHz -> 2 kHz • estimated event size ~ 30kb LHCb: a stroll from Trigger to Physics

  12. Level-0 Trigger strategy • select high ET / PT particles  hadrons / electrons / photons / p0’s / muons • reject complex / busy events  more difficult to reconstruct in HLT  take longer to reconstruct in HLT • reject empty events  uninteresting for future analysis L0 thresholds on ET / PT of candidates global event variables Pile-up system Calorimeter Muon system LHCb: a stroll from Trigger to Physics

  13. Scintillator Pad Detector (SPD) ECAL HCAL Pre-Shower Detector (PS) Level-0 sub-systems L0 Pile-up system: • 2 silicon planes upstream of nominal IP, part of the Vertex Locator (VELO) • Identifies multi-PV events: - 2-interactions crossings identified with efficiency ~60% and purity ~95% L0 Calorimeter : • Identify high-ET hadrons / e’s / g’s / p0’s • Electromagnetic and hadronic calorimeters - large energy deposits  ET in 2x2 cells • Scintillator Pad Detector (SPD) and Preshower (PS) - used for charged/electromagnetic nature of clusters, respectively L0 Muon System: • M1 – M5 muon stations (4 quadrants each) • σp/p ~ 20% for b-decays LHCb: a stroll from Trigger to Physics

  14. 1 MHz Level-0 Decision Unit Calorimeter • total ET in HCAL • SPD multiplicity • highest- ET candidates: h, e, g, 2 p0‘s Muon system • 2 m candidates per each of 4 quadrants Pile-up system • total multiplicity • # tracks in second vertex L0 Decision unit • cuts on global event variables • thresholds on the ET candidates L0DU report LHCb: a stroll from Trigger to Physics

  15. Global event cuts Cut Rate (MHz) S ET 5.0 GeV ~ 8.3 ~ 7 SPD multiplicity 280 hits ~ 13 Tracks in 2nd vertex 3 Pile-up multiplicity 112 hits Trigger Threshold (GeV) Approx. rate (kHz) Hadron 3.6 700 700 Electron 2.8 100 280 Photon 2.6 130 po local 4.5 110 po global 4.0 150 Muon 1.1 110 160 Di-muon 1.3 150 Level-0 bandwidth division Overall optimization given benchmark channels Global event variablesapplied first … Redundancy: Sub-triggers overlap … and then cuts on theET / PT candidates Decision Unit Variables Di-muon trigger is special - not subject to the global event selection - PTmm = PTm1 + PTm2 with PTm2 = 0 possible - “tags” clean B-signatures LHCb: a stroll from Trigger to Physics

  16. Level-0 performance studies Typical Level-0 performance Dedicated sub-triggers most relevant for each « channel type » Bs → Ds p B →pp B → J/Y(mm) Ks B → K*mm B → K*g B → J/Y(ee) Ks N.B.: efficiencies with respect to events selected offline LHCb: a stroll from Trigger to Physics

  17. ~2 kHz High Level Trigger Strategy Independent alleys: Follow the L0 triggered candidate Muon, Muon + Hadron, Hadron, ECal streams Partial Reconstruction: • A few tracks selected per alley (cuts e.g. on PT, Impact Parameter, mass) • full reconstruction done at the end of the alleys Summary Information: decision, type of trigger fired, info on what triggered LHCb: a stroll from Trigger to Physics

  18. Readout network Trigger Farm FE FE FE • Event Filter Farm with ~1800 nodes (estimated from 2005 Real-Time Trigger Challenge) • Sub-divided in 50 sub-farms • Readout from Level-0 at 1 MHz  50 Gb/s throughput • Scalable design  possible upgrade L0 trigger Timing and Fast Control CPU CPU CPU CPU CPU permanent storage HLT algos CPU time tested on a real farn  will fit in the size of the farm foreseen LHCb: a stroll from Trigger to Physics

  19. Muon Alley - Strategy Muon PreTrigger • Standalone m reconstruction:σp/p ~ 20% • VELO tracks reconstruction • Primary vertex reconstruction • Match VELO tracks and muons: σp/p ~ 5% L0-m Entry ~200 kHz Muon Pre-trigger ~20 kHz Muon Trigger • Tracking of VELO track candidates in the downstream T stations: σp/p ~ 1% • Refine m identification: match long (VELO-T) tracks and muons Muon Trigger ~1.8 kHz LHCb: a stroll from Trigger to Physics

  20. Muon Alley – Performance Muon PreTrigger • bμ~11% • Signal efficiency: ~88% ~20 kHz Muon Trigger • Single muon • PT> 3GeV and IPS> 3 • Bm content 60% • Dimuon • mass >0.5GeV and IP>100mm • J/y: mass>2.5GeV (no IP cut!) • Signal efficiency: ~87% < 1s of LHCb J/Ψ ~1.8 kHz dimuon mass (MeV) LHCb: a stroll from Trigger to Physics

  21. Bs→Dsp Bs→ DsK Exclusive Selections Exclusive selections • Use common available reconstructed and selected particles (Ds,D0, K*,Φ,..) • Wide B-mass windows (typically ~ 500 MeV) • Efficiency: e.g. ~90% for Bππ 4 tracks ~200 Hz Bs → fg f →KK Bs → ff p, K Ds→KKp Bs → Dsp Off-line B→ pp On-line B→ pp LHCb: a stroll from Trigger to Physics

  22. IV. Reconstruction and PID • Tracking: pattern recognition and fitting • Vertexing • RICH, Calo. and Muon reconstruction • Particle Identification (PID) LHCb: a stroll from Trigger to Physics

  23. Tracking strategy |B| (T) • A set of PR algorithms • Multi-pass strategy A typical event: • 26 long tracks  best quality for physics: good P & IP resolution • 11 upstream tracks  lower p, worse p resol., but useful in the RICH1 pattern recognition • 4 downtream tracks  improves Ks finding efficiency (good p resolution) • 5 seed/T tracks  used in the RICH2 pattern recognition • 26 VELO tracks  used in primary vertex reconstruction: good IP resolution B-field measured to ~ 0.03% precision! Z (cm) LHCb: a stroll from Trigger to Physics

  24. Pattern recognition - strategy VELO tracking (VELO seeds) Seeding (T seeds) Matching (long tracks) VELO-TT (upstream tracks) Forward tracking (long tracks) Ks tracking (downstream tracks) Clone killing Set of “best” tracks LHCb: a stroll from Trigger to Physics

  25. Pattern recognition - performance Performance on B-signal samples for “long” tracks: Efficiency ~ 95 % for p > 10 GeV Ghost rate ~16.7 % Dip due to beam-pipe material Efficiency Efficiency p (GeV) h LHCb: a stroll from Trigger to Physics

  26. Track fitting Tracks - pattern recognition attributes detector measurements to a track - modelled with straight-line segments, the (track) states Track fitting - determine the best track parameters and corresponding covariance matrix given the measurements on the track - take into account multiple scattering and energy losses in the detector material Description of a realistic detector with mis-alignments - introduction of trajectories to model “measurements” (concept adapted from BaBar) - trajectory = 3D curve in space (of arbitrary shape) - detector shapes locally described with trajectories - a track state (straight-line segment) can also be modeled as a trajectory - any trajectory can provide a parabolic approximation of its shape at any point along itself  closest approach track state – measurement becomes a general point-of-closest-approach problem! “banana-shaped” Outer Tracker modules LHCb: a stroll from Trigger to Physics

  27. Track fitting « à la Kalman » LHCb uses the Kalman filter technique: • Mathematically equivalent to least c2 method • Adds measurements recursively starting from a initial track estimate • Reconstructs tracks including multiple scattering and energy losses • We use a “bi-directional” fit … Bi-directional fit direction of the filter direction of the reverse filter track prediction filtered track track prediction filtered track Filter in both directions … smoothed result = weighted mean of both filters LHCb: a stroll from Trigger to Physics

  28. Fitting from a seed (track) state • Measurements (hit & error) • Seed State (position, direction & momentum) • Prediction (adaptive 5th order Runge-Kutta method solves motion in the inhomogeneous B) • Kalman Filtered (update prediction with Measurement info) LHCb: a stroll from Trigger to Physics

  29. Fitter: prediction and Kalman filter Velo TT IT&OT LHCb: a stroll from Trigger to Physics

  30. Fitter: Kalman smoother Velo TT IT&OT LHCb: a stroll from Trigger to Physics

  31. filtered state filtered state weighted mean Bi-directional fitting Velo TT IT&OT LHCb: a stroll from Trigger to Physics

  32. Track fitting performance Momentum resolution for “long” tracks dp/p = 0.35 – 0.55 % dp/p = 0.5% LHCb: a stroll from Trigger to Physics

  33. ,K Bs K K p Ds p  Primary vertex bt Vertexing performance Bs vertex resolution sz,core = 168 mm Primary vertex resolutions sx = 8 mm sz = 47 mm Typical resolutions LHCb: a stroll from Trigger to Physics

  34. RICH reconstruction RICH1 RICH2 Algorithm: • Input: all types of tracks available • Search for hits assuming different hypotheses • Global ring fit • Cherenkov angle resolutions in the range 0.6 – 1.8 mrad LHCb: a stroll from Trigger to Physics

  35. p identification K identification p e, m or p 100 100 p  p 80 80 K  K or p 60 60 K e, m, or p 40 40 20 20 pK or p K  p Particle Momentum (GeV/c) 0 0 Particle Momentum (GeV/c) RICH performace • Particle identification needed over momentum range 1-100 GeV/c • Ex.: need to distinguish Bd pp from other similar topology 2-body decays LHCb: a stroll from Trigger to Physics

  36. True Electrons backgrounds ECAL Magnet EPS(MeV) Calorimeter reconstruction (1/2) electrons: • Identification with e~95% based on - c2 of match calo. cluster energy – position of extrapolated track - Energy deposited in the preshower detector - Energy deposited in the hadronic calorimeter - Bremsstrahlung correction example LHCb: a stroll from Trigger to Physics

  37. Merged cluster Resolved cluster Bd→ π+π-π0 events Merged π0 Resolved π0 all π0s π0 from B all π0s π0 from B π0 mass (Mev/c²) π0 mass (Mev/c²) Calorimeter reconstruction (2/2) Neutral pions: • Resolved p0 : - reconstructed from isolated calo. photon clusters - mass resolution ~10 MeV, e~30% • Merged p0: - Several clustes merged into single (high energy) cluster - mass resolution ~15 MeV , e~20% LHCb: a stroll from Trigger to Physics

  38. eff ~ 94 % misid ~ 1.0 % Muon reconstruction • Extrapolate well-reconstructed tracks to the Muon system • Look for compatible hits in a “field of interest” around the extrapolated tracks Typical performances for b  J/Y(mm) X  ~ 10 MeV/c² em m ~ 94 % and ep  m ~ 1.0 % for tracks in Muon detector acceptance LHCb: a stroll from Trigger to Physics

  39. With PID With PID  invariant mass K invariant mass Particle Identification (PID) No PID Hadrons: • RICH1 and RICH2 Electrons & neutrals: • Calorimeter system Muons: • Muon system  invariant mass LHCb: a stroll from Trigger to Physics

  40. V. Physics analysis • Flavour tagging • Event selection & background estimation - example of the decay Bs Ds h LHCb: a stroll from Trigger to Physics

  41. Flavour tagging (1/2) • Purpose / need: • Determination of flavour of B-mesons at production (at primary vertex) • tagging gives (best estimate of) the flavour by examining the rest of the event • time-dependent analyses require flavour tagging • Quantifying the tagging performance : • tagging efficiency • wrong tag fraction • CP asymmetries diluted with dilution factor LHCb: a stroll from Trigger to Physics

  42. Dz/bgc = Dt K+ u u s s b b K - l + (e+, m+) Same-side tag l - (e-, m-) Uses flavour conservation in the hadronization around the Brec eD2  1% (B0) , 3% (Bs) eD2  5% Opposite side tag Assume: Flavour tagging (2/2) Bs0 rec t =0 Dt picoseconds after leaving the primary vertex, the reconstructed Bdecays. b-hadron PV LHCb: a stroll from Trigger to Physics

  43. ,K Bs K K Ds  Primary vertex bt Event selection: example of Bs Ds h (1/3) Bs→ Ds-p + Bs -> Ds∓ K± Requirements • Trigger on the B-decay of interest : high-PT tracks and displaced vertices • efficient trigger • Select the B-decay of interest while rejecting the background • good mass resolution • Tag the flavour of the B-decay • efficient tagging and good tagging power (small mistag rate) • Time-dependent measurements • good decay-time resolution LHCb: a stroll from Trigger to Physics

  44. Event selection: example of Bs Ds h (2/3) Bs→DsK : Bs reconstructed masssignal and main background Bs→Dsπ : Bs reconstructed masssignal and main background Bsmass resolution ~14 MeV LHCb note 2007-017 LHCb: a stroll from Trigger to Physics

  45. Event selection: example of Bs Ds h (3/3) (central values used for sensitivity studies) LHCb note 2007-017 LHCb: a stroll from Trigger to Physics

  46. VI. Physics sensitivity studiesThe example of Bs Ds h • Motivations • Sensitivity studies with RooFit LHCb: a stroll from Trigger to Physics

  47. Motivations (1/2) B-decay channels Bs→Ds-π+ and Bs→DsŦK± • Topology is very similar • Physics is different: • Bs→Ds-π+ can be used to measure Δms , ΔGs • CDF measurement Δms = 17.77 ± 0.1(stat.) ± 0.07(syst.) ps-1 • Bs→DsŦK± can be used to extract the CP angle g+fs • Standard Model prediction g ≈ 60° LHCb: a stroll from Trigger to Physics

  48. Motivations (2/2) Bs Dsp mode: • Flavour-specific decay (2 decay amplitudes only) Bs Ds K mode: • 4 decay amplitudes of interest: Bs, Bs -> Ds+K-, Ds-K+ -> 2 time-dependent asymmetries for the 2 possible final states • Ratio of amplitudes of order 1 -> large interference and asymmetry expected Bs→ Ds-p + Bs -> Ds∓ K± LHCb: a stroll from Trigger to Physics

  49. Physics sensitivity studies • LHCb Sensitivity studies • Measurement of g+fs with combined Bs Dsp and Bs  Ds K samples • Complete fit allows to obtain Dms, mistag rate, etc. • RooFit implementation for Toy MC: • Simultaneous Bs Dsp and Bs  Ds K fit: correlations taken into account • All decay rate equations included (see next page) • Using latest 2004 data challenge selection results (LHCb-note 2007-017) • Toy in propertime and mass includes: • propertime acceptance function (after trigger & selection) • per-event propertime error • smearing due to mis-tagging • background (rough estimate/description) • Fit with tagged events, and also with untagged Bs  Ds K events LHCb: a stroll from Trigger to Physics

  50. Decay rates with RooFit • Final state f : Ds-π+ or Ds-K+ • For charge conjugate final states: B → B , f → f , λf →λf , Af → Af p/q → q/p LHCb: a stroll from Trigger to Physics

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