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The LHCb experiment

The LHCb experiment. Walter Bonivento – I.N.F.N. Sezione di Cagliari - Italy. Why B physics at the LHC. At LHC start-up several precise measurements will be available from B-Factories and Tevatron to test the CKM paradigm of flavour structure and CP violation.

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The LHCb experiment

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  1. The LHCb experiment Walter Bonivento – I.N.F.N. Sezione di Cagliari - Italy The LHCb experiment

  2. Why B physics at the LHC At LHC start-up several precise measurements will be available from B-Factories and Tevatron to test the CKM paradigm of flavour structure and CP violation. However New Physics could still be hidden in mixing, in box and in penguin diagrams, realm of indirect discoveries. If NP will be found at LHC in direct searches, B Physics measurements will allow to understand its nature and flavour structure. The LHCb experiment

  3. Unitarity triangles • At the level of precision that will be probed by LHCb, there are two unitarity relations of the CKM matrix that are of interest: • Possible situation of the measurements when LHCb starts to take data: Differ at the percent levelphase of Vts b~240 g(SM)~650 χ(SM)~10  measurement of the angle g will be crucial The LHCb experiment

  4. Which B decays to measure the angles? a ~Vtd ~Vub* ~Vub* ~Vtd ~Vts g g b χ ~Vcb 2b+g g-2χ • and g g The LHCb experiment

  5. A complete program on B Physics includes: • Precise g determinations including from processes only at tree-level, in order to disentangle possible NP contributions • Several other measurements of CP phases in different channels for over-constraining the Unitarity Triangles BsDsK, B0D0K*0, B0pp & BsKK,… B0fKs, Bsff, ... B0rp, B0rr, … • Precise measurement of B0s-B0s mixing: Dms, DGs and phase fs. BsDsp, … BsJ/yf, BsJ/yh(’) • Search for effects of NP appearing in rare exclusive and inclusive B decays B0K*g, B0K*0l+l-, bsl+l-, Bsm+m-... The LHCb experiment

  6. K- few mm B0s K+ p p PYTHIA Why a forward detector for B physics at LHC • b and bbar are mostly produced at small angles wrt beam pipe AND correlated in one unit of rapidity  forward spectrometer to measure b decays and TAG them • large Lorenz boost large B meson average momentum ~ 80 GeV large average mean flight path ~cm  accurate measurement of proper time is possible (few % ) AND selection of B decays at TRIGGER level is possible • momentum distribution match particle ID capabilities of RICH detectors  ROOM is available for the detectors, contrary to cylindrical geometry • relatively low pT muon triggering possible because iron penetration depends here on pL which is large The LHCb experiment

  7. Detector requirements(I) Physics requirements(I) • Main constraint: the DELPHI cavern (20m) • Collision point in one side • Fixed target experiment design with dipole field magnet good analysing power for forward tracks • Acceptance: 250(300) mrad-10mrad defines the momentum range for the spectrometer and for particle ID Efficient particle identification : - p/K separation (1-->100 GeV) --> RICH ; also for flavor tagging, …) - electron and muon ID --> CALO + MUON (for B0(s) --> J/ψ X, flavour tagging, …) The LHCb experiment

  8. magnet Physics requirements(II) to measure the fast Bs oscillations,where A(mix)α cos(ΔmSτ), if ΔmS=20ps-1,  oscillation period is T=300fs  need a proper time resolution at least of σ(τ) ~< T/2π  σ(τ)/<τ>(1.5ps)~ few % But L= γβcτ = p/m cτ  σ(p)/<p> <few % and σ(L)/<L> <few % But average decay length ~7mm  need a vertex detector to measure it at the few % level (~200μm) exercise: try to reconstruct the argument arguing the path length in the lab frame in one oscillation period Background rejection mass (p and angular) resolution Rare decays with many tracks (up to 5) efficient tracking with low X0 (m.s. and γconversions) - tracker and magnet The LHCb experiment

  9. The experiment Single arm forward spectrometer 250/300 mrad v / h Acceptance 10 mrad pp collision side view The LHCb experiment

  10. Key issues • to avoid high number of interaction / bunch crossings : • L = 2 .1032cm-2s-1 for LHCb • --> simpler events (one interaction per bunch crossing dominates) and less radiation damage • for the detectors • σinelastic 80 mb and σbb 0.5 mb • --> need an efficient trigger (also on fully hadronic channels) • trigger strategy: • first level, hardware: large B mass large pT of B decay products; and selection of single interaction events • second level, software: large B lifetime  large impact parameters The LHCb experiment

  11. 100mb 230mb Comparison to other experiments 2007 LHCb 1y 2003 BdJ/yKS Bdpp • Enormous production rate at LHCb: ~ 1012 bb pairs per year much higher statistics than the current B factoriesBut more background from non-b events  challenging triggerand high energy  more primary tracks, tagging more difficult • But in addition, all b-hadron species are produced: B0, B+, Bs, Bc , Lb … • Only competition before LHC is from CDF+D0 (lower statistics, poorer PID) • ATLAS and CMS will only have lepton trigger, poor hadron identification BsJ/yf BsDsK σ(B) The LHCb experiment

  12. VELO  Vertex locator around the interaction region  Reconstruction of decay vertexes of b and c hadrons and IP for flavor tagging + fast response for L1 The LHCb experiment

  13. Δ02 IP r1 r2 track Δ01 VELO (II) • Design requirements and criteria: • Impact parameter resolution • L1 trigger fast stand alone patter recognition • MAIN IDEA: for B hadrons (IP)rz large but (IP)xy small the L1trigger first reconstructs in rz • and then in 3d ONLY the tracks with large IP •  strips with constant r and (in other sensors) radial strips with stereo angle of 10-200 multiple scattering in RF foils and detectors intrinsic resolution of the sensors small!!! • to have an equal contribtution from the 2 measured R points: σ2= σ1· r2/r1 strip pitch increasing linearly with radius small extrapolation factor The LHCb experiment

  14. Downstream Interaction region ~1m VELO(III) + some geometrical constraints: primary vertex σ(z)~5.6cm  ± 2 σ eta coverage required: ( 15-250mrad) - maximum wafer sizes 100mm - minimum safe radius 8mm • 21 silicon tracking stations placed along the beam direction • 2 retractable detector halves for beam injection periods (up to 30 mm) • an average track crosses 7 stations while <0.1% crosses <4 stations 21 stations Retractable detector halves The LHCb experiment

  15. VELO(IV) from simulation • up to 3GeV/c it is • multiple scattering • dominated! • lop p tracks limit the L1 performance!! 30 μm x=5% of X0 σ=8μm The LHCb experiment

  16. R-measuring sensor: (concentric strips) F–measuring sensor: (Radial strips with a stereo angle) VELO Sensor design • 2 sensor types: R and F • R measuring gives radial position • F measuring gives an approximate azimuthal angle • Varying strip pitch • 40 to 102 mm (R – sensor) • 36 to 97 mm (F – sensor) • First active silicon strip is 8.2 mm from the beam line • n+-on-n DOFZ silicon • minimises resolution and signal loss after type inversion • the high field side is always on the strip side in order to prevent loss of resolution and signal • Double metal layer for detector readout The LHCb experiment

  17. VELO in the Vacuum Double sided modules (1 x R and 1 x F sensor) 16 Beetle chips Silicon Sensor TPG* substrate with carbon fibre frame Secondary vacuum Chamber Retracting Detector Half Cooling contacts Carbon fibre paddle Silicon operating temperature -7oC The LHCb experiment *Thermalised Pyrolytic Graphite

  18. Illustration of Vdep … Vdep R/cm VELO environment • VELO sensors operate in a harsh non-uniform radiation environment • fluence to inner regions 1.3 x 1014neq./cm2 • fluence to outer regions 5 x 1012neq./cm2 • Estimated to survive 3 years The LHCb experiment

  19. Tracking system  Tracking system and dipole magnet to measure angles and momenta The LHCb experiment

  20. The spectrometer and the magnet x A particle of (pX, pY ,pZ) transversing (0,BY,0) receives a momentum kick of ΔpX=-e∫ BY dz and p=ΔpX/(sin αIN - sin αOUT) QUESTION: how to get pT? Then σP/p = 2* (σX/L)·p/ (e∫ BY dz) with L the lever arm (Kleiknecht, Phys Rep,84, pp 85-161(1982)) .( σP/p )MS ~√x/X0,independent of pminimise material!! To achieve σP/p ~0.5% at 100GeV/c, assuming some σX =100μ of detector point resolution, L~2.5m a bending power of ~4Tm is needed warm magnet: 2 Al coils + iron yoke excitation current : 2x2MA power dissipation: 4.2MW L(coil)=2H !!! but easy ramp up and possibility to revert the field to check systematics on B asymmetries… z αIN BY y z BY The LHCb experiment

  21. OT IT TT T3 T2 T1 The tracking chambers straw (=cannuccia) tubes; 5mm cell diameter Ar/CO2; light matrix nomex; light wrapping (Al) 4 layers/station (2 stereo) occupancy<7% type inversion NOT of concern here!! 4 layers/station (2 stereo) Cdet~50pF 1.3% of the area but 20% of the particles!!!! occupancy<0.5% The LHCb experiment

  22. Track reconstruction (I) TT T1-T3 VELO TT • reconstructed tracks • 72 on average in bb event : 26 long 11 upstream 4 downstream 26 VELO 5 T In BJKs 25% of Ks decay in the VELO acceptance 50% before the TT 25% downstream of TT The LHCb experiment

  23. Track reconstruction(II) • Example of reconstruction strategy: • for Long tracks • FORWARD TRACKING (90% of long tracks) • start from a VELO seed (straight lines, low B field, NO p information) • combined with T-seed (parabola, B information) • search for TT hits • BACKWARD TRACKING • from remaining T hits extrapolate back to VELO • all tracks refitted with Kalman filter (dowstream to upstream) The LHCb experiment

  24. Track reconstruction(III) Long tracks 98.7% of hits correctly assigned!! 13.3 VELO, 17(22) IT(OT), 4 TT The LHCb experiment

  25. Track reconstruction(IV) Long tracks Ks reconstruction in BJKs DD σ=4MeV LL σ LU multiple scattering dominated up to 100GeV ε=55-75% The LHCb experiment

  26. RICH  Two RICH detectors for charged hadron identification The LHCb experiment

  27. photon detectors radiator gas (n) mirrors beam pipe RICH (II) The LHCb experiment

  28. charged particle C RICH (III) if v>c/n or β>1/n The LHCb experiment

  29. charged particle C RICH (IV) particle mass! The LHCb experiment

  30. 2 RICH, 3 Radiators 1m 2m • RICH1upstream of the magnet • Aerogel (2 - ~10 GeV/c); n=1.03 • C4F10 (10 -~60 GeV/c); n=1.0014 • RICH2 downstream of the magnet • CF4 (16 – 100 GeV/c); n=1.0005 for low n needs a longer path for the charged particle The LHCb experiment

  31. Typical event Question: what are the Aerogel rings? The LHCb experiment

  32. Particle ID 3 radiators provide excellent pion/kaon separation ! The LHCb experiment

  33. Particle ID In BDsK Provide > 3s p–K separation for 3 < p < 80 GeV  / K separation Momentum (GeV/c) The LHCb experiment

  34. Particle ID In BDsK it is possible to tune the PID cut (efficiency/purity) depending on the specific physics analysis and for kaon/proton… The LHCb experiment

  35. Calorimeter system e h  Calorimeter system to identify electrons, hadrons and neutrals Important for the first level of the trigger The LHCb experiment

  36. Muon system m  Muon system to identify muons, also used in first level of trigger The LHCb experiment

  37. Trigger (I) • At LHC energies bbar events very similar to minimum bias except for 2 things: • high pT of decay products • detached secondary (and tertiary) vertexes The challenge: The 3 levels of the LHCb Trigger • Level-0 hardware trigger (10 MHz  1MHz ; 4μs latency) • Fully synchronous and pipe-lined (deadtime < 0.5%) • Pile-up System • Calorimeter and Muon high pTe, g, p0,m, or hadrons • Flexible L0 Decision unit • Level-1 software trigger (1MHz  40kHz ; max latency 1ms) • Partial read-out: Vertex Detector (VeLo), Trigger Tracker (TT) and L0 summary p info thanks to magnet fringe field!!! • High Level software trigger (HLT)(40kHz200Hzstorage; 10ms) • Full read-out: all detector data In 10 Mhz of crossings with visibile pp interaction 100kHz of bb pairs; only 15% will have one B with all decay products in the accepatance; and BR for CP violation are at 10-3 level!!! Common hardware The LHCb experiment

  38. Trigger (II) • How to determine the rejection level demanded by the L0? • Luminosity • L0 output rate • defines the minimum bias retention i.e. the rejection level The LHCb experiment ~ O(1) kHz

  39. Muon system and trigger(I) Triggering: OR of 5 stations minimum p of 5Gev (not pT!!!); rates varying from 100Hz/cm2 to 500kHz/cm2 higher than ATLAS or CMS Muon id. tagging and final state reconstruction high rate, high efficiency and ageing  MWPC and Triple-GEM for M1R1 Ar/CO2/CF4 gas mixtures track finding (straight line to IP) and pT calculation question to students: how is pt calculated from muon system alone? each station has a pad segmentation logical layout with F.O.I. (few pads in the bending plane..) The LHCb experiment

  40. Muon system and trigger (II) Main background: π and μ decays need to reduce by 50-100 standalone pT reconstruction ~ 20% trigger performance offline muon i.d. less efficient at low p due to multiple scattering and decays in flight The LHCb experiment

  41. Calorimeter system and trigger(I) In the muon trigger the signal dominates  the only parameter to control the trigger rate is pT For electron, completely different environment from the muons : background dominates!!! projective geometry: ECAL, SPD, PSD 4x4, 6x6 and 12x12 cm2 HCAL 13x13, 26x26 cm2 Preshower  e from π± (introduces a longitudinal segmentation in the calo) SPD  e from π0 irreducible background 12% of λI (suppressed at L1) The LHCb experiment

  42. Calorimeter system and trigger(II) R BJK 10% 25X0 Electro-magnetic: Shashlik type  1% , ε E WLS fibers pT P.M. e.g. DELPHI LumiMON performance of the electron trigger e.g. ATLAS Hadronic: iron-scintillating tiles with WLS fibers 80%  10% , 5.6λI E R Bπ π ε offline electron i.d. cluster  2x2 cells pT performance of the hadron trigger (essentially a pt cut)less efficient of e and μ The LHCb experiment

  43. PILE-UP VETO IP (95% of lumi) Why is it useful? vs cut vs luminosity The LHCb experiment

  44. HCAL trigger dominates MUON trigger dominates ECAL trigger dominates L0 performance less rejection OR from the other B! typically in one unit of rapidity The LHCb experiment

  45. L1 ingredients • Makes use of the • VELO 2D tracks IP • VELO+TT pT • L0 information The LHCb experiment

  46. Level-1 Decision Algorithm 1) generic algorithm (IP+pT of PT1 and PT2) + specific (level 0 signatures+ 3D track reconstruction ) Bandwidth division: Generic Single- muon Dimuon, general Dimuon, J/Psi Electron Photon Overlaps are absorbed in this direction The LHCb experiment

  47. Combined efficiency of L0 and L1 L0 efficiency L1 efficiency L0*L1 eff. The LHCb experiment

  48. Trigger Rates Overview HLT L1-confirmation HLT Full reconstruction Level-0 Level-1 5.6% 1% The LHCb experiment

  49. The Physics We concentrate here on few benchmark measurements driving the experiment design • B0s D-sπ+  ΔmS • B0d J/yKS  β • B0s J/yf χ and ΔΓS • B0s Ds K-+ γ The LHCb experiment

  50. Time-dependent decay rates B(BS) decay to a final state f: q/p=exp(-iφ)=exp(2iχ) (phase of Bs-Bsbar mixing) The LHCb experiment

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