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B physics in LHCb

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  1. B physics in LHCb Introduction: LHCb physics The experimental challenge Vertex reconstruction Particle ID Trigger Flavour tagging Systematic effects Hugo Ruiz – Winter meeting 2007 - Santiago

  2. Introduction

  3. CPV in the SM • SM introduces CP violation through: • the CKM matrix is complex • phases switch sign under CP: VijVij* • After requiring unitarity and removing non-physical phases, the values of Vij are no longer independent • 3 magnitudes + 1 phase fix the matrix: 4 parameters • All measurements related with electroweak quark transitions are coherent with the CKM picture of the SM: • BR, Dm and phases measured fit within a set of values of the 4 CKM parameters • In particular, unitarity triangle closes gently l = 0.2240±0.0036 A = 0.83±0.02 r = 0.168±0.029 h = 0.340±0.017 Wolfenstein parameterization, values from UTFit Hugo Ruiz – Winter Meeting Santiago 2007

  4. New Physics in B decays? • But there is room for NP in B meson decays • And after all, the CKM picture of CPV does not account for the presence of matter in the Universe… • Some quantities very sensitive to NP are yet to be measured or lacking precise measurement • Four examples accessible to LHCb in this talk: • c  arg(Vts)-pvia phase of Bs mixing • CKM fit prediction is very precise • g -arg(Vub) from tree decays • Tree processes assumed free of NP • Comparison with measurements from loop processes can reveal NP • Branching ratios of rare decays • Expect large contributions from NP models which fit rest of data • Angular distributions • Sensitive to non-SM operators in interactions Hugo Ruiz – Winter Meeting Santiago 2007

  5. 1. fs: current status Vts • Prediction from a global fit to CKM measurements (UT fit): fs= -0.037± 0.002 • Very small, so very sensitive to NP! • Recent D0 measurement: fs= -0.79±0.56(stat)+0.14-0.01(syst) • Note: no Bs produced in B factories • D0 used golden channel Bs→J/y(m+m-)f(K+K-) • The diagram for Bs oscillation in the SM is • The phase of the oscillation in the SM is given by: • fsSM -2  arg (Vts) -2c = -2l2h up to o(l6) Vts* Hugo Ruiz – Winter Meeting Santiago 2007

  6. 1. fs: The golden channel _ Bs0 • Bs(Bs)→J/y(m+m-)f(K+K-) can proceed directly or through mixing 2·arg(Vts) is only weak phase Bs0 Bs0 • Lifetime distributions of events with a Bs (Bs) at production show oscillation pattern • Oscillation phase is opposite and different phase in time evolution  ACP appears _ Tagged Bs Tagged Bs All experimental effects simulated hf = +, - 1 CP eigenstates Proper time (ps) Need flavour tagging Strong requirement on vertexing Hugo Ruiz – Winter Meeting Santiago 2007

  7. 1. fs: expectations in LHCb From Z. Ligeti et al hep-ph/0604112 Allowed regions CL > 0.90, 0.32, 0.05 • One nominal LHCb year (2 fb-1): • BR=3·10-5 3K events • s(fs)= 0.023 ( UT fit value: -0.037) • The measurement can be interpreted via a parameterization of NP effects: M12 = (1 + hse 2iss) MSM12 • MSM12= dispersive part of the BS mixing amplitude in the SM • Then Dms and fs can be used to constrain NP in the oscillation: 180o 2006, After first Dms measurement 90o ss Allowed region 0o 0.5 1.5 2.5 hs 180o fs= 0.04±0.03 90o ss LHCb, L=2fb-1 0o 0.1 0.3 0.5 hs 7 Hugo Ruiz – Winter Meeting Santiago 2007

  8. 2. g from tree processes • garg of (Vub)to o(l4) • Measurement (tree dacays only): g = (83 ± 19)o • From global fit of CKM params. (incl. loop processes!): (64.1 ± 4.6)o • One promising method for tree determination: measure BR of B- (K+p-)DK- and the charge-conjugated process • 2 diagrams (via D0 and via D0) contributing with similar amplitudes  large interference effects hence large CPV in the decay • No flavour tagging needed • No need to measure B lifetime   _ g arg(Vub) is the only weak phase involved 8 Hugo Ruiz – Winter Meeting Santiago 2007

  9. 2. g from tree processes ? ? • Buttoo manyunknowns: relative amplitudes and strong phases of each B, D decay ? ? • Need to measure BRs from additional D decay modes to extract g (ADS, GLW…) • Current status: B- (K+p-)DK- not yet seen in B factories • Present g sensitivity from B decays with higher BR, but less sensitivity • With LHCb 10fb-1, expected 3.5 K events • B+: 2500 with B/S ~1.5, B-: 1000 with B/S ~ 4.5* • sg = 3.6o(2.4o if combined with other tree decays) • Alternative method from ACP of BdK+K-, Bs p+p- with penguin contribution (NP?) gives sg= 4o Strong requirement on PId Hugo Ruiz – Winter Meeting Santiago 2007

  10. 2. g from tree processes  • Status of g from tree processes now and in ~ 2013: Current from tree processes only g From BDK, Tree process, LHCb10 fb-1  g g From UT fit, quantities affected by loops |Vub/Vcb| from semileptonic BRs NP! 10 10 10 31/05/2007 31/05/2007 This is what we know about CKM if we suspect NP in loop processes! Hugo Ruiz – Winter Meeting Santiago 2007

  11. 3. BR of Bsm+m- 5 BR (x10-9) SM prediction 3 Integrated Luminosity (fb-1) SM • Occurs via loops: • Small BR in SM: (3.4 ± 0.4) x 10-9 • Sensitive to NP! • Strongly enhanced by some SUSY models • Ex: up to x100 by CMSSM with parameters ‘preferred’ by anomalous m mag. moment in BNL. • Limit from Tevatron at 90% CL: • Current (1 fb-1)< 7·10-8 • Expected final (8 fb-1): < 2·10-8 • ~ x10 higher than SM! MSSM LHCb Sensitivity (signal+bkg is observed) LHCb: with L=2fb-1, 3s observation if SM value Yesterday’s talk by Diego Martinez

  12. 4. B0 K*0m+m- AFB, theory s = (m)2 [GeV2] b s SM • It is a rare decay: BR(SM) = (1.22+0.38-0.32) 10-6 • Decay seen in B factories, ~ no NP in BR • Angular distributions can reveal NP • Ex: AFB(s) between + and B direction in the +- rest-frame as a function of mmm2 d d Bd K* m g m AFB LHCb 2 fb-1: ~7k evtsB/S<0.5 s = (m)2 [GeV2] Hugo Ruiz – Winter Meeting Santiago 2007

  13. LHCb

  14. LHCb • The LHCb collaboration: • 619 scientists • 46 institutes • 14 countries • Spain: • Universidade de Santiago de Compostela • Universitat de Barcelona - Universitat Ramon Llull Hugo Ruiz – Winter Meeting Santiago 2007

  15. Detector overview Muon System RICHES: PID: K, separation VELO: primary vertex impact parameter displaced vertex PileUp System Interaction region Calorimeters: PID: e,, 0 Trigger Tracker: p for trigger and Ksreco Tracking Stations: p of charged particles Hugo Ruiz – Winter Meeting Santiago 2007

  16. A single-arm spectrometer? • (B), rad • (B), rad _ _ Direction of bb pairs: Detector acceptances: Pythiasbb • Within LHCb acceptance: ~ 1012 b hadrons per year • ~104 more than in B factories… but some disadvantages too pT (GeV) angle to beam _ h b and b very close in direction, important for flavour tagging! Hugo Ruiz – Winter Meeting Santiago 2007

  17. Experimental issues A typical bb event: high pT, IP tracks • Trigger • Hard work for hadron final states • Mass resolution: • Distinguish Bs from Bd • Reduce combinatorial bckgrd • Flavour tagging • B flavour at start of oscillation Dh~0.7 1 fm <L> ~ 8 mm Primary Vertex (PV): pp interaction Dh~1.4 B meson 1 IP B meson 2 • Vertex reconstruction: • Selection of B candidates (IP of tracks & displaced vertex) • Measurement of B lifetime • Particle identification: • Background reduction • Distinguish Bdpp, Bs KK Hugo Ruiz – Winter Meeting Santiago 2007

  18. Vertex reconstruction

  19. Luminosity ~ 1 mm • Luminous region(within 1 sigma): • With nominal LHC lumi(1034 cm-2s-1): 23 interactions per bunch crossing  23 PVs • Difficult to find secondary vertices! • LHCb needs a lower luminosity: • Chosen to maximize the probability of a single interaction: 2 – 5 · 1032 cm-2s-1 • 50 times lower than LHC design lumi • LHCb will probably reach its ‘design luminosity’ before ATLAS and CMS 5 cm Num. of pp collisions LHCb ~ Maximum for detector radiation Hugo Ruiz – Winter Meeting Santiago 2007

  20. VErtexLOcator: VELO 21 stations 1 m Silicon sensors Interaction region R sensors R sensor: pitch: 38 μm - 103μm thickness: 300μm φ sensor: pitch: 39 μm - 98μm thickness: 300 μm f sensors 8mm Hugo Ruiz – Winter Meeting Santiago 2007

  21. VELO VELO sensors • The closest to the beam, the less extrapolation distances, and the better IP and vertex resolution • But cannot go too close: at injection beams are separated, and VELO has to provide enough aperture • Solution: retractable detector • At 3 cm at beginning of fill • Moved to 8 mm when stable beams declared • Even though lower lumi, higher dose than ATLAS and CMS pixel detectors • Will have to replace in a few years RF Foil Hugo Ruiz – Winter Meeting Santiago 2007

  22. IP resolution • For trigger, VELO R-sensors allow for a fast search of high IP tracks in 2-D: PV dIP≃ 14mm ± 35 mm/pT Signal B IP 1/pTdistribution for B tracks R z Hugo Ruiz – Winter Meeting Santiago 2007

  23. Secondary vertex resolution Relevant resolution to identify B and measure its lifetime PV resol (~70 VELO tracks): σz=47 μm, σx=8 μm Dz z=168 m Bs→DsK B lifetime: z coordinate: • Typical s: 37 fs(2p·Dms-1~350 fs) • ATLAS: 83 fs, CMS: 77 fs • CDF ~ 87 fsfully reco decays PRL 242003 (2006) Bs→J/yf Hugo Ruiz – Winter Meeting Santiago 2007

  24. Particle identification

  25. K-p separation • B physics require separation between final states with p and K • Best example: extraction of g from ACP in BsK+K- and Bd p+p-. What happens if we are p-K blind? • If all tracks considered to be pions: m(B0) = 5279 MeV m(Bs) = 5367 MeV

  26. RICH detectors • Ring Imaging Cherenkov detectors measure angle of Cherenkov emission, a function of velocity of particles • Different radiating materials separate p-K in different ranges of momentum • LHCb has 3 radiators in 2 different detectors Hugo Ruiz – Winter Meeting Santiago 2007

  27. RICHes RICH 1 structure: RICH2 in the pit: Typical event (RICH2): HPD arrays out of acceptance Hugo Ruiz – Winter Meeting Santiago 2007

  28. Performance of RICHes Kaon identification: • Effect on Bdp+p-: Plot obtained using MC truth Hugo Ruiz – Winter Meeting Santiago 2007

  29. Calibration of PId • PId performance will be calibrated without using MC • Use D*+D0 p+, D0 K-p+ • BR: 3.810-4 (K-p+~400 times smaller) • “Golden” kinematics: (mD* - mD0) = 144.5 MeV • Purity on PId of each track is > 99% without using the RICH, by: • Kinematical cuts and charges • Veto in muon chambers and ECAL • Dedicated D* line in trigger: • 300 Hz with 50Hz of signal • To compare tracks from this sample with any other only a pT binning is needed Example: 1GeV<pT<1.2 GeV e (KK) D* calibration sample (300 s of data) All tracks in bb events p (GeV) Hugo Ruiz – Winter Meeting Santiago 2007

  30. MASS RESOLUTION

  31. Momentum measurement • Momentum is measured from curvature of tracks between VELO and main tracking stations • The LHCb (warm) magnet: • ∫B dL = 4 Tm • Field reversal to reduce syst. effects on CP asymmetries

  32. Tracking stations Outer Tracker : 450 cm 595 cm • Occupancy • (in an arbitrary # of events): Inner tracker Silicon detector, 198m pitch Next talk by Pablo Vázquez Outer tracker 4 layers of straws (0o,-5o,5o,0o) each Track 5mm straws e- e- e- 3 stations (T1 –T3) pitch 5.25 mm e- e- Hugo Ruiz – Winter Meeting Santiago 2007

  33. Tracking performance • Typical B track (p>12 GeV): • 20-50 hits • 98.7% correctly assigned • Efficiency >95% • Ghost rate <7% • Typical bb event: 6 m Note 1-D missing! Hugo Ruiz – Winter Meeting Santiago 2007

  34. Momentum and mass resolution Momentum resolution: • Mass resolution: Bs m+m- sm=18 MeV Combinatorial background Arbitrary scale!! dp/p ≃0.35%–0.55% p distribution for B tracks • N combinatorial background  sm! • CMS: 36 MeV (ms have large p) • ATLAS: 77 MeV (lower ∫BdL) Resolution dominated by multiple scattering (over detector resolution) up to 80 GeV Hugo Ruiz – Winter Meeting Santiago 2007

  35. Trigger

  36. The LHC environment Trigger an important issue! Particles reconstructed • Relevant rates: • LHC: 40 MHz, 2 bunches full: 30 MHz • At least 2 tracks in acceptance 10 MHz • bb:100 KHz • Decay of one B in acceptance:15 KHz • relevant decays BR ~10-4 – 10-9 Hugo Ruiz – Winter Meeting Santiago 2007

  37. Trigger overview 10 MHz Calo+ Muon system L0: hightpT+ not too busy • On custom boards • Fully synchr. (40 MHz), 4ms latency Pileup system 1 MHz High Level Trigger (HLT) In PC farm with ~1800 CPUs Refine pT measurement + IP cuts Reconstruct in(ex)clusive decays Whole detector Full detector = full flexibility, but no time to process everything for every event! Average latency: 2 ms (ATLAS, CMS: ~ 100 Hz, 1Mb/evt) ~2KHz, ~35Kb/evt Hugo Ruiz – Winter Meeting Santiago 2007

  38. L0 ET triggers • Fast search for ‘high’ pT particles • Calorimeter: look for high ET candidates in three categories: e±, g and p0 • In regions of 2x2 cells • Particle identification from • ECAL / HCAL energy • PS and SPD information • Muons: • Straight line search in M2-M5 • Look for compatible hits in M1 • Momentum measurement 20% Scintillator Pad Detector (SPD) ECAL HCAL Pre-Shower Detector Interactionregion Hugo Ruiz – Winter Meeting Santiago 2007

  39. L0 performance • Bandwidth share: • Efficiency (off-line selected evts): LHCb only e ~ 50 % L0 is the bottle-neck for hadronic channels Hugo Ruiz – Winter Meeting Santiago 2007

  40. Trigger: HLT 1MHz Hadr. alley ECAL alley Muon alley Ex(in)clusive selections (~ relaxed offline selections) … 8KHz HLT 2KHz Disk Hugo Ruiz – Winter Meeting Santiago 2007

  41. Example: di-hadron alley • L0 hadron: 700 KHz • Reconstruct Velo, match to L0 object, IP cut (~75mm): 250 kHz (~2 cands.) • Reconstruct T tracker, match VELO track, pT>2GeV: 40 kHz (~1.2 cands.) • Select VELO tracks with IP forming good vertex with 1st candidate • Match them to T stations and cut at pT>1 GeV: 5-8 kHz (~1 cand. vertex) • Then enter ex(in)clusive selections (rate reduced by a factor 100)

  42. Bandwidth share Calibration and evaluation/reduction of systematics Hugo Ruiz – Winter Meeting Santiago 2007

  43. Trigger performance • Overall efficiencies (on offline reconstructed evts): Unbiased B • The inclusive B sample: • 900 Hz of B  mX, 550 Hz true • From the accompanying B meson: ~ 1.5109 fully contained, m-tagged and decay-unbiased B mesons / 2fb-1 • Tagging enhanced: eeff ~ 0.15 • This trigger only: factor of ~10higher yield in BB than B-factories for data mining (but worse backgrounds) PV Trigger m (BR xx) Hugo Ruiz – Winter Meeting Santiago 2007

  44. Flavour tagging

  45. Flavour tagging • Flavour tagging: determination of the flavour of the signal B at production • Needed for all measurements involving oscillations • At a hadroncollider, information can be obtained from: Selection High pT, low IP, close to signal B hadron from fragmentation or B** decay (K±, p±) Same side (SS) Signal B m±, e± PV High pT, high IP Opposite side (OS) Tagging B Dx K± Displaced vertex vertex charge (weighted on track pT)

  46. Wrong tags… • Flavour tagging algorithms are not perfect! • Backgrounds in tagger selections • The tagging B can oscillate incoherently (unlike in B-factories): • 40% B±,10% baryons: no oscillation  • 40% Bd:Dmd ~ Gd oscillated 17.5% • 10% Bs:Dms >> Gs  oscillated 50%  • Characterization of tagging algorithms: • etag: fraction of events with a tag • w  NW/(NW+NR): wrong tag fraction • eeff  etag(1-2w)2: effective tagging efficiency • Indicates the reduction in number of events that would account for the same statistical degradation as the fraction of wrong tags Average mixing probability: 13% Hugo Ruiz – Winter Meeting Santiago 2007

  47. Tagging performance • Typical performance for hadronic decays: Babar/BELLE: ~ 30% Hugo Ruiz – Winter Meeting Santiago 2007

  48. Systematic effects on tagging

  49. CP asymmetries and tagging ACPmeas = DtagDres ACPtrue •  is a first order correction to CP asymmetries: exp [-(m t)2/2], only relevant for Bs (1-2w) • dw dACP uncertainty on the physics parameter that we want to extract  we needsmall w anddw! • Required precision: imposing that dACPinduced by w is half the statistical error for 2fb-1: • fsfrom BsJ/yf:dw/w < 15% • g from BsDsK: dw/w < 2% • MC cannot be used to reach such calibration of w (unlike in B factories) because of several effects: • Uncertainty on bb production mechanism • Asymmetry in interaction with matter, B(*,**)hadron composition… Hugo Ruiz – Winter Meeting Santiago 2007

  50. Control channels • Idea: accumulate high statistics in flavour-specific modes • w can be extracted by: • B±: just comparing tagging with observed flavour • Bd and Bs: fitting known oscillation B/S~0.2–0.8 Hugo Ruiz – Winter Meeting Santiago 2007